National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Padre Island National Seashore

Natural Resource Condition Assessment

Natural Resource Report NPS/PAIS/NRR—2014/747
ON THE COVER

Shorebirds resting on Malaquite Beach at Padre Island National Seashore.

Photograph by: Shannon Amberg, SMUMN GSS 
Padre Island National Seashore

Natural Resource Condition Assessment

Natural Resource Report NPS/PAIS/NRR—2014/747

Shannon Amberg

Andy Nadeau

Kathy Kilkus

Sarah Gardner

Barry Drazkowski

GeoSpatial Services

Saint Mary’s University of Minnesota

890 Prairie Island Road 

Winona, Minnesota 55987

January 2014

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, 

Colorado, publishes a range of reports that address natural resource topics. These reports are of 

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screen readers, please email irma@nps.gov. 

Please cite this publication as:

Amberg, S., A. Nadeau, K. Kilkus, S. Gardner, and B. Drazkowski. 2014. Padre Island National 

Seashore: Natural Resource Condition Assessment. Natural Resource Report NPS/PAIS/NRR—

2014/747. National Park Service, Fort Collins, Colorado.

NPS 613/123431, January 2014
iii

Contents 

Page

Figures............................................................................................................................................. v

Tables............................................................................................................................................. ix

Plates............................................................................................................................................ xiii

Photos............................................................................................................................................ xv

Appendices.................................................................................................................................. xvii

Executive Summary..................................................................................................................... xix

Acknowledgments........................................................................................................................ xxi

Acronyms and Abbreviations ..................................................................................................... xxii

Chapter 1 NRCA Background Information .................................................................................... 1

Chapter 2 Introduction and Resource Setting ................................................................................. 5

2.1 Introduction........................................................................................................................ 5

2.1.1 Enabling Legislation .................................................................................................. 5

2.1.2 Geographic Setting..................................................................................................... 5

2.1.3 Visitation Statistics.................................................................................................... 6

2.2 Natural Resources.............................................................................................................. 6

2.2.1 Ecological Units and Watersheds .............................................................................. 6

2.2.2 Resource Descriptions ............................................................................................... 7

2.2.3. Resource Issues Overview ........................................................................................ 9

2.3 Resource Stewardship...................................................................................................... 14

2.3.1 Management Directives and Planning Guidance..................................................... 14

2.3.2 Status of Supporting Science ................................................................................... 14

Literature Cited...................................................................................................................... 16

Chapter 3 Study Scoping and Design ........................................................................................... 19

3.1 Preliminary Scoping ........................................................................................................ 19

3.2 Study Design.................................................................................................................... 20

3.2.2 General Approach and Methods................................................................................... 25
iv

Contents (continued)

Page

Literature Cited...................................................................................................................... 30

Chapter 4 Natural Resource Conditions ....................................................................................... 31

4.1 Terrestrial Vegetative Communities................................................................................ 33

4.2 Algal Mats of Mud Flats (Wind Tidal Flats)................................................................... 49

4.3 Seagrass Community ....................................................................................................... 59

4.4 Emergent Wetland and Pond Communities..................................................................... 70

4.5 Migratory Bird Species.................................................................................................... 86

4.6 Resident Bird Species.................................................................................................... 109

4.7 Colonial Waterbirds....................................................................................................... 122

4.8 Coyotes.......................................................................................................................... 159

4.9 Small Mammals............................................................................................................. 166

4.10 Macroinvertebrates...................................................................................................... 176

4.11 Reptiles........................................................................................................................ 193

4.12 Sea Turtles................................................................................................................... 201

4.13 Amphibians.................................................................................................................. 218

4.14 Water Quality............................................................................................................... 227

4.15 Air Quality................................................................................................................... 256

4.16 Dark Night Skies.......................................................................................................... 269

4.17 Coastal Dunes and Beaches......................................................................................... 291

Chapter 5 Discussion .................................................................................................................. 312

5.1 Component Data Gaps................................................................................................... 312

5.2 Component Condition Designations.............................................................................. 314

5.3 Park-wide Condition Observations................................................................................ 316

Appendices.................................................................................................................................. 322
v

Figures

Page

Figure 1. Relative coastal vulnerability for PAIS........................................................................ 13

Figure 2. Symbols used for individual component assessments with condition or 

concern designations along the vertical axis and trend designations along the 

horizontal. ..................................................................................................................................... 27

Figure 3. A geographic profile of Padre Island with typical vegetation communities. ............... 34

Figure 4. Upper and Lower Laguna Madre in relation to PAIS .................................................. 60

Figure 5. Number of wetlands and ponds in PAIS by elevation.................................................. 75

Figure 6. Wetland and pond area in PAIS by elevation............................................................... 76

Figure 7. Major North American migratory flyways................................................................... 87

Figure 8. Zoogeographic regions of the world; shaded areas represent transition areas 

between regions ............................................................................................................................ 90

Figure 9. Number of Neotropical migrant species observed on the PAIS Christmas 

Bird Count from 1974-1990.......................................................................................................... 93

Figure 10. Padre Island peregrine falcon survey sighting rates from 1995-2012........................ 96

Figure 11. Number of resident bird species observed on the PAIS Christmas Bird 

Count from 1974-1990................................................................................................................ 111

Figure 12. The habitat zones used to classify bird distribution in Chapman (1984). ................ 114

Figure 13. Laughing gull abundance on the central coast of Texas from 2003-2011................ 134

Figure 14. Distribution of bird species along the Gulf beach as reported by Chapman 

et al. (1984). ................................................................................................................................ 135

Figure 15. Est. number of breeding pairs (all species) observed at the island group 

614-341 from 1973-2011. ........................................................................................................... 137

Figure 16. Change in vegetative cover on Marker 81 (referred to in this component as 

island 614-345) spoil island in PAIS .......................................................................................... 143

Figure 17. Trapping locations where coyotes were documented within PAIS.......................... 161

Figure 18. Locations of the four main sampling sites used during the Baccus (1977) 

study in PAIS. ............................................................................................................................. 167

Figure 19. Locations of mammal trap transects during the Frey and Jones (2008) 

survey in PAIS. ........................................................................................................................... 168

Figure 20. Distribution of dominant macroinvertebrate taxa along the Gulf beaches of 

PAIS............................................................................................................................................ 183
vi

Figures (continued)

Page

Figure 21. Location of the three freshwater ponds in PAIS....................................................... 230

Figure 22. Fluctuations in daily DO observations at Baffin Bay and Bird Island data 

loggers in the Laguna Madre in PAIS, August 2008 through August 2009............................... 234

Figure 23. Average monthly water temperatures (°C) at three sampling locations in 

the upper Laguna Madre in PAIS between 2003 and 2005 ........................................................ 235

Figure 24. Average monthly water temperatures (°C) at three sampling locations in 

the lower Laguna Madre in PAIS between 2003 and 2005. ....................................................... 236

Figure 25. Fluctuations in daily water temperature (°C) observations from Baffin Bay 

and Bird Island data loggers in the Laguna Madre in PAIS, August 2008 through 

August 2009................................................................................................................................ 237

Figure 26. Fluctuations in daily pH observations from Baffin Bay and Bird Island 

data loggers in the Laguna Madre in PAIS, August 2008 through August 2009 ....................... 239

Figure 27. Average monthly salinity values (ppt) from three water quality stations in 

the Laguna Madre, August 2003 through July 2005................................................................... 240

Figure 28. Average monthly salinity values (ppt) from three stations in the lower 

Laguna Madre in PAIS, August 2003 through July 2005........................................................... 240

Figure 29. Fluctuations in daily salinity values (ppt) from Baffin Bay and Bird Island 

data loggers in the Laguna Madre in PAIS, August 2008 through August 2009. ..................... 241

Figure 30. Turbidity event lasting several days captured at the Bird Island data logger 

in the Laguna Madre in PAIS between 30 August and 5 September 2008................................. 243

Figure 31. Short-term spike in turbidity (approximately 7 hours) as captured at the 

Bird Island data logger in the Laguna Madre in PAIS on 25 June 2009. ................................... 243

Figure 32. Annual average concentrations of sulfate (SO4), nitrate (NO3), and 

ammonium (NH4) in precipitation (mg/L) in Corpus Christi, Texas, 2002-2006 ...................... 260

Figure 33. Annual 4th highest 8-hour maximum ozone (O3) concentrations (ppb) in 

the PAIS region, 1995-2012 ....................................................................................................... 261

Figure 34. Annual particulate matter (PM2.5) concentrations (weighted annual mean) 

near PAIS, 2000-2012................................................................................................................. 262

Figure 35. Artificial sky brightness in southeastern Texas........................................................ 269

Figure 36. Grayscale representation of sky luminance from a location in Joshua Tree 

National Park .............................................................................................................................. 272

Figure 37. False color representation of Figure 2 after a logarithmic stretch of pixel 

values .......................................................................................................................................... 273
vii

Figures (continued)

Page

Figure 38. Contour map of anthropogenic sky glow at a location in Joshua Tree 

National Park, analogous to Figure 37 with natural sources of light subtracted ........................ 274

Figure 39. Contour map of sky brightness in fisheye projection, Big Shell Beach, 30 

March 2009 ................................................................................................................................. 277

Figure 40. Anthropogenic light measured at Big Shell Beach, 30 March 2009, in 

panoramic projection. ................................................................................................................. 278

Figure 41. Contour map of sky brightness in fisheye projection, Bird Island Boat 

Ramp, 30 March 2009. The sky dome to the north is Corpus Christi, TX................................. 281

Figure 42. Anthropogenic light measured at Bird Island Boat Launch, 30 March 

2009, in panoramic projection. ................................................................................................... 282

Figure 43. Contour map of sky brightness in fisheye projection; Bird Island Boat 

Ramp on the left, and the North Rim of Grand Canyon National Park on the right................... 283

Figure 44. Contour map of sky brightness in fisheye projection; Bird Island Boat 

Ramp on the left, and Santa Monica Mountains National Recreation Area on the 

right............................................................................................................................................. 284

Figure 45. Contour map of sky brightness in fisheye projection; Big Shell Beach on 

the left, and Bird Island Boat Launch on the right...................................................................... 285

Figure 46. Geomorphologic formations of Padre Island National Seashore, showing 

the major zones in land formation across a barrier island. ......................................................... 293

Figure 47. Average annual change in shoreline position (m/yr) from Aransas Pass to 

the north boundary of Padre Island National Seashore............................................................... 297

Figure 48. Net rates of long-term change in shoreline position for the lower Texas 

Gulf of Mexico, including PAIS, calculated from shoreline positions from 1930 

through 2007 ............................................................................................................................... 298

Figure 49. Trend in mean sea level rise recorded at Padre Island, TX, 1958 to 2006............... 305

Figure 50. Trend in mean sea level rise recorded at Port Mansfield, TX, 1963 to 

2006............................................................................................................................................. 305

Figure 51. Trend in mean sea level rise recorded at Rockport, TX, 1948-2006........................ 306

Figure 52. Symbols used for individual component assessments with condition or 

concern designations along the vertical axis and trend designations along the 

horizontal. ................................................................................................................................... 316
viii
ix

Tables 

Page

Table 1.Monthly temperature and precipitation normals for PAIS................................................ 6

Table 2. Vulnerability rankings and ranges of values for the six selected PAIS 

variables........................................................................................................................................ 12

Table 3. GULN Vital Signs selected for monitoring in PAIS...................................................... 15

Table 4. Padre Island National Seashore natural resource condition assessment 

framework..................................................................................................................................... 22

Table 5. Scale for a measure’s significance level in determining a components overall 

condition. ...................................................................................................................................... 25

Table 6. Scale for condition level of individual measures. .......................................................... 26

Table 7. Area of terrestrial vegetation mapping units, according to surficial geology 

GIS data ........................................................................................................................................ 36

Table 8. Non-native species documented in PAIS....................................................................... 38

Table 9. Seagrass species composition (in hectares) and percent cover of vegetated 

areas in the lower and upper Laguna Madre in 1998.................................................................... 61

Table 10. Seagrass species composition, in the lower and upper Laguna Madre in 

2011, represented by mean percent coverage ............................................................................... 61

Table 11. Total coverage (in hectares) and percent cover of bare and vegetated 

bottoms in the lower and upper Laguna Madre in 1998 ............................................................... 62

Table 12. Major National Wetland Inventory (NWI) classifications (Cowardin 1979) 

by area in PAIS. ............................................................................................................................ 72

Table 13. Area of map classes that contain wetland and pond communities, according 

to GIS landcover data.................................................................................................................... 73

Table 14. Area of wetland and pond mapping units, according to surficial geology 

GIS data ........................................................................................................................................ 73

Table 15. Vascular plants documented in the salt marsh and salty sands habitats of 

PAIS.............................................................................................................................................. 74

Table 16. Additional plant species documented in PAIS wetlands.............................................. 75

Table 17. Number of Neotropical migrant individuals observed on the PAIS 

Christmas Bird Count from 1974-1990 ........................................................................................ 92

Table 18. Neotropical migrant species observed during the Chapman (1984) coastal 

bird surveys................................................................................................................................... 94
x

Tables (continued)

Page

Table 19. Neotropical migrant species and abundance results from the Blacklock et 

al. (1998) surveys and banding efforts.......................................................................................... 95

Table 20. Migratory raptor species identified on the NPS Certified Species List ....................... 96

Table 21. Waterfowl species observed in PAIS during several avian monitoring 

reports. .......................................................................................................................................... 98

Table 22. Waterfowl abundance as recorded on the PAIS CBC from 1974-1990....................... 99

Table 23. Resident bird species observed during the Chapman (1984) coastal bird 

surveys. ....................................................................................................................................... 112

Table 24. Resident bird species observed during the 1993 and 1995 surveys of the 

Gulf and Laguna Madre coasts. .................................................................................................. 113

Table 25. Resident raptor species in PAIS................................................................................. 115

Table 26. Resident raptor species observed during the PAIS CBC (1974-1990) ...................... 116

Table 27. Families of waterbirds as defined by the USGS Patuxent Wildlife Research 

Center's Waterbird Monitoring Partnership................................................................................ 122

Table 28. Average monthly abundance and diversity of colonial waterbirds in PAIS 

from October 1980 to June 1981 ................................................................................................ 127

Table 29. Colonial waterbird species observed on the Gulf coast and the Laguna 

Madre during the 1993-94 and 1994-95 surveys. ....................................................................... 129

Table 30. Abundance of colonial waterbirds during the 1993-94 survey on the Gulf 

coast ............................................................................................................................................ 130

Table 31. Average annual abundance of breeding birds in PAIS, 2011 abundance 

values, and the overall percent change observed in 2011 compared to the average 

abundance of the species............................................................................................................. 132

Table 32. Distribution of protected colonial waterbird species during the 1993-94 

bird surveys of the PAIS Gulf beach; habitat types are defined in text, and in Figure 

14................................................................................................................................................. 136

Table 33. Predation accessibility and pressure for several historic colonial waterbird 

nesting colonies in the PAIS region............................................................................................ 141

Table 34. Shannon-Wiener Diversity Indices for rodent species of the four main 

sampling sites in PAIS................................................................................................................ 170

Table 35. Small mammal species collected in PAIS.................................................................. 171
xi

Tables (continued)

Page

Table 36. Macroinvertebrate species collected in three Laguna Madre environments 

that border PAIS ......................................................................................................................... 178

Table 37. Distribution of benthic macroinvertebrates on tidal flats during three 

studies by Withers (1993, 1996, 1998)....................................................................................... 179

Table 38. Distribution and abundance (number of individuals) of benthic 

macroinvertebrates by zone along Gulf beaches within PAIS.................................................... 181

Table 39. Mean benthic macroinvertebrate density (per m2) by study site and by 

major beach zone (across all study sites).................................................................................... 184

Table 40. Comparison of mean macroinvertebrate densities (#/m2) between three 

sampling efforts at Closed Beach, PAIS..................................................................................... 184

Table 41. Crustacean macroinvertebrate density (log of total number per liter of pond 

water) for three ponds within PAIS ............................................................................................ 184

Table 42. Abundance of macroinvertebrate taxa recorded by Withers (1996) at two 

sites within PAIS......................................................................................................................... 186

Table 43. Benthic macroinvertebrate species richness and diversity by beach zone................. 187

Table 44. Macroinvertebrate species documented by Sissom et al. (1990) in three 

PAIS ponds. ................................................................................................................................ 187

Table 45. Reptiles found in PAIS as documented by NPS (2012b)........................................... 194

Table 46. Number of Kemp’s ridley sea turtle nests located along the Texas coast, 

1996-2012 ................................................................................................................................... 205

Table 47. The total number of Kemp’s ridley, green, and loggerhead sea turtle nests 

confirmed in PAIS and along the Texas coast, 2004-2012......................................................... 206

Table 48. The total number of Kemp’s ridley, green, and loggerhead sea turtle eggs 

recorded in nests that were collected and taken to the PAIS incubation facility or 

Texas corrals and total number of hatchlings released to the Gulf of Mexico, 2004-

2012............................................................................................................................................. 207

Table 49. Amphibians that have been confirmed in PAIS......................................................... 219

Table 50. Texas Commission on Environmental Quality standards for surface-water 

quality ......................................................................................................................................... 229

Table 51. TCEQ 2002 water quality criteria for nutrients in saltwater estuaries in 

PAIS............................................................................................................................................ 229

Table 52. Dissolved oxygen values (mg/L) for three freshwater ponds located in 

PAIS, September 1989 through August 1990............................................................................. 233
xii

Tables (continued)

Page

Table 53. Summary of dissolved oxygen observations (mg/L) from three water 

quality stations in the Laguna Madre in PAIS, including minimum, maximum, and 

mean values, and number of exceedances. ................................................................................. 233

Table 54. DO observations collected in the lower and upper Laguna Madre in 2011, 

including minimum, maximum, and mean values (mg/L).......................................................... 233

Table 55. Water temperature values (°C) for three freshwater ponds located in PAIS, 

September 1989 through August 1990. ...................................................................................... 234

Table 56. Summary of water temperature observations (°C) from three water quality 

stations in the Laguna Madre in PAIS, including minimum, maximum, mean values, 

and exceedances.......................................................................................................................... 235

Table 57. pH values for three freshwater ponds located in PAIS, September 1989 

through August 1990................................................................................................................... 237

Table 58. Summary of pH observations from three water quality stations in the 

Laguna Madre in PAIS, including minimum, maximum, and mean values, and 

number of exceedances............................................................................................................... 238

Table 59. Summary of pH observations from the upper and lower Laguna Madre, 

including minimum, maximum, and mean values collected in 2011 ......................................... 238

Table 60. Salinity values (ppt) for three freshwater ponds located in PAIS, September 

1989 through August 1990.......................................................................................................... 239

Table 61. Summary of salinity observations from the upper and lower Laguna Madre, 

including minimum, maximum, and mean values collected in 2011 ......................................... 241

Table 62. Turbidity observations (NTU) for three freshwater ponds located in PAIS, 

September 1989 through August 1990 ....................................................................................... 242

Table 63. Summary of turbidity observations (NTU) from three water quality stations 

in the Laguna Madre in PAIS, including minimum, maximum, and mean values, and 

number of exceedances............................................................................................................... 242

Table 64. Nutrient ranges and exceedances from six water sampling locations in the 

upper and lower Laguna Madre in PAIS, August 2003 and August 2005 ................................. 244

Table 65. National Park Service Air Resources Division air quality index values.................... 258

Table 66. Sky quality photometric report for Big Shell Beach, 30 March 2009 ....................... 276

Table 67. Sky quality photometric report for Bird Island Boat Ramp, 30 March 2009 ............ 280

Table 68. Identified data gaps or needs for the featured components in PAIS.......................... 312
xiii

Plates

Page

Plate 1. The three watersheds that occur within park boundaries................................................ 18

Plate 2. Example of a vegetation map created for an energy development proposal................... 44

Plate 3. Extent of grassland and dune (sparse) vegetation in the northern half of PAIS ............. 45

Plate 4. Extent of grassland and dune (sparse) vegetation in the southern half of PAIS ............. 46

Plate 5. Extent of terrestrial vegetation mapping units, according to NPS surficial geology data

....................................................................................................................................................... 47

Plate 6. Mapped oak motte locations in the northern portion of PAIS, with an inset showing an 

aerial photo of one of the mottes................................................................................................... 48

Plate 7. The extent of wind tidal flats in PAIS (USGS 2009), and the approximate locations of 

Withers’ (1996,1998) tidal flats sites along the Laguna Madre.................................................... 58

Plate 8. Percent cover of all seagrass in PAIS.............................................................................. 68

Plate 9. Mean canopy height (cm) of all seagrass in PAIS. ......................................................... 69

Plate 10. Wetlands and ponds in PAIS according to NWI data ................................................... 81

Plate 11. Extent of wetland and pond vegetation classes in PAIS according to Laine and Ramsey 

(1998)............................................................................................................................................ 82

Plate 12. Extent of wetland and pond mapping units, according to NPS surficial geology data . 83

Plate 13. Sample map showing approximate wetland and pond elevations in a northern portion 

of PAIS.......................................................................................................................................... 84

Plate 14. Sample map showing approximate wetland elevations in a central portion of PAIS. .. 85

Plate 15. PAIS Christmas Bird Count survey area (1974-1990)................................................ 108

Plate 16. PAIS Christmas Bird Count survey area (1974-1990)................................................ 121

Plate 17. Location of the 12 dredge spoil islands used by nesting colonial waterbirds in PAIS.

..................................................................................................................................................... 149

Plate 18. Land cover of PAIS' spoil mounds.............................................................................. 150

Plate 19. Dredge spoil island 614-341 in PAIS.......................................................................... 151

Plate 20. Dredge spoil island 614-342 in PAIS.......................................................................... 152
xiv

Plate 21. Dredge spoil island 614-343 in PAIS.......................................................................... 153

Plate 22. Dredge spoil island 614-344 in PAIS.......................................................................... 154

Plate 23. Dredge spoil island 614-345 in PAIS.......................................................................... 155

Plate 24. Dredge spoil island 614-346 in PAIS.......................................................................... 156

Plate 25. Dredge spoil island 614-347 in PAIS.......................................................................... 157

Plate 26. Dredge spoil island 614-360 in PAIS.......................................................................... 158

Plate 27. Transect locations for Rocha’s (1995) Gulf beaches (Sites 1-4) and approximate 

locations of Withers’ (1996,1998) tidal flats sites along the Laguna Madre.............................. 192

Plate 28. Trap locations used during 2002-2003 inventory of reptiles and amphibians in PAIS

..................................................................................................................................................... 200

Plate 29. Trapping locations used during the 2002-2003 PAIS inventory................................. 225

Plate 30. Amphibian species found during the 2002-2003 PAIS inventory .............................. 226

Plate 31. Locations of the long-term water quality sampling stations featured in NPS (2003), 

NPS (2012c) NPSTORET database, and the NPS (2013) Gulf Network monitoring effort on the 

Laguna Madre in PAIS ............................................................................................................... 252

Plate 32. Dissolved oxygen values in the Laguna Madre from 2011 (left) and 2012 (right) based 

on monitoring surveys conducted by Wilson and Dunton (2012). ............................................. 253

Plate 33. pH values in the Laguna Madre from 2011 (left) and 2012 (right) based on monitoring 

surveys conducted by Wilson and Dunton (2012)...................................................................... 254

Plate 34. Values in the Laguna Madre from 2011 (left) and 2012 (right) based on monitoring 

surveys conducted by Wilson and Dunton (2012)...................................................................... 255

Plate 35. Dark night sky sampling locations within PAIS; both sites were surveyed on 30 March 

2009............................................................................................................................................. 290

Plate 36. Location of NOAA sea level rise monitoring stations relative to PAIS. .................... 311
xv

Photos 

Page

Photo 1. Aerial photo of a Laguna Madre spoil island................................................................... 8

Photo 2. A grassland in PAIS, with vegetated dunes in the background ..................................... 33

Photo 3. Dune vegetation at PAIS, with bare ground exposed .................................................... 37

Photo 4. A grassland in PAIS with heavy litter accumulation due to lack of fire........................ 39

Photo 5. A vegetation sampling transect through a PAIS grassland............................................ 40

Photo 6. Dry Algal mats on mud flats in PAIS ............................................................................ 49

Photo 7. Inundated wind-tidal flats with ORV tire tracks............................................................ 53

Photo 8. Seagrass bed in the Laguna Madre ................................................................................ 59

Photo 9. Waterfowl on Pond B along Park Road 22.................................................................... 70

Photo 10. Reddish egrets.............................................................................................................. 86

Photo 11. Black-necked stilt....................................................................................................... 109

Photo 12. Brown pelicans (Pelecanus occidentalis) flying along the Gulf coast of 

PAIS............................................................................................................................................ 122

Photo 13. American white pelican nesting colony on Pelican Island (614-345) in the 

Laguna Madre ............................................................................................................................. 133

Photo 14. Coyote (Canis latrans)............................................................................................... 159

Photo 15. Spotted ground squirrel (Spermophilus spilosoma)................................................... 166

Photo 16. Hispid cotton rat (Sigmodon hispidus)....................................................................... 166

Photo 17. A ghost crab at PAIS.................................................................................................. 176

Photo 18. Texas scarlet snake (Cemophora coccinea lineri)..................................................... 193

Photo 19. Keeled earless lizard .................................................................................................. 195

Photo 20. Kemp’s ridley sea turtle............................................................................................. 201

Photo 21. Hawksbill sea turtle entangled in rope....................................................................... 208

Photo 22. Green turtle with fibropapillomatosis........................................................................ 210

Photo 23. Green tree frog (Hyla cinerea)................................................................................... 218

Photo 24. The Gulf of Mexico shore along PAIS ...................................................................... 227

Photo 25. Pond B located near PAIS Park Headquarters (seen in the background). ................. 231
xvi

Photos (continued)

Page

Photo 26. Emergent dunes in PAIS (left); beach zone adjacent to the water’s edge in 

PAIS (right)................................................................................................................................. 291

Photo 27. View of the fore-island dune ridge as seen from the Gulf beach in PAIS 

(left); atop the fore-island dune ridge looking south down the length of the fore-island 

ridge parallel to the beach (right)................................................................................................ 292

Photo 28. An active blowout dune in the northern part of PAIS (left); vegetated backisland dunes, or stabilized dunes, as viewed from the grasslands in the northern part 

of PAIS (right) ............................................................................................................................ 292

Photo 29. Big Shell Beach at PAIS............................................................................................ 293

Photo 30. Imagery of Mansfield Channel jetties at the southern boundary of PAIS 

shows the flow of sediments through the channel and away from the littoral nearshore 

zone............................................................................................................................................. 303
xvii

Appendices

Page

Appendix A. Native plant species documented in PAIS. ........................................................... 322

Appendix B. Plant species documented in interior/grassland habitats....................................... 324

Appendix C. List of macroinvertebrate species and respective microhabitat types 

observed during a study of PAIS between 1991 and 1992. ........................................................ 327

Appendix D. List of macroinvertebrate species observed at two former oil and gas 

development sites that have been restored in PAIS.................................................................... 329

Appendix E. List of macroinvertebrate species observed at three southerly wind-tidal 

flats in PAIS between 1997 and 1998......................................................................................... 331

Appendix F. Vascular plants within 60 m (200 ft) of the three ponds sampled at PAIS............ 332

Appendix G. Algae documented in three PAIS ponds by Sissom et al. (1990). ........................ 334

Appendix H. Fungal species documented in three PAIS ponds by Florence Oxley................... 336

Appendix I. Neotropical migrant species that are confirmed in PAIS ....................................... 337

Appendix J. Resident bird species identified on the PAIS Certified Species List...................... 341

Appendix K. Resident bird species observed on the PAIS CBC from 1974-1990..................... 343

Appendix L. Colonial waterbird species that have been identified in PAIS, and appear 

on the NPS Certified Species List............................................................................................... 345

Appendix M. Summary of small mammal species present or probably present within 

PAIS............................................................................................................................................ 347

xix

Executive Summary

The Natural Resource Condition Assessment (NRCA) Program aims to provide documentation 

about the current conditions of important park natural resources through a spatially explicit, 

multi-disciplinary synthesis of existing scientific data and knowledge. Findings from the NRCA 

will help Padre Island National Seashore (PAIS) managers to develop near-term management 

priorities, engage in watershed or landscape scale partnership and education efforts, conduct park 

planning, and report program performance (e.g., Department of the Interior’s Strategic Plan 

“land health” goals, Government Performance and Results Act).

The objectives of this assessment are to evaluate and report on current conditions of key park 

resources, to evaluate critical data and knowledge gaps, and to highlight selected existing 

stressors and emerging threats to resources or processes. For the purpose of this NRCA, staff 

from the National Park Service (NPS) and Saint Mary’s University of Minnesota – GeoSpatial 

Services (SMUMN GSS) identified key resources, referred to as “components” in the project. 

The selected components include natural resources and processes that are currently of the 

greatest concern to park management at PAIS. The final project framework contains 17 resource 

components, each featuring discussions of measures, stressors, and reference conditions.

This study involved reviewing existing literature and, where appropriate, analyzing data for each 

natural resource component in the framework to provide summaries of current condition and 

trends in selected resources. When possible, existing data for the established measures of each 

component were analyzed and compared to designated reference conditions. A weighted scoring 

system was applied to calculate the current condition of each component. Weighted Condition 

Scores, ranging from zero to one, were divided into three categories of condition: low concern, 

moderate concern, and significant concern. These scores help to determine the current overall 

condition of each resource. The discussions for each component, found in Chapter 4 of this 

report, represent a comprehensive summary of current available data and information for these 

resources, including unpublished park information and perspectives of park resource managers, 

and present a current condition designation when appropriate. Each component assessment was 

reviewed by PAIS resource managers and NPS Gulf Coast Network staff.

Existing literature, short- and long-term datasets, and input from NPS and other outside agency 

scientists support condition designations for components in this assessment. However, in a 

number of cases, data were unavailable or insufficient for several of the measures of the featured 

components. In other instances, data establishing reference condition were limited or unavailable 

for components, making comparisons with current information inappropriate or invalid. In these 

cases, it was not possible to assign condition for the components. Current condition was not able 

to be determined for 12 of the 17 components (70.5%) due to these data gaps.

For the five components with sufficient available data, the overall condition varied. These 

components include colonial waterbirds, water quality, air quality, dark night skies, and sea 

turtles. All five components were determined to be of moderate concern based on available data. 

Sea turtles, air quality, and dark night skies all had stable trends, while colonial waterbirds was 

determined to have a declining trend in resource condition. Detailed discussion of these 

designations is presented in Chapters 4 and 5 of this report. 
xx

Several park-wide threats and stressors influence the condition of priority resources in PAIS. 

Those of primary concern include extreme weather events (e.g., hurricanes, tropical storms, and 

drought), relative sea level rise, energy development within and around the park (i.e., oil and gas 

drilling, wind farms), and human impacts (i.e., vehicular and pedestrian traffic). Understanding 

these threats, and how they relate to the condition of these resources, can help the NPS prioritize 

management objectives and better focus conservation strategies to maintain the health and 

integrity of park ecosystems.
xxi

Acknowledgments

We acknowledge Padre Islands National Seashore staff for the technical expertise provided 

during scoping, through multiple stages of review, and via phone and email; specifically, Jim 

Lindsay, Wade Stablein, and Dr. Donna Shaver. Gulf Coast Inventory and Monitoring Network 

staff, including Martha Segura, Joe Meiman, Whitney Granger, and Jeff Bracewell provided 

logistical insight, support, and critical review of interim documents. Diana Del Angel, Coastal 

Geoscientist for the Harte Research Institute of Texas A&M Corpus Christi, provided review of 

the coastal dunes and beaches component. Ellen Porter of the NPS - Air Resources Division, 

provided review of the air quality component. R. Dale McPherson, Southeast Region Natural 

Resource Condition Assessment Coordinator, helped with project implementation and document 

reviews. Jeff Albright, National Natural Resource Condition Assessment Coordinator, provided 

program guidance. Thank you to all others who assisted the development of this document.
xxii

Acronyms and Abbreviations 

ABC – American Bird Conservancy

ARD – NPS Air Resources Division

ARK – Animal Rehabilitation Keep

ATVs – All-terrain vehicles

BBS – Breeding Bird Survey

CAA – Clean Air Act of 1977

CASTNet – Clean Air Status and Trends Network

CBBEP – Coastal Bend Bays & Estuaries Program

CBC – Christmas Bird Count

C-CAP – Coastal Change and Analysis Project

CCD – Charge-coupled Device

CL – Condition Level

CVI – Coastal Vulnerability Index

DAM – Water Impoundments Database

DEM – Digital Elevation Model

DO – Dissolved Oxygen

DRINKS – Drinking Water Supplies Database

Dv – Deciviews

EIS – Environmental Impact Statement

EPA – Environmental Protection Agency 

GAGES – Water Gages Database

GIS – Geographic Information System

GIWW – Gulf Intracoastal Waterway

GPRA – Government Performance and Results Act
xxiii

Acronyms and Abbreviations (continued)

GRI – Geologic Resources Inventory

GULN – Gulf Coast Network

HAB – Harmful Algal Bloom

I&M - Inventory and Monitoring

IFD – Industrial Facilities Discharge

IMPROVE – Interagency Monitoring of Protected Visual Environments Program

IRMA – NPS Integrated Resource Management Applications

ITIS – Integrated Taxonomic Information System

LiDAR – Light Detection and Ranging

LLM – Lower Laguna Madre

MHW – Mean High Water

MLW – Mean Low Water

MSL – Mean Sea Level

MW – Milliwatt

NAAQS – National Ambient Air Quality Standards

NMFS – U.S. National Marine Fisheries Service

NADP – National Atmospheric Deposition Program

NOAA - National Oceanic and Atmospheric Administration

NPS - National Park Service

NRCA – Natural Resource Condition Assessment

NST – Night Sky Team

NVCS - National Vegetation Classification Standard

NWI - National Wetland Inventory

O3 – Ozone
xxiv

Acronyms and Abbreviations (continued)

ORV – Off-road Vehicle

PAIS - Padre Island National Seashore

PM – Particulate Matter

ppb – Parts per Billion

RF3 – River Reach File

RSS – Resource Stewardship Strategy 

SL – Significance Level

SMUMN GSS – Saint Mary’s University of Minnesota, Geospatial Services 

SSAP – Shoreline Shape and Projection Program

STORET – Storage and Retrieval

STSSN – Sea Turtle Stranding and Salvage Network

TCEQ – Texas Commission on Environmental Quality

TED – Turtle Excluder Device

TCWS – Texas Colonial Waterbird Society

TPWD – Texas Parks and Wildlife Department

TSS – Total Suspended Solids

ULM – Upper Laguna Madre

USACE – United States Army Corps of Engineers

USFWS – United States Fish and Wildlife Service 

USGS – United States Geological Survey

UTMSI – University of Texas Marine Science Institute

VOCs – Volatile Organic Compounds

WCS – Weighted Condition Score

WHSRN – Western Hemisphere Shorebird Reserve Network
xxv

Acronyms and Abbreviations (continued)

ZLM – Zenith Limiting Magnitude

1

Chapter 1 NRCA Background Information

Natural Resource Condition Assessments (NRCAs) evaluate current conditions for a subset of 

natural resources and resource indicators in national park units, hereafter “parks.” NRCAs also 

report on trends in resource condition (when possible), identify critical data gaps, and 

characterize a general level of confidence for study findings. The resources and indicators 

emphasized in a given project depend on the park’s resource setting, status of resource 

stewardship planning and science in identifying high-priority indicators, and availability of data 

and expertise to assess current conditions for a variety of potential study resources and 

indicators.

NRCAs represent a relatively new approach to 

assessing and reporting on park resource conditions. 

They are meant to complement—not replace—

traditional issue- and threat-based resource 

assessments. As distinguishing characteristics, all 

NRCAs:

• are multi-disciplinary in scope;

1

• employ hierarchical indicator frameworks;

2

• identify or develop reference conditions/values 

for comparison against current conditions;3

• emphasize spatial evaluation of conditions and GIS (map) products;

4

• summarize key findings by park areas; and5

• follow national NRCA guidelines and standards for study design and reporting products.

Although the primary objective of NRCAs is to report on current conditions relative to logical 

forms of reference conditions and values, NRCAs also report on trends, when appropriate (i.e., 

when the underlying data and methods support such reporting), as well as influences on resource

 1 The breadth of natural resources and number/type of indicators evaluated will vary by park. 

2 Frameworks help guide a multi-disciplinary selection of indicators and subsequent “roll up” and reporting of data 

for measures  conditions for indicators  condition summaries by broader topics and park areas 

3 NRCAs must consider ecologically-based reference conditions, must also consider applicable legal and regulatory 

standards, and can consider other management-specified condition objectives or targets; each study indicator can be 

evaluated against one or more types of logical reference conditions. Reference values can be expressed in qualitative 

to quantitative terms, as a single value or range of values; they represent desirable resource conditions or, 

alternatively, condition states that we wish to avoid or that require a follow-on response (e.g., ecological thresholds 

or management “triggers”).

4 As possible and appropriate, NRCAs describe condition gradients or differences across a park for important natural 

resources and study indicators through a set of GIS coverages and map products. 

5 In addition to reporting on indicator-level conditions, investigators are asked to take a bigger picture (more 

holistic) view and summarize overall findings and provide suggestions to managers on an area-by-area basis: 1) by 

park ecosystem/habitat types or watersheds, and 2) for other park areas as requested.

NRCAs Strive to Provide...

Credible condition reporting 

for a subset of important 

park natural resources and 

indicators

Useful condition summaries 

by broader resource 

categories or topics, and by 

park areas
2

conditions. These influences may include past activities or conditions that provide a helpful 

context for understanding current conditions, and/or present-day threats and stressors that are 

best interpreted at park, watershed, or landscape scales (though NRCAs do not report on 

condition status for land areas and natural resources beyond park boundaries). Intensive causeand-effect analyses of threats and stressors, and development of detailed treatment options, are

outside the scope of NRCAs.

Due to their modest funding, relatively quick timeframe for completion, and reliance on existing 

data and information, NRCAs are not intended to be exhaustive. Their methodology typically 

involves an informal synthesis of scientific data and information from multiple and diverse 

sources. Level of rigor and statistical repeatability will vary by resource or indicator, reflecting 

differences in existing data and knowledge bases across the varied study components.

The credibility of NRCA results is derived from the data, methods, and reference values used in 

the project work, which are designed to be appropriate for the stated purpose of the project, as 

well as adequately documented. For each study indicator for which current condition or trend is 

reported, we will identify critical data gaps and describe the level of confidence in at least 

qualitative terms. Involvement of park staff and National Park Service (NPS) subject-matter 

experts at critical points during the project timeline is also important. These staff will be asked to 

assist with the selection of study indicators; recommend data sets, methods, and reference 

conditions and values; and help provide a multi-disciplinary review of draft study findings and 

products.

NRCAs can yield new insights about current park resource conditions but, in many cases, their 

greatest value may be the development of useful documentation regarding known or suspected 

resource conditions within 

parks. Reporting products can 

help park managers as they 

think about near-term 

workload priorities, frame data 

and study needs for important 

park resources, and 

communicate messages about 

current park resource 

conditions to various 

audiences. A successful NRCA 

delivers science-based 

information that is both 

credible and has practical uses 

for a variety of park decision 

making, planning, and partnership activities.

However, it is important to note that NRCAs do not establish management targets for study 

indicators. That process must occur through park planning and management activities. What an 

NRCA can do is deliver science-based information that will assist park managers in their 

ongoing, long-term efforts to describe and quantify a park’s desired resource conditions and 

Important NRCA Success Factors

Obtaining good input from park staff and other NPS 

subject-matter experts at critical points in the 

project timeline 

Using study frameworks that accommodate 

meaningful condition reporting at multiple levels 

(measures  indicators  broader resource topics 

and park areas)

Building credibility by clearly documenting the data 

and methods used, critical data gaps, and level of 

confidence for indicator-level condition findings 
3

management targets. In the near term, NRCA findings assist strategic park resource planning6

and help parks to report on government accountability measures.

7 In addition, although in-depth 

analysis of the effects of climate change on park natural resources is outside the scope of 

NRCAs, the condition analyses and data sets developed for NRCAs will be useful for park-level 

climate-change studies and planning efforts.

NRCAs also provide a useful complement to rigorous NPS science support programs, such as the 

NPS Natural Resources Inventory & Monitoring (I&M) Program.

8 For example, NRCAs can 

provide current condition estimates and help establish reference conditions, or baseline values,

for some of a park’s vital signs monitoring indicators. They can also draw upon non-NPS data to 

help evaluate current conditions for those same vital signs. In some cases, I&M data sets are 

incorporated into NRCA analyses and reporting products.

Over the next several years, the NPS plans to fund a NRCA project for each of the approximately 

270 parks served by the NPS I&M Program. For more information on the NRCA program, 

visit http://nature.nps.gov/water/nrca/index.cfm

 6 An NRCA can be useful during the development of a park’s Resource Stewardship Strategy (RSS) and can also be 

tailored to act as a post-RSS project.

7 While accountability reporting measures are subject to change, the spatial and reference-based condition data 

provided by NRCAs will be useful for most forms of “resource condition status” reporting as may be required by the 

NPS, the Department of the Interior, or the Office of Management and Budget. 

8 The I&M program consists of 32 networks nationwide that are implementing “vital signs” monitoring in order to 

assess the condition of park ecosystems and develop a stronger scientific basis for stewardship and management of 

natural resources across the National Park System. “Vital signs” are a subset of physical, chemical, and biological 

elements and processes of park ecosystems that are selected to represent the overall health or condition of park 

resources, known or hypothesized effects of stressors, or elements that have important human values.

NRCA Reporting Products...

Provide a credible, snapshot-in-time evaluation for a subset of 

important park natural resources and indicators, to help park 

managers:

Direct limited staff and funding resources to park areas and natural 

resources that represent high need and/or high opportunity situations

(near-term operational planning and management)

Improve understanding and quantification for desired conditions for the 

park’s “fundamental” and “other important” natural resources and 

values

(longer-term strategic planning)

Communicate succinct messages regarding current resource conditions 

to government program managers, to Congress, and to the general 

public 

(“resource condition status” reporting) 

5

Chapter 2 Introduction and Resource Setting

2.1 Introduction

2.1.1 Enabling Legislation

Padre Island National Seashore (PAIS) is the longest stretch of undeveloped barrier island in the 

world (Cooper et al. 2005). On 28 September 1962, PAIS was authorized by Congress and was 

established

...in order to save and preserve, for purposes of public recreation, benefit, and 

inspiration, a portion of the diminishing seashore of the United States that remains 

undeveloped the Secretary of the Interior shall take appropriate action in the public 

interest toward the establishment of the following described lands and waters as the Padre 

Island National Seashore (Public Law 87-711).

2.1.2 Geographic Setting

PAIS is approximately 52,609 ha (130,000 ac) and is located near the southern tip of Texas on 

the coast of the Gulf of Mexico. The barrier island that makes up PAIS is a relatively young

landform in geological terms, as deposits found on the unit date back only a few thousand years 

(Weise and White 1980). Gulf currents and tidal movements formed this barrier island, in part, 

by pushing sand on to the mainland beach ridge (Weise and White 1980).

Most of the National Seashore is less than 6.1 m (20 ft) above sea level. On the southern end of 

the National Seashore, the erosion rate is higher due to a drier climate and lack of vegetation; as 

a result, the dune ridge in this area is low and segmented (Weise and White 1980). There are 

areas of the park that are as high as 15.2 m (50 ft) above sea level. The northern fore-island dune 

ridge is an example of one of these elevated areas in the park, and was largely created by 

prevailing southeasterly winds (Weise and White 1980). 

The Laguna Madre, a portion of which is also part of the park, separates PAIS from the Texas 

mainland. The Laguna Madre is approximately 200 km (124 mi) long, stretching from Corpus 

Christi Bay to the Mexican border. The Laguna is divided into two separate sections by a 20-km 

(12.4 mi) stretch of sand and mudflats that rarely flood (Onuf 2002). The upper section is 

approximately 80 km (50 mi) long, reaching up to Corpus Christi Bay, while the lower section is 

95 km (59 mi) long, extending nearly to the Mexican border. The average depth of both portions 

of the lagoon is 1 m (3.3 ft) (Onuf 2002). It is one of five coastal environments in the world 

known for its hypersaline water (Onuf 2002). The high salinity of the Laguna is attributed to 

limited runoff and few freshwater systems that empty into the Laguna Madre (Frey and Jones 

2008).

The gulf climate is influenced by several regional factors including the El Niño-La Niña

oscillation. Regional drought events can be linked with La Niña events, while El Niño causes 

cooler winter and spring temperatures as well as unusually high winter precipitation (Segura et 

al. 2007). 

Winter in PAIS is mostly mild due to the warm Gulf of Mexico waters, and temperatures 

typically remain just below 20 °C (68 °F) (Table 1). The summer months in PAIS are typically 
6

hot, with average high temperatures in July and August of 32.3 °C (90.1 °F) and 32.4 °C (90.4

°F), respectively (Table 1).

Table 1.Monthly temperature and precipitation normals for PAIS (Station TX417704, Rockport, Texas) 

(http://www.climate-charts.com/USA-Stations/TX/TX417704.php). Jan Feb Mar Apr May Jun Jul

Aug

Sep

Oct

Nov

Dec

Annual

Average Temperature (°C)

Max 17.2 18.6 21.8 24.9 28.1 31.0 32.3 32.4 31.2 27.6 22.6 18.7 25.5

Min 7.2 9.1 13.5 17.4 21.6 24.6 25.3 25.1 22.9 18.5 12.8 8.4 17.2

Average Precipitation (cm)

Total 6.1 5.5 6.0 5.3 9.3 8.9 6.2 8.0 14.0 10.7 6.5 4.9 7.6

2.1.3 Visitation Statistics

In 2012, PAIS had 573,855 visitors to the park (NPS 2012); most visits occurred between May 

and August (NPS 2012). The average number of recreational visitors to the park from 2004-2012 

was 634,135 visitors (NPS 2012). The Gulf beach and the Laguna Madre provide areas that 

allow visitors to fish, camp, bird watch, and swim. Some popular activities on the Gulf beach 

include riding bicycles, four-wheel drive vehicle use, collecting seashells, picnicking, and 

observing the hatching of sea turtles. Windsurfing and hunting are popular activities on the 

Laguna Madre (NPS 2011b). 

2.2 Natural Resources

2.2.1 Ecological Units and Watersheds

PAIS is part of the Environmental Protection Agency’s (EPA) Western Gulf Coastal Plain Level 

III Ecoregion. This ecoregion stretches south from the Mississippi River delta in Louisiana, and 

extends southwest down the Gulf coast to the southern tip of Texas. According to the EPA 

(2010, p. 7),

The principal distinguishing characteristics of the Western Gulf Coastal Plain are its 

relatively flat coastal plain topography and mainly grassland potential natural vegetation. 

Inland from this region the plains are older, more irregular, and have mostly forest or 

savanna-type vegetation potentials. Largely because of these characteristics, a higher 

percentage of the land is in cropland than in bordering ecological regions. Urban and 

industrial land uses have expanded greatly in recent decades, and oil and gas production 

is common.

The EPA divides the Level III Ecoregions into smaller Level IV Ecoregions. In PAIS, the 

Western Gulf Coastal Plain Ecoregion includes one Level IV Ecoregion: the Laguna Madre 

Barrier Islands and Coastal Marshes (EPA 2010). PAIS lies within three watersheds, all of which 

are part of the Texas-Gulf Water Resource Region (USGS 2013). The three watersheds present 

in the park are the North Laguna Madre Watershed, the Central Laguna Madre Watershed, and 

the South Laguna Madre Watershed (Plate 1, USGS 1994).
7

2.2.2 Resource Descriptions

Vegetation

PAIS supports approximately 400 plants species from more than 75 plant families (NPS 2010). 

Little vegetation is found on the park’s beaches due to steady winds, rising and receding tides, 

and violent storm activity. However, a few salt-tolerant plant species have established on the 

beaches including glasswort (Salicornia spp.), sea purslane (Halimione portulacoides), and sea 

dropseed (Sporobolus heterolepis). Active dunes also largely lack vegetation due to strong winds 

and constantly shifting sand; the few species that do grow on active dunes include beach croton 

(Croton monanthogynus), sedges (Cyperaceae spp.), and sea oats (Uniola paniculata) (NPS 

2010).

The foredunes of the park are a region of the beach that has been stabilized due to the 

establishment of vegetation on the elevated sands. Representative species of this region include 

bitter panicgrass (Panicum amarum), sea oats, gulfdune paspalum (Paspalum monostachyum), 

prairie senna (Senna obtusifolia), and morning glory (Ipomoea sagittata) (NPS 2010). 

The low coastal grasslands make up the majority of the interior of the park. This area is 

composed mainly of grass species such as seashore saltgrass (Distichlis spicata), red lovegrass 

(Eragrostis secundiflora), bushy bluestem (Andropogon glomeratus), and marshhay cordgrass 

(Spartina patens). Other species, such as phlox (Phlox subulata), ragweed (Ambrosia 

artemisiifolia), gulfdune paspalum, and sea oats occur in this region of the park.

Species such as cattails (Typha spp.), sedges, starrush whitetop (Rhynchospora colorata), 

marshhay cordgrass, and gulfdune paspalum are found in the pond and marsh habitats across the 

park. Vegetation in these habitats is very tolerant to inundations of both salt and freshwater 

because of the variable length of time the land is submerged. On the western side of the island, 

the wind tidal flats support species that are both flood- and saltwater-tolerant, including sea 

heliotrope (Heliotropium curassavicum), sea blight (Suaeda australis), sea dropseed, glasswort, 

and sea purslane.

The Laguna Madre is dominated by marine grasses, particularly widgeon grass (Ruppia 

maritima), manatee grass (Syringodium filiforme), shoal grass (Halodule wrightii), and halophila 

(Halophila johnsonii). Spoil islands, areas in the Laguna Madre created by dredging activity, 

support ragweed, sea dropseed, glasswort, and dune paspalum (NPS 2010).

Mammals

Twenty-four species of terrestrial mammals have been documented in PAIS. Common native 

mammals in the park include spotted ground squirrels (Spermophilus spilosoma), Gulf coast 

kangaroo rats (Dipodomys compactus), Texas pocket gophers (Geomys personatus), black-tailed 

jackrabbits (Lepus californicus), coyotes (Canis latrans), and white-tailed deer (Odocoileus 

virginianus) (NPS 2013). Less common native mammals include the Virginia opossum 

(Didelphis virginiana), American badger (Taxidea taxus), and the seldom seen bobcat (Lynx 

rufus) (NPS 2013). In addition to the native species found in PAIS, there are three non-native 

mammals: the black rat (Rattus rattus), house mouse (Mus musculus), and nilgai (Boselaphus 

tragocamelus) (NPS 2013).
8

Birds

PAIS has confirmed the presence of 370 bird species within park boundaries (NPS 2013), of 

which approximately 40% are migratory species. Common migratory species found in PAIS 

include the indigo bunting (Passerina cyanea), northern waterthrush (Parkesia noveboracensis), 

Lincoln’s sparrow (Melospiza lincolnii), Swainson’s thrush (Catharus ustulatus), and the gray 

catbird (Dumetella carolinensis) (Blacklock 1998, NPS 2013). Substantially fewer resident bird 

species (67) occur in the park (NPS 2013). Examples of resident bird species in the park include 

the northern pintail (Anas acuta), eastern meadowlark (Sturna magna), and the burrowing owl 

(Athene cunicularia).

The Laguna Madre is a unique and biologically rich habitat that supports a variety of waterfowl, 

marine, and resident/migratory species. Colonial waterbirds are a unique group of birds that nest 

in large numbers on islands in the Laguna Madre. This group of birds is one of the more studied 

avian assemblages in the park, and has been monitored annually by the Texas Colonial 

Waterbird Society (TCWS) since 1974. Examples of common colonial waterbirds seen in PAIS 

include the American white pelican (Pelecanus erythrorhynchos), laughing gull (Leucophaeus 

atricilla), and the royal tern (Sterna maxima).

Herptiles

According to the NPS Certified Species List, 25 species of reptiles are found within PAIS 

boundaries (NPS 2013). While lizards, turtles, and American alligators (Alligator 

mississippiensis) are found in the park, snakes are the most diverse reptile group in the park (15 

species documented). Common snakes in the park include the western diamondback rattlesnake 

(Crotalus atrox), the Texas glossy snake (Arizona elegans arenicola), and the Gulf coast ribbon 

snake (Thamnophis proximus orarius).

Considerably fewer amphibian species occur in PAIS, as only five species have been 

documented in the park (NPS 2013). Species such as the green tree frog (Hyla cinerea), spotted 

chorus frog (Pseudacris clarkii), and Hurter’s spadefoot toad (Scaphiopus hurterii) are typically 

observed near the park’s 

ephemeral ponds (Cooper et al. 

2005).

Islands in the Park

The original dredging of the 

Gulf Intracoastal Waterway 

(GIWW) created 27 spoil 

islands in the Laguna Madre,

ranging from less than 1 ha to 

16.2 ha in size (NPS 1996, 

2001). To maintain navigable 

depths, the GIWW and 

associated channels are redredged every 1-3 years (NPS 

2001). Oil and gas companies 

also dredge channels to reach 

their facilities throughout the 

Photo 1. Aerial photo of a Laguna Madre spoil island (from Weise 

and White 1980).
9

Laguna Madre. Some of the resulting dredge spoil may be placed on existing spoil islands along 

the waterway or may be used to create new islands. Vegetation becomes established on many 

spoil islands from seeds carried by the wind, water, or nesting birds (NPS 2001). 

The dredge spoil islands within the park are of high importance to nesting colonial waterbirds. 

Within PAIS boundaries, there are 11 man-made dredge spoil islands in the Laguna Madre that 

have acted as the primary nesting habitat for colonial waterbird species; several of these islands 

support thousands of nesting pairs from many different species. For many species (e.g., 

American white pelican and black skimmer [Rynchops niger]), the colony sites in PAIS represent 

>25% of their coast-wide population (TCWS 2011). However, the unstable nature of these manmade islands has become evident in recent years, as several of the islands have experienced

elevated rates of erosion. 

The National Audubon Society and the Coastal Bend Bays and Estuaries Program (CBBEP), in 

cooperation with the U.S. Fish and Wildlife Service (USFWS), have spent large amounts of 

money to re-establish islands in the Laguna Madre (not in PAIS) that have historically acted as 

rookery islands but have since been lost due to erosion (Wade Stablein, PAIS Biological 

Technician, email communication, 27 March 2013). The colonial waterbird nesting populations 

are monitored annually in the park by the TCWS, and the data are provided to the park for 

analysis of potential trends in species abundance and island health.

There are also two natural islands within PAIS’ administrative boundaries, North and South Bird 

Islands. These islands have not supported nesting colonial waterbirds for some time, but should 

the dredge spoil islands in the Laguna continue to erode at an elevated rate, these naturally 

occurring islands could see an increase in colonial waterbird use (Stablein, email 

communication, 27 March 2013).

2.2.3. Resource Issues Overview

Changes in the Laguna Madre

The Gulf of Mexico supports as much as 50% of the United States’ seagrass population (Beck et 

al. 2007). These seagrass communities have experienced dramatic changes in recent years, as 

vegetation on the bottom of the lagoon has decreased by almost 3,000 ha [1,214 ac] from 1960 to 

1998 (approximately 4% of the entire lagoon bottom) (Onuf 2002). According to Beck et al. 

(2007), loss of lagoon vegetation is due to climate changes, variations in water level, erosion of 

sediment, turbidity, and physical removal by dredging or other human-induced methods. The 

salinity of the lagoon has decreased by half since the 1960s (Pulich and Onuf 2002), with several 

changes on the landscape found to be the main reasons for this shift, including the dredging of 

the GIWW, the opening of the “Land Cut” between the upper and lower Laguna Madre, and the 

creation of the Mansfield Channel at the south end of the park. As a result, plant composition has 

transitioned to more salt-intolerant plants, such as manatee grass and turtle grass (Thalassia 

testudinum). This transition has stressed the shoal grass population, as shoal grass’s composition 

decreased from 64% to 40% in the time following dredging of the GIWW (Onuf 2002). A 

decrease in shoal grass density is likely to affect other species, as shoal grass is an important 

food source for the redhead duck (Aythya americana) and many other waterfowl species (Onuf 

2002, Pulich and Onuf 2002).
10

Non-native Species

While vegetative sampling efforts in PAIS have only recorded two non-native plant species 

(buffelgrass [Pennisetum ciliare] and annual rabbitsfoot grass [Polypogon monspeliensis]; 

Drawe et al. 1981, Nelson et al. 2000), James Lindsay (written communication, October 2012)

indicates that PAIS is home to over 40 species of non-native plant species (See Chapter 4.1 for a 

list of all non-native plant species). While the exact locations and extent of these non-native plant 

species has not been mapped, these species have the potential to outcompete native species, and 

may result in a non-native dominated landscape.

Non-native plant species are prevalent on the park’s dredge spoil islands. Several of these 

species were planted on the islands by cabin owners prior to PAIS assuming ownership of the 

islands; plants were placed on the islands in order to stabilize the soil and prevent rapid erosion 

(Stablein, written communication, 15 March 2013). Examples of non-native plant species that 

were planted on these islands include Brazilian peppertrees (Schinus terebinthifolius), oleander 

(Nerium oleander), bermudagrass (Cynodon dactylon) and saltcedar (Tamarix ramosissima). The 

presence of non-native plant species on these islands is of substantial concern to the nesting 

colonial waterbirds, as several of these nesting species require bare ground or a specific degree 

of vegetative cover in order to successfully nest.

A non-native wildlife species in the park that is uncommon is the nilgai. Nilgai, a species of 

antelope from Pakistan and northern India, were introduced to Texas in the first half of the 20th

century (NatureServe 2011), and occasionally can be found roaming the grasslands of the park. 

In the 1980s, Lochmiller and Sheffield (1989) reported a nilgai population of approximately 

10,000 in southeastern Texas. It was later reported that the population grew to around 15,000 

(Schmidly 2004). 

Hurricanes and Tropical Storms

Hurricanes and tropical storms strike the Gulf Coast of Texas at an average rate of 0.67 storms 

per year, or approximately two storms every 3 years; most of these weather events occur between 

late spring and early fall, with prime storm activity occurring from August through October 

(Weise and White 1980). When wind velocities reach 119 km/hr (74 mi/hr), tropical storms 

become categorized as hurricanes; the most severe hurricanes have sustained winds in excess of 

322 km/hr (200 mi/hr) (Weise and White 1980). Such strong winds can cause significant 

geomorphological change to the beach and dunes and impact vegetation communities. During 

storm events, vegetation can be uprooted and large amounts of sand can be displaced inland or 

into the bay or lagoon (Weise and White 1980). An equally destructive force is the tidal storm 

surge that accompanies hurricanes. These surges expend tremendous energy, causing erosion and 

transporting and depositing huge amounts of coastal sediment in the process (Weise and White

1980). 

Barrier islands along the central Gulf coast of Texas serve as the first land feature in the path of 

hurricane storm surges and high winds. The well-developed foredune ridge along the Gulf side 

of PAIS provides a significant defense for the mainland against damaging winds and tidal storm 

surges; the barrier helps to block much of the tidal inflow and dissipate wave energy that could 

cause substantial damage to shorelines (Weise and White 1980). However, the barrier islands in 

the path of such storms experience significant changes in this process. Storm surges do not 

always stop at the beach; they can wash across through low-lying areas and flood into washover 
11

channels and tidal flats on the backside of the dune ridge, carrying sediments inland and toward 

the Laguna Madre (Weise and White 1980). PAIS has experienced significant coastal and island 

changes due to tropical storms and hurricanes in the last 60 years, making the barrier island a 

highly dynamic landscape.

Harmful Algal Blooms (HABs)

Red tides are unusually high concentrations of algal blooms, specifically Karenia brevis (NPS 

2006), and are a naturally occurring phenomenon in marine ecosystems. Red tides are 

characterized by a discoloration of water (usually reddish in color) and an ammonia-like smell. 

Blooms are pushed toward shore by the tides and become a problem not only in open water, but 

also on shorelines and coastal areas that are protected from wind and tides (NPS 2006). Red tide 

events are a cause for concern because the microscopic algae release a neurotoxin that paralyzes 

fish, makes seafood unsafe for consumption, and can cause difficulty in breathing for visitors and 

park staff (NPS 2006, EPA 2012). Although little is known about what instigates a red tide, the 

most common time of year for red tides is between August and February. It has been 

hypothesized that high temperatures and periods of little to no wind or rain create favorable 

conditions for red tides (NPS 2006), but it is unclear what other conditions contribute to events.

Relative Sea Level Rise

Being surrounded by water on both sides of the park, relative sea level rise is a concern for the 

park. According to Douglas (1997), sea level around the earth has increased an average of 18 cm 

(7.1 in) over the last century. Meehl et al. (2007) estimate that changing climates will cause 

global sea levels to rise approximately 20-60 cm (7.9-23.6 in) during the 21st century (assuming 

the polar ice sheets remain stable). Sea level rise could impact the beaches of PAIS through 

increased rates of shoreline erosion, inundation of groundwater aquifers with saltwater, flooding 

of wetlands and estuaries, as well as damage to park infrastructure (Pendleton et al. 2004).

Pendleton et al. (2004) conducted a sea-level rise vulnerability assessment at PAIS in 2004. A 

coastal vulnerability index (CVI) was used in order to estimate and map the potential effects that 

future sea-level rise would have on the park; areas of the park that were most vulnerable to the 

physical effects of sea-level rise were highlighted in this report. Coastal evolution is a process 

that involves unique shoreline variables. In order to more accurately predict the effects that sealevel rise may have at the park, Pendleton et al. (2004, p. 3-4) focused on six variables that 

strongly influence PAIS’s coastal evolution:

1. Geomorphology;

2. Historical shoreline change rate;

3. Regional coastal slope;

4. Relative sea-level change;

5. Mean significant wave height;

6. Mean tidal range.

The vulnerability of each of these variables was assessed using a 5-tiered ranking system (Table 

2).
12

Table 2. Vulnerability rankings and ranges of values for the six selected PAIS variables (table modified from Pendleton et al. 2004).

Variables Very Low (1) Low (2) Moderate (3) High (4) Very High (5)

Geomorphology

Rocky cliffed coasts, 

fjords

Medium cliffs, 

indented coasts

Low cliffs, glacial 

drift, alluvial plains

Cobble beaches, 

estuary, lagoon

Barrier beaches, sand 

beaches, salt marsh, 

mud flats, deltas, 

mangrove, coral reefs

Shoreline 

Erosion/Accretion (m/yr) > 2.0 1.0 - 2.0 -1.0 - 1.0 -2.0 - -1.0 < -2.0

Coastal Slope (%) > 1.20 1.20 - 0.90 0.90 - 0.60 0.60 - 0.30 < 0.30

Relative Sea-level 

Change (mm/yr) < 1.8 1.8 - 2.5 2.5 - 3.0 3.0 - 3.4 > 3.4

Mean Wave Height (m) < 0.55 0.55 - 0.85 0.85 - 1.05 1.05 - 1.25 > 1.25

Mean Tide Range (m) > 6.0 4.0 - 6.0 2.0 - 4.0 1.0 - 2.0 < 1.0
13

Figure 1 displays the relative coastal vulnerability of PAIS as determined by Pendleton et al. 

(2004)’s CVI analysis. The areas of the park with very high vulnerability to sea-level rise 

(represented as red sections in Figure 1) include shorelines located in washover canals and dunes 

where shoreline erosion is the highest. The central portion of PAIS is least vulnerable to sealevel rise, primarily due to high levels of shoreline accretion and elevated dune ridges (Pendleton 

et al. 2004).

Figure 1. Relative coastal vulnerability for PAIS. The innermost color bar is the CVI, while the remaining 

six color bars are separated based on the variables outlined earlier in this text (Image reproduced from 

Pendleton et al. 2004).
14

2.3 Resource Stewardship

2.3.1 Management Directives and Planning Guidance

The broad resource management goals of PAIS, according to NPS (1996, p. 9-10), are to:

1. Preserve, understand, protect, and manage the cultural and natural resources of the park 

within naturally functioning ecosystems, consistent with cultural resource preservation;

2. Provide the means and opportunity for people to study, understand and enjoy the 

resources of the seashore without compromising the resources or ethnographic values.

From a natural resource-specific viewpoint, the park’s established natural resource management 

objectives are to:

1. Restore and sustain natural ecosystem conditions and processes unimpaired from human 

influence, to the degree practical given landscape, insuring visitor safety, and cultural 

resource constraints;

2. Carry out a management program, which preserves and restores resource conditions and 

values defined by law and policy and is compatible with cultural resource objectives;

3. Preserve a comprehensive natural resource base for its innate value to promote scientific 

and educational interest (NPS 1996, p. 10).

Furthermore, all park management activities are prioritized based on where they fall within the 

following management strategy (NPS 1996, p. 4):

1. Minimize the loss of resources/information;

2. Meet legal requirements;

3. Mitigate management and visitor caused impacts;

4. Mitigate external impacts.

2.3.2 Status of Supporting Science

The Gulf Coast Inventory and Monitoring Network (GULN) identifies key resources for each of 

its parks network-wide. The identified resources, called “Vital Signs”, are used to determine the 

overall health of the parks. In 2007, the GULN completed and released a Vital Signs Monitoring 

Plan (Segura et al. 2007); Table 3 shows the GULN Vital Signs selected for monitoring in PAIS.
15

Table 3. GULN Vital Signs selected for monitoring in PAIS (Segura et al. 2007).

Category GULN Vital Sign

Category 

1

Category 

2

Category 

3

No 

Monitoring 

Planned

Air and 

Climate

Ozone X

Air Contaminants X

Weather/Climate X

Geology 

and Soils

Coastal Dynamics X

Subsidence/Relative Sea Level Rise X

Soil Biota X

Soil Chemistry X

Soil Compaction X

Soil Structure and Stability X

Water

Groundwater Hydrology X

Water Chemistry X

Water Nutrients X

Water Toxics X

Biological 

Integrity

Non-native Vegetation X

Non-native Animals X

Freshwater Wetland Communities X

Marine and Estuarine Submerged Aquatic Veg. X

Forest Health X

Marine Invertebrates X

Terrestrial Invertebrates X

Marine and Estuarine Fish X

Amphibians X

Non T&E Reptiles X

Migratory Birds X

Resident Birds X

Non T&E Small Mammals X

Terrestrial Vegetation X

T&E Rare Birds X

T&E Rare Plats X

T&E Rare Reptiles X

Human Use Visitor Usage X

Landscapes 

(Ecosystem 

Pattern and 

Processes)

Fire and Fuel Dynamics X

Land Cover/Land Use X

Soundscape X

A Category 1 represents Vital Signs for which the network will develop protocols and implement monitoring. B Category 2 represents Vital Signs that are monitored by PAIS, another NPS program, or by another federal or state 

agency using other funding. 

C Category 3 represents high-priority Vital Signs for which monitoring will likely be done in the future.
16

Literature Cited

Beck, M. W., W. L. Kruczynski, and P. F. Sheridan. 2007. Conclusions. Pages 255-263 in

Seagrass status and trends in the northern Gulf of Mexico: 1940-2002. Handley, L., D.

Altsman, and R. DeMay (eds.). U.S. Geological Survey Scientific Investigations Report 

2006-5287, Reston, Virginia.

Climate-Charts. 2010. Climate, global warming, and daylight charts and data: Rockport, Texas, 

USA. Climate-Charts Online. http://www.climate-charts.com/USAStations/TX/TX417704.php (accessed 13 February 2012).

Cooper, R. J., S. B. Cederbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore: final report. University of Georgia, Athens, GA.

Douglas, B. C. 1997. Global sea rise, a redetermination. Surveys in Geophysics 18:279-292.

Environmental Protection Agency (EPA). 2010. Primary distinguishing characteristics of Level 

III and IV Ecoregions of the continental United 

States. http://www.epa.gov/wed/pages/ecoregions/level_iii_iv.htm (accessed 25 March 

2013).

Environmental Protection Agency (EPA). 2012. The facts about harmful algal blooms. United 

States Environmental Protection Agency 

Online http://water.epa.gov/polwaste/nps/outreach/upload/HABs_final_12-6-12.pdf

(accessed 9 January 2013).

Meehl, G. A., T. F. Stocker, W. D. Collins, P. Friedlingstein, A. T. Gaye, J. M. Gregory, A. 

Kitoh, R. Knutti, J. M. Murphy, A. Noda and others. 2007. Global climate projections In

Climate change 2007: the physical basis, edited by S. Solomon, D. Qin, M. Manning, Z. 

Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller, 747-845. Volume contribution 

of working group I to the Fourth Assessment Report of the Intergovernmental Panel on 

Climate Change. Cambridge University Press Cambridge, UK and New York, New York.

National Park Service (NPS). 1974. Padre Island National Seashore, Texas: master plan. 

National Park Service, Washington, D. C.

National Park Service (NPS). 1996. Padre Island National Seashore resources management plan. 

National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS). 2001. Padre Island National Seashore: Oil and gas management 

plan. National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS). 2006. Padre Island National Seashore: other life forms. U.S. 

Department of the Interior. http://www.nps.gov/pais/naturescience/otherlifeforms.htm

(accessed 21 February 2012).

National Park Service (NPS). 2009. Padre Island National Seashore: Fish. U.S. Department of 

the Interior. http://www.nps.gov/pais/naturescience/fish.htm (accessed 16 February 2012).
17

National Park Service (NPS). 2010. Padre Island National Seashore: Plants. U.S. Department of 

the Interior. http://www.nps.gov/pais/naturescience/plants.htm (accessed 16 February 2012). 

National Park Service (NPS). 2011a. Padre Island National Seashore: Birds. U.S. Department of 

the Interior. http://www.nps.gov/pais/naturescience/birds.htm (accessed 16 February 2012).

National Park Service (NPS). 2011b. Padre Island National Seashore: Things to do. U.S. 

Department of the Interior. http://www.nps.gov/pais/planyourvisit/things2do.htm (accessed 

15 February 2012).

National Park Service (NPS). 2012. Padre Island NS annual park visitation 

report. https://irma.nps.gov/Stats/ (accessed 29 April 2013).

National Park Service (NPS). 2013. NPSpecies online 

database. https://irma.nps.gov/App/Species/Search (accessed 16 January 2013).

Pendleton, E. A., E. R. Thieler, S. J. Williams, and R. L. Beavers. 2004. Coastal vulnerability 

assessment of Padre Island National Seashore (PAIS) to sea-level rise. U.S. Geological 

Survey Open-File Report 2004-1090. U.S. Geological Survey, Reston, Virginia.

Segura, M., R. Woodman, J. Meiman, W. Granger, and J. Bracewell. 2007. Gulf Coast Network 

Vital Signs monitoring program. Natural Resource Report NPS/GULN/NRR—2007/015 

Version 1.1. National Park Service, Lafayette, Louisiana.

Texas Colonial Waterbird Society (TCWS). 2011. Summary of colonial waterbird counts for 

2011 for the Central Texas 

Coast. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/tcws/documents/2011_C

entral_Coast.pdf (accessed 12 February 2013).

U.S. Geological Survey (USGS). 1994. 1:250,000-scale hydrologic units of the United States. 

ArcGIS data file.

U.S. Geological Survey (USGS). 2013. Science in your watershed: water resource 

regions. http://water.usgs.gov/wsc/map_index.html (accessed 25 March 2013).

Weise, B. R., and W. A. White. 1980. Padre Island National Seashore: a guide to the geology, 

natural environments, and history of a Texas Barrier Island. Texas Bureau of Economic 

Geology, University of Texas at Austin, Texas.
18

Plate 1. The three watersheds that occur within park boundaries (USGS 1994).
19

Chapter 3 Study Scoping and Design

This NRCA is a collaborative project between the NPS and Saint Mary’s University of 

Minnesota Geospatial Services (SMUMN GSS). Project stakeholders include the PAIS resource 

management team and GULN Inventory and Monitoring Program staff. Before embarking on the 

project, it was necessary to identify the specific roles of the NPS and SMUMN GSS. Preliminary 

scoping meetings were held, and a task agreement and a scope of work document were created 

cooperatively between the NPS and SMUMN GSS.

3.1 Preliminary Scoping

A preliminary scoping meeting was held on 12-13 December 2011. At this meeting, SMUMN 

GSS and NPS staff confirmed that the purpose of the PAIS NRCA was to evaluate and report on 

current conditions, critical data and knowledge gaps, and selected existing and emerging 

resource condition influences of concern to PAIS managers. Certain constraints were placed on 

this NRCA, including the following:

• Condition assessments are conducted using existing data and information;

• Identification of data needs and gaps is driven by the project framework categories;

• The analysis of natural resource conditions includes a strong geospatial component;

• Resource focus and priorities are primarily driven by PAIS resource management;

This condition assessment provides a “snapshot-in-time” evaluation of the condition of a select 

set of park natural resources that were identified and agreed upon by the project team. Project 

findings will aid PAIS resource managers in the following objectives:

• Develop near-term management priorities (how to allocate limited staff and funding

resources);

• Engage in watershed or landscape scale partnership and education efforts;

• Consider new park planning goals and take steps to further these;

• Report program performance (e.g., Department of Interior Strategic Plan “land health” 

goals, Government Performance and Results Act [GPRA]).

Specific project expectations and outcomes included the following:

• For key natural resource components, consolidate available data, reports, and spatial 

information from appropriate sources including PAIS resource staff, the NPS Integrated 

Resource Management Application (IRMA) website, Inventory and Monitoring Vital 

Signs, and available third-party sources. The NRCA report will provide a resource 

assessment and summary of pertinent data evaluated through this project.
20

• When appropriate, define a reference condition so that statements of current condition 

may be developed. The statements will describe the current state of a particular resource 

with respect to an agreed upon reference point.

• Clearly identify “management critical” data (i.e., those data relevant to the key 

resources). This will drive the data mining and gap definition process.

• Where applicable, develop GIS products that provide spatial representation of resource 

data, ecological processes, resource stressors, trends, or other valuable information that

can be better interpreted visually.

• Utilize “gray literature” and reports from third party research to the extent practicable.

3.2 Study Design

3.2.1 Indicator Framework, Focal Study Resources and Indicators

Selection of Resources and Measures

As defined by SMUMN GSS in the NRCA process, a “framework” is developed for a park or 

preserve. This framework is a way of organizing, in a hierarchical fashion, bio-geophysical 

resource topics considered important in park management efforts. The primary features in the 

framework are key resource components, measures, stressors, and reference conditions. 

“Components” in this process are defined as natural resources (e.g., birds), ecological processes 

or patterns (e.g., natural fire regime), or specific natural features or values (e.g., geological 

formations) that are considered important to current park management. Each key resource 

component has one or more “measures” that best define the current condition of a component 

being assessed in the NRCA. Measures are defined as those values or characterizations that 

evaluate and quantify the state of ecological health or integrity of a component. In addition to 

measures, current condition of components may be influenced by certain “stressors,” which are 

also considered during assessment. A “stressor” is defined as any agent that imposes adverse 

changes upon a component. These typically refer to anthropogenic factors that adversely affect 

natural ecosystems, but may also include natural processes or disturbances such as floods, fires, 

or predation (adapted from GLEI 2010). 

During the PAIS NRCA scoping process, key resource components were identified by NPS staff 

and are represented as “components” in the NRCA framework. While this list of components is 

not a comprehensive list of all the resources in the park, it includes resources and processes that 

are unique to the park in some way, or are of greatest concern or highest management priority in 

PAIS. Several measures for each component, as well as known or potential stressors, were also

identified in collaboration with NPS resource staff.

Selection of Reference Conditions

A “reference condition” is a benchmark to which current values of a given component’s 

measures can be compared to determine the condition of that component. A reference condition 

may be a historical condition (e.g., flood frequency prior to dam construction on a river), an 
21

established ecological threshold (e.g., EPA standards for air quality), or a targeted management 

goal/objective (e.g., a bison herd of at least 200 individuals) (adapted from Stoddard et al. 2006).

Reference conditions in this project were identified during the scoping process using input from 

NPS resource staff. In some cases, reference conditions represent a historical reference before

human activity and disturbance was a major driver of ecological populations and processes, such 

as “pre-fire suppression.” In other cases, peer-reviewed literature and ecological thresholds 

helped to define appropriate reference conditions.

Finalizing the Framework

An initial framework was adapted from the organizational framework outlined by the H. John 

Heinz III Center for Science’s “State of Our Nation’s Ecosystems 2008” (Heinz Center 2008). 

This initial framework was presented to park resource staff to stimulate meaningful dialogue 

about key resources that should be assessed. Significant collaboration between SMUMN GSS 

analysts and NPS staff was needed to focus the scope of the NRCA project and finalize the 

framework of key resources to be assessed. 

The NRCA framework was finalized in March 2012, following acceptance from NPS resource 

staff. It contains a total of 17 components (Table 4) and was used to drive analysis in this NRCA. 

This framework outlines the components (resources), most appropriate measures, known or 

perceived stressors and threats to the resources, and the reference conditions for each component

for comparison to current conditions.
22

Table 4. Padre Island National Seashore natural resource condition assessment framework.
23

Table 4. Padre Island National Seashore natural resource condition assessment framework (continued).
24

Table 4. Padre Island National Seashore natural resource condition assessment framework (continued).
25

3.2.2 General Approach and Methods

This study involved gathering and reviewing existing literature and data relevant to each of the 

key resource components included in the framework. No new data were collected for this study; 

however, where appropriate, existing data were further analyzed to provide summaries of 

resource condition or to create new spatial representations. After all data and literature relevant 

to the measures of each component were reviewed and considered, a qualitative statement of 

overall current condition was created and compared to the reference condition when possible.

Data Mining

The data mining process (acquiring as much relevant data about key resources as possible) began 

at the initial scoping meeting, at which time PAIS staff provided data and literature in multiple 

forms, including: NPS reports and monitoring plans, reports from various state and federal 

agencies, published and unpublished research documents, databases, tabular data, and charts. 

GIS data were provided by NPS staff. Additional data and literature were also acquired through 

online bibliographic literature searches and inquiries on various state and federal government 

websites. Data and literature acquired throughout the data mining process were inventoried and 

analyzed for thoroughness, relevancy, and quality regarding the resource components identified 

at the scoping meeting.

Data Development and Analysis

Data development and analysis was highly specific to each component in the framework and 

depended largely on the amount of information and data available for the component and 

recommendations from NPS reviewers and sources of expertise including NPS staff from PAIS

and the GULN. Specific approaches to data development and analysis can be found within the 

respective component assessment sections located in Chapter 4 of this report.

Scoring Methods and Assigning Condition

Significance Level

A set of measures are useful in describing the condition of a particular component, but all 

measures may not be equally important. A “Significance Level” represents a numeric 

categorization (integer scale from 1-3) of the importance of each measure in assessing the 

component’s condition; each Significance Level is defined in Table 5. This categorization allows 

measures that are more important for determining condition of a component (higher significance 

level) to be more heavily weighted in calculating an overall condition. Significance Levels were 

determined for each component measure in this assessment through discussions with park staff 

and/or outside resource experts.

Table 5. Scale for a measure’s significance level in determining a components overall condition.

Significance Level

(SL) Description

1 Measure is of low importance in defining the condition of this component.

2 Measure is of moderate importance in defining the condition of this 

component.

3 Measure is of high importance in defining the condition of this component.
26

Condition Level

After each component assessment is completed (including any possible data analysis), SMUMN 

GSS analysts assign a Condition Level for each measure on a 0-3 integer scale (Table 6). This is 

based on all the available literature and data reviewed for the component, as well as 

communications with park and outside experts.

Table 6. Scale for condition level of individual measures.

Condition Level

(CL) Description

0 Of NO concern. No net loss, degradation, negative change, or alteration.

1 Of LOW concern. Signs of limited and isolated degradation of the component.

2 Of MODERATE concern. Pronounced signs of widespread and uncontrolled 

degradation.

3 Of HIGH concern. Nearing catastrophic, complete, and irreparable degradation 

of the component.

Weighted Condition Score

After the Significance Levels (SL) and Condition Levels (CL) are assigned, a Weighted 

Condition Score (WCS) is calculated via the following equation:

WCS = ∑ SLi ∗ CLi

# of measures

i=1

3 ∗ ∑ SLi

# of measures

i=1

The resulting WCS value is placed into one of three possible categories: condition of no to low 

concern (WCS = 0.0 – 0.33); condition of moderate concern (WCS = 0.34 - 0.66); and condition 

of significant concern (WCS = 0.67 to 1.00). Figure 2 displays all of the potential graphics used 

to represent a component’s condition in this assessment. The colored circles represent the 

categorized WCS; red circles signify a significant concern, yellow circles a moderate concern 

and green circles a condition of low concern. Gray circles are used to represent situations in 

which SMUMN GSS analysts and park staff felt there was currently insufficient data to make a 

statement about the condition of a component. The arrows inside the circles indicate the trend of 

the condition of a resource component, based on data and literature from the past 5-10 years, as 

well as expert opinion. An upward pointing arrow indicates the condition of the component has 

been improving in recent times. A right-pointing arrow indicates a stable condition or trend and 

an arrow pointing down indicates a decline in the condition of a component in recent times. 

These are only used when it is appropriate to comment on the trend of condition of a component. 

A gray, triple-pointed arrow is reserved for situations in which the trend of the component’s 

condition is currently unknown.
27

Figure 2. Symbols used for individual component assessments with condition or concern designations 

along the vertical axis and trend designations along the horizontal.

Preparation and Review of Component Draft Assessments

The preparation of draft assessments for each component was a highly cooperative process 

among SMUMN GSS analysts and PAIS and GULN staff. Though SMUMN GSS analysts rely 

heavily on peer-reviewed literature and existing data in conducting the assessment, the expertise 

of NPS resource staff also plays a significant and invaluable role in providing insights into the 

appropriate direction for analysis and assessment of each component. This step is especially 

important when data and/or literature are limited for a resource component.

The process of developing draft documents for each component began with a detailed phone or 

conference call with an individual or multiple individuals considered local experts on the 

resource components under examination. These conversations were a way for analysts to verify 

the most relevant data and literature sources that should be used and also to formulate ideas 

about current condition with respect to the NPS staff opinions. Upon completion, draft 

assessments were forwarded to component experts for initial review and comments.

Development and Review of Final Component Assessments

Following review of the component draft assessments, analysts used the review feedback from 

resource experts to compile the final component assessments. As a result of this process, and 

based on the recommendations and insights provided by PAIS resource staff and other experts,

the final component assessments represent the most relevant and current data available for each 

component and the sentiments of park resource staff and resource experts. 

Format of Component Assessment Documents

All resource component assessments are presented in a standard format. The format and structure 

of these assessments is described below.
28

Description

This section describes the relevance of the resource component to the park and the context within 

which it occurs in the park setting. For example, a component may represent a unique feature of 

the park, it may be a key process or resource in park ecology, or it may be a resource that is of 

high management priority in the park. Also emphasized are interrelationships that occur among 

the featured component and other resource components included in the NRCA.

Measures

Resource component measures were defined in the scoping process and refined through dialogue 

with resource experts. Those measures deemed most appropriate for assessing the current 

condition of a component are listed in this section, typically as bulleted items.

Reference Conditions/Values

This section explains the reference condition determined for each resource component as it is 

defined in the framework. Explanation is provided as to why specific reference conditions are 

appropriate or logical to use. Also included in this section is a discussion of any available data 

and literature that explain and elaborate on the designated reference conditions. If these 

conditions or values originated with the NPS experts or SMUMN GSS analysts, an explanation 

of how they were developed is provided.

Data and Methods

This section includes a discussion of the data sets used to evaluate the component and if or how 

these data sets were adjusted or processed as a lead-up to analysis. If adjustment or processing of 

data involved an extensive or highly technical process, these descriptions are included in an 

appendix for the reader or a GIS metadata file. Also discussed is how the data were evaluated 

and analyzed to determine current condition (and trend when appropriate). 

Current Condition and Trend

This section presents and discusses in-depth key findings regarding the current condition of the 

resource component and trends (when available). The information is presented primarily with 

text but is often accompanied by detailed maps or plates that display different analyses, as well 

as graphs, charts, and/or tables that summarize relevant data or show interesting relationships. 

All relevant data and information for a component is presented and interpreted in this section.

Threats and Stressor Factors

This section provides a summary of the threats and stressors that may impact the resource and 

influence to varying degrees the current condition of a resource component. Relevant stressors 

were described in the scoping process and are outlined in the NRCA framework. However, these 

are elaborated on in this section to create a summary of threats and stressors based on a 

combination of available data and literature, and discussions with resource experts and NPS

natural resources staff. 

Data Needs/Gaps

This section outlines critical data needs or gaps for the resource component. Specifically, what is 

discussed is how these data needs/gaps, if addressed, would provide further insight in 

determining the current condition or trend of a given component in future assessments. In some 

cases, the data needs/gaps are significant enough to make it inappropriate or impossible to 
29

determine condition of the resource component. In these cases, stating the data needs/gaps is 

useful to natural resources staff seeking to prioritize monitoring or data gathering efforts.

Overall Condition 

This section provides a qualitative summary statement of the current condition that was 

determined for the resource component using the WCS method. Condition is determined after 

thoughtful review of available literature, data, and any insights from NPS staff and experts, 

which are presented in the Current Condition and Trend section. The Overall Condition section 

summarizes the key findings and highlights the key elements used in determining and justifying 

the level of concern, if any, that analysts attribute to the condition of the resource component. 

Also included in this section are the graphics used to represent the component condition.

Sources of Expertise

This is a listing of the individuals (including their title and affiliation with offices or programs) 

who had a primary role in providing expertise, insight, and interpretation to determine current 

condition (and trend when appropriate) for each resource component.

Literature Cited

This is a list of formal citations for literature or datasets used in the analysis and assessment of 

condition for the resource component. Note, citations used in appendices and plates referenced in 

each section (component) of Chapter 4 are listed in that section’s “Literature Cited” section.
30

Literature Cited

Great Lakes Environmental Indicators Project (GLEI). 2010. Glossary, 

Stressor. http://glei.nrri.umn.edu/default/glossary.htm (accessed 31 January 2013).

The H. John Heinz III Center for Science, Economics, and the Environment. 2008. The state of 

the nation’s ecosystems 2008: Measuring the land, waters, and living resources of the United 

States. Island Press, Washington, D.C.

Stoddard. J. L., D. P. Larsen, C. P. Hawkins, R. K. Johnson, and R. J. Norris. 2006. Setting 

expectations for the ecological condition of streams: the concept of reference condition. 

Ecological Applications 16(4):1267-1276.
31

Chapter 4 Natural Resource Conditions

This chapter presents the background, analysis, and condition summaries for the 17 key resource 

components in the project framework. The following sections discuss the key resources and their 

measures, stressors, and reference conditions. The summary for each component is arranged 

around the following sections:

1. Description

2. Measures 

3. Reference Condition 

4. Data and Methods 

5. Current Condition and Trend (including threats and stressor factors, data needs/gaps, and 

overall condition) 

6. Sources of Expertise 

7. Literature Cited

The order of components follows the project framework (Table 4):

4.1 Terrestrial Vegetative Communities

4.2 Algal Mats on Mud Flats

4.3 Seagrass Community

4.4 Emergent Wetland and Pond Communities

4.5 Migratory Birds 

4.6 Resident Birds 

4.7 Colonial Water Birds 

4.8 Coyotes 

4.9 Small Mammals

4.10 Macroinvertebrates

4.11 Reptiles

4.12 Sea Turtles

4.13 Amphibians

4.14 Water Quality

4.15 Air Quality

4.16 Dark Night Skies

4.17 Coastal Dunes and Beaches
32
33

4.1 Terrestrial Vegetative Communities

Description

The terrestrial vegetative 

communities of PAIS can 

generally be divided into 

two broad categories: 

vegetated dunes and interior 

grasslands. Little vegetation 

is found on the beach due to 

winds, tides, and storms; 

those species that are 

present must be salt tolerant

(NPS 2010). Vegetation 

stabilizes dune formations

by rooting into the sand and 

holding it in place (Baccus 

et al. 1977). Common plant 

species in the dunes include 

seaoats, bitter panicgrass, gulfdune paspalum, and morning-glories (Ipomoea spp.) (Baccus 

1977, NPS 2010). Much of the island’s interior is dominated by grassland, with species such as 

red lovegrass, bushy bluestem, shore little bluestem (Schizachyrium scoparium var. littorale),

cordgrasses (Spartina spp.), and coastal indigo (Indigofera miniata), as well as gulfdune 

paspalum and seaoats (Carls et al. 1991, NPS 2010). Several relict live oak (Quercus fusiformis) 

mottes also occur within PAIS (Nelson et al. 2000). A geographic profile of the island and its 

vegetation types is shown in Figure 3. 

Measures

• Native plant species richness

• Percent coverage of grassland and dune vegetation

• Percent of bare ground

• Extent of oak motte areas

• Presence vs. absence of non-native species

Photo 2. A grassland in PAIS, with vegetated dunes in the background 

(photo by Shannon Amberg, SMUMN GSS).
34

Figure 3. A geographic profile of Padre Island with typical vegetation communities (NPS 2001, adapted from Weise and White 1980). MSL = 

mean sea level.
35

Reference Conditions/Values

The reference condition for the park’s terrestrial vegetative communities will be the condition 

prior to intensive cattle ranching/grazing (late 1800s through 1971). Historical records from early 

explorers of the island suggest that it was “originally well covered with tall grass” (Recenthin 

and Passey 1967, as cited in Kattner 1973, p. 1).

Data and Methods

Descriptions of the park’s vegetative communities from the 1970s and early 1980s were found in 

Kattner (1973), Baccus et al. (1977), and Drawe et al. (1981). The park’s vegetation was also 

sampled and described by Carls et al. (1991) and Nelson et al. (2000), who focused particularly 

on the Big Ball Hill Region south of the Visitor Center. Drawe and Kattner (1978) and Lonard et 

al. (2004) studied the effects of fire on the park’s terrestrial vegetation, while McAtee (1974) and 

McAtee and Drawe (1981) explored the impacts of vehicle and pedestrian traffic at PAIS. 

The first landcover classification for the park was completed by Laine and Ramsey (1998), and 

provides information on the extent of grassland and sparse (i.e., dune) vegetation in PAIS based 

on mid-1990s aerial photography. Laine and Ramsey (1998) followed classification and 

interpretation protocols developed by the National Oceanic and Atmospheric Administration

(NOAA) Coastal Change and Analysis Project (C-CAP) program and used map descriptions that 

fit National Vegetation Classification Standard (NVCS) guidelines. Since 2000, several 

vegetation maps of selected areas within the park have been created for energy development 

proposals (Plate 2). However, these maps cover just small areas and are not reflective of 

conditions in the park as a whole. In 2007, the NPS created a surficial geology GIS map for 

PAIS, based primarily on 2003 aerial photography (NPS 2012). Map classifications are loosely 

based on land cover and include four classes that could be considered terrestrial vegetative 

communities (NPS 2007). This map and associated data are considered the most up-to-date and 

accurate information regarding the distribution of terrestrial vegetative communities in PAIS.

Current Condition and Trend

Native Plant Species Richness

Several researchers have sampled the terrestrial vegetative communities of PAIS and 

documented species richness by habitat type. These studies occurred in different areas of the 

park with some differences in methodologies and are therefore not directly comparable, but 

provide a broad picture of native species richness within PAIS. Species lists for the dunes and 

interior grasslands are included as Appendix A.

A total of 115 native plant species have been documented in the dune and grassland habitats of 

PAIS by the sources discussed in this assessment (Appendix B), including two species (Flaveria 

brownii and Sporobolus tharpii) endemic to western Gulf of Mexico barrier islands (Nelson et al. 

2000). While the dune and grassland habitats have many plant species in common, the grasslands 

appear to be slightly more diverse. Approximately 100 plant species have been documented in 

PAIS grasslands, while just less than 70 species are known to occur in the dunes.

Percent Coverage of Grassland and Dune Vegetation

Some of the first estimates of grassland and dune vegetation coverage in PAIS come from the 

1998 landcover mapping and classification project, based on mid-1990s aerial photography 
36

(Laine and Ramsey 1998). The landcover classes that include these two communities are 

described by Laine and Ramsey (1998) as follows:

• Grassland - areas containing dune hummocks that are densely vegetated with grasses

including shore little bluestem, cordgrass species, gulfdune paspalum, bushy bluestem, 

and others.

• Sparse vegetation - sand dunes of varying height that are sparsely vegetated with shore 

little bluestem, sea oats, bitter panicgrass, other grasses and forbs.

According to this landcover GIS data, grassland covered 5,549.2 ha (13,712.4 ac) or 

approximately 10.5% of the park while dune or sparse vegetation covered 2,392.8 ha (5,912.7 

ac), approximately 4.5% of the park (Plate 3 and Plate 4). 

In 2007, the NPS created a surficial geology map for PAIS, based primarily on 2003 aerial 

photography (NPS 2012a). This classification system included three terrestrial vegetation 

mapping units: fore-island dune ridge, sparsely vegetated barrier flat, and vegetated barrier flat. 

All grassland area would be included in the latter category, while dune vegetation includes the 

first two units and perhaps some of the vegetated barrier flats as well. The vegetated barrier flat 

unit included several wetland subunits, which were excluded from this area analysis (but will be 

included in section 4.6 of this assessment). The non-wetland portions of these three mapping 

units cover a total of 12,250 ha (30,272.6 ac) within the park. The area of each map unit is shown 

in Table 7 and Plate 6.

Table 7. Area of terrestrial vegetation mapping units, according to surficial geology GIS data (NPS 2007).

Map Unit Area (ha) % of park

Fore-island dune ridge 176.4 0.3

Sparsely vegetated barrier flat 998.5 1.9

Vegetated barrier flat (non-wetland) 11,076.0 21.0

Total 12,250.9 23.2

Percent of Bare Ground

Very few studies have addressed the percentage of bare ground in PAIS terrestrial vegetative 

communities. Only Drawe et al. (1981) documented the percent of bare ground in various 

terrestrial habitats. Their study included three habitats that could be considered terrestrial, nonwetland communities: coastal dunes, low coastal sands, and shoregrass flats. Of these three 

habitats, the coastal dunes exhibited the most bare ground with 52% cover. The shoregrass flats 

followed with 45% bare ground, while the low coastal sands habitat consisted of just 18% bare 

ground (Drawe et al. 1981). Vegetation sampling during the summer of 2012 collected some data 

regarding bare ground coverage, but this has not yet been analyzed or published.
37

Photo 3. Dune vegetation at PAIS, with bare ground exposed (photo by Shannon Amberg, SMUMN 

GSS).

Extent of Oak Motte Areas

According to Kattner (1973), Padre Island historically supported an extensive live oak forest. In 

the early 1970s, several sizable oak mottes (clonal groves) existed in the northern portion of the 

island, with a few smaller mottes in the southern part (Kattner 1973). These are believed to be 

remnants of the historic live oak forest (NPS 1996). To date, PAIS staff have mapped the 

locations of two remaining oak mottes identified as sensitive resources in the northern portion of 

the park. These are shown in Plate 6. According to 2009 NAIP imagery, these stands are 

approximately 600 and 900 m2 in size. Other stands of live oak likely occur further south in the 

park but have not been investigated thoroughly by PAIS staff or researchers (Stablein, e-mail 

communication, 29 August 2012).

Presence vs. Absence of Non-Native Species

Forty-eight non-native plant species have been documented within PAIS boundaries (Table 8) 

(NPS 2012b; Lindsay, written communication, October 2012). However, vegetation sampling 

efforts in the park’s terrestrial vegetative communities have only recorded two non-natives: 

Buffelgrass and annual rabbitsfoot grass (Drawe et al. 1981, Nelson et al. 2000). The locations 

and extent of these non-native species have not been mapped.
38

Table 8. Non-native species documented in PAIS (NPS 2012b; Lindsay, written communication, October 

2012).

Scientific name Common name Scientific name Common name 

Pennisetum glaucum pearl millet Chenopodium ambrosioides Mexican tea

Sonchus asper spiny sowthistle Cyperus rotundus nutgrass

Sonchus oleraceus common sowthistle Bothriochloa ischaemum var.

 songarica 

king ranch bluestem

Cakile lanceolata ssp.

 pseudoconstricta

coastal searocket Arundo donax giant reed

Nasturtium officinale watercress Avena fatua wild oat; flaxgrass

Amaranthus viridis slender amaranth Cynodon dactylon bermudagrass

Spergularia echinosperma brittleseed sandspurry Dactyloctenium aegyptium Egyptian grass

Bromus catharticus rescuegrass Dichanthium annulatum Kleberg’s bluestem

Chloris canterai var. canterai Paraguayan windmill 

grass

Digitaria bicornis Asian crabgrass

Chloris gayana Rhodes grass Parapholis incurva curved sicklegrass

Digitaria sanguinalis hairy crabgrass Paspalum dilatatum dallisgrass

Eragrostis barrelieri Mediterranean lovegrass Pennisetum ciliare buffelgrass

Lolium perenne perennial ryegrass Medicago polymorpha bur clover

Polypogon monspeliensis annual rabbitsfoot grass Melilotus albus white sweet-clover

Sorghum halepense Johnson grass Melilotus indicus annual yellow 

sweetclover

Leucaena leucocephala white leadtree Nicotiana glauca tree tobacco

Lantana camara largeleaf lantana Citrullus colocynthis colocynth

Anagallis arvensis scarlet pimpernel Citrullus lanatus var. citroides watermelon

Tribulus terrestris puncturevine Tamarix chinensis five-stamen tamarisk

Cucumis melo cantaloupe Tamarix gallica French tamarisk

Tamarix aphylla Athel tamarisk Tamarix ramosissima salt cedar

Tamarix canariensis Canary Island tamarisk Schinus terebinthifolius Brazilian peppertree

Echinochloa crus-galli barnyard grass Nerium oleander oleander

Chenopodium album lambsquarters Casuarina equisetifolia beach sheoak

Threats and Stressor Factors

Threats to the park’s terrestrial vegetative communities include human traffic (pedestrian and 

vehicle), oil and gas exploration and development, drought, altered fire regime, and insects and 

disease. Both pedestrian and vehicle traffic can destroy dune vegetation and impact soils, which 

can affect species composition and create harsher microenvironments for remaining plants 

(McAtee and Drawe 1974). Traffic also decreases aboveground and root production, as well as 

plant cover and diversity, with reductions in cover related to the intensity and type of traffic 

(McAtee and Drawe 1981). Plant communities in areas with heavy traffic are typically in earlier 

successional stages than those in similar undisturbed areas (McAtee and Drawe 1981).

Fire was likely a natural and common historic occurrence on Padre Island. However, fires would 

have become less frequent after human settlement, particularly with the introduction of grazing, 

which reduces the fine fuels necessary to carry fire. Since the establishment of PAIS and 

cessation of grazing, plant litter accumulation (Photo 4) and wildfire occurrence in these areas of 

litter build-up have become a problem for NPS staff (Drawe and Kattner 1978). Fires that occur 

in areas with excess built-up fuel can be particularly threatening to park resources and visitors. 

Research suggests that prescribed burning may be helpful in decreasing litter accumulation and 
39

wildfire risk, without harming the park’s terrestrial vegetative communities (Drawe and Kattner 

1978, Lonard et al. 2004). 

Photo 4. A grassland in PAIS with heavy litter accumulation due to lack of fire (photo by Shannon 

Amberg, SMUMN GSS).

Oil and gas development is one of the greatest threats to the park’s natural resources (NPS 1996). 

According to the park’s most recent oil and gas management plan (NPS 2001), vegetation 

disturbance due to drilling pad, access road, and other construction is predicted to impact 45-101 

ha (113-250 ac) of PAIS over the next 20 years. The two mapped live oak mottes have been 

designated as sensitive resources, and no disturbance will be allowed within 152 m (500 ft) of 

the mottes (NPS 2001). Carls et al. (1990) analyzed the vegetation around oil and gas 

development sites and access roads in PAIS, and determined that the greatest impacts on 

vegetation were hard surfacing of sites and alteration of site elevation (given the low relief of the 

island’s ecosystem). A total of 18.6 ha were disturbed by 24 drilling/processing sites and 8.9 ha 

by 17 access roads at the time of the Carls et al. (1990) survey. The secondary dunes and 

vegetated flats were the most impacted habitat in the park, with 17 oil and gas sites affecting this 

vegetation. The primary dunes habitat was impacted by only three sites (Carls et al. 1990). 

Vegetation communities could also be impacted by potential oil, gas, and other hazardous 

substance leaks or spills (NPS 2001).

Data Needs/Gaps

There are many data needs related to the park’s terrestrial vegetative communities. These include 

a more in-depth plant species inventory (e.g., species lists by plant community), the current and 

historic coverage of grassland and dune vegetation, the extent of non-native plant species within 

the park, and how these non-natives are impacting native communities and ecological processes 

(e.g., fire, erosion). Data are also needed regarding bare ground coverage within the park’s 

vegetative communities. A survey of 91 vegetation field plots in the park during the summer of 
40

2012 may provide some of this 

information (Photo 5); an analysis of 

this work has not yet been published 

(Arnie Peterson, GULN Vegetation 

Mapping Coordinator, e-mail 

communication, 9 November 2012). 

Further information regarding the 

isolated oak mottes would also be 

useful. Potential research topics 

include change in motte extent over 

time, genetic status (e.g., are the 

individual mottes genetically unique 

and, if so, could they hybridize?) 

(Cooper et al. 2005), and factors 

threatening their continued existence. 

Overall Condition

Native Species Richness

The project team defined the 

Significance Level for native species 

richness as a 3. A total of 115 native 

species have been confirmed in the 

park’s terrestrial vegetative communities by the sources discussed in this assessment. There is no 

evidence that species richness has changed during the past several decades; however, these 

habitats have not been sampled since the early 2000s. Due to this lack of recent data, a Condition 

Level was not assigned for this measure.

Percent Coverage of Grassland and Dune Vegetation

The project team defined the Significance Level for percent coverage of grassland and dune 

vegetation as a 3. A comparison of landcover (Laine and Ramsey 1998) and surficial geology 

(NPS 2007) data suggest that overall terrestrial vegetative cover has increased in PAIS over time. 

However, since surficial geology mapping did not specifically separate grassland and dune 

vegetation, any changes in the relative percentage of these two cover types cannot be determined 

(Condition Level = N/A). In the absence of disturbance, grasslands may replace dune vegetation 

over time.

Percent of Bare Ground

The project team defined the Significance Level for percent of bare ground as a 3. Very little 

information is available regarding bare ground coverage within the park’s terrestrial vegetative 

communities. Therefore, a Condition Level could not be assigned for this measure.

Extent of Oak Motte Areas

The project team defined the Significance Level for extent of oak motte areas as a 3. Only two 

oak mottes have been mapped within the park and information on their extent is very limited. A 

Condition Level could not be assigned at this time.

Photo 5. A vegetation sampling transect through a PAIS 

grassland (NPS photo).
41

Presence vs. Absence of Non-Native Species

The project team defined the Significance Level for presence vs. absence of non-native species as 

a 3. While 48 non-native species have been confirmed within PAIS boundaries, their locations 

(e.g., presence in specific community types) and extents have not been studied. As a result, a 

Condition Level was not assigned for this measure.

Weighted Condition Score

A Weighted Condition Score was not calculated for PAIS terrestrial vegetative communities, as 

Condition Levels were not assigned for any of the measures. The current condition and trend of 

this component is unknown. 

Sources of Expertise

Wade Stablein, Biological Technician, PAIS

Arnie Peterson, Vegetation Mapping Coordinator, GULN

James Lindsay, Chief of Science and Resource Management, PAIS
42

Literature Cited

Baccus, J. T., J. K. Horton, and P. D. Carangelo. 1977. A study of beach and dunes floral and 

faunal interrelations as influenced by recreational and user impact on Padre Island National 

Seashore. National Park Service, Southwest Region, Santa Fe, New Mexico.

Boss Operating Company, LLC. 2007. Description of habitat and vegetation types: Dunn 

McCampbell 16-1, 16-2, & 16-3. Boss Operating Company, San Antonio, Texas.

Carls, E. G., R. I. Lonard, and D. B. Fenn. 1990. Impact of oil and gas operations on the 

vegetation of Padre Island National Seashore, Texas, USA. Ocean and Shoreline 

Management 14:85-104.

Carls, E. G., R. I. Lonard, and D. B. Fenn. 1991. Notes on the vegetation and flora of North 

Padre Island, Texas. The Southwestern Naturalist 36(1):121-125.

Cooper, R. J., S. B. Cedarbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore. National Park Service, Gulf Coast Network, Lafayette, Louisiana.

Drawe, D. L., and K. R. Kattner. 1978. Effect of burning and mowing on vegetation of Padre 

Island. The Southwestern Naturalist 23(2):273-278.

Drawe, D. L., K. R. Kattner, and W. H. McFarland. 1981. Vegetation and soil properties of five 

habitat types on North Padre Island. Texas Journal of Science 33:145-157.

Kattner, K. R. 1973. Secondary successional vegetation on Padre Island National Seashore. 

Thesis. Texas A&I University, Kingsville, Texas.

Laine, S. C., and E. R. Ramsey. 1998. Landcover classification for Padre Island National 

Seashore. U.S. Geological Survey, National Wetlands Research Center, Lafayette, Lousiana.

Lonard, R. I., F. W. Judd, E. H. Smith, and C. Yang. 2004. Recovery of vegetation following a 

wildfire in a barrier island grassland, Padre Island National Seashore, Texas. The 

Southwestern Naturalist 49(2):173-188.

McAtee, J. W., and D. L. Drawe. 1974. A preliminary study of human impact on the vegetation 

and microclimate of the beach and foredunes of Padre Island National Seashore. National 

Park Service, Southwest Region, Santa Fe, New Mexico.

McAtee, J. W., and D. L. Drawe. 1981. Human impact on beach and foredune vegetation of 

North Padre Island, Texas. Environmental Management 4(6):527-538. 

National Park Service (NPS). 1996. Padre Island National Seashore resources management plan. 

National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS). 2001. Oil and gas management plan: Padre Island National 

Seashore. National Park Service, Padre Island National Seashore, Texas.
43

National Park Service (NPS) - Geologic Resources Division. 2007. Digital Surficial Units of 

Padre Island National Seashore and Vicinity, Texas. paissur.shp ArcGIS data layer. National 

Park Service (NPS), Geologic Resources Division, Denver, Colorado.

National Park Service (NPS). 2010. Padre Island National Seashore: 

Plants. http://www.nps.gov/pais/naturescience/plants.htm (accessed 16 February 2012).

National Park Service (NPS) - Geologic Resources Division. 2012a. Padre Island National 

Seashore: GRI ancillary map information document. National Park Service (NPS), Geologic 

Resources Division, Denver, Colorado.

National Park Service (NPS). 2012b. Certified species list for vascular plants in Padre Island 

National Seashore. Obtained at https://irma.nps.gov/App/Species/Search (accessed 14 August 

2012).

Nelson, A. D., J. R. Goetze, I. G. Negrete, V. E. French, M. P. Johnson, and L. M Macke. 2000. 

Vegetational analysis of four communities in the Big Ball Hill region of Padre Island 

National Seashore. The Southwestern Naturalist 45(4):431-442.

Recenthin, C. A., and H. Passey. 1967. The vegetation of Padre Island National Seashore. U.S. 

Department of Agriculture, Soil Conservation Service, Temple, Texas. 
44

Plate 2. Example of a vegetation map created for an energy development proposal (Boss Operating 

Company 2007).
45

Plate 3. Extent of grassland and dune (sparse) vegetation in the northern half of PAIS (Laine and 

Ramsey 1998).
46

Plate 4. Extent of grassland and dune (sparse) vegetation in the southern half of PAIS (Laine and 

Ramsey 1998)
47

Plate 5. Extent of terrestrial vegetation mapping units, according to NPS surficial geology data (NPS 

2007)
48

Plate 6. Mapped oak motte locations in the northern portion of PAIS, with an inset showing an aerial 

photo of one of the mottes.
49

4.2 Algal Mats of Mud Flats (Wind Tidal Flats)

Description

The wind tidal flats found in PAIS represent a unique habitat; instead of being formed by the 

tides, these areas are cyclically flooded and exposed in response to wind processes (Withers and 

Tunnell 1998). The wind tidal flats ecosystem is the second largest ecosystem in the park, 

spanning 21.7% (11,447 ha [28,287 ac]) of the park’s land (NPS 2013). The wind tidal flats in 

the Laguna Madre are vulnerable due to their proximity to the GIWW because it puts them at a 

higher risk for petroleum contamination, which would adversely affect algae productivity, 

waterfowl, and commercial 

fisheries in the area (Withers 

1993). 

Blue-green algae (more 

appropriately referred to as 

Cyanobacteria) are a group of 

photosynthetic bacteria that are 

the primary producers on the 

flats. Globally, cyanobacteria 

account for 20-30% of the 

Earth’s photosynthetic 

productivity (Pisciotta et al. 

2010). 

Some parts of the wind tidal 

flats are frequently inundated 

by water from the Laguna 

Madre. In these areas, 

extensive algal mats are often observed. A large accumulation of blue-green algae is referred to 

as an algal mat. These mats cover a majority of the soil on the wind-tidal flats in the park, and 

can look like foam, scum, or a layer of film floating on water or soil surfaces. The thickness of 

these mats varies; in some areas, the algal mat is thick enough to trap hydrogen sulfide gas, 

which is released from the soil. Pores then form in the soil, causing the surface of the soil to rise 

(USDA et al. 2005). 

The algal mats on these flats contribute the majority of productivity within the estuary, making 

them important feeding grounds for shorebirds; over 20 species of shorebirds (e.g., red knots 

[Calidris canutus]) occupy the flats seasonally (Withers and Tunnell 1998). Other birds utilize 

this habitat as well, including the federally protected piping and snowy plovers (Charadrius

melodus, C. alexandrinus), reddish egret (Egretta rufescens), white-tailed hawk (Buteo

albicaudatus), and peregrine falcon (Falco peregrinus) (Withers and Tunnell 1998). Benthic 

invertebrates inhabit the algal mat and provide the link between the primary producers and the 

consumers (Cooper et al. 2005). Plate 7 displays the extent of wind-tidal flats in PAIS (USGS 

2009).

Photo 6. Dry Algal mats on mud flats in PAIS (Photo by Shannon 

Amberg, SMUMN GSS).
50

Measures

• Extent of algal mats

• Density and richness of benthic macroinvertebrates

Reference Conditions/Values

The reference condition for the algal mats on tidal flats in PAIS would be the extent of algal mats 

and the density and richness of benthic macroinvertebrates that occupy the algal mats found at 

the Nine-mile Hole tidal flats, just south of the land-cut tidal flats.

Data and Methods

Withers (1993) conducted a study on abundance and distribution of benthic macroinvertebrates 

on a blue-green algal flat along the northern boundary of PAIS. The algal flat was divided into 

four microhabitats: intertidal, wet, dry or damp areas, and open water. Macroinvertebrates were 

sampled in three of the four microhabitats twice a month between 1991 and 1992. The open 

water microhabitat was not sampled for macroinvertebrate abundance or distribution.

Withers (1996) studied the macroinvertebrates of several tidal flats on the Laguna Madre side of 

PAIS. From November 1995 to March 1996, macroinvertebrate communities were studied at two 

former drilling sites south of Malaquite Beach to explore the impact of drilling and restoration 

activities on these organisms. Withers (1996) looked at diversity and density at the restored sites 

and in nearby control areas.

Withers (1998) investigated the biological productivity of three southerly wind-tidal flats in 

PAIS between 1997 and 1998. The study included taking core samples from each wind-tidal flat 

to determine the density of benthic macroinvertebrates. The three flats were located at 

Yarborough Pass, Dunn Ranch, and at Mile Marker 45. Samples were collected from 

Yarborough Pass between November 1997 and March 1998. The Dunn Ranch site was sampled 

between December 1997 and February 1998, and the Mile Marker 45 site was sampled between 

December 1997 and April 1998. 

Laine (1998) created a landcover classification for PAIS, which included wind tidal flats. Each 

landcover class description matches guidelines set by the National Vegetation Classification 

System (NVCS). Twelve general class descriptions were used, including wind tidal flat. A threepart map was created using a total of 479 randomly sampled points to classify landcover within 

PAIS.

Gilbeaut (2010) conducted a study on the transition of estuarine wetland habitat caused by sealevel rise. GIS models were used to measure the loss of tidal flats on Mustang Island and North 

Padre Island (north of PAIS). A digital elevation model (DEM) was used for elevation reference. 

The model created estuarine wetland habitat types, which includes tidal flats, based on elevation.

The models allow for the adjustment of elevation of the island to simulate sea level rise.
51

Current Condition and Trend

Extent of Algal Mats

Laine (1998)

Laine (1998) classified 11,298 ha (27,918.40 ac); 21.75% of the National Seashore) as wind tidal 

flats in PAIS. Algal mats likely do not cover the entire extent of the wind tidal flats, but will only 

occur in these areas within the park. The flats that become infrequently inundated by wind action 

are ideal habitat for algal mats. Plate 7 displays the extent of wind tidal flats in PAIS according 

to Lain (1998).

Withers (1998)

Withers (1998) studied two tidal flat sites (Yarborough Pass, Dunn Ranch) with extensive algal 

mats covering the surface. The third site (Mile Marker 45) was said to have little algal covering 

on the surface that still becomes inundated. Some algal mats were also present on the dry areas 

of Mile Marker 45. Plate 7 displays the approximate locations of Withers’ (1998) tidal flats sites 

along the Laguna Madre.

Gilbeaut (2010)

Gilbeaut (2010) concluded that Mustang Island lost approximately 57% of its tidal flat habitat 

between 1950 and 2004. As a result of sea-level rise, the low tidal flats became open water 

during inundation periods. Open water is unsuitable for the growth of blue-green algal mats 

because algal mats require high levels of light penetration to grow and light levels are reduced in 

deeper water. While this study may not have been in PAIS, the wind tidal flats around both Padre 

and Mustang Islands are vulnerable to many of the same factors. It is probable that PAIS windtidal flats experienced similar changes during this time. 

Density and Richness of Benthic Macroinvertebrates

The macroinvertebrate community of PAIS is discussed in detail in Chapter 4.10 of this 

document. For this assessment, only a discussion of the studies that sampled the wind tidal flats 

is included.

Withers (1993)

Withers (1993) documented 38 macroinvertebrate taxa (including tanaids, insects, and 

polychaetes) in the algal mat/wind tidal flat area on the park’s northern boundary (Appendix C). 

Of the three sampling areas in the study (damp/dry, wet, intertidal), the intertidal sampling area 

had the highest species richness value (31 sp.), while the damp/dry sampling area had the lowest 

observed species richness (15 sp.) (Appendix C). 

Density ranged greatly throughout the three microhabitats, and throughout the year (0 –

70,000/m2). The intertidal and wet sampling locations had the greatest density values (reaching 

70,000 organisms/m2), while the damp sampling location only had a peak density of 16 

organisms/m2 (Figures 2a, 3a, and 4a in Withers 1993). Tanaids showed the highest density in all 

three sampling locations (Withers 1993).

Withers (1996)

Withers (1996) explored the distribution of macroinvertebrates at two former oil and gas 

development sites that had been restored (See Plate 7). At the Texaco site, six species were found 
52

within the restored area, six in an adjacent control area, and four within tire tracks (Withers

1996, Appendix D). Only insects were found in tire tracks and no mollusks were found at the 

site. At Yarborough Pass, 27 macroinvertebrate species occurred in the restored area, 13 in the 

control area, and only six within tire tracks (Withers 1996). Nearly all species found at the 

control and tire track locations were also present in the restored area (Appendix D).

Withers (1996) noted low densities of most macroinvertebrates at two tidal flats south of 

Malaquite Beach. On average, less than one polychaete, tanaid, and amphipod were detected per 

5.4 x 5 cm sediment core from the Texaco site. Insect larvae were slightly more common, 

reaching a density of 1.2 per core at control sites in November (Withers 1996). Densities were 

higher at Yarborough Pass. Peak mean monthly insect density was 15 organisms per core and 

peak tanaid density was around 100 organisms per core. Polychaete, amphipod, and mollusk 

densities never exceeded four organisms per core (Withers 1996).

Withers (1998)

Withers (1998) collected 31 species representing four phyla from three tidal flats in 1998. The 

Yarborough Pass tidal flat had the highest species richness value (25 sp.), while the Dunn Ranch 

and Mile Marker 45 tidal flats had 14 and 13 species, respectively (Appendix E).

The highest density of benthic macroinvertebrates was recorded at Mile Marker 45. Thirteen 

species were recorded at this site, and the density was given for the two most abundant phyla of 

benthic macroinvertebrates. Polychaete density peaked in February at this site with 

approximately 1,300 organisms/m2

, while mollusk density peaked in March 1998 with 

approximately 250 organisms/m2.

Twenty-five macroinvertebrate species were observed at the Yarborough Pass site. Polychaete 

and tanaid densities peaked in two separate months, as peak polychaete densities were about 350 

organisms/m2 and 550 organisms/m2 for January and March, respectively; tanaid densities were 

the highest in January and March with approximate densities of 11,000 organisms/m2 and 15,000 

organisms/m2, respectively. Dunn Ranch had the lowest density during the study, as polychaete 

and tanaid densities did not exceed 60 organisms/m2 for any of the days sampled. The insect 

density peaked in January with about 190 organisms/m

2

. Most macroinvertebrate densities 

seemed to peak in March, perhaps due to a spring flooding of the tidal flats. 

Threats and Stressor Factors

There are several threats to the park’s algal mats on mud flats (wind tidal flats). PAIS staff 

identified seven stressors that have occurred or are presently occurring in the park. Those 

stressors include off-road vehicle (ORV) and boat disturbances, marine debris, oil/contaminant 

spills, oil and gas exploration and development, sea-level rise, sedimentation, impaired water 

quality and/or quantity (water quality is discussed in detail in Chapter 4.14 of this report), and 

desiccation of the wind tidal flats (a natural process). 

Off-road vehicle disturbance is a major stressor to the wind-tidal flats in PAIS. Tire tracks create 

ditches and trenches that alter the surface hydrology of the wind-tidal flats (NPS 2013). Vehicles 

can compact soil and cause other habitat destruction on the sensitive beaches and tidal flats of 

PAIS. This can impact macroinvertebrate communities, as shown in Withers (1996), where 

fewer species were found in tire tracks than in restored and control areas. The wind-tidal flats are 
53

inundated as a result of 

northerly winds across the 

Laguna Madre, which, when 

flooded, allows blue-green algae 

mats to grow. The trenches 

caused by ORVs prevent the 

natural flow of water across the 

wind-tidal flats (NPS 2012). 

These algal communities are 

fragile and very static, often 

taking a long time to heal after 

vehicular disturbance (Photo 7). 

As an example of how static 

these areas of PAIS are, damage 

from vehicular traffic seen in 

aerial photographs taken in 

the1930s and 1940s is still 

apparent in modern satellite imagery (Stablein, written communication, 13 May 2013). Further 

complicating this threat is the fact that there are no known methods for restoration once these 

communities are damaged (Stablein, written communication, 13 May 2013).

Marine debris can cause light attenuation, deplete oxygen, and alter physical habitat by changing 

its structure. Marine debris has caused habitat degradation by harming (e.g., smothering, 

fragmenting), “habitat forming species” such as the algal mats; this process may result in 

reduced macroinvertebrate populations (Donohue et al. 2001, Asoh et al. 2004, Chiappone et al. 

2005, as cited by EPA 2011, p. 9). Algal mats also require shallow waters with a low degree of 

light attenuation, so increased light attenuation may stop algal mats from forming (NPS 2012). 

This may also reduce the density and richness of macroinvertebrates that inhabit the algal mats.

Oil spills and oil/gas exploration and development can be a threat to the algal mats on the mud 

flats in PAIS. Oil spills may cause habitat degradation and poor water quality, and exposure to 

oil can cause mortality among bivalves, amphipods, and polychaetes (Withers and Tunnell 

1998). Oil and gas drilling sites have also altered suitable habitat. According to Carls et al. 

(1990), drilling sites have affected the elevation near wind tidal flats. If the change in elevation 

were to cause the flats to flood unevenly or not flood in areas, this could cause a reduction of 

extent of algal mats in these drier areas.

Sea-level rise poses a threat to the algal mats on wind tidal flats because it may cause permanent 

inundation if it were to exceed the aggradation rates (Morton and Holmes 2009). According to 

Glick et al. (2007), sea level rise will cause the extent of wind-tidal flats to decrease, and as a 

result the extent of algal mats will also decrease due to loss of suitable habitat. Sedimentation 

may be a threat to algal mats on PAIS. Aggradation is a natural process of sediment 

accumulation caused by wave action; however, over time this process may cause the area of 

wind tidal flats to decrease due to increased elevation. Tidal flats decreased in area in the Laguna 

Madre/Corpus Christi Bay region between the 1950s and 1980; most of the lost tidal flats were 

replaced by open water or seagrass beds (Withers and Tunnell 1998).

Photo 7. Inundated wind-tidal flats with ORV tire tracks (Photo 

by Shannon Amberg, SMUMN GSS).
54

Poor water quality can negatively affect algal mats and the organisms that inhabit them. Water 

with elevated levels of nitrogen and phosphorous can be harmful to aquatic organisms such as 

macroinvertebrates (USGS 2013). Eutrophication is a result of excess nutrients in the water, and 

it can cause harmful algal blooms (red tide) and deplete dissolved oxygen levels, which threaten 

fish and macroinvertebrate populations (USGS 2013). Drastic declines in mollusk, echinoderm, 

crustacean, and polychaete populations have been noted in the Gulf of Mexico following red 

tides (Dupont and Coy 2008).

Data Needs/Gaps

There are limited data for the extent of algal mats on wind-tidal flats and the density and richness 

of macroinvertebrates in PAIS. Laine (1998) and Withers (1998) are more than 10 years old, and 

therefore outdated. While the Gilbeaut (2010) study is current, it only focuses on the wind-tidal 

flats of Mustang Island, which is north of PAIS. Withers (1993, 1996, 1998) conducted a shortterm study on benthic invertebrates occurring in the wind-tidal flats. According to Withers 

(1998, p. 32), short-term studies of benthic macroinvertebrate communities “tend to be 

inadequate for proper characterization.” The macroinvertebrate studies that have occurred in and 

near PAIS have typically provided single-year “snapshots” of the community at particular 

locations, which have not been revisited over time. Resampling the sites discussed in this 

assessment would provide insight into whether or not the macroinvertebrate community has 

changed over time (i.e., changes in diversity, density, etc.). Longer-term studies of several years 

would also provide a more detailed picture of the park’s macroinvertebrate populations.

Overall Condition

Extent of Algal Mats

The project team defined the Significance Level for extent of algal mats as a 3. It is difficult to 

get an exact extent of algal mats from the Withers (1998) study; however, if algal mats occupy 

most of the wind-tidal flats identified in the Laine (1998) document, the algal mats may cover 

approximately 21% of the park. Gilbeaut (2010) executed a GIS model that estimated a 57% loss 

in tidal flat extent on Mustang Island between 1950 and 2004, which may mean that the tidal 

flats in PAIS also suffered a similar loss. There are no studies that give the exact extent of the 

algal mats on the wind-tidal flats in PAIS. As a result, a Condition Level could not be determined 

for this measure. 

Density and Richness of Benthic Macroinvertebrates

The Significance Level for density and richness of benthic macroinvertebrates is a 3. As 

described in the macroinvertebrates assessment (Chapter 4.10), a Condition Level could not be 

assigned for this measure because of a lack of current data. The historic data collected by 

Withers (1993, 1996, 1998) could be used as a baseline of comparison for future condition 

assessments.
55

Weighted Condition Score

The Weighted Condition Score for algal mats on mud flats was not calculated due to a lack of 

current data for all measures. 

Sources of Expertise

Wade Stablein, Biological Technician, PAIS

James Lindsay, Chief of Science and Resource Management, PAIS
56

Literature Cited

Asoh, K., T. Yoshikawa, R. Kosaki, and E. Marschall. 2004. Damage to cauliflower coral by 

monofilament fishing lines in Hawaii. Conservation Biology 17:170-180.

Carls, E. G., R. I. Lonard, and D. B. Fenn. 1990. Impact of oil and gas operations on the 

vegetation of Padre Island National Seashore, Texas, USA. Ocean and Shoreline 

Management 14:85-104.

Chiappone, M., H. Dienes, D. Swanson, and S. Miller. 2005. Impacts of lost fishing gear on coral 

reef sessile invertebrates in the Florida Keys National Marine Sanctuary. Biological 

Conservation 121(2):221-230.

Cooper, R. J., S. B. Cederbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore: Final report. Warnell School of Forest Resources, University of 

Georgia, Athens, Georgia.

Donohue, M. J., R. C. Boland, C. M. Sramek, and G. A. Antonelis. 2001. Derelict fishing gear in 

the Northwestern Hawaii Islands: diving surveys and debris removal in 1999 confrim threat 

to coral reef ecosystems. Marine Pollution Bulletin 42(12):1301-1312.

Dupont, J. M., and C. Coy. 2008. Only the strong will survive: red tides as communitystructuring forces in the eastern Gulf of Mexico. University of South Florida College of 

Marine Science, St. Petersburg, Florida.

Environmental Protection Agency (EPA). 2011. Marine debris in the North Pacific: a summary 

of existing information and identification of data gaps. U.S. Environmental Protection 

Agency, San Francisco, California.

Gilbeaut, J. C., E. Barraza, and B. Radosavljevic. 2010. Estuarine wetland habitat transition 

induced by relative sea-level rise on Mustang and North Padre Islands, Texas: Phase 1. 

Publication CBBEP – 64, Project Number – 0822. Coastal and Marine Geospatial 

Laboratory, Harte Research Institute, Texas A&M University, Corpus Christi, Texas.

Glick, P., J. Clough, and B. Nunley. 2007. Sea-level rise and coastal habitats in the Pacific 

Northwest: an analysis for Puget Sound, southwestern Washington, and northwestern 

Oregon. National Wildlife Federation, Western Natural Resource Center, Seattle, 

Washington.

Laine, S. C., and E. W. Ramsey III. 1998. Landcover classification for Padre Island National 

Seashore. Final report. U.S. Geological Survey, National Wetlands Research Center, 

Lafayette, Louisiana.

Morton, R. A., and C. W. Holmes. 2009. Geological processes and sedimentation rates of windtidal flats, Laguna Madre, Texas. U.S. Geological Survey, Austin, Texas.

National Park Service (NPS). 2012. Public comment sought on reclamation of wind-tidal flats in 

the area of Yarborough Pass and Back Island road. National Park Service, Corpus Christi, 

Texas.
57

National Park Services (NPS). 2013. Park staff have installed 140 bollards at Yarborough Pass 

and 81 bollards at various openings in the foredune ridge south of Big Shell Beach. 

November 4, 2011. http://www.nps.gov/pais/parknews/newsreleases.htm (accessed 12 April 

2013).

Pisciotta, J. M., Y. Zou, and L.V. Baskakov. 2010. Light-dependent electrogenic activity of 

cyanobacteria. PLoS One Online http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2876029/

(accessed 12 April 2013).

U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), and 

National Park Service (NPS). 2005. Soil survey of Padre Island National Seashore, Texas: 

special report. United States Department of Agriculture, National Resources Conservation 

Service, and United States Department of the Interior, National Park Service. Padre Island 

National Seashore, Texas. 

U.S. Geological Survey (USGS). 2009. Spatial vegetation data. GIS Data. Padre Island National 

Seashore, Texas. 

U.S. Geological Survey (USGS). 2013. The effects of urbanization on water 

quality. http://ga.water.usgs.gov/edu/urbanmet.html (accessed 1 March 2013). 

Withers, K. 1993. Study to determine the abundance and distribution of benthic invertebrates and 

shorebirds on a North Padre Island blue-green algal flat. National Park Service, Corpus 

Christi, Texas.

Withers, K. 1996. An evaluation of recovery of benthic invertebrate communities in vehicle 

tracks and restored oil and gas impacted areas on wind-tidal flats in the Upper Laguna 

Madre, Padre Island National Seashore, Texas. Environmental Consulting, Sinton, Texas.

Withers, K. 1998. Biological productivity of southerly wind-tidal flats within Padre Island 

National Seashore, Texas. Center for Coastal Studies, Texas A&M University, Corpus 

Christi, Texas. 

Withers, K., and J. W. Tunnell. 1998. Identification of tidal flat alteration and determination of

effects on biological productivity of these habitats within the coastal bend. Corpus Christi

Bay National Estuary Program, Corpus Christi, Texas.
58

Plate 7. The extent of wind tidal flats in PAIS (USGS 2009), and the approximate locations of Withers’ 

(1996,1998) tidal flats sites along the Laguna Madre.
59

4.3 Seagrass Community

Description

Seagrass communities are ecologically important because they provide food and protection to 

marine mammals, birds, fish, and invertebrates, including redhead ducks, red drum (Sciaenops 

ocellatus), and green sea 

turtles (Chelonia mydas) 

(Handley et al. 2007). 

According to Blair and 

White (1997), seagrass 

communities are also 

considered ecological 

indicators of estuarine 

water quality because they 

are sensitive to factors 

such as nutrient loading 

and eutrophication.

PAIS is located on the 

southeastern coast of 

Texas, in the western

portion of the Gulf of 

Mexico. Over 50% of the 

U.S. seagrass distribution 

and approximately 5% of 

the world’s seagrass distribution occurs in the Gulf of Mexico (Green and Short 2003, as cited by 

Handley et al. 2007). The most widespread seagrass beds in the western Gulf of Mexico are 

found in the Laguna Madre (Onuf 1996). The Laguna Madre supports a variety of seagrass 

species, including turtle grass, manatee grass, star grass (Halophila engelmannii), widgeon grass, 

and shoal grass (Onuf 2007).

Measures

• Species composition

• Percent cover

• Canopy height

Reference Conditions/Values

Historical accounts indicate that seagrass beds/meadows covered nearly the entire lagoon bottom 

of the lower Laguna Madre in 1965 (Onuf 2007). However, data recording the changes to the 

community over time are very limited. A long-term monitoring strategy has been recently (2011) 

initiated by the GULN through a task agreement with researchers at the University of Texas 

Marine Science Institute (UTMSI) in Port Aransas, Texas. This will provide insight on current 

community condition; however, several years of data will be needed to determine condition 

trends. There is debate among seagrass experts on what characterizes a “healthy” seagrass 

community and what changes in communities over time may suggest about the community’s 

Photo 8. Seagrass bed in the Laguna Madre (Photo by Dr. Ken 

Dunton, http:// www.texasseagrass.org).
60

ability to persist and thrive (Wilson and 

Dunton 2011). Thus, no reference 

condition has been defined for the seagrass 

community in the Laguna Madre in PAIS. 

The comprehensive survey efforts by 

GULN could serve as a reference 

condition for future research efforts.

Data and Methods

Onuf (2007) documented seagrass status 

and trends in the Laguna Madre. Sampling 

in the lower Laguna Madre consisted of 22 

east-west transects that were set at 0.6 m 

(2 ft) intervals. Sampling in the upper 

Laguna Madre consisted of transects set at 

0.3 to 1.5 m (1 ft to 5 ft) intervals and 

perpendicular to the long axis of the 

lagoon. Four sediment core samples were 

retrieved from each transect. The study 

documented species present in each core. 

Earlier survey source maps were digitized 

for an analysis of historical trends. Figure 

4 displays the two segments of the Laguna 

Madre.

Wilson and Dunton (2011) surveyed areas 

near PAIS as part of a monitoring program 

for seagrass in Texas. Sampling station 

locations were distributed among three estuary systems, including the Laguna Madre, which was 

separated into two segments, Upper Laguna Madre (ULM) and Lower Laguna Madre (LLM). 

The LLM sampling sites were all located just outside of the park boundaries. Observational data 

was obtained through field surveys and displayed using GIS to analyze the relationships between 

study parameters and seagrass habitat. 

Current Condition and Trend

Species Composition

Onuf (2007) documented several changes in seagrass community composition since 1965. 

Species composition has been dynamic, with the historically dominant species (shoal grass) 

being replaced by manatee grass and turtle grass. By 1998, shoal grass coverage was at just 46%. 

Turtle grass dominated the southern portion of the lower Laguna Madre, with fringes of shoal 

grass along the shore and west side of the lagoon. Shoal grass was also present in the middle 

region of the lower Laguna Madre, where it dominated the shore area. Manatee grass was found 

at both the north and south ends of the lower Laguna Madre, splitting around the deep bare areas. 

The upper Laguna Madre’s vegetative area only covered 63% of the permanently flooded areas. 

Shoal grass was the dominant species with 92% coverage in the vegetated area. Manatee grass 

was found on the north end of the basin, covering 6% of the vegetated area. Star grass was 

Figure 4. Upper and Lower Laguna Madre in relation 

to PAIS (Onuf 2007).
61

confined to the deep fringe or mid region of the lagoon; it covered 1% of the basin. Widgeon 

grass, also covering only 1%, was only located in the shallow flats by the GIWW. Table 9

displays the species composition in the lower and upper Laguna Madre in 1998.

Table 9. Seagrass species composition (in hectares) and percent cover of vegetated areas in the lower 

and upper Laguna Madre in 1998 (Onuf 2007).

Lower Laguna Madre Upper Laguna Madre

Seagrass Species Area (Hectares) Percent Cover Area (Hectares) Percent Cover

Shoal grass 21,118 45.7% 20,553 91.6%

Manatee grass 12,861 27.9% 1,452 6.5%

Turtle grass 11,132 24.1% N/A 0%

Star grass 1,063 2.3% 307 1.3%

Widgeon grass N/A 0% 132 0.6%

Total Vegetated 46,174 100% 22,444 100%

Wilson and Dunton (2011) documented slightly different seagrass compositions for the two 

sections of the Laguna Madre. There were five and six seagrass species identified in the lower 

and upper Laguna Madre, respectively. The seagrass composition in the upper Laguna Madre 

consists of shoal grass, manatee grass, turtle grass, widgeon grass, and star grass. Unlike the 

upper basin, there is no star grass present in the lower Laguna Madre. The lower Laguna Madre 

contains shoal grass, manatee grass, turtle grass, and widgeon grass. Table 10 displays the 

seagrass compositions (represented by mean percent coverage) in both sections of the Laguna 

Madre.

Table 10. Seagrass species composition, in the lower and upper Laguna Madre in 2011, represented by 

mean percent coverage (Wilson and Dunton 2011).

Seagrass Species Lower Laguna Madre Upper Laguna Madre

Shoal grass 25.5 ± 35.7% 60.9 ± 36.9%

Manatee grass 1.4 ± 7.6% 13.6 ± 26.1%

Turtle grass 18 ± 33.4% 0.1 ± 0.6%

Widgeon grass 0.6 ± 3.5% 0.3 ± 1.9%

Star grass N/A 0.3 ± 1.5%

Total Vegetated 45.9 ± 40% 75.2 ± 30.9%

Percent Cover

Onuf (2007) documented changes in seagrass coverage since 1965. The lower Laguna Madre has 

lost vegetative area over the years. In 1965, it was reported that nearly the entire lagoon floor of 

the lower Laguna Madre was composed of seagrasses. There was an approximate 21% decrease 

in seagrass cover between the 1960s and 1970s. In 1998, the percent cover of seagrass in the 

lower and upper Laguna Madre was 67% and 63%, respectively. The percent cover of vegetated 

bottoms in Laguna Madre as a whole has decreased by 4%, between 1965 and 1998. Bare areas 

in the lower lagoon have increased from 9,181 ha to 22,761 ha between the 1960s and 1998. The 

bare area in the upper basin has decreased from 20,826 ha to 12,950 ha between the 1960s and 

1998. 
62

Table 11 displays the coverage (in hectares) and percent cover in the lower and upper Laguna 

Madre.

Table 11. Total coverage (in hectares) and percent cover of bare and vegetated bottoms in the lower and 

upper Laguna Madre in 1998 (Onuf 2007).

Cover Type Lower Laguna Madre Upper Laguna Madre

Bare 22,761 (33%) 12,950 (36. 6%)

Total Vegetated 46,174 (67%) 22,444 (63.4%)

Total 68,935 35,394

Wilson and Dunton (2011) recorded the percent cover of seagrass in the Laguna Madre. The 

mean percent cover for upper and lower Laguna Madre was 75.2 ± 30.9% and 45.9 ± 40.0%, 

respectively. Approximately 32% of the sampling stations recorded less than 10% seagrass 

coverage in the lower Laguna Madre. In the upper Laguna Madre, there were approximately 10 

sampling stations (6.9%) that recorded less than 10% seagrass coverage. There were roughly 48 

stations in the lower basin that recorded areas devoid of vegetation; only seven stations in the 

upper basin recorded areas with no vegetation.

Canopy Height

Wilson and Dunton (2011) documented canopy heights in the lower and upper Laguna Madre. 

Recorded canopy heights ranged between 3 and 51 cm. Mean canopy heights were 15.7 ± 9.8 cm 

and 20.1 ± 8.8 cm in the lower and upper Laguna Madre, respectively. Greater canopy heights in 

the lower Laguna Madre seem to be concentrated in the bottom portion of the basin; greater 

canopy heights were dispersed throughout the upper Laguna Madre. This difference may have 

occurred because the lower Laguna Madre has a higher percent of areas with no vegetation, 

which may count against the average canopy height. It is important to note that canopy height 

varies among seagrass species, which makes it difficult if not impossible to determine the health 

of the seagrasses through gross canopy height alone.

Threats and Stressor Factors

The park has identified several potential threats to the park’s seagrass communities, including 

light attenuation, unbalanced physiochemical water quality parameters, nutrient loading in the 

estuary, oil and gas exploration and development, and hazardous materials. 

Light attenuation or reduction in water transparency is a stressor on seagrass communities, since 

they require a certain amount of light to grow. According to Duarte et al. (2004), reduced water 

transparency is a primary cause of seagrass loss. Reduced water transparency could be caused by 

nutrient loading or turbidity. The extent of water transparency can also limit the depth at which 

seagrass grows (Dunton 2010, FDEP 2011). Bare areas in the lagoon may cause a negative 

feedback loop because non-vegetated areas are unstable and apt to resuspension of sediment 

caused by wind-driven waves; this can lead to a decline in seagrass area by preventing future 

growth in these areas (Onuf 1994, as cited by Teeter 2002). Light attenuation or reduction in 

transparency/clarity can be caused by a number of factors, some having short-term effects and 

others having long-term or extended effects. Dredging and channel construction cause sediments 

to become suspended and allow little light through to the plants; however, most suspended 

sediments settle in a short amount of time (Pulich et al. 1997). It is believed that nutrient 

enrichment/loading is another stressor to seagrass, as it encourages algal blooms (phytoplankton) 
63

(Pulich et al. 1997) This algae may grow on the surface of seagrass leaves causing shading from 

available light (Martha Segura, written communication, 11 April 2013). Algal blooms have the 

ability to cause light attenuation in a short time period, and they can remain in an area for a long 

period. An algal bloom called a brown tide was recorded in the upper Laguna Madre between 

1990 and 1995. According to Onuf (1996), there was a significant reduction in seagrass biomass 

between 1991 and 1993. Additional research has also shown that seagrass communities are 

adversely affected by unrelenting brown tide events (Pulich et al. 1997, as cited by Cooper et al.

2005). 

Excess total suspended solids (TSS) and chlorophyll A are threats to seagrass because they 

reduce water clarity. Total suspended solids are mostly sediments that become resuspended by 

processes such as wind and waves. Chlorophyll A levels represent the abundance of 

phytoplankton present (Dunton 2012). Reduced water clarity puts a strain on seagrass 

photosynthesis (EPA 2012). Seagrass depth can be limited by even low levels of TSS (Burd and 

Dunton 2000, as cited by Teeter 2002).

According to Handley et al. (2004), the health of seagrass beds is dependent on water quality. 

Imbalances in water quality parameters, including dissolved oxygen (DO), pH, temperature, and 

salinity can cause stress on seagrass beds. Some seagrass species are less tolerant to water quality 

fluctuations than others. Manatee grass is rather intolerant to saline fluctuations, restricting it to 

the upper Laguna Madre because of its low saline variation (Wilson and Dunton 2011). Species 

composition shifts have also been known to occur as a result of water quality imbalances 

(Cooper et al. 2005). 

Nutrient loading is a threat resulting from human population growth and development near the 

coast. A surplus of nutrients causes an imbalance and favors fast-growing organisms, such as 

phytoplankton and macroalgae (Dunton 2010). The high nutrient concentrations can also cause a 

shift in vegetation, typically favoring widgeongrass and seaweeds (Fourqurean and Rutton 2003, 

as cited by Dunton 2010).

Oil and gas exploration and development is another threat caused by anthropogenic activity. The 

risk of chemical spills increases in areas with oil and gas development, such as barge traffic in 

the GIWW or offshore oil rigs. Oil could possibly be carried into the Laguna Madre by currents, 

winds, or storms that follow a spill (SDWF 2012). If oil comes in contact with seagrass beds, it 

can become trapped in the sediment and kill the seagrass (STC 1995).

Prop scarring also threatens the seagrass communities. Boat propellers can physically damage 

seagrass leaves and roots or even uproot the plants (TPWD 2012). Most prop scarring occurs in 

shallow waters (< 1 m) which are favorable for seagrasses (PAIS 2001). According to Martin 

(2008), prop scarring is an increasing threat in the PAIS area due to increased recreational 

boating as a result of coastal population growth.

Data Needs/Gaps

There are limited long-term data on seagrass in PAIS. While current data exist, long-term studies 

are not available. The imagery used by Wilson and Dunton (2011) is outdated (from 2004) and 

could cause inconsistencies, so updated imagery is recommended for future seagrass mapping. 

The long-term monitoring program to assess trends in seagrass community composition by the 
64

GULN through the University of Texas Marine Science Institute in Port Aransas will provide 

substantial insight into how the community is changing over time. The 2012 data from this 

relatively young monitoring effort was not yet available at the time this assessment was 

developed, but would provide further insight into the current condition of seagrass communities 

in the Laguna Madre.

Overall Condition

Species Composition

The project team defined the Significance Level for species composition as a 3. The species 

compositions in both sections of the Laguna Madre have been shifting since the mid 1960s. 

Shoal grass, for example, has decreased by approximately 31,000 ha (76,603 ac) in the lower 

basin, but has increased approximately 8,000 ha (19,768 ac) in the upper basin. It is not clear if 

the shift in species composition is an indication of degrading health of the system or a natural 

phenomenon. It should be noted that even though the species composition has been shifting, all

of the seagrass species that are expected to be in the Laguna Madre are still there (Martha 

Segura, written communication, 11 April 2013). More research is needed to understand this. Due 

to the reference condition being undefined, a Condition Level was not assigned for this measure.

Percent Cover

The Significance Level for percent cover is a 3. The percent cover has been changing in both the 

upper and lower Laguna Madre over the last few decades. In the 1960s, the bottom of the lower 

Laguna Madre was almost entirely covered with seagrass beds. Wilson and Dunton (2011) 

discovered that approximately 32% and 7% of the sampling stations recorded less than 10% 

seagrass coverage in the lower and upper Laguna Madre, respectively. Wilson and Dunton 

(2011) specify that it is currently difficult to determine if the seagrass community in the Laguna 

Madre is actually expanding or contracting at this time based on limited comprehensive field 

evaluations over time. However, they note that 2011 data compared to delineations of seagrass 

habitat by NOAA in 2004 and 2007 indicate that many seagrass beds are in decline. Further 

monitoring would provide a more complete understanding of any change over time. Due to the 

reference condition being undefined and the lack of data to determine trends over time, a 

Condition Level was not assigned for this measure.

Canopy Height

The project team defined the Significance Level for this measure as a 3. According to Wilson and 

Dunton (2011), seagrass community canopy heights ranged from 3 to 51 cm (1.18 to 20.1 in) 

throughout the lagoon. The average canopy height was greater in the upper lagoon basin (20.1 ± 

8.8 cm) than in the lower lagoon basin (15.7 ± 9.8 cm). As was previously mentioned, canopy 

height varies among seagrass species, which makes it difficult if not impossible to determine the 

health of the seagrasses through gross canopy height alone. With the absence of a reference 

condition, and the variability in canopy heights between different species and communities, a 

Condition Level was not assigned for this measure.
65

Weighted Condition Score

A Weighted Condition Score for the PAIS seagrass community could not be calculated at this 

time due to an undefined reference condition and limited data on the characteristics and 

condition of the seagrass community in the Laguna Madre adjacent to PAIS.

Sources of Expertise

Martha Segura, I & M Program Coordinator, GULN

Joe Meiman, Hydrologist, GULN

Wade Stablein, Biological Technician, PAIS

Dr. Ken Dunton, Professor, Department of Marine Science, University of Texas Marine Science 

Institute, Port Aransas
66

Literature Cited

Burd, A. B., and K. H. Dunton. 2000. Field verification of a light-driven model of biomass

changes in the seagrass Halodule wrightii. University of Texas, Austin, Texas.

Environmental Protection Agency (EPA). 2012. Total solids. U.S. Environmental Protection 

Agency. http://water.epa.gov/type/rsl/monitoring/vms58.cfm (accessed 6 November 2012).

Duarte, C. M., N. Marba, and R. Santos. 2004. What may cause the loss of seagrass. Pages 24-32 

in Borum, J., C. M. Duarte, D. Krause-Jensen, and T. M. Greve, eds. European seagrasses: an 

introduction to monitoring and management. Monitoring and Managing of European 

Seagrasses, Hilleroed, Denmark.

Dunton, K. 2012. Texas coastal bays and estuaries report card. The Univeristy of Texas Marine 

Science Institute. http://texasseagrass.org/documents/SeagrassReportCardPDF.pdf (accessed 

6 November 2012).

Florida Department of Environmental Protection (FDEP) 2011. What are 

seagrasses? http://www.dep.state.fl.us/coastal/habitats/seagrass/ (accessed 1 November 

2012).

Green, E. P., and F. T. Short. 2003. World atlas of seagrasses. University of California Press, 

Berkeley, California.

Handley, L., D. Altsman, and R. DeMay, eds. 2007. Seagrass status and trends in the northern 

Gulf of Mexico: 1940-2002. U.S. Geological Survey Scientific Investigations Report 2006-

5287 and U.S. Environmental Protection Agengy 855-R-04-003. U.S. Geological Survey, 

Reston, Virginia, and U.S. Environmental Protection Agency, Washington, D.C.

Onuf, C. P. 1996. Biomass patterns in seagrass meadows of the Laguna Madre, Texas. Bulletin 

of Marine Science 58(2):404-420.

Onuf, C. P. 2007. Laguna Madre. Pages 29-40 in Handley, L., D. Altsman, and R. DeMay, eds. 

Seagrass status and trends in the northern Gulf of Mexico: 1940–2002. U.S. Geological 

Survey Scientific Investigations Report 2006-5287 and U.S. Environmental Protection 

Agency 855-R-04-003. U.S. Geological Survey, Reston, Virginia, and U.S. Environmental 

Protection Agency, Washington, D.C.

Safe Drinking Water Foundation (SDWF). 2012. Oil spills. Safe Drinking Water 

Foundation. http://www.safewater.org/PDFS/resourcesknowthefacts/Oil_Spills.pdf (accessed 

6 November 2012).

Sea Turtle Conservancy (STC). 1995. Sea turtle threats: oil spills. Sea Turtle Conservancy, 

Gainesville, Florida. http://www.conserveturtles.org/seaturtleinformation.php?page=oilspills

(accessed 6 November 2012).

Teeter, A. M. 2002. Sediment transport in wind-exposed shallow, vegetated aquatic systems. 

Dissertation. Louisiana State University and Agricultural and Mechanical College, Baton 

Rouge, Louisiana.
67

Texas Parks and Wildlife (TPWD). 2012. Seagrass conservation plan for Texas. Texas Parks and 

Wildlife, Resource Protection Division. http://www.tpwd.state.tx.us/publications/pwdpubs/

media/pwd_bk_r0400_0041.pdf (accessed 6 November 2012).

Wilson, C. J., and K. H. Dunton. 2011. Assessment of seagrass habitat quality and plant 

physiological condition in Texas coastal waters. The University of Texas at Austin Marine 

Science Institute, Port Aransas, Texas.
68

Plate 8. Percent cover of all seagrass in PAIS (Wilson and Dunton 2011).
69

Plate 9. Mean canopy height (cm) of all seagrass in PAIS (Wilson and Dunton 2011).
70

4.4 Emergent Wetland and Pond Communities

Description

The inland areas of PAIS, between the Gulf dunes and Laguna Madre beaches, support wetlands 

and freshwater ponds that are vital to the island’s wildlife. These areas provide habitat for 

aquatic invertebrates, fish, amphibians, and waterfowl, as well as a water source for other birds 

and mammals (Photo 9; NPS 1996, White et al. 2006). On barrier islands such as Padre Island, 

wetlands and ponds form in flat areas left behind migrating dunes. These low-lying areas, 

scoured by wind and sand, are called deflation flats or troughs (Hunter et al. 1972, as cited in 

Weise and White 1980). Wetlands and ponds are more common in the north and central portions 

of PAIS, where precipitation is slightly higher than in the south, allowing wetlands to hold water 

for most of the year (Weise and White 1980, NRCS 2007). 

Photo 9. Waterfowl on Pond B along Park Road 22 (photo by Shannon Amberg, SMUMN GSS).

Vegetation around the lowest emergent wetlands typically includes southern cattail (Typha 

domingensis), spikerushes, (Eleocharis spp.), common threesquare 

(Schoenoplectus pungens), seashore paspalum (Paspalum vaginatum), coastal waterhyssop 

(Bacopa monnieri), and sweetscent (Pluchea odorata var. odorata). Species common in slightly 

higher wetlands with less frequent standing water are cordgrass (Spartina spp.), flatsedges 

(Cyperus spp.), bushy seaoxeye (Borrichia frutescens), paspalum (Paspalum spp.), largeleaf 

pennywort (Hydrocotyle bonariensis), and frog fruit (Phlya sp.) (White et al. 2006). Additional 

wetland species noted in PAIS include starrush whitetop, marsh fimbry (Fimbristylis castanea), 

rushes (Juncus spp.), and smartweed (Polygonum sp.) (Drawe et al. 1981, White et al. 2006).

Measures

• Extent

• Native species composition

• Wetland/pond elevation
71

Reference Conditions/Values

Since little is known about the island’s wetlands and ponds, a reference condition has not been 

established. The ephemeral nature of many of the park’s wetlands and a lack of historic 

information make this difficult to determine.

Data and Methods

Descriptions of the park’s ecological communities, including wetlands, from the 1970s and early 

1980s were found in Baccus et al. (1977), Weise and White (1980), and Drawe et al. (1981). In 

1989-1990, Sissom et al. (1990) completed a baseline survey of three ponds in PAIS. They 

sampled chemical and physical parameters and surveyed both the flora and fauna around these 

ponds. The first landcover classification for the park was completed by Laine and Ramsey 

(1998), and provides information on the extent of wetland vegetation in PAIS. Laine and Ramsey 

(1998) followed classification and interpretation protocols developed by NOAA C-CAP and 

used map descriptions that fit NVCS guidelines. 

White et al. (2006) studied the status of wetlands on south Texas barrier islands, which included 

PAIS. They compared aerial photos from the 1950s, 1979, and 2002-2004 to identify changes in 

wetland extent over time. National Wetland Inventory (NWI) GIS data from the USFWS 

provides wetland information for the park. USFWS interpreted color infrared aerial imagery 

from the 1990s to generate this data. It provides an indication of the prevalence and distribution 

of wetland types (according to the Cowardin et al. 1979 classifications) throughout the park. In 

2007, the NPS created a surficial geology GIS map for PAIS, based primarily on 2003 aerial 

photography (NPS 2012). Map classifications are loosely based on land cover and include four 

classes that could be considered wetlands and/or ponds (NPS 2007). This map and associated 

data are considered the most up-to-date and accurate information regarding wetland and pond 

distribution in PAIS.

Since little data exist for wetland/pond elevation, SMUMN GSS analysts used ArcGIS to 

determine the approximate elevations (above sea level) of wetlands and ponds throughout PAIS. 

First, a set of points was created by using the centroid of each NWI wetland polygon (one point 

per polygon), ensuring that points were actually inside their respective polygons. Then, 

elevations for each point were extracted from a 30 m (98 ft) DEM created as part of a NOAA 

Tsunami Inundation Gridding Project (Taylor et al. 2008). Finally, the point elevations were 

spatially joined to the NWI wetland polygons, resulting in an elevation attribute within the 

wetland polygon layer. 

Current Condition and Trend

Extent

According to NWI GIS data, approximately 2,298.9 ha (5,681 ac) or just over 4% of the park is 

classified as palustrine (non-tidal) wetland (Table 12, Plate 10). The majority of this wetland area 

(2,205 ha) supports persistent emergent vegetation (PEM1).
72

Table 12. Major National Wetland Inventory (NWI) classifications (Cowardin 1979) by area in PAIS.

NWI 

Code NWI Description Total wetland 

area (ha)

PEM1A Palustrine, emergent, persistent, temporary flooded 992.3

PEM1C Palustrine, emergent, persistent, seasonally flooded 1,040.8

PEM1F Palustrine, emergent, persistent, semipermanently flooded 169.5

PEM1Khs Palustrine, emergent, persistent, artificially flooded, diked/impounded 2.6

PUBF Palustrine, unconsolidated bottom*, semipermanently flooded 4.5

PUBKx Palustrine, unconsolidated bottom, artificially flooded, excavated 0.9

PUSA Palustrine, unconsolidated shore, temporary flooded 45.8

PUSC Palustrine, unconsolidated shore, seasonally flooded 42.5

Total 2,298.9

*Areas classified as “unconsolidated bottom” or “unconsolidated shore” have <30% vegetative cover 

White et al. (2006) found that their North Padre Island study area supported more palustrine 

marsh (356 ha) than any other island studied. The extent of palustrine marsh on North Padre 

fluctuated over time, decreasing from the mid-1950s to 1979 (275 ha to 258 ha), and then 

increasing between 1979 and the early 2000s. Losses between the 1950s and 1979 were 

attributed primarily to replacement by uplands, although some area was lost to migrating dunes 

(White et al. 2006).

The 1998 landcover classification project (Laine and Ramsey 1998) identified three landcover 

classes that could include inland wetlands or ponds: inland water, emergent wetland, and

unconsolidated shore. These classes are described by Laine and Ramsey (1998, p. 6) as follows

• Inland water - Semipermanent areas of standing freshwater that are found parallel to the

foredune ridge along the interior of the island. Depending on depth, vegetation may

consist of species such as largeleaf pennywort.

• Emergent wetland - Shallow depressions that are inundated with freshwater from rain

events or saltwater from tropical storms. These areas are vegetated with bulrush, cattails, 

black willow, gulfdune paspalum, and largeleaf pennywort.

• Unconsolidated Shore - Areas adjacent to washover channels and inland water areas 

consisting of fine sands with little to no vegetation. If vegetation is present, it is sparse

with species such as cattails or bulrush.

The last two classes (emergent wetland and unconsolidated shore), however, were not separated 

into estuarine and palustrine systems. Therefore, the extent of inland/palustrine wetlands cannot 

be precisely determined from these data. These extents are shown in Table 13 and Plate 11, but it 

should be noted that, with the exception of the inland water class, these extents are 

overestimates.
73

Table 13. Area of map classes that contain wetland and pond communities, according to GIS landcover 

data (Laine and Ramsey 1998)

Map class Area (Ha)

Inland water 947.3

Emergent wetland* 7,882.9

Unconsolidated shore* 2,613.3

*These classes include both inland (palustrine) and shore (estuarine) communities, and therefore 

overestimate inland wetland and pond community coverage.

The 2007 PAIS surficial geology map includes several wetland and pond mapping units: 

temporarily flooded brackish to fresh marsh, seasonally flooded brackish to fresh marsh, 

semipermanently flooded brackish to fresh marsh, and water. According to GIS data, these four 

map units together cover 2,614 ha (6,459.6 ac) of the park. The area of each map unit is 

presented in Table 14 and Plate 12.

Table 14. Area of wetland and pond mapping units, according to surficial geology GIS data (NPS 2007).

Map unit Area (Ha)

Temporarily flooded brackish to fresh marsh 549.8

Seasonally flooded brackish to fresh marsh 1,266.2

Semipermanently flooded brackish to fresh marsh 599.6

Water 198.5

Total 2,614.1

Native Species Composition

While several sources have described the plant species characteristic of the park’s wetland and 

pond communities, a comprehensive study of wetland species composition is not available. 

Drawe et al. (1981) sampled 10 vegetation transects in PAIS in 1972-1973. They divided the 

island into five habitat types; while wetlands were not identified as a specific habitat, two of the 

habitat types (salt marsh and salty sands) were described as inundated to the point that sampling 

was not possible during four of the six study seasons (Drawe et al. 1981). These two habitat 

types likely included the park’s wetland and pond communities. A list of plant species identified 

in the salt marsh and salty sands communities is presented in Table 15. More species may be 

present but were not documented because of inaccessibility due to standing water during the 

majority of sampling periods. 
74

Table 15. Vascular plants documented in the salt marsh and salty sands habitats of PAIS (Drawe et al. 

1981). Some scientific and common names were updated to match those accepted by the Integrated 

Taxonomic Information System (ITIS).

Scientific name Common name Salt marsh Salty sands

Hydrocotyle bonariensis water pennywort x

Juncus marginatus grassleaf rush x

Sisyrinchium sagittiferum spearbract blue-eyed grass x

Salicornia bigelovii dwarf saltwort x

Blutaparon vermiculare silverhead x x

Sesuvium portulacastrum shoreline seapurslane x

Limonium carolinianum lavender thrift x

Heliotropium curassavicum 

 var. curassavicum

salt heliotrope x

Heterotheca subaxillaris camphorweed x

Erigeron procrumbens Corpus Christi fleabane x

Conoclinium betonicifolium betonyleaf thoroughwort x

Rhynchospora colorata starrush whitetop x

Eleocharis geniculata Canada spikesedge x

Eleocharis montevidensis spike sedge; sand spikerush x

Fimbristylis castanea marsh fimbry x x

Schoenoplectus pungens 

 var. longispicatus

common threesquare x

Croton capitatus hogwort x

Croton glandulosus sand croton x

Croton punctatus gulf croton x

Spiranthes vernalis spring lady’s tresses x

Eragrostis secundiflora red lovegrass x

Paspalum monostachyum gulfdune paspalum x

Spartina patens saltmeadow cordgrass x

Sporobolus virginicus seashore dropseed x x

Phyla nodiflora frog fruit x

Total 15 13

During their survey of three ponds in PAIS, Sissom et al. (1990) documented all vascular plant 

species within 60 m (200 ft) of the ponds. Across all three ponds, Sissom et al. (1990) recorded 

46 native and three non-native species (Appendix F). Sissom et al. (1990) also sampled algae in 

the ponds and obtained fungal species lists from another researcher studying these organisms in 

the three ponds. Fungal samples were found on foam, live and decaying plant matter, and other 

organic debris in the ponds. These algal and fungal species lists are also included in Appendix G 

and Appendix H. A total of 27 algal and 32 fungal taxa were identified (Sissom et al. 1990).

Additional species reported in PAIS wetlands but not recorded by Drawe et al. (1981) or Sissom 

et al. (1990) are listed in Table 16. All together, just over 70 native plant species have been 

documented in PAIS wetland and pond communities.
75

Table 16. Additional plant species documented in PAIS wetlands (White et al. 2006).

Scientific name Common name Scientific name Common name 

Schoenoplectus californicus California bulrush Chloracantha spinosa spiny chlorocantha

Paspalum vaginatum seashore paspalum Panicum spp. panicgrasses

Spartina spartinae gulf cordgrass Andropogon glomeratus bushy blustem

Borrichia frutescens bushy seaoxeye Cynodon dactylon* bermudagrass

* indicates non-native species

Wetland/Pond Elevation

The presence of freshwater in the park’s wetlands and ponds is largely determined by the 

elevation of the wetland or pond and the depth of the underlying groundwater table at any given 

time of the year (Stablein, phone communication, 29 August 2012). As groundwater levels 

fluctuate, the elevation of a wetland or pond can help managers predict whether the feature will 

contain water; water bodies at higher elevations are more likely to dry up if groundwater levels 

drop, while those at lower elevations continue to hold water. To date, the only published 

wetland/pond elevation data available for the park are from Sissom et al. (1990)’s three study 

ponds. The elevations at the deepest points of Ponds A, B, and C were 3.5 m (11.6 ft), 2.5 m (8.1 

ft), and 3.1 m (10.3 ft) respectively (Sissom et al. 1990).

SMUMN GSS analysts used a DEM to determine the approximate elevations of wetlands and 

ponds throughout PAIS (see Plate 13 Plate 14). Analysis of the resulting elevation data shows 

that nearly 75% of wetlands and ponds in PAIS are between 2 and 4 m (6.6-13.1 ft) in elevation 

(Figure 5). However, the total wetland/pond area is more widely distributed between 1 and 5 m 

(3.3-16.4 ft) in elevation (Figure 6).

Figure 5. Number of wetlands and ponds in PAIS by elevation.

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8 9

Number of wetland/pond polygons

Elevation (m)
76

Figure 6. Wetland and pond area in PAIS by elevation. 

Threats and Stressor Factors

Threats to the park’s wetlands and ponds include non-native plant species, oil and gas 

exploration and development, drought, sand migration, storm surge, and saltwater inundation. 

Non-native plants, such as giant reed (Arundo donax), threaten to outcompete native species for 

limited resources in wetland areas, eventually replacing the natives. Aggressive native species, 

such as common reed (Phragmites sp.), may also threaten wetland species diversity (one known 

Phragmites location in the park is shown in Plate 10). Extreme weather events, including 

droughts and hurricanes, can negatively impact the wetlands and ponds of PAIS. Hurricanes are 

one of the greatest concerns for the park’s ecological communities, as their effects can be 

devastating (Cooper et al. 2005). On average, the Texas coast is struck by 0.67 storms per year, 

or approximately two storms every 3 years (Weise and White 1980). Storm surge is perhaps one 

of the most significant threats associated with hurricanes. These waters erode and transport large 

amounts of sediment and can even breach the dunes, allowing saltwater to reach inland areas of 

the island (Weise and White 1980). High winds, during hurricanes and at other times, have the 

potential to blow sand from the dunes and beaches into wetlands and ponds; this sand could 

eventually accumulate and reduce the wetland or pond’s capacity to hold water. For example, 

Sissom et al. (1990) reported that one of their study ponds was filling with sand and would 

eventually be converted to a marsh. 

According to NPS (1996, p. 19), “Oil/natural gas exploration and production pose a greater 

threat to the natural resources of Padre Island National Seashore than any other type of activity 

that takes place in or near the park.” Incidents such as hydrocarbon spills, improper drilling mud 

disposal, and sump discharges can have long-term cumulative effects on the park’s groundwater. 

Groundwater is shallow at PAIS and any contamination has the potential to impact wetland and 

pond communities (NPS 1996).

Data Needs/Gaps

Cooper et al. (2005) identified several data needs for wetlands and ponds at PAIS. These include 

how wetlands have changed over time, if any wetlands have become degraded and are in need of 

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8 9

Area of wetland/pond polygons (ha)

Elevation (m)
77

restoration, and how or if park development and oil/gas exploration are affecting wetlands and 

surface water regime. A more comprehensive native plant survey specific to the park’s wetlands 

and ponds would also be helpful in assessing community condition. Repeated analysis of 

wetland/pond elevation in the future will help determine if any changes (i.e., sedimentation and 

filling of wetlands) are occurring over time. 

Overall Condition

Extent

The project team assigned this measure a Significance Level of 3. The extent of wetland and 

pond communities in PAIS varies greatly depending on moisture availability (e.g., precipitation). 

While NWI and NPS surficial geology data provide some indication of wetland extent within 

PAIS, it is not enough to assess the current condition of this measure. White et al. (2006)’s

assessment of barrier island wetlands suggests that palustrine marsh area on North Padre Island 

has remained relatively stable over time. However, the most recent imagery used in their 

assessment was 2002-2004, which is nearly a decade ago. As a result of a lack of comparable 

information over time, a Condition Level was not assigned. 

Native Species Composition

The species composition measure was also assigned a Significance Level of 3. Wetland and pond 

native species composition has not been comprehensively surveyed. The available information is 

limited, both temporally and spatially. As a result, a Condition Level could not be determined for 

this measure.

Wetland/Pond Elevation

This measure was assigned a Significance Level of 3. Prior to this assessment, published 

elevation data was only available for three ponds in the park at one point in time (Sissom et al. 

1990). SMUMN GSS analysts used a DEM to create a GIS data layer with the approximate 

elevations of wetlands and ponds throughout PAIS. This information may be used as a baseline 

for future assessments, but at this time there is not enough data to determine a Condition Level

for this measure. 
78

Weighted Condition Score

A Weighted Condition Score was not calculated for PAIS emergent wetland and pond 

communities, since Condition Levels could not be determined for any of the measures.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS

Joe Meiman, Hydrologist, GULN
79

Literature Cited

Baccus, J. T., J. K. Horton, and P. D. Carangelo. 1977. A study of beach and dunes floral and 

faunal interrelations as influenced by recreational and user impact on Padre Island National 

Seashore. National Park Service, Southwest Region, Santa Fe, New Mexico.

Cooper, R. J., S. B. Cedarbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore. National Park Service, Gulf Coast Network, Lafayette, Louisiana.

Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe. 1979. Classification of wetlands and 

deepwater habitats of the United States. U.S. Fish and Wildlife Service, Washington, D.C.

Drawe, D. L., K. R. Kattner, and W. H. McFarland. 1981. Vegetation and soil properties of five 

habitat types on North Padre Island. Texas Journal of Science 33:145-157.

Hunter, R. E., R. L. Watson, G. W. Hill, and K. A. Dickinson. 1972. Modern depositional 

environments and processes, northern and central Padre Island, Texas. Pages 1-17 in Padre 

Island National Seashore field guide: Gulf Coast Association of Geological Societies, 

convention field trip, October 14, 1972. Gulf Coast Association of Geological Societies, 

Austin, Texas.

Laine, S. C., and E. R. Ramsey. 1998. Landcover classification for Padre Island National 

Seashore. U.S. Geological Survey, National Wetlands Research Center, Lafayette, Lousiana.

National Park Service (NPS). 1996. Padre Island National Seashore resources management plan. 

National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS) - Geologic Resources Division. 2007. Digital Surficial Units of 

Padre Island National Seashore and Vicinity, Texas. paissur.shp ArcGIS data layer. National 

Park Service (NPS), Geologic Resources Division, Denver, Colorado.

National Park Service (NPS) - Geologic Resources Division. 2012. Padre Island National 

Seashore: GRI ancillary map information document. National Park Service (NPS), Geologic 

Resources Division, Denver, Colorado.

Natural Resource Conservation Service (NRCS). 2007. Soil survey of Padre Island National 

Seashore, Texas: special report. Natural Resource Conservation Service, Washington, D.C., 

and National Park Service, Fort Collins, Colorado.

Sissom, S. L., R. D. Koehn, D. Lemke, R. R. Smith, C. Caudle, and F. Oxley. 1990. A baseline 

study of three ponds within the Padre Island National Park. National Park Service, Padre 

Island National Seashore, Corpus Christi, Texas.

Taylor, L. A., B. W. Eakins, K. S. Carignan, R. R. Warnken, T. Sazonova, and D. C. Schoolcraft. 

2008. Digital elevation model of Corpus Christi, Texas: Procedures, data sources, and 

analysis. NOAA Technical Memorandum NESDIS NGDC-11. National Geophysical Data 

Center, Boulder, Colorado. Available online 

at http://www.ngdc.noaa.gov/dem/squareCellGrid/download/401.
80

U.S. Fish and Wildlife Service (USFWS). 2012. TX_wetlands.gdb. ArcGIS geodatabase. 

Downloaded from http://www.fws.gov/wetlands/Data/State-Downloads.html (accessed 13 

July 2012).

Weise, B. R., and W. A. White. 1980. Padre Island National Seashore: A guide to the geology, 

natural environments, and history of a Texas barrier island. Texas Bureau of Economic 

Geology, Austin, Texas.

White, W. A., T. A. Tremblay, R. L. Waldinger, and T. R. Calnan. 2006. Status and trends of 

wetland and aquatic habitats on Texas barrier islands, coastal bend. Texas General Land 

Office, Austin, Texas, and the National Oceanic and Atmospheric Administration, 

Washington, D.C.
81

Plate 10. Wetlands and ponds in PAIS according to NWI data (USFWS 2012). The location of a 

potentially problematic Phragmites stand is also shown in the far left map.
82

Plate 11. Extent of wetland and pond vegetation classes in PAIS according to Laine and Ramsey (1998).

The emergent wetland and unconsolidated shore classes include both estuarine and palustrine areas and 

therefore overestimate the true extent of inland/palustrine wetlands. 
83

Plate 12. Extent of wetland and pond mapping units, according to NPS surficial geology data (NPS 2007).

Temp. = temporarily, Seas. = seasonally, Semiperm. = semipermanently.
84

Plate 13. Sample map showing approximate wetland and pond elevations in a northern portion of PAIS.
85

Plate 14. Sample map showing approximate wetland elevations in a central portion of PAIS.
86

4.5 Migratory Bird Species

Description

Bird populations often act as excellent indicators of an ecosystem’s health (Morrison 1986, Hutto 

1998, NABCI 2009). Birds are often highly visible components of ecosystems, and bird 

communities typically reflect the abundance and distribution of other organisms with which they 

co-exist (Blakesley et al. 2010). Migratory birds serve as excellent ecological indictors because a 

disturbance adversely affecting 

any of the habitats used by these 

species (e.g., stopover, wintering, 

or breeding habitats) can cause 

declines in populations and a 

decrease in species’ reproductive 

success (Hilty and Merenlender 

2000, Zöckler 2005). 

The unique ecosystems and 

physical formations of the Gulf 

and Laguna Madre coasts in PAIS 

provide bird species with ideal 

stopover and overwintering 

habitats (Chaney et al. 1993a), 

and represent a vital area for 

many migratory bird species in 

North and South America. The 

American Bird Conservancy 

(ABC) has identified PAIS as a 

Globally Important Bird Area. In 

order to be listed as a Globally 

Important Bird Area, a site must, during some part of the year, contain habitat that supports:

1. A significant population of an endangered or threatened species;

2. A significant population of a U.S. WatchList species;

3. A significant population of a species with a limited range, or

4. A significantly large concentration of breeding, migrating or wintering birds, including 

waterfowl, seabirds, wading birds, raptors or landbirds (ABC 2010).

In addition to being listed as a Globally Important Bird Area by the ABC, PAIS has also been 

identified by the Western Hemisphere Shorebird Reserve Network (WHSRN) as a part of the 

Laguna Madre, a Site of International Importance (WHSRN 2009). Criteria for being listed as a 

Site of International Importance include:

1. At least 100,000 shorebirds annually, or

2. At least 10% of the biogeographic population of a species (WHSRN 2009).

Photo 10. Reddish egrets (photo by Joel Reynolds, USFWS).
87

PAIS has confirmed the presence of more than 350 species of birds, of which approximately 

40% are migratory species (NPS 2013b). PAIS is located along one of the major migration 

flyways in North America (Figure 7), and many species, such as the red knot and the black tern 

(Chlidonias niger), pass through the park on their way from wintering grounds in the south to 

breeding grounds in the north. PAIS acts as an important over-wintering area for several 

migratory species, such as the ruby-crowned kinglet (Regulus calendula) and the black-bellied 

plover (Pluvialis squatarola), as these species spend the winter months along PAIS’s coastlines 

before returning to their breeding grounds in the spring. Migratory birds that nest in the park 

include the long-billed curlew (Numenius americanus), as well as least tern (Sternula antillarum) 

and Wilson’s plover (Charadrius wilsonia), both which nest along the tidal flats.

Figure 7. Major North American migratory flyways. PAIS is located at a bottleneck of the Central and 

Mississippi Flyways (NPS 2013b).

One of the more iconic overwintering species is the redhead. Seventy-five percent of the global 

redhead populations overwinter in the Laguna Madre (Weller 1964, Roetker 2003, James 2006), 

and the overwintering birds are almost entirely dependent upon shoalgrass rhizomes. The 

overwintering redheads also utilize the three freshwater ponds found in the interior of the park. 

PAIS is also home to several species of conservation concern, such as the piping plover, reddish 
88

egret (Photo 10), wood stork (Mycteria americana), and swallow-tailed kite (Elanoides 

forficatus); these species are listed as threatened or endangered by the state of Texas or by the 

USFWS (NPS 2013c). Two migratory species in PAIS, the red knot and Sprague’s pipit (Anthus 

spragueii), are candidate species for federal protection and listing under the Endangered Species 

Act. 

Long-distance migratory species are highly informative indicator species, as their overall health 

depends on several different ecosystems. Global Christmas Bird Count (CBC) data indicate 

significant declines in migratory bird numbers in recent years (Peterjohn and Sauer 1999, 

Vicerky and Herkert 2001). The red knot is one of the park’s longest distance migrants, as it 

migrates south to the park from its circumpolar breeding habitats; the coastal red knot habitat in 

PAIS is among the highest quality habitats for this species in Texas (Stablein, written 

communication, 13 May 2013). There are six recognized subspecies of red knots, and it is likely 

that PAIS is home to the C. c. rufa and C. c. roselaari subspecies, with the C. c. rufa subspecies 

being the most commonly observed (Stablein, written communication, 13 May 2013). 

Monitoring of long distance migratory species populations (such as the red knot) as they pass 

through or overwinter in PAIS may help managers to develop a better understanding of the 

overall health of not only the PAIS ecosystem, but also the other ecosystems that these bird 

species rely on. 

Measures

• Neotropical species abundance and diversity

• Raptor abundance

• Waterfowl abundance

Reference Conditions/Values

A reference condition for migratory bird species in PAIS has not been defined. No annual 

monitoring of these groups of birds occurs in the park, and a contemporary baseline survey could 

be used to define condition. There are limited contemporary and historic data for this component, 

and no reference condition was used in the assessment of condition for this component.

Data and Methods

While there have been several bird surveys in the park (see Chaney et al. 1993a, 1993b, 1995a, 

1995b; Ginter 2004; Lawson 2009), these surveys did not identify any Neotropical migrants and 

their results are not discussed here. Chapter 4.6 uses and summarizes the data from these surveys 

when appropriate.

The NPS Certified Bird Species List (NPS 2013a) (Appendix I) for PAIS was used for this 

assessment; this list represents all of the bird species confirmed as present in the park. For this 

component, only bird species considered migratory were included. SMUMN GSS removed 

resident and colonial waterbird species from this list, as these groups are discussed separately in 

Chapters 4.6 and 4.7 of this document. 

The Christmas Bird Count conducted in PAIS was part of the International Christmas Bird 

Count, which started in 1900 and is coordinated internationally by the Audubon Society. The 
89

PAIS CBC was conducted annually from 1974-1990. Multiple volunteers surveyed a 24-km (15-

mi) diameter on one day, typically between 14 December and 5 January. The center point of the 

24-km diameter was 27.3333oN, -97.3333oW (Plate 15). Unlike the surveys that occur during the 

breeding season (such as the North American Breeding Bird Survey [BBS]), the CBC surveys 

overwintering and resident birds that are not territorial and singing. The total number of species 

and individuals were recorded each year. Recently, a new CBC circle was established with a 

center point located in the center of 9-mile Hole. Staff members of the NPS, TPWD, USFWS, 

CBBEP, National Audubon Society, Texas A&M University – Corpus Christi, and volunteers 

from the public perform this CBC in the winter. Data from this new CBC are not yet available 

from the National Audubon Society. 

Chapman (1984) investigated the effects of the 1979 IXTOC I oil spill on the coastal bird species 

of Mustang and Padre Islands. One of the survey areas (area B) was located within PAIS, and 

extended from Malaquite Beach in the north, to Mansfield Channel in the south. From October 

1980 to June 1981, bird censuses were conducted from a four-wheel drive vehicle that was 

driven along the beaches, and only birds on the beaches were counted (i.e., no flying or 

swimming birds were counted). Stops were made every 16 km (10 mi), and researchers were 

allowed 15 minutes to scan the area for birds that may have been missed from the vehicle.

Blacklock et al. (1998) mist-netted and banded Neotropical migrants in PAIS during the 1998 

spring migration. Net locations were near the PAIS ranger station and near a wetland and dune 

habitat along Bird Island basin. Blacklock et al. (1998) also investigated Neotropical migrant 

habitat use in the park by setting up line-transects in three habitat types: dunes, wetlands, and 

grasslands. These transects were surveyed throughout the 1998 spring migration.

Seegar et al. (2012) summarized the annual peregrine falcon migration survey on South Padre 

Island in 2012. South Padre Island is one of the major overwintering areas for the Arctic 

peregrine falcon (F. p. tundrius), and surveys, banding, and blood sample collections have 

occurred during migration (spring and fall) in the area for over 50 years. The long-term 

objectives of the South Padre Island surveys are:

• Monitoring population trends and migration phenology through band returns and 

sightings;

• Maintaining a banded population to continue this monitoring;

• Sampling blood from captured individuals for DNA level genetic analyses to identify the 

regional make-up of the Padre migrants;

• Identifying migratory pathways, breeding areas and critical wintering areas for the 

migrant Padre population through band returns and locations of radio marked falcons 

tracked by satellite (Seegar et al. 2012, p. 2).

Surveys in the spring of 2012 spanned from 10-24 April, while fall surveys in 2012 were from 25 

September to 23 October. The southern boundary of the survey area was near the northern end of 

U.S. Highway 100, while the northern boundary was the Mansfield Channel. Despite the fact that 
90

these surveys did not take place in PAIS, the results reported by Seegar et al. (2012) likely apply 

to the park due to the extremely close proximity of the two areas.

Survey participants used all-terrain vehicles (ATVs) to survey the study area from dawn to dusk, 

and recorded time, species, age, sex, location, and activity when falcons were sighted (Seegar et 

al. 2012). When a falcon was sighted in an area that allowed a safe, successful trapping effort, 

researchers would make a capture attempt. If a falcon was successfully captured, it was banded 

(U.S. Geological Survey [USGS] bands), had 2 mL of blood withdrawn, and had an axillary 

feather sample taken; a full description of banding protocol is available in Seeger et al. (2012).

Current Condition and Trend

Neotropical Species Abundance and Diversity

Nearctic-Neotropic migrants, hereafter Neotropical migrants, are bird species that breed in the 

temperate latitudes of the U.S. and Canada, but migrate to the tropical latitudes of Central and 

South America in the winter months (Figure 8, TPWD 2013). Stotz et al. (1996) estimate that 

approximately 420 bird species are classified as Neotropical migrants, and 333 of these species 

have been recorded in Texas (TPWD 2013). TPWD (2013) estimates that nearly half of all the 

documented bird species in Texas are Neotropical migrants.

Figure 8. Zoogeographic regions of the world; shaded areas represent transition areas between regions 

(TPWD 2013).

PAIS is located in a unique area, as it lies very close to the perceived boundary between the 

Nearctic and Neotropical zoogeographic regions. Because of this, several Neotropical nesting 

species (e.g., great kiskadee [Pitangus sulphuratus]) also call PAIS home. For this measure of 

the assessment, however, only Neotropical migrant species will be discussed. TPWD (2013) has

identified all Neotropical migrant species that are known to occur in Texas, and Appendix I 

indicates which of these species are present in PAIS.

NPS Certified Species List
91

The NPS Certified Bird Species List (accessible from: https://irma.nps.gov/App/Species/Search) 

confirms the presence of 370 bird species within PAIS. Of the 370 species identified, 216 species 

are identified as migratory species, and 127 species are Neotropical migrants (Appendix I).

PAIS Christmas Bird Count (1974-1990)

The Christmas Bird Count conducted in PAIS was part of the International Christmas Bird 

Count, which started in 1900 and is coordinated internationally by the Audubon Society. The 

PAIS CBC was conducted annually from 1974-1990. Multiple volunteers surveyed a 24-km (15-

mi) diameter on one day, typically between 14 December and 5 January. Unlike the surveys that 

occur during the breeding season (such as the North American BBS), the CBC surveys 

overwintering and resident birds that are not territorial and singing. The total number of species 

and individuals were recorded each year. Recently, a new CBC circle was established with a 

center point located in the center of 9-mile Hole. Staff members of the NPS, TPWD, USFWS, 

CBBEP, National Audubon Society, Texas A&M University – Corpus Christi, and volunteers 

from the public perform this CBC in the winter. Data from this new CBC are not yet available 

from the National Audubon Society. SMUMN GSS adjusted the yearly data for these surveys in 

order to only display the observed Neotropical migrants (Table 17). The highest number of 

observed individuals occurred during the 1975-76 survey when 527 individuals were observed 

(Table 17). The 1974-75 survey had the highest number of observed species (14) (Figure 9).
92

Table 17. Number of Neotropical migrant individuals observed on the PAIS Christmas Bird Count from 1974-1990. Data retrieved 

from http://audubon2.org/cbchist/count_table.html.

Species 1974

-75

1975

-76

1976

-77

1977

-78

1978

-79

1979

-80

1980-

81

1981-

82

1982-

83

1983-

84

1984-

85

1985-

86

1986-

87

1987-

88

1988-

89

1989-

90

semipalmated plover 31 1 22 22 23 22 1 2 2

Wilson's plover 5 3

black-bellied plover 88 138 21 108 137 11 9 109 83 45 69 20 127 187 58 13

red knot 25 279 9 92 289 1 75 21 4 129 19

white-rumped sandpiper 4

least sandpiper 185 56 113 103 38 14 69 160 158 3 25 10 69

semipalmated sandpiper 6 3

whimbrel 1

Wilson's phalarope 4

solitary sandpiper 2 1 1

indigo bunting 1

Cassin's sparrow 9 7 2 8 1 2 1 1

Lincoln's sparrow 21 7 18 33 1 21 6 2 23 12 16 13 30

barn swallow 1

gray catbird 1 1 1

yellow-throated warbler 1 9 1

black-throated green 

warbler 2

palm warbler 1 1

orange-crowned warbler 30 28 15 48 9 23 5

house wren 12 5 18 7 2 3 5 1

common nighthawk 1

Total 414 527 197 424 493 59 121 371 310 64 246 20 163 219 148 15
93

Figure 9. Number of Neotropical migrant species observed on the PAIS Christmas Bird Count from 1974-

1990. Data retrieved from http://audubon2.org/cbchist/count_table.html.

There was a considerable degree of variation in the number of observed Neotropical migrant 

species and individuals throughout the duration of the PAIS CBC (Table 17, Figure 9). The CBC 

data available for these species can only be interpreted with caution, as count data are largely 

dependent upon the effort of the observers and may not always provide an accurate depiction of a 

species’ abundance in PAIS.

Chapman (1984) 

Chapman (1984) surveyed the Padre and Mustang Island areas to investigate the effect that the 

IXTOC oil spill had on the coastal bird population. The study area of this project stretched from 

Aransas Pass in the north to Brazos Santiago Pass in the south. All of PAIS was included in the 

survey area, although the number of species identified during the survey was not isolated to 

specific regions. Because of this, the species observed during this survey must be analyzed and 

interpreted with caution.

Similar to the transformations made to the CBC data for this assessment, the Chapman (1984) 

data were organized to include only the Neotropical migrant species that were observed. After 

re-organization, Chapman (1984) reported nine Neotropical migrants, most of which were 

sandpiper or plover species (Table 18). No abundance or diversity data were reported for these 

species.

0

2

4

6

8

10

12

14

16

1974-75 1975-76 1976-77 1977-78 1978-79 1979-80 1980-81 1981-82 1982-83 1983-84 1984-85 1985-86 1986-87 1987-88 1988-89 1989-90

Number of Species Observed

Years
94

Table 18. Neotropical migrant species observed during the Chapman (1984) coastal bird surveys.

Species

semipalmated plover

Wilson's plover

black-bellied plover

upland sandpiper

red knot

pectoral sandpiper

Baird's sandpiper

least sandpiper

buff-breasted sandpiper

Blacklock et al. (1998)

Banding and survey efforts during the 1998 spring migration identified 447 Neotropical migrant 

birds of 53 different species (Table 19). Five species (indigo bunting, northern waterthrush, 

Lincoln’s sparrow, Swainson’s thrush, and the gray catbird) accounted for more than 50% of the 

captured birds (Table 19). Blacklock et al. (1998) indicated that there were not significant 

differences in species abundance between the dune or wetland habitats, although more individual 

birds were netted in the wetland habitats.
95

Table 19. Neotropical migrant species and abundance results from the Blacklock et al. (1998) surveys 

and banding efforts.

Common Name # of Individuals Common Name # of Individuals

yellow-billed cuckoo 1 ovenbird 3

blue grosbeak 4 northern waterthrush 64

painted bunting 10 American redstart 2

indigo dunting 75 Tennessee warbler 13

rose-breasted grosbeak 6 blue-winged warbler 1

black-headed grosbeak 1 Nashville warbler 17

dickcissel 5 Canada warbler 2

Lincoln's sparrow 37 hooded warbler 7

barn swallow 3 Wilson's warbler 1

northern rough-winged swallow 1 scarlet tanager 5

hooded oriole 1 summer tanager 10

Baltimore oriole 18 house wren 1

orchard oriole 9 veery 3

gray catbird 29 gray-cheeked thrush 7

bay-breasted warbler 3 Swainson's thrush 34

cerulean warbler 1 wood thrush 1

yellow-throated warbler 2 eastern wood-pewee 2

blackburnian warbler 1 least flycatcher 3

chestnut-sided warbler 2 Acadian flycatcher 4

yellow warbler 22 great crested flycatcher 2

blackpoll warbler 3 eastern kingbird 2

black-throated green warbler 4 warbling vireo 16

worm-eating warbler 2 red-eyed vireo 9

yellow-breasted chat 11 Chuck-will's-widow 1

black-and-white warbler 7 lesser nighthawk 1

Kentucky warbler 5 common highthawk 1

prothonotary warbler 1

Blacklock et al. (1998) also refers to a 1997 banding survey that was similar to the 1998 banding 

efforts; however, the 1997 summary report is not on file at PAIS and a hard or digital copy of the 

report has not been located. Despite this, Blacklock et al. (1998, p. 2) states that: “transect data 

from the present study [1998] strongly support the conclusions from the 1997 study that burned 

areas do not receive nearly as much use by Neotropical migrants as unburned habitats.” 

Blacklock et al. (1998) also suggests that Neotropical migrants likely favor the wetland habitats 

in the park, but that enough evidence to make that conclusion was not yet available.

Raptor Abundance

The NPS Certified Species List (NPS 2013a) identifies 11 migratory raptor species as present in 

the park (Table 20). Unfortunately, few studies have focused exclusively on raptors in PAIS, and 

the studies that have focused on this avian group have dealt almost exclusively with the 

peregrine falcon. As an important over-wintering site for the Arctic peregrine falcon, surveys of 
96

this species in PAIS during the spring and fall allow researchers to monitor population trends, 

migratory characteristics, contaminants, and diseases (Seegar et al. 2012).

Table 20. Migratory raptor species identified on the NPS Certified Species List (NPS 2013a).

Scientific Name Common Name

Accipiter cooperii Cooper's hawk

Accipiter striatus sharp-shinned hawk

Buteo jamaicensis red-tailed hawk

Buteo lineatus red-shouldered hawk

Buteo platypterus broad-winged hawk

Buteo swainsoni Swainson's hawk

Elanoides forficatus swallow-tailed kite

Ictinia mississippiensis Mississippi kite

Cathartes aura turkey vulture

Coragyps atratus black vulture

Falco peregrinus peregrine falcon

During the spring 2012 survey, Seegar et al. (2012) recorded 301 peregrine falcon sightings and 

captured 37 birds during 145 survey hours in the field. The sighting rate for spring 2012 (20.76 

birds/10 survey hours) was among the highest recorded during recent years, and was higher than 

the 18-year average (17.53 + 4.16 birds/10 survey hours) (Figure 10). Seven of the captured birds 

had been previously banded in the Padre Island area, and one bird was a migrant from the

Maryland coast (Seegar et al. 2012).

Figure 10. Padre Island peregrine falcon survey sighting rates from 1995-2012 (PEFA = peregrine 

falcon). Solid lines represent trend lines for the fall and spring surveys (figure reproduced from Seeger et 

al. 2012).

The fall 2012 peregrine falcon survey resulted in 1,002 observations and 228 captures during 484 

survey hours in the field (Seegar et al. 2012). Only seven of the captured individuals had been 

previously banded (all were Padre Island area bands). The sighting rate for fall 2012 (20.70 

birds/10 survey hours) was higher than the 19-year average (18.73 + 4.37 birds/10 survey hours) 

for fall sightings (Figure 10). 
97

Waterfowl Abundance

Very few studies have focused specifically on the migratory waterfowl population of PAIS. 

Several of the aforementioned bird studies (e.g., Chapman 1984; Chaney et al 1993b, 1995b; 

CBC data) have documented the presence of waterfowl species during their monitoring. The 

species that have been observed in the park during these monitoring efforts are reported in Table 

21.
98

Table 21. Waterfowl species observed in PAIS during several avian monitoring reports.

Species NPS 

(2013)

Chapman 

(1984)

Chaney et 

al. (1993b)

Chaney et 

al. (1995b) CBC

mottled duck X X X X X

northern pintail X X X X X

blue-winged teal X X X X X

northern shoveler X X X X

gadwall X X X X X

American wigeon X X X X X

canvasback X X X X

redhead X X X X X

lesser scaup X X X X X

bufflehead X X X X X

red-breasted merganser X X X X X

fulvous whistling duck B X X

black-bellied whistling duck B X X

greater white-fronted goose X X X

snow goose X X X

green-winged teal X X X X

common goldeneye X X X

mallard X X X

long-tailed duck (oldsquaw) A X X X

surf scoter X X X

Canada goose X X

wood duck B X X

cinnamon teal X X

ring-necked duck X X

greater scaup X X

black scoter X X

hooded merganser X X

ruddy duck X X

American black duck X

Ross's goose X

Harlequin Duck A X

white-winged scoter X

A = Vagrant species B = Migratory species

The CBC efforts in the park (from 1974-1990) were the only source of abundance data for many 

waterfowl species in PAIS. While this data is now outdated, it does provide some context for 

what the historic migratory baseline values may be for the identified species. The results from 

the CBC efforts are outlined in Table 22.
99

Table 22. Waterfowl abundance as recorded on the PAIS CBC from 1974-1990.

Species

1974-

75

1975-

76

1976-

77

1977-

78

1978-

79

1979-

80

1980-

81

1981-

82

1982-

83

1983-

84

1984-

85

1985-

86

1986-

87

1987-

88

1988-

89

1989-

90

greater white-fronted goose 19 171 27 5 138 58 8 8 7 109

snow goose 1217 1830 5930 1664 1272 2659 2959 2354 553 578 729 1685 354 260 226 101

Canada goose 1690 844 2227 547 6 421 2290 405 30 82 5 373 6 47 252

wood duck 2 4

gadwall 90 167 164 105 23 349 665 54 3 91 3 21 253 19 92

American wigeon 5115 281 110 417 343 180 408 50 43 11 14 5 101 29 21

mallard 6 39 109 77 84 89 53 148 5 2 8 24 1

mottled duck 128 99 361 246 150 182 102 216 33 57 35 109 54 50 2

blue-winged teal 21 13 121 40 85 302 70 100 31 4 8 20 5 2

cinnamon teal 7 2 5 11 10 3 1 17

northern shoveler 340 330 216 568 326 635 674 462 59 327 331 54 551 44 58 41

northern pintail 14266 6165 14346 3140 2813 4835 8290 932 2306 8293 2170 1630 924 199 1849 1372

green-winged teal 577 473 285 596 58 1003 387 246 180 600 23 95 162 53 3

canvasback 13 9 1 13 16 5 11 46 8 4 8 3

redhead 20044 7551 10934 4210 6606 14724 2564 5951 1946 1112 8669 4856 627 776 4921 304

ring-necked duck 1 13 1 4 1

greater scaup 11 1 2173

lesser scaup 8085 6073 12990 5001 4856 2210 6547 3753 3006 56 632 233 278 1156 733

surf scoter 3 1

black scoter 2

long-tailed duck (oldsquaw) 4

bufflehead 926 372 1202 246 354 1030 406 249 288 243 184 153 223 139 141 261

common goldeneye 55 74 5 3 3 1

hooded merganser 4 2 16 10 1 37 3 6

red-breasted merganser 50 1 124 159 36 83 22 23 3 4 24 9 7

ruddy duck 40 36 130 81 343 813 118 8 10 57 10 41 4 4 1

Total 52641 24514 49376 17141 17355 29697 25659 14963 8524 11483 12312 9358 3981 1774 10770 3099
100

During the CBC efforts, there were three species that dominated in terms of annual abundance: 

the northern pintail, redhead, and the lesser scaup (Aythya affinis). These species accounted for 

almost 75% of all individuals observed during the PAIS CBC. There was a considerable amount 

of annual variation in these species (and all species observed during the CBC) (Table 22), and as 

mentioned previously, CBC data must be interpreted with caution as these data are dependent 

upon the number of researchers participating in the count, the skill-base of the researchers, and 

the level of effort displayed by the researchers.

As displayed in the CBC data (Table 22), the redhead is a prominent species in PAIS during the 

winter months. Large congregations of redheads (approximately 75% of the global population) 

overwinter in the Laguna Madre, as the Laguna holds large amounts of shoalgrass; shoalgrass 

comprises 70-92% of the overwintering redhead’s diet (Koenig 1969, McMahan 1970, Cornelius 

1977, Marsh 1979, Woodin 1996, James 2006). With such a large concentration of a single 

species in a single location foraging on a specific food, management efforts can be very focused. 

Redhead populations have been increasing annually since the 1980s (James 2006); Wilkins and 

Otto (2002) indicated that breeding redhead populations reached near record numbers in the 

early 2000s. There is concern whether the Laguna Madre can support the ever-growing redhead 

population, as shoalgrass populations in the Laguna are heavily impacted by several threats:

1. Heavy foraging in previous winters by redheads and other waterfowl species;

2. Active dredging of the GIWW, which increases turbidity and can bury shoalgrass 

communities;

3. Altered salinity regime in the Laguna, primarily stemming from runoff and freshwater 

input;

4. Propeller scarring from both recreational and commercial vessels (James 2006).

While the global redhead population appears to be of low concern, their continued presence in 

PAIS and the Laguna Madre is not as clear. Aggregations of wintering redheads have been 

known to abandon historic wintering areas when submerged aquatic vegetation populations 

become depleted (this has been documented in the Chesapeake Bay [Perry et al. 1981] and in the 

Laguna Madre de Tamaulipas, Mexico [Saunders and Saunders 1981]). Managers of PAIS and 

agencies that operate in the Laguna should pay close attention to any trends in both the 

abundance of wintering redheads and the population of shoalgrass in the Laguna. 

Threats and Stressor Factors

Migratory bird species face deteriorating habitat conditions along their migratory routes and 

wintering grounds. Most of the birds that breed in the United States spend winters in the 

Neotropics (MacArthur 1959); deforestation rates in these wintering grounds have occurred at an 

annual rate up to 3.5% (Lanly 1982). While forest and habitat degradation does occur in the 

United States, it does not approach the level of degradation seen in the tropics (WRI 1989). 

Furthermore, Robbins et al. (1989) supported the suggestion that deforestation in the tropics has 

a more direct impact on Neotropical migrant populations than deforestation and habitat loss in 

the United States.
101

Grassland degradation is also an issue for migratory birds in the PAIS area, as less than 1% of 

the original coastal grasslands of the Texas coast is considered to be in pristine condition 

(Smeins et al. 1991). Lawson (2009) reports a significant portion of the remaining pristine

coastal grasslands in Texas is located in PAIS. Nationwide, over 97% of the native grasslands in 

the United States have been lost, primarily due to land conversion to agricultural fields (NABCI 

2011).

As urban areas continue to develop and grow, modern alterations to the landscape often foster 

competition between native and non-native bird species. Human-made structures may fragment a 

landscape and reduce its continuity; often as these changes occur, non-native bird species are 

able to inhabit the areas. Marzluff (2001, pp. 26-28) states that, 

The most consistent effects of increasing settlement were increases in non-native species 

of birds, increases in birds that use buildings as nest sites (e.g., swallows and swifts), 

increases in nest predators and nest parasites (brown-headed cowbirds [Molothrus ater]), 

and decreases in interior- and ground-nesting species.

Being located along two major migratory routes, PAIS is frequently home to migratory “fallout” 

events. Fallout is when migratory birds descend to the ground in large numbers following a 

disturbance of some kind. While exhaustion is one of the most common causes of fallout, many 

factors can influence a species’ migration pattern and cause fallout events. Examples of these 

factors include food availability (Niles et al. 1996), the presence of a large desert (Berthold 

1993) or open body of water (Alerstam 1990), topographic features (Berthold 1993, Bishop 

1997), or weather events (Alerstam 1990, Niles et al. 1996). 

In PAIS, weather-related fallout events are the most common, as hurricanes and strong 

thunderstorms may occur along the coast during migratory periods. Spring fallout events in 

Texas typically occur after strong, fast-moving cold-fronts move across the coast and into the 

Gulf of Mexico during the middle of the day (HAS 2013). The heavy rain and wind that 

accompany these cold fronts force migratory birds to the ground to avoid exhaustion. Migratory 

species that reach PAIS via a transoceanic flight (across the Gulf of Mexico) typically avoid 

periods of unfavorable weather, and large-scale movements often coincide with favorable wind 

conditions (Richardson 1976, Williams et al. 1977, Williams 1985, Moore et al. 1995, Butler 

2000), whereas birds migrating over landmasses tend to ground when wind and weather 

conditions deteriorate (Butler 2000). 

While extreme weather events such as hurricanes, tropical storms, and severe thunderstorms 

represent a significant threat to PAIS’s migratory birds, drought is also a source of stress for 

migratory birds. Drought is a major threat to most of the natural resources in PAIS. Not only do 

periods of drought remove many sources of standing water in PAIS (particularly the Bird Island 

Basin area), but these periods also affect availability of food for migratory birds. Drought may 

reduce forage items such as insects and plant species (Smith 1982), and could lead to starvation 

for many breeding birds in the park. Drought could also interrupt or alter the migratory patterns 

of species (Zeng 2003, Dai et al. 2004, Gordo 2007). 

Human disturbance is a common threat to animal populations across the planet, and in the PAIS 

area, one of the more pressing issues is oil and gas exploration and development. These 
102

operations often fragment the landscape, and the traffic and human activity associated with the 

sites may further disrupt the bird species in the area. Oil and gas extraction has taken place on 

Padre Island since before the park’s establishment in 1961 (Lawson 2009). Seventy oil/gas 

operations occurred within the administrative boundaries of the park between 1951 and 1981

(NPS 2005). The number of active operations in PAIS has declined since that era, and as of 

March 2013, there were six active pads within the park and 11 active wells. Two more pads are 

scheduled for restoration in the future, and only two operators (soon to be one operator) are 

active in the park (Stablein, written communication, 13 May 2013). While no new wells are 

being planned, exploration and development could occur at any time.

Data Needs/Gaps

Prior to 2013, there was no annual monitoring program for the land birds (whether migratory, 

resident, or raptor species) in PAIS. However, the GULN is beginning both a wintering and 

breeding landbird survey that will document both migratory and resident bird species. The 

establishment of these annual bird surveys and the resumption of the CBC efforts will likely 

provide the park with survey results that would be descriptive for both the resident and migratory 

species of the park. PAIS’s location provides managers with a unique opportunity to survey 

several unique species during the migratory periods, and long-term trend data would allow for a 

greater understanding of the health of not only PAIS’s habitat, but also the breeding and 

wintering areas for Neotropic migrants. Point counts will provide more detail regarding the 

breeding bird population of the park. The migratory bird population in PAIS serves as an 

excellent indicator of the entire park’s health, and routine winter area search surveys will provide 

managers with a better understanding of health of both the migratory bird population and the 

many PAIS ecosystems.

No formal survey for migratory raptors exists in the park. It is likely that raptors could be 

surveyed in conjunction with Neotropical migrants in the park; results of these surveys could be 

assessed as a separate measure (i.e., raptor population) or the raptors could be merged and 

analyzed as part of the Neotropical migrant measure. 

Overall Condition

Neotropical Species Abundance and Diversity

The project team defined the Significance Level for Neotropical species abundance and diversity 

as a 3. Because of its location along two major migratory flyways, PAIS is home to hundreds of 

different species that are not widely distributed in the U.S. There are not enough historic baseline 

data to assign this measure a Condition Level. Continuation/resumption of the Blacklock et al. 

(1998) survey would allow managers to detect any trends and to better understand species 

abundance and diversity in the park, although these surveys would need to be replicated for 

several years to have meaningful data. 
103

Raptor Abundance

The Significance Level for raptor abundance is a 3. PAIS likely represents a high-priority area for 

migratory raptor species, especially the peregrine falcon. However, there are no established 

raptor monitoring programs in the park, and there is not enough historic baseline data to assign 

this measure a Condition Level. Creation of annual surveys would allow managers to detect any 

trends and to better understand the species composition and abundance in the park.

Waterfowl Abundance

The Significance Level for waterfowl abundance was defined as a 3. The abundant coastline and 

wetland habitats in the park serve as important wintering and stopover grounds for migratory 

waterfowl, especially the redhead. However, there is no contemporary abundance data for this 

measure, and a Condition Level cannot be assigned at this time. Monitoring of the overwintering 

species and the health of their forage species’ populations will be essential in accurately defining 

the condition of this measure in the future. Special attention should be given to the overwintering 

redheads in the Laguna Madre, as this group represents a substantial percentage of the global 

population.

Weighted Condition Score

A Weighted Condition Score for PAIS’s migratory bird species was not assigned. Few data exist 

regarding the abundance and diversity of the park’s Neotropical migrants or raptor species. The 

establishment of annual surveys in the park would allow these measures to be accurately 

assessed in the future.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
104

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102:163–167.

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108

Plate 15. PAIS Christmas Bird Count survey area (1974-1990).
109

4.6 Resident Bird Species

Description

Resident bird species are birds that remain in one area throughout the year and do not migrate 

(Mettke-Hofmann et al. 2009). Unlike migratory birds, trends in resident bird species’ 

populations are likely due to changes occurring in their immediate habitat or ecosystem, and (in 

theory) it is possible to study all of their population processes directly throughout the year 

(Koskimies 1989). 

Excluding migratory species and colonial 

waterbirds (which are discussed in Chapters 

4.5 and 4.7, respectively), NPS (2013) 

identifies 67 resident bird species in PAIS; 

these include waterfowl, raptors, shorebirds, 

and passerine species. Several of the resident 

species in the park, such as the northern 

pintail and the black-necked stilt (Himantopus 

mexicanus, Photo 11), are dependent upon the 

coastal wetlands/shorelines, while other 

species, such as the eastern meadowlark, rely 

on the grasslands of the park’s interior.

While migratory and colonial waterbird 

species often overshadow the resident bird 

species of PAIS, this particular assemblage of 

birds may serve as a valuable ecological 

indicator, as they are year-round residents of 

the park, and their health and abundance are 

dependent solely upon PAIS ecosystems. As 

previously stated in this report, PAIS has been 

identified as both a Globally Important Bird 

Area (ABC 2010), and as part of the Laguna 

Madre, a Site of International Importance 

(WHSRN 2009). Monitoring of the resident 

bird populations in PAIS may help managers to protect the vital bird habitats present in the park.

Measures

• Species abundance and diversity

• Species distribution

• Raptor abundance

Photo 11. Black-necked stilt (TPWD photo).
110

Reference Conditions/Values

A reference condition for resident bird species in PAIS has not been defined. No annual 

monitoring of these birds occurs in the park, and a contemporary baseline survey could be used 

to define condition. There are limited contemporary and historic data for this component;

therefore, no reference condition was used in the assessment of condition for this component.

Data and Methods

This component focuses exclusively on the resident bird species of the park, and does not discuss 

any migratory or colonial waterbird species (these species are discussed in Chapter 4.5 and 

Chapter 4.7 of this document, respectively). A species was classified as a resident bird based on 

the NPS Certified Bird Species List (NPS 2013, Appendix J), and by using the species’ life 

history traits and established range as described by the Cornell University Lab of Ornithology’s 

All About Birds online database (http://www.allaboutbirds.org).

The Christmas Bird Count conducted in PAIS was part of the International Christmas Bird 

Count, which started in 1900 and is coordinated internationally by the Audubon Society. The 

PAIS CBC was conducted annually from 1974-1990. Multiple volunteers surveyed a 24-km (15-

mi) diameter on one day, typically between 14 December and 5 January. The center point of the 

24-km diameter was 27.3333oN, -97.3333oW (Plate 16). Unlike the surveys that occur during the 

breeding season (such as the North American BBS), the CBC surveys overwintering and resident 

birds that are not territorial and singing. The total number of species and individuals were 

recorded each year. Recently, a new CBC circle was established with a center point located in 

the center of 9-mile Hole. Staff members of the NPS, TPWD, USFWS, CBBEP, National 

Audubon Society, Texas A&M University – Corpus Christi, and volunteers from the public 

perform this CBC in the winter. Data from this new CBC are not yet available from the National 

Audubon Society. 

Chapman (1984) investigated the effects of the 1979 IXTOC I oil spill on the coastal bird species 

of Mustang and Padre Islands. All of PAIS was included in the survey area, although the number 

of species identified during the survey was not isolated to specific regions. Because of this, the 

species observed during this survey must be analyzed and interpreted with caution. From 

October 1980 to June 1981, bird censuses were conducted from a four-wheel drive vehicle that 

was driven along the beaches, and only birds on the beaches were counted (i.e., no flying or 

swimming birds were counted). Stops were made every 16 km (10 mi), and researchers were 

allowed 15 minutes to scan the area for birds that may have been missed from the vehicle. Only 

resident bird species that were observed are reported in this component.

EcoServices, Inc. conducted annual bird surveys along both the Gulf and Laguna Madre beaches 

from 1992-1995 (Chaney et al. 1993a, b, 1995a, b). These surveys focused on identifying 

protected bird species and determining the abundance and distribution of these species (Chaney 

et al. 1993b). The results of the surveys were summarized annually and released as internal NPS 

documents. At the midpoint of the project, more funds became available and the surveys were 

expanded to include all bird species (not just protected species). Data regarding protected species 

were collected in July 1992, and from October to April of 1992-93; data for all species were 

collected in July 1992 and from January to April of 1993. Unfortunately, much of the data 

collected in this report has been lost, as the only known physical copy has been lost. The data 
111

and information that were available from these studies are reported here, and were adjusted for 

this component to only include resident bird species in PAIS.

Current Condition and Trend

Species Abundance and Diversity

NPS Certified Species List

The NPS Certified Bird Species List (accessible from: https://irma.nps.gov/App/Species/Search) 

confirms the presence of 370 bird species within PAIS. Of the 370 species, 67 are identified as 

resident species (Appendix J).

PAIS Christmas Bird Count (1974-1990)

The most abundant data source for resident bird species’ abundance was PAIS’s annual CBC, 

which ran continuously from 1974 through 1990; this survey was discontinued in 1990 and has 

not been resumed in the years following. SMUMN GSS adjusted the yearly data for these 

surveys in order to display only the observed resident species (Appendix K). The highest number 

of observed individuals occurred during the 1977-78 survey when 5,835 individuals were 

observed (Appendix K). This survey year featured an unusually high number of American coots 

(Fulica americana) (900 individuals), as well as the highest number of northern bobwhite 

(Colinus virginianus) individuals (240) recorded during the PAIS CBC. The 1974-75 and 1975-

76 surveys had the highest number of observed species with 41 (Figure 11).

Figure 11. Number of resident bird species observed on the PAIS Christmas Bird Count from 1974-1990. 

Data retrieved from http://audubon2.org/cbchist/count_table.html.

Five resident bird species were responsible for over 70% of the individuals observed during the 

PAIS CBC; these species were the American coot, mourning dove (Zenaida macroura), red0

5

10

15

20

25

30

35

40

45

1974-75 1975-76 1976-77 1977-78 1978-79 1979-80 1980-81 1981-82 1982-83 1983-84 1984-85 1985-86 1986-87 1987-88 1988-89 1989-90

Number of Resident Species

Year
112

winged blackbird (Agelaius phoeniceus), eastern meadowlark, and the brown-headed cowbird 

(Appendix K). The red-winged blackbird was observed every year during the CBC, and was the 

most numerous of all the resident bird species, with more than 10,000 observations over the 

course of the park’s CBC (Appendix K).

There was a considerable degree of variation in the number of observed resident bird species and 

individuals throughout the duration of the PAIS CBC (Appendix K, Figure 11). The CBC data 

available for these species can only be interpreted with caution, as count data are largely 

dependent upon the effort and number of the observers, and may not always provide an accurate 

depiction of a species’ abundance in PAIS.

Chapman (1984)

The data presented in Chapman (1984) were organized to include only the resident species that 

were observed. After re-organization, Chapman (1984) reported 10 resident bird species (Table 

23). No abundance or diversity data were reported for these species.

Table 23. Resident bird species observed during the Chapman (1984) coastal bird surveys.

Scientific Name Common Name

Tachybaptus dominicus least grebe

Podilymbus podiceps pied-billed grebe

Buteo albicaudatus white-tailed hawk*

Falco sparverius American kestrel*

Colinus virginianus northern bobwhite

Fulica americana American coot

Haematopus palliatus American oystercatcher

Charadrius alexandrinus snowy plover

Charadrius vociferus killdeer

Eremophila alpestris horned lark

* Indicates a raptor species

Chaney et al. (1993a, b; 1995a, b)

The 1993 and 1995 surveys of the Gulf and Laguna Madre coasts of PAIS resulted in the 

observation of 17 resident bird species (Table 24). Eleven species were observed during the Gulf 

surveys, and 13 species were observed on the Laguna Madre surveys (Table 24); only three 

species (snowy plover, turkey vulture [Cathartes aura], and American kestrel [Falco 

sparverius]) were observed on all four surveys. No abundance or diversity estimates were 

reported by Chaney et al. (1993a, b; 1995a, b).
113

Table 24. Resident bird species observed during the 1993 and 1995 surveys of the Gulf and Laguna 

Madre coasts (Chaney et al. 1993a, b; 1995a, b).

Gulf Laguna Madre

Scientific Name Common Name

Chaney 

et al. 

(1993a)

Chaney 

et al. 

(1995a)

Chaney 

et al. 

(1993b)

Chaney 

et al. 

(1995b)

Dendrocygna bicolor fulvous whistling-duck X X

Buteo jamaicensis red-tailed hawk* X

Charadrius alexandrinus snowy plover X X X X

Cathartes aura turkey vulture* X X X X

Falco sparverius American kestrel* X X X X

Colinus virginianus northern bobwhite X

Dendrocygna autumnalis black-bellied whistling-duck X

Podilymbus podiceps pied-billed grebe X X

Fulica americana American coot X X

Eremophila alpestris horned lark X X X

Sturnella magna eastern meadowlark X X

Charadrius vociferus killdeer X X X

Buteo albicaudatus white-tailed hawk* X X

Molothrus ater brown-headed cowbird X

Quiscalus mexicanus great-tailed grackle X

Haematopus palliatus American oystercatcher X

Polyborus plancus crested caracara* X

* Indicates a resident raptor species

Species Distribution

Few data exist regarding the distribution of resident bird species in PAIS. The bird surveys that 

have documented distribution have been focused on migratory shorebirds (Chapman 1984), 

colonial waterbirds (TCWS 2011a, b), or priority shorebird species (Chaney et al. 1993a, b; 

1995a, b). Further complicating analyses, the Chaney et al. (1993a; 1995a, b) documents have 

been lost or damaged in the years following completion. Without these complete documents, an 

accurate summary of the data is not possible.

Chapman (1984)

Chapman (1984) surveyed the coastal shorebird population of the park, and did not record many 

resident bird species (Table 23); this study focused only on the beach habitat zones of the park 

(Figure 12). Because of this, the conclusions reached in this report must be interpreted with 

caution as they do not apply to all of the resident bird population, nor do they apply to all of the 

habitat types in PAIS.
114

Figure 12. The habitat zones used to classify bird distribution in Chapman (1984).

Among the bird species that frequent the beach habitat, Chapman (1984) found that the foreshore 

region (Figure 12) of the beaches at PAIS supported the highest concentration of birds, while the 

backshore (which is almost always dry) was the least used habitat. Perhaps more importantly, 

Chapman (1984) noted that birds were not uniformly distributed along the beaches, and that 

three factors affected the distribution of birds in the study area:

1. Beach substrate composition;

2. The presence of storm-tidal passes;

3. The location of tar mats, fresh tar balls, or mousse (Chapman 1984, p. 30).

Big Shell and Little Shell Beaches had noticeably lower numbers of bird species and individuals

(Chapman 1984). This trend was likely due to the unique shell-hash substrate of these two 

beaches; most of the survey area in Chapman (1984) consisted of a fine-sand substrate. The 

shell-hash substrate supports low numbers of invertebrates and other important forage species for 

shorebirds, which is probably why these beaches were poorly represented in the survey.

Raptor Abundance

There have been no studies in PAIS that focused exclusively on the park’s resident raptor 

population. Several studies have observed resident raptor species, however, and these results are 

summarized below. 

NPS Certified Species List
115

The NPS Certified Bird Species List confirms the presence of 13 native raptor species within 

PAIS (Table 25). The white-tailed hawk is listed as a threatened species by the State of Texas 

(TPWD 2013), while the aplomado falcon, is federally and state-listed as an endangered species 

(TPWD 2013).

Table 25. Resident raptor species in PAIS (NPS 2013).

Scientific Name Common Name

State 

Status

Federal 

Status

Buteo albicaudatus white-tailed hawk T

Buteo jamaicensis red-tailed hawk

Buteo lineatus red-shouldered hawk

Elanus leucurus white-tailed kite

Parabuteo unicinctus Harris' hawk

Cathartes aura turkey vulture

Coragyps atratus black vulture

Falco femoralis aplomado falcon E E

Falco sparverius American kestrel

Polyborus plancus crested caracara

Athene cunicularia burrowing owl

Bubo virginianus great horned owl

Tyto alba barn owl

T = Threatened

E = Endangered

PAIS Christmas Bird Count (1974-1990)

The CBC was the only data source that recorded abundance values for each survey, and 1,947 

individual resident raptors from nine species were observed during the CBC in PAIS from 1974-

1990 (Table 26). Only the white-tailed hawk was observed during every year of the CBC, while 

the turkey vulture was the most abundant raptor species (Table 26). The inaugural CBC (1974-

1975) resulted in the highest number of raptor species observed (nine), while the 1979-80 and

1980-81 surveys resulted in the highest number of observed raptor individuals (Table 26).
116

Table 26. Resident raptor species observed during the PAIS CBC (1974-1990). Data retrieved from http://audubon2.org/cbchist/count_table.html.

Species

1974

-75

1975

-76

1976

-77

1977

-78

1978

-79

1979

-80

1980

-81

1981

-82

1982

-83

1983

-84

1984

-85

1985

-86

1986

-87

1987

-88

1988

-89

1989

-90

black vulture 14 2 4 12 2 2 5 4 2

turkey vulture 149 40 50 107 5 242 261 14 29 14 29 73 42

Harris's hawk 4 11 3 5

white-tailed hawk 11 6 10 13 4 14 20 6 2 1 8 5 6 5 7 4

red-tailed hawk 11 5 1 15 1 16 12 2 2 1 5 1 4 1

crested caracara 3 7 4 10 12 20 1 2 2 2

American kestrel 68 31 42 68 11 82 91 30 30 11 6 25 16 6 22

great horned owl 2 1 3 5 2 1 2 1

burrowing owl 2 1 1
117

Chapman (1984)

Only two raptor species (the white-tailed hawk and American kestrel) were observed during the 

Chapman (1984) survey (Table 23). No abundance data were recorded.

Chaney et al. (1993a, b; 1995a, b)

Five resident raptor species were observed during both the Gulf and Laguna coast bird surveys 

(Chaney et al. 1993a, b; 1995a, b; Table 24). Only the turkey vulture and the American kestrel 

were observed during both of the Gulf and Laguna Madre surveys (1993, 1995). The red-tailed 

hawk (Buteo jamaicensis) was only observed on the Gulf coast, while the crested caracara was 

only observed on the Laguna Madre coast (Table 24).

Threats and Stressor Factors

Because resident bird species do not migrate, they are entirely dependent upon the PAIS 

ecosystem for their sustenance. As a result, stressors on the PAIS landscape become stressors to 

the resident bird population; a decline in the resident bird population is likely indicative of a 

much larger issue occurring on the island. Currently, the major threats facing the PAIS resident 

bird population include human disturbance (e.g., oil and gas exploration/development, habitat 

alteration/loss), invasive and non-native species, predation, and drought. 

Human disturbance exists in many forms in PAIS, with boating, fishing, vehicle travel on the 

Gulf beach, and wildlife viewing all being popular visitor activities. However, several sources of 

human disturbance come from sources not tied to visitor activity, such as commercial boat traffic 

on the GIWW and oil exploration and developments in the park. These represent significant 

threats to the resident birds. The GIWW supports some of the heaviest industrial water traffic in 

the world (DOI 1989), and there are currently six active drilling pads within the park and 11 

active wells. Two more pads are scheduled for restoration in the future (pending funding), and 

only one operator is active in the park (Stablein, written communication, 13 May 2013). While 

no new wells are being planned, exploration and development could occur at any time. The 

establishment of additional oil sites, coupled with the increase in vehicle and foot traffic, could 

fragment or eliminate several habitat areas of the park’s resident birds.

Drought represents a major threat to the resident bird population of the park. Periods of drought 

often reduce the water levels in critical bird habitats such as Bird Island Basin and the freshwater 

ponds on the island. Drought may also reduce the availability of prey or forage species (Smith 

1982). 

Data Needs/Gaps

Prior to 2013, there was no annual monitoring program for the land birds (whether migratory, 

resident, or raptor species) in PAIS. However, the GULN is beginning both a wintering and 

breeding landbird survey that will document both migratory and resident bird species. The 

establishment of these annual bird surveys and the resumption of the CBC efforts will likely 

provide the park with survey results that would be descriptive for both the resident and migratory 

species of the park. Point counts will provide more detail regarding the breeding bird population 

of the park. The resident bird population in PAIS serves as an excellent indicator of the entire 

park’s health, and routine winter area search surveys will provide managers with a better 

understanding of the health of both the resident bird population and the many PAIS ecosystems.
118

Overall Condition

Species Abundance and Diversity

The project team defined the Significance Level for this measure as a 3. However, there are not 

enough historic or contemporary abundance data to assign this measure a Condition Level. 

Species Distribution

The project team defined the Significance Level for species distribution as a 3. Very few studies 

have focused on the distribution of the resident bird species in the park. Without this descriptive 

data, an assessment of condition for this measure is not possible; therefore, a Condition Level

was not assigned.

Raptor Abundance

The Significance Level for raptor abundance is a 3. No raptor-specific survey has been conducted 

in the park, and the only records for this group come from sightings reported in various shorebird 

and waterbird surveys. Until baseline abundance data for this measure are obtained, a Condition 

Level cannot be assigned.

Weighted Condition Score

A Weighted Condition Score for PAIS’s resident bird species was not assigned. Few data exist 

regarding the abundance, diversity, or distribution of the park’s resident birds; the establishment 

of annual surveys in the park would allow these measures to be accurately assessed in the future.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
119

Literature Cited

American Bird Conservancy (ABC). 2010. Globally important bird areas of the United 

States. http://www.abcbirds.org/abcprograms/domestic/iba/index.html (accessed 9 February 

2013).

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1993a. Bird use of the Padre Island National 

Seashore Gulf Beach from Sept. 1992 - Aug. 1993. National Park Service unpublished 

report, Contract 1443PX749092188. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1993b. Laguna Madre bird project from 

Yarborough Pass to Mansfield Channel during July 1992 through April 1993. National Park 

Service unpublished report, contract 1443PX7000092582. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1995a. Gulf beach survey: northern boundary 

to Yarborough Pass, Padre Island National Seashore October 1994 – September 1995. 

National Park Service unpublished report. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1995b. Laguna Madre bird survey, 

Yarborough Pass to Northern Boundary, Padre Island National Seashore, August 1994 to 

August 1995. National Park Service unpublished report, Contract 1443PX749094190.

Corpus Christi, Texas.

Chapman, B. R. 1984. Seasonal abundance and habitat-use patterns of coastal bird populations 

on Padre and Mustang Island barrier beaches (following the Ixtoc I oil spill). U.S. Fish and 

Wildlife Service. OBS-83/31, Slidell, Louisiana.

Cornell Lab of Ornithology. 2013. All about birds website. http://www.allaboutbirds.org/

(accessed 9 February 2013).

Koskimies, P. 1989. Birds as a tool in environmental monitoring. Annales Zoologici Fennici 26: 

153-166.

Mettke-Hofmann, C., S. Lorentzen, E. Schlicht, J. Schneider, and F. Werner. 2009. Spatial 

neophilia and spatial neophobia in resident and migratory warblers (Sylvia). Ethology 115: 

482-492.

National Park Service (NPS). 2013. NPSpecies online 

database. https://irma.nps.gov/App/Species/Search (accessed 10 March 2013).

Smith, K. G. 1982. Drought-induced changes in avian community structure along a montane 

sere. Ecology 63(4):952-961.

Texas Colonial Waterbird Society (TCWS). 2011a. Summary of colonial waterbird counts for 

2011 for the Central Texas 

Coast. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/tcws/documents/2011_C

entral_Coast.pdf (accessed 12 February 2013).
120

Texas Colonial Waterbird Society (TCWS). 2011b. Colonial waterbirds on the Lower Texas 

Coast. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/tcws/documents/2011_L

ower_Coast.pdf (accessed 12 February 2013).

Texas Parks & Wildlife Department (TPWD). 2013. Nongame and Rare Species Program: 

federal and state listed bird 

species. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/texas_rare_species/liste

d_species/birds.phtml (accessed 15 February 2013).

U.S. Department of the Interior (DOI). 1989. U.S. Fish and Wildlife Service news release. U.S. 

Department of the Interior, Washington D.C.

Western Hemisphere Shorebird Reserve Network (WHSRN). 2009. WHSRN 

sites. http://www.whsrn.org/whsrn-sites (accessed 4 March 2013).
121

Plate 16. PAIS Christmas Bird Count survey area (1974-1990).
122

4.7 Colonial Waterbirds

Description

Colonial waterbirds are a unique group of bird species that gather in large numbers to nest at a 

specific location during the breeding season (Buckley and Buckley 1979, Kushlan 1986). These 

colony sites may consist of several unique species; waterbirds typically observed at these 

colonies include herons, gulls, terns, pelicans, egrets, and many others. Table 27 lists those 

families classified as waterbirds according to the Patuxent Wildlife Research Center’s Waterbird 

Monitoring Partnership (USGS 2001). For the purpose of this assessment, only species that are 

listed in Table 27 are discussed in this component. 

Table 27. Families of waterbirds as defined by the USGS Patuxent Wildlife Research Center's Waterbird 

Monitoring Partnership (USGS 2001).

Family Species

Pelecanidae pelicans

Phalacrocoracidae cormorants

Ahhingidae darters

Fregatidae frigatebirds

Ardeidae herons, egrets, bitterns

Threskiornithidae ibises, spoonbills

Laridae gulls, terns, skimmers

PAIS is home to two very unique coastal ecosystems, both of which are used by the park’s 

waterbirds. On the east side of Padre Island is the Gulf of Mexico (Gulf) shoreline, while the 

west side of the island is bordered by the Laguna Madre. The Gulf shoreline is comprised of 

firmly packed white sand and is bordered by a narrow dune ridge. The Gulf coast provides 

important foraging and staging grounds for birds, especially during the migratory period. 

However, few, if any, waterbird colonies exist on the Gulf coast beach (TCWS 2011a, 2011b).

Photo 12. Brown pelicans (Pelecanus occidentalis) flying along the Gulf coast of PAIS (Photo by 

Shannon Amberg, SMUMN GSS).
123

The Laguna Madre is a hypersaline lagoon that can be 1.5 to 3 times as salty as the ocean, and is 

a very sensitive ecosystem (NPS 2012). Unlike the turbulent nature of the Gulf coast, the Laguna 

Madre has hardly any flow of sediments. Beginning in 1942, the U.S. Army Corps of Engineers 

(USACE) began extending the GIWW within the Laguna Madre from Corpus Christi, Texas to 

the Mexican border (Chaney et al. 1978). To allow for navigation throughout the GIWW, the 

channel needed to be dredged to a depth of 3.7 m (12 ft) and a width of 38.1 m (125 ft) (Chaney 

et al. 1978). Dredge material was frequently disposed of along land banks; however, there were 

several instances in the Laguna Madre where this disposal method was not possible. According 

to Chaney et al. (1978, p. 10), “For those channels that pass through deep water areas, dredged 

material is deposited in sub-aqueous banks; but in shallow bays or lagoons, the deposits 

commonly form emergent domes, or islands.”

Within PAIS boundaries, there are 11 man-made dredge spoil islands in the Laguna Madre that 

have acted as nesting islands for colonial waterbird species (Island 614-344 is discussed in this 

document, but is located just outside of the park boundary) (Plate 17). Several of these islands 

support over 1,000 birds of many different species. For many species (e.g., American white 

pelican and black skimmer), the colony sites in PAIS represent >25% of their coast-wide 

populations (TCWS 2011a). Annual monitoring of these colonies provides PAIS managers with 

valuable information regarding the health of these unique bird populations.

Measures

• Species abundance and diversity

• Species distribution

• Nesting success

Reference Conditions/Values

A reference condition for colonial waterbirds in PAIS has not been defined. Although there are 

no NPS-initiated waterbird surveys, the TCWS has monitored the rookery islands of the park for 

the past 36 years. While there is a high degree of variability in annual abundance, yearly 

comparisons to the 36-year average for each colony island may serve as an appropriate reference 

condition for some measures.

Data and Methods

The NPS Certified Bird Species List (NPS 2013a) (Appendix L) for PAIS was used for this 

assessment; this list represents all of the confirmed bird species present in the park. For this 

component, only bird species considered colonial waterbirds (as defined in Table 27 and 

Appendix L) were included. SMUMN GSS removed migratory and resident species from this 

list, as these species are discussed separately in Chapters 4.5 and 4.6 of this document. 

Chapman (1984) investigated the effects of the 1979 IXTOC I oil spill on the coastal bird species 

of Mustang and Padre Islands. One of the survey areas (area B) was located within PAIS, and 

extended from Malaquite Beach in the north, to Mansfield Channel in the south. From October 

1980 to June 1981, bird censuses were conducted from a four-wheel drive vehicle that was 

driven along the beaches, and only birds on the beaches were counted (i.e., no flying or 
124

swimming birds were counted). Stops were made every 16 km (10 mi), and researchers were 

allowed 15 minutes to scan the area for birds that may have been missed from the vehicle.

EcoServices Annual Bird Surveys (1992-95)

EcoServices conducted annual bird surveys along both the Gulf and Laguna Madre beaches from 

1992-1995 (Chaney et al. 1993a, b, 1995a, b). These surveys were summarized annually and 

released as internal documents to the NPS. Although the surveys on the Gulf likely only 

identified non-breeding colonial waterbirds, their observations are reported in this document. 

Chaney et al. (1993a) was the first of these annual surveys, and monthly bird surveys were 

completed along the Gulf beach of PAIS from September 1992 to August 1993. The survey area 

included the beach areas of the park from Mansfield Channel north to the upper end of the closed 

beach area, for a total length of 103 km (64 mi). A total of 51,205 birds of 56 species were 

identified during the study; of those species identified, 22 were colonial waterbirds. Only the 

results that applied to the waterbirds of PAIS are reported in this component.

Chaney et al. (1993b, p. 1) continued the surveys, and focused the study to “determine the 

abundance and distribution of protected bird species within the mudflats bordering the Laguna 

Madre shoreline from Yarborough Pass to Mansfield Channel.” This section of the park does not 

contain any rookeries, and it is likely that only foraging or vagrant colonial waterbirds were 

observed in these areas (Stablein, written communication, 11 April 2013). At the midpoint of the 

project, more funds became available and the surveys were expanded to include all bird species 

(not just protected species). Data regarding protected species were collected in July 1992, and 

from October to April of 1992-93; data for all species were collected in July 1992, and from 

January to April of 1993. Unfortunately, much of the data collected in this report has been lost, 

as the only known physical copy has been lost.

Chaney et al. (1995a) replicated the bird surveys of Chaney et al. (1993a), surveying the Gulf 

beach from October 1994 through September 1995. This study utilized a similar methodology as 

its predecessor, and recorded 36,697 birds of 52 species. Similarly, Chaney et al. (1995b) 

replicated the bird surveys conducted on the Laguna Madre coast that were completed by Chaney 

et al. (1993b). These surveys were completed monthly from August 1994 to August 1995, and 

recorded 66,895 birds of 93 species; 25 species were identified as colonial waterbirds. 

Unfortunately, much of the data collected in these reports has been lost, as the only known 

physical copies have been damaged/lost.

The TCWS is made up of Audubon Texas, CBBEP, Texas A&M University – Kingsville, Texas 

General Land Office, Texas Parks & Wildlife Department (TPWD), The Nature Conservancy, 

USFWS, the NPS, and various volunteers. TPWD has taken on the responsibility of being the 

point of contact for managing and accessing data, until a better way of managing and serving the 

data is identified and managed. According to TPWD (2013), the mission of the TCWS is to 

“monitor, promote research, and inform management of colonial waterbird populations in 

Texas.” Annual surveys are conducted at each of the 12 dredge spoil islands that have 

historically supported breeding colonial waterbirds. Surveys for some islands date back to 1973, 

while other islands had initial surveys conducted as recently as 2003. TCWS provides annual 

regional reports for the upper, central, and lower coasts of Texas (TCWS 2011a, b); PAIS is 

located on both the central and lower coasts, and the data for the park are split into two separate 

summary reports.
125

For data regarding the extent and cover of the spoil islands, Laine and Ramsey (1998) completed 

a landcover classification for the park which provides information on the extent of various 

vegetation communities and non-vegetated areas on PAIS spoil islands based on mid-1990s 

aerial photography. Laine and Ramsey (1998) followed classification and interpretation 

protocols developed by the NOAA C-CAP program and used map descriptions that fit NVCS 

guidelines. In 2007, the NPS created a surficial geology GIS map for PAIS, based primarily on 

2003 aerial photography (NPS 2012b). Map classifications are loosely based on land cover and 

divide spoil island communities into two classes: vegetated spoil mound and barren spoil mound 

(NPS 2007).

Current Condition and Trend

The colonial waterbirds that nest in the park are heavily dependent upon the unique dredge spoil 

islands and do not nest on the mainland of PAIS. According to surficial geology GIS data (NPS 

2007), the total area of the spoil islands (vegetated and barren spoil mounds) within the PAIS 

boundary (based on 2003 aerial imagery) is just under 490 ha (1,208 ac). However, this includes 

some spoil deposits on Padre Island itself. Using 2009 aerial imagery and the ArcGIS measure 

tool, SMUMN analysts estimated the area of the 18 islands studied by Chaney et al. (1978). The 

total area of all islands was approximately 80 ha (197.7 ac), a 6 ha (14.9 ac) reduction from the 

1970s total (Chaney et al. 1978). 

The availability of mineral earth or bare ground is important for many species of ground-nesting 

sea and shorebirds. No data have been published regarding the overall percentages of vegetative 

cover and bare ground on the park’s spoil islands. According to the park’s surficial geology GIS 

data, vegetated spoil mound covers approximately 440 ha (1087 ac) or nearly 90% of the park’s 

spoil islands (total area = 490 ha [1,211 ac]) (NPS 2007). The barren spoil mound class covers 

approximately 49 ha (121 ac) or around 10% of the total spoil island area (NPS 2007). However, 

bare ground patches do occur within the vegetated spoil mound class; the size of these areas and, 

therefore, overall percentages of vegetative cover and bare ground, have not been measured.

The 1998 landcover classification (Laine and Ramsey 1998) identified three vegetated cover 

classes on spoil mounds: emergent wetland, grassland, and sparse vegetation. The emergent 

wetland and grassland classes make up 33 ha (81.5 ac) and 186 ha (460 ac) of the spoil mounds, 

respectively (Laine and Ramsey 1998; Plate 18). These numbers include some spoil mounds on 

Padre Island, not just the separate spoil islands in the Laguna Madre. The sparse vegetation class 

covers 155 ha of the spoil mounds, but it is unknown what percent of this landcover class is 

actually vegetated.

A fourth class identified on the 1998 landcover classification (Laine and Ramsey 1998) was 

unconsolidated shore, described as “consisting of fine sands with little to no vegetation” (Laine 

and Ramsey 1998, p. 6). According to landcover GIS data, unconsolidated shores make up 

approximately 162 ha of the park’s spoil mounds. However, this includes some spoil mound on 

Padre Island as well as the separate spoil islands. The 155 ha (383 ac) of spoil mounds covered 

with sparse vegetation, as discussed above, likely includes substantial patches of bare ground 

(Laine and Ramsey 1998; Plate 18).
126

Species Abundance and Diversity

NPS Certified Bird Species List

The NPS Certified Bird Species List (NPS 2013a) contains 43 colonial waterbird species 

belonging to seven different families (Appendix L). This list, however, does not allow for an 

analysis of annual abundance or diversity, as no data are collected. This list simply represents a 

synthesis of confirmed reports of bird species occurring within PAIS boundaries.

Chapman (1984)

Chapman (1984) documented the seasonal abundance of coastal bird populations along the Texas 

Gulf coast. The maximum abundance values for all coastal bird species occurred during the 

spring and fall migration periods, with peak abundance occurring in the fall of 1980 (186 

birds/km) (Chapman 1984). Chapman (1984) did not focus specifically on colonial waterbirds, 

although 10 colonial waterbird species were identified during the surveys. These species, along 

with their estimated abundance values, are displayed in Table 28.
127

Table 28. Average monthly abundance and diversity of colonial waterbirds in PAIS from October 1980 to June 1981. Data represents mean birds 

per km (+ standard deviation). Data modified from Chapman et al. (1984).

Month

Species Oct Nov Dec Jan Feb Mar Apr May June

Monthly

Average

royal tern 1.4 (1.1) 2.4 (1.6) 1.5 (0.9) 1.1 (0.1) 3.6 (0.9) 1.6 (0.2) 7.8 (1.7) 4.5 (2.3) 3.4 (1.5) 3.03

Caspian tern 1 (0.6) 1.8 (1.3) 0.2 (0.2) --- --- 0.2 (0.1) 0.1 (0.0) --- 0.1 (0.0) 0.57

Forster's tern 0.8 (0.8) 1.8 (1.0) 1 (0.7) 1.7 (0.3) 4.4 (1.5) 0.3 (0.3) 0.1 (0.0) --- --- 1.44

sandwich tern 0.3 (0.1) 0.5 (0.5) 0.5 (0.5) 0.1 (0.1) 0.1 (0.1) 0.4 (0.3) 3.8 (1.4) 0.8 (0.5) 1.3 (1.2) 0.87

least tern --- --- --- --- --- --- 1.7 (0.5) 2.2 (0.6) 1.3 (0.2) 1.73

black tern --- --- --- --- --- --- --- 0.9 (0.9) 2.2 (2.2) 1.55

laughing gull 7.1 (3.3) 4.8 (1.9) 3.7 (1.8) 2.3 (0.8) 2.6 (0.8) 11.8 (5.8) 12 (0.4) 9.3 (0.5) 9.1 (2.8) 6.97

herring gull 0.2 (0.1) 0.9 (0.2) 1.1 (0.3) 1.1 (0.1) 2 (0.7) 1.7 (0.3) 1.5 (0.8) 0.1 (0.1) --- 1.08

ring-billed gull 2.4 (0.5) 5 (2.3) 4.5 (2.7) 4.3 (0.6) 6.2 (2.2) 5 (3.4) 1 (0.1) 0.1 (0.1) --- 3.56

great blue 

heron 0.5 (0.3) 0.3 (0.0) 0.3 (0.1) 0.1 (0.1) 0.2 (0.1) --- 0.1 (0.0) 0.1 (0.1) 0.1 (0.1) 0.21

# of species 8 8 8 7 7 7 9 8 7 7.67
128

Chapman (1984) found the abundance of colonial waterbirds to be higher during the fall and 

spring months, with most species experiencing a notable drop in abundance during the winter 

season (Table 28). A few notable examples of this trend are the royal tern (increased from 1.6 

birds/km in March, to 7.8 birds/km in April), and the laughing gull (increased from 2.6 birds/km 

in February, to 11.8 birds/km in March) (Table 28). With only one calendar year surveyed by 

Chapman (1984), it is impossible to detect any trends in abundance. Density values for all 

coastal birds were reported in Chapman et al. (1984), but as this component of the NRCA 

focuses only on colonial waterbird species, the reported density values in Chapman et al. (1984) 

would be inaccurate.

Chaney et al. (1993a, 1995a) (Gulf Coast Surveys)

Chaney et al. (1993a, 1995a) investigated avian usage of PAIS’s Gulf beach; Chaney et al. 

(1993a) surveyed the beach from 1992-93, while Chaney et al. (1995a) surveyed the beach from 

1994-95. Species observed on the Gulf coast are assumed to be using the coast for foraging or 

migration, as no nesting colonies are present on the Gulf side of PAIS (Stablein, written 

communication, 11 April 2013).

During the 1992-93 survey of the Gulf beach, Chaney et al. (1993a) observed 22 colonial 

waterbird species (Table 29). A total of 14,210 colonial waterbird individuals were observed 

during the survey (Table 30). However, prior to February 1993 only “protected” bird species 

were counted; resident gull and tern species were not counted for the duration of the survey (an 

estimated 11,200 black terns were observed, but not reported, in September 1992). Because of 

these discrepancies, the total number of birds reported in Table 29 and Table 30 are not true 

reflections of how many birds may have used the Gulf beach for the 1992-93 season. No 

diversity estimates were calculated by Chaney et al. (1993a). 
129

Table 29. Colonial waterbird species observed on the Gulf coast and the Laguna Madre during the 1993-

94 and 1994-95 surveys.

Gulf Laguna Madre

Common Name

Chaney

et al.

(1993a)

Chaney

et al.

(1995a)

Chaney

et al.

(1993b)

Chaney

et al.

(1995b)

brown pelican X X X X

double-crested cormorant X X X X

great blue heron X X X X

snowy egret X X X X

little blue heron X X X X

tricolored heron X X X X

reddish egret X X X X

cattle egret X X X X

yellow-crowned night heron X X X X

laughing gull X X

ring-billed gull X X X X

herring gull X X X X

lesser black-backed gull X

gull-billed tern X X X X

Caspian tern X X X X

royal tern X X X X

sandwich tern X X X X

common tern X X X X

Forster's tern X X X X

least tern X X X X

black tern X X X X

black skimmer X X X X

American white pelican X X

neotropic cormorant X X X

great egret X X

green-back heron (green heron) X

black-crowned night-heron X

white ibis X X X

white-faced ibis X

roseate spoonbill X X

Franklin's gull X

magnificant frigatebird X

Totals 22 23 30 25
130

Table 30. Abundance of colonial waterbirds during the 1993-94 survey on the Gulf coast. Dates on the left side of the dotted line (Sept 1993 – Jan 

1994) represent the period that documented only priority bird species in the park. Dates after the dotted line represent the survey period when all 

species were recorded, not just priority species (Chaney et al. 1993a).

Month

Species Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug Total

brown pelican 24 69 233 26 12 304 91 19 70 15 32 84 978

magnificent frigatebird 3 3

double-crested cormorant 62 181 830 222 109 114 1 1,519

great blue heron 2 1 1 6 4 1 16 31

little blue heron 6 8 2 1 17

tricolored heron 2 1 9 12

reddish egret 1 24 10 3 2 40

reddish egret white 1 1 1 3

cattle egret 32 114 284 11 441

yellow-crowned night-heron 2 1 3

royal tern 1 1

common tern 3 3

Forster's tern 12

least tern 1,405 1 21 1,348 840 646 957 988 6,206

black tern 1 1 223 823 1,477 2,428 4,953

Total 1,432 159 425 860 234 413 263 1,494 1,452 1,490 2,469 3,532 14,210
131

Chaney et al. (1995a) surveyed the Gulf beach in PAIS monthly from October 1994 to 

September 1995. Twenty-three colonial waterbird species were observed during the monthly 

surveys (Table 29); however, the physical report has been lost in the years following its 

completion, and alternate copies of the report have not been located. Because of this, an accurate 

summary and analysis of the data from this research is not possible. 

Chaney et al. (1993b, 1995b) Laguna Madre Surveys

Chaney et al. (1993b) surveyed the Laguna Madre side of Padre Island from July 1992 to April 

1993. The survey methods largely replicated the survey efforts of Chaney et al. (1993a) on the 

Gulf side of the island. However, much of the report has been damaged or lost in the years since 

its completion, and a full analysis of the results is not possible at this time. All that can 

accurately be reported from the incomplete document are the 30 species of colonial waterbirds 

observed during the monthly surveys (Table 29).

Chaney et al. (1995b) surveyed the Laguna Madre side of Padre Island from August 1994 to 

August 1995. Similar to Chaney et al. (1993b, 1995a), the final report is damaged and does not 

have complete data to report on. All that can accurately be reported is that 25 colonial waterbird 

species were observed on the Laguna side of the island during the survey period (Table 29). 

Texas Colonial Waterbird Society Annual Monitoring

The TCWS annually surveys the islands of PAIS and records abundance based on the estimated 

number of breeding pairs for each species observed on the islands. In this section, islands 614-

341a, b, c, and d are reported on as though they constitute a single island (as was done in NPS 

2013a); the same procedure was used for islands 614-342a, and b. Plate 19-Plate 26 present the 

2011 abundance data in relation to the 36-year average for each of the eight islands surveyed.

Of the nine islands surveyed in 2011, four had zero nesting colonial waterbirds (614-341, Plate 

19; 614-344, Plate 22: 614-346, Plate 24; 614-347, Plate 25). Island 614-345 had the highest 

number of breeding pairs (780). However, 740 of those pairs were American white pelicans; the 

other species that were observed on the island were observed in lower numbers than their 

historical average (Plate 23). 

When the TCWS survey results were summarized for the entire park, seven of the 22 identified 

colonial waterbird species exhibited a positive percent change in breeding pair abundance when 

comparing 2011 park-wide survey results to the previous 36-year average (Table 31). Twelve 

colonial waterbirds exhibited a negative percent change in breeding pair abundance, and three 

species (brown pelican, Forster’s tern [Sterna forsteri], and sooty tern [Sterna fuscata]) were not 

observed frequently enough to determine a trend (trends were only determined for species that 

averaged > 1 breeding pair/year) (Table 31). 
132

Table 31. Average annual abundance of breeding birds in PAIS, 2011 abundance values, and the overall 

percent change observed in 2011 compared to the average abundance of the species. Italicized entries 

were not observed frequently enough to determine trend.

Species

Park-wide avg # of 

breeding pairs

2011 # of 

breeding pairs

% 

Change 

white pelican 130.86 740 465

brown pelican 0.77 20 n/a

great blue heron 5.39 19 252

great egret 5.89 0 -100

snowy egret 13.02 3 -77

little blue heron 3.12 1 -68

tricolored heron 17.49 3 -83

reddish egret 9.04 10 11

cattle egret 111.92 0 -100

black crowned night heron 2.38 0 -100

white ibis 1.32 0 -100

white-faced ibis 13.59 0 -100

roseate spoonbill 15 0 -100

laughing gull 389.02 252 -35

gull-billed tern 14.01 60 328

Caspian tern 8.3 81 876

royal tern 46.9 14 -70

sandwich tern 49.01 16 -67

Forster's tern 4.24 67 1,481

least tern 0.96 0 n/a

sooty tern 0.18 0 n/a

black skimmer 16.29 135 729

The Caspian tern (Sterna caspia) and the black skimmer exhibited the highest percentage 

increase in abundance in 2011, with the Caspian tern increasing 876% and the black skimmer 

increasing by 729% (Table 31). The American white pelican had the highest increase in terms of 

number of estimated breeding pairs observed in 2011 (740 breeding pairs observed), up 609 pairs 

(465%) from the 36-year average (Table 31). The 740 breeding pairs of American white pelicans 

were observed on island 614-345 (commonly referred to as “Pelican Island”, Photo 13) and were 

the only colony of breeding pelicans observed during the TCWS (2011a) survey of the central 

coast of Texas.
133

Photo 13. American white pelican nesting colony on Pelican Island (614-345) in the Laguna Madre 

(image from TCWS 2011a).

Several species typically observed in PAIS (e.g., cattle egret [Bubulcus ibis], roseate spoonbill 

[Ajaia ajaja], and white-faced ibis [Plegadis chihi]) were not recorded during the 2011 survey. 

Aside from these species, the tricolored heron [Egretta tricolor] (-83%) and the snowy egret 

[Egretta thula] (-77%) exhibited the largest percent decrease in breeding pair abundance.

The laughing gull was observed more than any other colonial waterbird during the TCWS 

surveys (Table 31, Plate 19-Plate 26). However, breeding pair abundance values for the species 

in PAIS were lower in 2011 compared to the 36-year historical average, down 137 pairs (-35%; 

Table 31). TCWS (2011a) reported a similar trend during 2011 surveys of the central coast of 

Texas. Laughing gull abundance values were lower along the central coast in 2011, down 7,649 

pairs (-22%) compared to the 8-year historic average, and down 8,099 pairs from 2010 (-22%; 

Figure 13).
134

Figure 13. Laughing gull abundance on the central coast of Texas from 2003-2011 (TCWS 2011a). 

Species Distribution

Beaches along islands have several distinct zones, each characterized by unique features. Bird 

species will utilize these zones for feeding, staging, and breeding, with some zones being favored 

more than others. Chaney et al. (1993a, p. 4) defined five habitat zones when recording their 

species-specific observations: 

1. Nearshore – feeding or resting in or over the water close to shore;

2. Foreshore – feeding or loafing in the bare, wet area of the beach from the water 

line (swash zone) to the berm;

3. Berm – feeding or loafing on the first ridge of sand and debris caused by the high 

tide, usually dry above and wet underneath;

4. Backshore – a usual loafing area of dry sand and debris between the berm and the 

vegetated foredune ridge;

5. Aerial – groups of birds flying north or south that were obviously migrating, 

usually offshore.

A visual representation of these habitat types was presented previously in Figure 12. Chapman et 

al. (1984) used similar terminology in their assessment of bird distribution, simplifying the 

number of habitat types to only foreshore, backshore, and berm.

Chapman et al. (1984)

Chapman et al. (1984) found that most of the birds in the study area were concentrated along the 

foreshore region (Zone 2 on Figure 12) (Figure 14). This area was used primarily as a feeding 

and loafing area for several species, with the majority of species congregating near the high-tide 

swash line (Chapman et al. 1984). The dry backshore (Zone 4 on Figure 12) was the least used 

habitat type (Figure 14). This zone was frequented most by shorebirds foraging on garbage and 

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

2003 2004 2005 2006 2007 2008 2009 2010 2011

# of Nesting Pairs

Year

Laughing Gull

Previous 8 year avg.
135

food left by human campers. Note that the data in Figure 14 includes all bird species observed 

during Chapman et al. (1984)’s bird surveys; colonial waterbird distribution was not isolated by 

the authors.

Figure 14. Distribution of bird species along the Gulf beach as reported by Chapman et al. (1984).

Along the length of the barrier islands surveyed in Chapman et al. (1984), birds were not evenly 

distributed, as several beaches exhibited low bird abundance throughout the study. Two of these 

areas, Big Shell Beach and Little Shell Beach, were located in PAIS; the shell substrate of these 

beaches had low-density prey species populations that could not support foraging colonial 

waterbirds. No birds were observed breeding on the beaches monitored during the study.

Chaney et al. (1993a) (Gulf Coast Surveys) 

Although Chaney et al. (1993a) did not focus on colonial waterbirds, the report did find some 

overall trends in distribution along the island. Along the entire survey of the open beach area (the 

portion of PAIS that begins in the north at North Beach and ends at Mansfield Channel in the 

south), Chaney et al. (1993a) found that the distribution of birds was uneven. The areas of the 

Gulf beach with large shell substrate (e.g., Big Shell Beach – Mile 17.1-28.8) had fewer birds 

present than areas of the beach with washover channels. Washover channels are not permanent, 

but appeared to be selected by shorebirds for feeding, nesting, and roosting. 

Chaney et al. (1993a) only reported the species distribution for protected species, and a complete 

analysis of colonial waterbird distribution is not possible. However, five colonial waterbird 

species were identified as protected and their distribution data is presented in Table 32.

0

10

20

30

40

50

60

70

80

90

100

Oct Nov Dec Jan Feb Mar Apr May Jun

Percentage

Month

Foreshore

Berm

Backshore
136

Table 32. Distribution of protected colonial waterbird species during the 1993-94 bird surveys of the PAIS 

Gulf beach; habitat types are defined in text, and in Figure 14. (Chaney et al. 1993a).

Habitat Type

Species 1 2 3 4 5 Total

brown pelican 410 462 3 103 978

reddish egret 28 14 1 43

least tern 169 4,207 1,576 254 6,206

double-crested cormorant 675 844 1,519

black tern 399 4,106 388 41 19 4,953

Total 1,681 9,633 1,968 295 122 13,699

Brown pelicans appeared to favor the nearshore and foreshore habitats, and Chaney et al. (1993a) 

notes that they were among the easiest species to identify in flight, perhaps indicating why they 

have the highest abundance in the aerial habitat type (Table 32). Similarly, double-crested 

cormorants (Phalacrocorax auritus) were only found in the nearshore and foreshore habitat types 

(Table 32). Overall, the most individuals were identified in the foreshore habitat (9,633 

individuals); the least tern and the black tern accounted for 86% of the observations in this 

habitat type (Table 32). No birds were observed breeding on the coasts surveyed by Chaney et al. 

(1993a)

Texas Colonial Waterbird Society Annual Monitoring

The TCWS survey data allow for the distribution measure to be analyzed in a variety of ways 

(e.g., island species richness, island abundance). When looking at the species richness averages 

of each island (i.e., on average, how many species of colonial waterbirds are found at an island in 

a given year), the 614-341 island group had the highest value (12 species/year). Island 614-346 

exhibited the lowest species richness value over the 36-year period (2.21 species/year). The 

majority of the TCWS surveyed islands (five out of eight islands [62.5%]) exhibited a decrease 

in species richness in 2011; including each individual sub island from the 614-341 and 614-342 

sub groups, eight of the 12 islands (66.7%) exhibited a decline in species richness.

The 614-341 island group had the highest average number of breeding bird pairs (4,573; Figure 

15), and the highest recorded number of breeding pairs in one breeding season (15,341 in 1978; 

Figure 15). However, since 2008, only 13 breeding pairs have been observed on the island group 

(NPS 2013a). Island 614-346 had the lowest average number of breeding bird pairs (17.4), and 

has not had any nesting birds observed on the island since 2006 (NPS 2013a); Plate 24 reveals 

that much of the island is now submerged or has eroded away.
137

Figure 15. Est. number of breeding pairs (all species) observed at the island group 614-341 from 1973-

2011 (NPS 2013a).

American white pelicans have only been observed breeding on two islands in the park (614-341 

and 614-345). Island 614-345 is commonly referred to as Pelican Island, due to the large 

numbers of pelicans that have bred on the island in the past. 2011 marked the largest number of 

breeding pelicans for this island (740 pairs). Similarly, the brown pelican has only been observed 

breeding on the 614-342 island group; the breeding abundance of brown pelicans in PAIS is 

substantially lower when compared to the American white pelican. Cattle egrets have been 

observed breeding on islands 614-360, 614-345, and 614-341, with island 614-341 supporting

the largest group of breeding cattle egrets in the park, averaging almost 379 pairs a year (Plate 

19).

Laughing gulls are the most abundant colonial waterbird in the park, and have been observed 

breeding on all of the surveyed islands in the park (Plate 19-Plate 26). While they are distributed 

across the spoil islands in the park, laughing gulls have occurred in the highest numbers at the 

island 614-341 cluster (36-year average = 3,163; Plate 19). Other species that have been 

observed breeding on all of the surveyed islands include the tricolored heron, gull-billed tern

(Sterna nilotica), Caspian tern, royal tern, Forster’s tern, and the black skimmer.

Nesting Success

Nesting success is typically defined as the mean number of young fledged for each nesting 

attempt, although Erwin and Custer (1982) suggest that it may be more desirable to estimate the 

number of young fledged to each nesting female. In PAIS, there have been no estimates of 

colonial waterbird nesting success; without these data, an assessment of current condition is not 

possible for this measure.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

Est. # of Breeding Pairs

Year

Average = 4572.68
138

Threats and Stressor Factors

Several threats exist for the colonial waterbird population in PAIS. These threats range from 

global threats like climate change, sea level rise, and subsidence, to more local threats such as 

erosion and wind farm developments. 

One threat to the colonial waterbird populations of PAIS are wind farms and future wind farm 

developments in south Texas. Wind turbines are suspected to be a direct cause of mortality in 

bird species, as the rotating blades on a wind turbine can strike flying birds. The extent to which

mortalities occur in bird species is likely dependent upon several factors, namely the species of 

birds in the area, the height of the turbine (i.e., higher turbines lead to more mortalities), and the 

elevation of the wind farm above sea level (de Lucas et al. 2008). Elevation equates to larger 

diameter for the turbine’s rotor; the larger the rotor, the faster the tip speed, even at slow 

rotations per minute. 

Recent efforts to develop alternative energy sources have resulted in more wind farm 

development across the planet (de Lucas et al. 2008). However, the exact effect that these wind 

farms have on birds is still poorly understood. Some studies have found that wind farms are 

responsible for no more mortalities than other human-made structures (e.g., buildings, 

communication towers) (Osborn et al. 2000), while other studies have found that turbines are 

responsible for unusually high numbers of bird mortalities (Smallwood and Thelander 2007). 

Two onshore wind energy developments have occurred in Kenedy County, Texas, on the 93,078 

ha (230,000 ac) Kenedy Ranch. This ranch lies across the Laguna Madre from the park, just a 

few kilometers west of the park boundary. One of these developments, the 283 megawatt (MW)

Gulf Wind Farm, developed by Babcock and Brown but sold to Pattern Energy Group in 2010, is 

located across approximately 2,954 ha (7,300 ac) of the John G. and Marie Stella Kenedy 

Memorial Foundation portion of the Kenedy Ranch. The Gulf Wind Farm project consists of 118 

Mitsubishi turbines. The other wind energy development on the Kenedy Ranch is the 404 MW 

Peñascal (I and II) Wind Project, which operates on the John G. Kenedy Jr. Charitable Trust 

property. The Peñascal Wind Project consists of 168 Mitsubishi MHI 92 wind turbines, each 

with a capacity of 2.4 MW. This project was completed in April 2010 by Iberdrola, and 

continues to be owned and operated by Iberdrola (Stablein, written communication, 10 May 

2013).

Another onshore wind energy project occurring near PAIS is Los Vientos (I and II) Wind Energy 

Project, located approximately 24 km (15 mi) southwest of the park in Willacy County, Texas. 

This 400 MW development is owned by Duke Energy Renewables, and consists of 171 turbines

made up of 84 Mitsubishi 2.4 MW turbines and 87 Siemens 2.7 MW turbines. This large project 

was scheduled for completion in December 2012, but continues to be developed and was nearing 

completion in March 2013. One other notable wind project occurring within the park’s general 

vicinity is the 200 MW Magic Valley Wind Farm, constructed by Blattner Energy, and operated 

by E.ON. The Magic Valley Wind Farm is located 8 km (5 mi) east of Raymondville, TX, or 40 

km (25 mi) west of PAIS, and is made up of 112 Vesta 1.8 MW turbines, whose output is 

contracted to local energy producer, AEP (Stablein, written communication, 10 May 2013).

To the north of PAIS is the Papalote Creek Wind Farm. Completed in December 2010, this 380 

MW onshore project is made up of 196 turbines. The first phase consisted of 109 Vestas 1.65 
139

MW turbines, online in the fall of 2009, followed up by 87 Siemens 2.3 MW turbines a little 

over one year later (Stablein, written communication, 10 May 2013).

Two proposals for large offshore (from the park) wind farms have been submitted over the past 6

years; however, one of these proposed developments was abandoned (Stablein, written 

communication, 10 May 2013). Superior Renewable Energy began plans for one of these 

offshore developments in 2003, but after Babcock and Brown acquired Superior in 2006, and 

some preliminary planning and public scoping in 2006, the project was soon dropped. In 2011, 

Baryonyx Corporation proposed a development of offshore turbines, which included large 

turbines directly east of the park’s visitor center; however, these plans have been changed to 

relocate the proposed site further south (Stablein, written communication, 10 May 2013). The 

park has agreed to join the USACE in co-authoring the environmental impact statement (EIS) for 

this development. It is believed that the original proposed locations were not carried forward due 

to conflict with air space designated to the U.S. Navy. Moving turbines further south off South 

Padre Island includes potential for shared air space with the approved development of SpaceX 

launch zone, Boca Chica Beach in Cameron County, Texas.

While not currently being constructed in the U.S., the latest turbines are now larger than 7.0 

MW; the diameter of the turbine’s rotor can be more than 165 m (541 ft), with hub heights as tall 

as 110 to 120 m (361-394 ft). Offshore turbines are quickly approaching more than 200 m (656 

ft) in total height. It is expected within a few years that offshore turbines will be generating at 

least 10.0 MW, have rotors up to 180 m (590 ft) in diameter, and hub heights as tall as 150 m 

(492 ft). Installation of hundreds of these offshore turbines is possible within 8 km (5 mi) of the 

park in the Gulf of Mexico waters (Stablein, written communication, 10 May 2013).

Another threat to the park’s colonial waterbirds are fire ants (Solenopsis geminata, S. invicta) 

that were accidentally brought to the U.S. on a shipping vessel from South America, and have 

since spread rapidly across the southeastern U.S. (Wetterer and Moore 2005). These ant species 

are well-known predators of bird hatchlings, and ground nesting bird species are particularly 

vulnerable. Many colonial waterbird species are ground nesters, and the presence of fire ants on 

nesting islands represents a significant threat. Fire ants on dredge spoil islands in the Laguna 

Madre have previously been found to have significant impacts on the breeding success of several 

colonial waterbird species, particularly the snowy egret, laughing gull, and great blue heron 

(Ardea herodias) (Mrazek 1974). Monitoring of fire ant abundance on islands that support large 

waterbird colonies may help to explain potential decreases in abundance, changes in distribution, 

or a change in reproductive success.

Human disturbance represents a significant threat to colonial waterbird species, as these species 

are particularly vulnerable to human intrusion (Manuwal 1978). Recreational activities, such as 

boating, fishing, and wildlife viewing have the potential to flush waterbirds from their nests, 

leaving their young/eggs vulnerable. With the GIWW passing near several of the nesting 

colonies, boat traffic represents a significant threat to these islands. Any human interaction at the 

islands during nesting season could result in nest abandonment, and a dramatic reduction in 

nesting success. Furthermore, human disturbance in the form of oil spills and marine debris also 

represent significant threats to the survival and reproduction of colonial waterbirds. Several manmade structures (e.g., pilings and signage) in the Laguna Madre often serve as excellent perch 
140

locations for raptor species, and may result in increased raptor abundance and predation on 

colonial waterbirds.

The primary mammalian predators of nesting colonies are likely coyotes and raccoons (Procyon 

lotor). These predators likely gain access to the nesting islands by swimming or wading across 

from the Texas mainland or from an adjacent barrier island (Coste and Skoruppa 1989). Islands 

that support nesting colonial waterbirds are most at risk to raccoons and coyotes if they are in 

close proximity to large islands or to the mainland of Texas; low water levels also facilitate 

predator’s access to the colonies (Coste and Skoruppa 1989). Coyotes are the largest documented 

predators in PAIS that are able to successfully inhabit the barrier islands in Texas (Snodgrass 

1997), and have been documented hunting or scavenging along the small spoil islands of the 

Laguna Madre (Frey and Jones 2008). Raccoons are opportunistic predators, and frequently 

forage for insects, herptiles, small birds, and nuts/fruits. Raccoons have proven to be major 

predators on Virginia barrier island populations of colonial waterbirds (Erwin et al. 2001), and 

the establishment of large raccoon populations on lands adjacent to colonial waterbird nesting 

colonies could have significant impacts on the nesting success of these species.

Coste and Skoruppa (1989) documented the bird colonies along the Texas coast that were 

experiencing disturbance from mammalian predators. Several of the identified colonies were 

located in or near PAIS, and the primary predators observed were raccoons and coyotes. Table 

33 displays the name and approximate location of the colonies, how accessible they are to 

predators, and how successful potential predator control efforts may be if instituted. These data 

are nearly 25 years old, and care should be taken when interpreting the table, as several factors 

may have changed in the years since the report.
141

Table 33. Predation accessibility and pressure for several historic colonial waterbird nesting colonies in the PAIS region (Coste and Skoruppa 

1989).

Colony Name Class Longitude Latitude

Predator 

Accessibility Predators Present

Potential for 

Predator 

Control?

Mkr 37 Spoil (NM75) W of GIWW 1 27°33' -97°17' Medium Coyotes/Raccoons Fair

North of Bird Island (NM 87-91) 1 27°32' -97°17' Medium Coyotes/Raccoons Fair

North Bird Island 3 27°31' -97°17' High Coyotes/Raccoons Poor

West Side Spoil Islands 3 27°30' -97°17' High Cats/Coyotes Poor

South Bird Island 1 27°30' -97°18' High Coyotes Poor

South of South Bird Is. 1 27°29' -97°19' Low Badger/Coyote Good

Mkr 103-117 spoil (NM 207-221) 1 27°18' -97°24' Medium Raccoons Fair

South Baffin Bay Islands (4 islands) 1 27°15' -97°24'

3 islands are 

high, 1 island 

is low Coyotes

3 islands are 

poor, 1 island is 

good

Mkr 87 (178) Diked Spoil 1 27°27' -97°23' Medium Coyotes/Raccoons Fair

Spoil from cut east of Potrero Grande 3 27°06' -97°26' High Coyotes/Raccoons/Rabbits Poor

Spoil east of Potrero Cortado 3 27°04' -97°26' High Coyotes/Raccoons Poor

South Land Cut - North Section 1 26°47' -97°27' Medium Coyotes Fair

South Land Cut - South Section 1 26°47' -97°27' Medium Coyotes Fair

Mansfield Odd Spoil 1 26°39' -97°23' Medium Coyotes/Raccoons Poor

NE Mansfield Intersection 1 26°34' -97°24' Low Coyotes/Raccoons Good

SW Mansfield Intersection 1 26°33' -97°25' Medium Coyotes/Raccoons Fair

Green Hill Spoil Island 1 26°30' -97°23' Medium Coyotes/Raccoons Fair

Mansfield Channel Spoil West section 2 26°33' -97°23' High Raccoons Poor

Green Island 1 26°24' -97°19' Medium Raccoons Fair

Class 1: Site is essential for continued breeding by one or a few species with narrow habitat requirements, highly restricted distribution, or low abundance in Texas. Alternatively, the 

site is highly desirable for a diverse assemblage of colonial waterbirds, important to a few species with somewhat specialized nesting requirements and restricted range in Texas, or an 

important component of a bay system nesting complex.

Class 2: Site is important to only a few nesting pairs of species with broad distribution.

Class 3: Site has minimal or no known current use for waterbird breeding but is important for consideration in management to stimulate future breeding.
142

Non-native plants were introduced to some spoil island communities by people who built cabins 

on the islands prior to park establishment (NPS 2001), and represent a major threat to nesting 

colonial waterbirds in the park. Examples of non-native plants documented on park spoil islands 

include oleander, Brazilian peppertree, saltcedar, and bermudagrass (Chaney et al. 1978, NPS 

1996). Bufflegrass is also present on several of the spoil islands, and, like many of the other nonnative plant species, often prevents the bare-ground or mineral earth nesting colonial species 

(e.g., black skimmers and tern species) from establishing nests on the spoil islands. 

Erosion in the PAIS area is a significant concern for colonial waterbirds, as some islands in the 

area that used to support rookeries and colonies have completely eroded away (Coste and 

Skoruppa 1989). No data have been gathered on erosion or accretion rates around the park’s 

spoil islands. However, anecdotal information indicates that many of these islands are eroding 

(Smith 2002, Chaney and Blacklock 2005). In a study of Laguna Madre rookery islands, Smith 

(2002) identified two spoil islands within PAIS (identified as LM 63A and Marker 81 [referred 

to in this component as 614-345]) that had lost area to erosion between 1975 and 1995. Marker 

81 (island 614-345) spoil island was approximately 1.7 ha (4.2 ac) in size during the 1970s 

(TCWS 1982), but was measured at just 1.1 ha (2.8 ac) in 1986 (Coste and Skoruppa 1989). The 

landcover of this island is shown in Figure 16. The bare ground that once provided habitat for 

ground-nesting birds has become intertidal flats, no longer suitable for nesting (Smith 2002).
143

Figure 16. Change in vegetative cover on Marker 81 (referred to in this component as island 614-345) 

spoil island in PAIS (Smith 2002). Note that large areas that were vegetated are now intertidal, meaning 

they are at least temporarily under water, due to erosion.

Data Needs/Gaps

Continued monitoring of the colonial waterbird colonies is needed to observe future trends and 

patterns in the breeding species of the park. Although difficult to obtain in colonial nesting 

species, a survey or estimate of nesting success for nesting species would also help managers to 

better understand recruitment into the regional and global populations.

In addition to the annual waterbird colony surveys, monitoring of the erosion rates on the nesting 

islands may be beneficial to managers. Erosion rates in the past few decades have elevated, as is 

represented in Figure 16. Heavy boat traffic in the Laguna, paired with the unstable nature of the 

dredge spoil put these islands at high risk. Although knowing erosion rates may be beneficial to 

managers, whether or not the NPS can take preventative/reactive actions to this threat is a much 

larger issue. The dredge spoil islands represent some of the last remaining breeding habitat for 

colonial waterbird species, as the creation of the GIWW and development along the Laguna has 

displaced many of the birds from their natural habitat (i.e., natural islands). The spoil islands predate the park by almost 15 years, and could be considered a historic component of the park 
144

(James Lindsay, email communication, 25 March 2013). While the islands are not a natural 

feature of the landscape, PAIS managers are essentially tasked to ensure that the colonial 

waterbird species do not pass into regional extirpation. It may be worthwhile for PAIS managers 

to investigate potential avenues to add material to rookery islands within the park; James Lindsay 

indicated (email communication, 25 March, 2013) that the USFWS and the State of Texas are 

pursuing BP restoration funds to add material to rookery islands in the Laguna (including islands 

within PAIS).

Overall Condition

Species Abundance and Diversity

The project team defined the Significance Level for this measure as a 3. The TCWS datasets have 

documented the abundance of colonial waterbirds in the Laguna Madre for over 30 years. This 

data provides useful information to analyze trends in the overall colonial waterbird population 

and in individual species. As displayed in Table 31, over half of the observed species (12 out of 

22) exhibited a decline in breeding pair abundance in 2011. Three species that were observed in 

PAIS frequently during the TCWS surveys (cattle egret, roseate spoonbill, and white-faced ibis) 

were not observed breeding in the park in 2011. The most numerous colonial waterbird during 

the course of the TCWS surveys has been the laughing gull. However, 2011 abundance numbers 

declined 35 percent compared to the 36-year average; this trend was reported throughout the 

central coast of Texas (TCWS 2011a). The 2011 results, combined with the numerous threats 

that these populations face on the dredge spoil islands in the park, warranted a Condition Level

of 2 for the species abundance and diversity measure.

Species Distribution

Species distribution was assigned a Significance Level of 3. The unique landscape of PAIS has 

been largely shaped by natural events and occurrences; however, the natural islands and habitat 

in the Laguna Madre that are typically utilized by colonial waterbirds have been developed by 

humans or degraded by human activity. The only suitable nesting locations for colonial 

waterbirds in the park are found on man-made dredge spoil islands. The TCWS has actively 

monitored the breeding locations of colonial waterbirds for over 30 years, and the park has a 

substantial data set that allows researchers and managers to see where species have typically 

nested (and in what numbers) throughout the years. Because of issues like erosion, predation, 

and sea-level rise, the stability of the colonial waterbird populations on these dredge spoil islands 

is of moderate concern to managers; a Condition Level of 2 was assigned to the measure of 

species distribution.

Nesting Success

The project team defined the Significance Level for nest success as a 3. However, no studies have 

documented the nesting success of colonial waterbirds in PAIS, and a Condition Level was not 

assigned.

Weighted Condition Score

The Weighted Condition Score for PAIS’s colonial waterbirds is 0.667, which is on the 

borderline of moderate and high concern. After analysis of the historic data, and the threats and 

stressors that are likely to affect the dredge spoil islands in the near future, SMUMN GSS 

determined that the moderate concern designation was most appropriate at this time. It is 
145

important to note that because of its borderline Weighted Condition Score, it will be important 

for PAIS managers to closely monitor the condition of these species in the future. Further 

degradation of the breeding islands or declines in the breeding pair abundance values may dictate 

a future designation of high concern. 

Sources of Expertise

James Lindsay, Chief of Science and Resource Management, PAIS

Wade Stablein, Biological Technician, PAIS
146

Literature Cited

Buckley, P. A., and F. G. Buckley. 1979. What constitutes a waterbird colony? Reflections from 

the northeastern United States. Proceedings 1978 Conference. Colonial Waterbird Group 3: 

1-15.

Chaney, A. H., and G. W. Blacklock. 2005. Colonial waterbird and rookery island management 

plan. Coastal Bend Bays and Estuaries Program, Corpus Christi, Texas.

Chaney, A. R., B. R. Chapman. J. P. Kargas, D. A. Nelson, R. R. Schmidt, and L. C. Thebeau. 

1978. The use of dredged material islands by colonial seabirds and wading birds in Texas. 

Technical Report D-78-8. U.S. Army Engineer Waterways Experiment Station, Vicksburg, 

Mississippi. 

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1993a. Bird use of the Padre Island National 

Seashore Gulf Beach from Sept. 1992 - Aug. 1993. National Park Service unpublished 

report, Contract 1443PX749092188. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1993b. Laguna Madre bird project from 

Yarborough Pass to Mansfield Channel during July 1992 through April 1993. National Park 

Service unpublished report, contract 1443PX7000092582. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1995a. Gulf beach survey: northern boundary 

to Yarborough Pass, Padre Island National Seashore October 1994 – September 1995. 

National Park Service unpublished report. Corpus Christi, Texas.

Chaney, A. H., G. W. Blacklock, and S. G. Bartels. 1995b. Laguna Madre bird survey, 

Yarborough Pass to Northern Boundary, Padre Island National Seashore, August 1994 to 

August 1995. National Park Service unpublished report, Contract 1443PX749094190.

Corpus Christi, Texas.

Chapman, B. R. 1984. Seasonal abundance and habitat-use patterns of coastal bird populations 

on Padre and Mustang Island barrier beaches (following the Ixtoc I oil spill). U.S. Fish and 

Wildlife Service. OBS-83/31, Slidell, Louisiana.

Coste, R. L., and M. K. Skoruppa. 1989. Colonial waterbird rookery island management plan for 

the south Texas coast. Center for Coastal Studies, Corpus Christi State University, Corpus 

Christi, Texas.

de Lucas, M., G. F. E. Janss, D. P. Whitfield, and M. Ferrer. 2008. Collision fatality of raptors in 

wind farms does not depend on raptor abundance. Journal of Applied Ecology 45:1695-1703.

Erwin, R. M., and T. W. Custer. 1982. Estimating reproductive success in colonial waterbirds: an 

evaluation. Colonial Waterbirds 5:49-56.

Erwin, R. M., B. R. Truitt, and J. E. Jimenez. 2001. Ground-nesting waterbirds and mammalian 

carnivores in the Virginia barrier island region: running out of options. Journal of Coastal 

Research 17(2):293-296.
147

Frey, J. K., and G. D. Jones. 2008. Mammal inventory of Padre Island National Seashore. Final 

report. Department of Fishery and Wildlife Sciences, New Mexico State University, Las 

Cruces, New Mexico.

Hudson, J. M. 2011. Houston Audubon Society’s letter to the U.S. Army Corp of Engineers 

regarding the Baryonyx Corporation’s proposed wind farm. Unpublished letter. Houston 

Audubon Society, Houston, Texas.

Kushlan, J. A. 1986. Colonies, surveys, and sites: the terminology of colonial waterbird research. 

Colonial Waterbirds 9:119-120.

Laine, S. C., and E. R. Ramsey. 1998. Landcover classification for Padre Island National 

Seashore. U.S. Geological Survey, National Wetlands Research Center, Lafayette, Lousiana.

Manuwal, D. A. 1978. Effects of man on marine birds: a review. Pages 140-160 in Wildlife and 

people: the proceedings of the John S. Wright Forestry Conference. Department of Forestry 

and Natural Resources and the Cooperative Extension Service, Purdue University, Indiana.

Mrazek, R. W. 1974. The relationship of the fire ant (Solenopsis geminata) (Fabricius) to the 

young of birds nesting on two spoil islands in the Laguna Madre. Thesis. Texas A & I 

University, Kingsville, Texas. 

National Park Service (NPS). 1996. Padre Island National Seashore resources management plan. 

National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS). 2001. Oil and gas management plan: Padre Island National 

Seashore. National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS) - Geologic Resources Division. 2007. Digital Surficial Units of 

Padre Island National Seashore and Vicinity, Texas. paissur.shp ArcGIS data layer. National 

Park Service (NPS), Geologic Resources Division, Denver, Colorado.

National Park Service (NPS). 2012a. Padre Island National Seashore coasts/shorelines 

website. http://www.nps.gov/pais/naturescience/coasts.htm (accessed 16 January 2013).

National Park Service (NPS) - Geologic Resources Division. 2012b. Padre Island National 

Seashore: GRI ancillary map information document. National Park Service (NPS), Geologic 

Resources Division, Denver, Colorado.

National Park Service (NPS). 2013a. NPSpecies online 

database. https://irma.nps.gov/App/Species/Search (accessed 16 January 2013).

National Park Service (NPS). 2013b. PAIS colonial waterbird count 1973-2011.xlsx. Excel 

Spreadsheet. Received from Wade Stablein. March 2013.

Osborn, R. G., K. F. Higgines, R. E. Usgaard, C. D. Dieter, and R. D. Neiger. 2000. Bird 

mortality associated with wind turbines at the Buffalo Ridge Wind Resource Area, 

Minnesota. The American Midland Naturalist 143(1):41-52.
148

Smallwood, K. S., and C. Thelander. 2007. Bird mortality in the Altamont Pass Wind Resource 

Area, California. The Journal of Wildlife Management 72(1):215-223.

Smith, E. H. 2002. Colonial waterbirds and rookery islands. Pages 182-197 in Tunnell, J. W., 

and F. W. Judd. Laguna Madre of Texas and Tamaulipas. Texas A&M University Press, 

College Station, Texas.

Snodgrass, K. 1997. Food habits of coyotes (Canis latrans) on barrier islands. Thesis. Texas 

A&M University-Corpus Christi. Corpus Christi, Texas.

Texas Colonial Waterbird Society (TCWS). 2011a. Summary of colonial waterbird counts for 

2011 for the Central Texas 

Coast. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/tcws/documents/2011_C

entral_Coast.pdf (accessed 12 February 2013).

Texas Colonial Waterbird Society (TCWS). 2011b. Colonial waterbirds on the Lower Texas 

Coast. http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/tcws/documents/2011_L

ower_Coast.pdf (accessed 12 February 2013).

Texas Parks & Wildlife Department (TPWD). 2013. Partnership: Texas Colonial Waterbird 

Society (TCWS) 

website. http://www.tpwd.state.tx.us/huntwild/wild//wildlife_diversity/tcws/ (accessed 12 

February 2013).

U.S. Geological Survey (USGS). 2001. Waterbird Monitoring Partnership: what is a 

waterbird? http://www.pwrc.usgs.gov/cwb/whatis/ (accessed 12 February 2013).

U.S. Department of Defense (DoD). 2012. Public scoping meeting and preparation of 

environmental impact statement for Baryonyx Corporation, Inc.’s proposed wind farm, 

offshore, Willacy and Cameron Counties, TX. Federal Register 77(50):15088-15089. 

Available from http://www.gpo.gov/fdsys/pkg/FR-2012-03-14/pdf/2012-6128.pdf (accessed 

8 October 2012).

U.S. Department of the Interior (DOI). 1989. U. S. Fish and Wildlife Service News Release. U.S. 

Department of the Interior, Washington D.C.

Wetterer, J. K., and J. A. Moore. 2005. Red imported fire ants (Hymenoptera: Formicidae) at 

gopher tortoise (Testundines: Testutudinae) burrows. Florida Entomologist 88(4):349-354.
149

Plate 17. Location of the 12 dredge spoil islands used by nesting colonial waterbirds in PAIS.
150

Plate 18. Land cover of PAIS' spoil mounds (Laine and Ramsey 1998).
151

Plate 19. Dredge spoil island 614-341 in PAIS. These islands were identified as a single unit in TCWS 

reporting, but are shown as four sub islands in this plate.
152

Plate 20. Dredge spoil island 614-342 in PAIS. These islands were identified as a single unit in TCWS 

reporting, but are shown as two sub islands in this plate.
153

Plate 21. Dredge spoil island 614-343 in PAIS.
154

Plate 22. Dredge spoil island 614-344 in PAIS.
155

Plate 23. Dredge spoil island 614-345 in PAIS.
156

Plate 24. Dredge spoil island 614-346 in PAIS.
157

Plate 25. Dredge spoil island 614-347 in PAIS.
158

Plate 26. Dredge spoil island 614-360 in PAIS.
159

4.8 Coyotes

Description

Coyotes (Photo 14) are highly adaptive, top-level predators found throughout the continental 

United States (Bekoff and Gese 2003). They are widespread across Texas and are reported as the 

most common native large carnivore in southeastern Texas (Schmidly 2004). There are 19 

recognized subspecies of coyote, and PAIS is 

home to the lower Rio Grande subspecies 

(Canis latrans microdon). Coyotes are 

documented as the largest predator in PAIS 

able to successfully inhabit the barrier islands 

of Texas (Snodgrass 1997). They have been 

observed frequently traversing the coastal 

grasslands and prairies throughout the park 

(Frey and Jones 2008).

Dog-like in appearance, coyotes have thick, 

grayish fur and a long, bushy tail (Bekoff 

1977). The size of adult coyotes varies by 

gender (males are typically larger) and 

geographic region, but an average adult 

coyote has a body length of about 1-1.5 m (3.28-4.92 ft) (Hall and Kelson 1959, Bekoff 1977, 

Bekoff and Gese 2003). They are primarily active in early evening and at night, but are also 

sporadically active during the day (Bekoff 1977). 

Coyotes are opportunistic predators, and prey species will vary depending on fluctuations in prey 

base abundance (Bekoff 1977, Windberg and Mitchell 1990, Bekoff and Gese 2003). In southern 

Texas, coyotes have been observed preying upon rabbits, rodents, fruit, insects, and carrion 

(TPWD 2012a). In PAIS, coyotes have been observed hunting or scavenging along the beaches, 

mudflats, and occasionally on the small spoil islands in the Laguna Madre (Frey and Jones 

2008). They are known to eat several species of mammals (mostly rodents), birds, reptiles, fish, 

crabs, insects, fruits, and seeds (Snodgrass 1997). 

Measures

• Population density 

• Distribution

Reference Conditions/Values

The ideal reference condition is population densities and distribution prior to the intensive 

grazing and settlement of North Padre Island that occurred during the late 1800s through 1971. 

There is very little historic information that identifies coyote population parameters on Padre 

Island during this time.

Data and Methods

Very little historic information quantifies coyote densities or distribution for the reference 

period. Bailey (1905) reported the presence of coyotes on Padre Island and other nearby barrier 

Photo 14. Coyote (Canis latrans) (USGS photo).
160

islands in the late 1800s and early 1900s, but did not characterize densities or specific 

distribution of coyotes on the island. Few studies document the status or health of coyote

populations in PAIS up to present times. The studies that do exist are primarily biological 

inventories of the park, and only address the presence/absence of species rather than describe 

distribution or population densities. 

Frey and Jones (2008) examined mammal populations in the coastal prairie ecosystem of PAIS. 

The objectives of the research were to 1) conduct a baseline inventory of small mammals present 

in the park, 2) describe the habitat associations of small mammal populations, 3) determine 

factors that structure small mammal populations in the park, and 4) characterize patterns in 

species diversity and community structure. Standardized Sherman trap transects were utilized to 

evaluate small mammal presence on the island, while sign (scat, tracks, etc.), direct observation, 

and motion-sensing cameras were used to document the presence of larger mammals, including 

coyotes. 

Current Condition and Trend

Population Density

No current estimate of population density exists for coyotes in PAIS. Density of coyote 

populations is likely to be largely dependent upon food and habitat availability. Snodgrass (1997) 

noted that coyotes on barrier islands, such as PAIS, tend to be more solitary than coyotes on the 

mainland because of the seasonal or limited availability of mammalian prey items. 

Frey and Jones (2008) recorded locations of coyote observations during a survey of small 

mammals in PAIS, but did not specifically quantify densities through intensive trapping. Based 

on the number of coyotes observed, Frey and Jones (2008) suggested that coyote densities 

appeared high in PAIS, although additional species-specific trapping studies are needed to 

quantify this observation. 

Distribution

Very little research investigating coyote distribution in PAIS has been completed to date. Frey 

and Jones (2008) documented the presence and relative distribution of coyotes in PAIS by 

mapping coyote sightings during the 2005 through 2007 field seasons. Locations of all coyote 

sightings, both by direct researcher observation or images captured on motion-sensing cameras, 

were mapped to determine presence throughout the park; coyote sign (i.e., scat and tracks) was

also documented when encountered. 

Ninety-one of 214 images from motion-sensing cameras placed throughout the park documented 

coyotes. During the Frey and Jones (2008, p. 74) inventory, researchers stated that they 

“frequently saw coyote tracks traversing transects of Sherman traps, adjacent to mudflats, along 

the beach, and on most small islands in the Laguna Madre”. Figure 17 shows the trap locations 

where coyotes were documented in the park. However, it could not be verified if individual 

coyote observations were unique individuals or if some individuals were observed repeatedly 

during the study. Likewise, the distance that individuals traveled throughout the park was not 

determined. 
161

Figure 17. Trapping locations where coyotes were documented within PAIS (Frey and Jones 2008). The 

section to the left shows locations from the northern boundary of the park to beach mile 28.5. The section 

on the right shows locations from beach mile 28.5 to the southern boundary of the park. Cala = Canis 

latrans.

Threats and Stressor Factors

Outbreaks of harmful algal blooms (HABs), specifically red tides, have been identified as a 

threat to coyote populations in PAIS, particularly when events are sustained. HABs arise when 

annual blooms of naturally occurring microscopic algae grow out of control. The algae in red 

tides is made up of the dinoflagellate Karenia brevis and is found primarily in the Gulf of 

Mexico (NOAA 2012b). K. brevis produces a neurotoxin, known as brevetoxin, which at high 

concentrations can affect the central nervous systems of fish, shellfish, birds, and marine and 

terrestrial mammals, as well as sicken people who eat contaminated molluscan shellfish (NOAA 

2012a, TPWD 2012b). When temperature, salinity, and nutrients reach certain levels in the Gulf 
162

of Mexico, exponential increases in K.brevis algae occur (TPWD 2012b). While the precise 

combination of factors that cause a red tide event is not clear, it is believed that high 

temperatures in addition to lack of rainfall and wind play a significant role in overgrowth of red 

tide algae (TPWD 2012b). In 2009, PAIS experienced a sustained red tide event that also 

coincided with an extended drought period for the region. During this time, a number of coyotes 

in the park were observed displaying symptoms of neurological problems, such as ataxia (loss of 

coordination) and partial paralysis of limbs; at least 12 coyotes were discovered dead during and 

in the weeks following this event, all of which were determined to have died from brevetoxin 

poisoning. 

Drought is also a threat to the coyote population in PAIS as periods of drought remove many 

sources of water, as well as significantly affect the availability of prey items. Starvation during 

crashes of prey sources may be a substantial mortality factor for coyotes across their range 

(Bekoff and Gese 2003).

Coyotes are susceptible to a wide variety of diseases and parasites (Bekoff 1977). External 

parasites, such as various species of fleas, ticks, lice, and internal parasites, such as cestodes, 

roundworms, intestinal worms, hookworms, whipworms, heartworms, pinworms, thorny-headed 

worms, and coccidia fungus have been known to affect coyotes across the extent of their range 

(Bekoff 1977). Diseases such as tularemia, distemper, rabies, and bubonic plague also are 

documented in coyote populations (Beckoff 1977). Disease and parasitic outbreaks within a 

small population can hinder recruitment of young and overall population sustainability.

Human activity has been identified as a leading cause of death in adult coyotes (Windberg et al. 

1985, Gese et al. 1989, Windberg 1995). A possible threat to coyote populations in PAIS is 

coyote-human conflicts, including collisions with vehicles while crossing roadways. Another 

possible impact is the habituation of coyotes to humans, particularly in park settings where 

refuse stations and visitor supplies are associated with opportunistic food sources. In the last 7 

years, a total of five coyotes have died as a result of collision with vehicles on park roads; two or 

three individuals have had to be removed as a result of habituation to humans (Stablein, email 

communication, 29 November 2012). Historically, poaching of coyotes for pelts or reduction of 

large predators in the area has been a threat to coyote populations. Although coyotes within PAIS 

are protected from hunting, poaching or hunting that occurs outside of park boundaries may 

affect the health of the population in the park.

Data Needs/Gaps

An investigation into the current coyote population density and distribution is needed for PAIS. 

In addition, assessments detailing disease prevalence or impacts and reproductive success would 

provide valuable insights into population conditions in the park. Without this information, an 

assessment of current condition of the coyote population is not possible.

Overall Condition

Population Density

The project team defined the Significance Level for population density as a 3. However, due to 

the lack of data regarding this measure, a Condition Level was not assigned.
163

Distribution

The project team defined the Significance Level for distribution as a 3. Very limited data exist for 

coyote distribution in PAIS; thus, a Condition Level was not assigned to this measure.

Weighted Condition Score

Information regarding the coyote population at PAIS is not available at this time. Without 

detailed information regarding the population density and distribution of coyotes in the PAIS 

area, a Weighted Condition Score cannot be assigned.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
164

Literature Cited

Bailey, V. 1905. Biological survey of Texas. North American Fauna 25:1-222.

Bekoff, M. 1977. Canis latrans. Mammalian Species 79:1-9.

Bekoff, M., and E. M. Gese. 2003. Coyote (Canis latrans). In G. A. Feldhammer, B. C. 

Thompson, and J. A. Chapman (eds.). Wild mammals of North America: Biology, 

management and conservation (2nd ed.). John Hopkins University Press, Baltimore, 

Maryland.

Frey, J. K., and G. D. Jones. 2008. Mammal inventory of Padre Island National Seashore. Final 

report. Department of Fishery and Wildlife Sciences, New Mexico State University, Las 

Cruces, New Mexico.

Gese, E. M., O. J. Rongstad, and W. R. Mytton. 1989. Changes in coyote movements due to 

military activity. Journal of Wildlife Management 53:334-339.

Hall, E. R., and K. R. Kelson. 1959. Mammals of North America. Ronald Press, New York, New 

York.

National Oceanic and Atmospheric Administration (NOAA). 2012a. Ocean Facts: A “red tide” is 

a common term used for a harmful algal 

bloom. http://oceanservice.noaa.gov/facts/redtide.html (accessed 20 August 2012).

National Oceanic and Atmospheric Administration (NOAA). 2012b. Brevetoxin and Florida red 

tides. http://www.nmfs.noaa.gov/pr/pdfs/health/brevetoxin.pdf (accessed 20 August 2012).

Schmidly, D. J. 2004. Mammals of Texas. Revised edition. Texas Parks and Wildlife 

Department, University of Texas Press, Austin, Texas. 

Sherrod, L., and D. O. Brown. 1989. Padre Island National Seashore vegetation history: Final 

report. Horizon Environmental Services, Inc., Austin, Texas.

Texas Parks and Wildlife Department (TPWD). 2012a. Coyote (Canis latrans). Texas Parks and 

Wildlife Department. http://www.tpwd.state.tx.us/huntwild/wild/species/coyote/ (accessed 3 

May 2012).

Texas Parks and Wildlife Department (TPWD). 2012b. Red tide in Texas: Frequently asked 

questions. Texas Parks and Wildlife Department. http://www.tpwd.state.tx.us/landwater/

water/environconcerns/hab/redtide/faq.phtml (accessed 20 August 2012).

Windberg, L. A., H. L. Anderson, and R. M. Engeman. 1985. Survival of coyotes in southern 

Texas. Journal of Wildlife Management 49:301-307.

Windberg, L. A., and C. D. Mitchell. 1990. Winter diets of coyotes in relation to prey abundance 

in southern Texas. Texas Journal of Mammalogy 71:439-447.
165

Windberg, L. A. 1995. Demography of a high-density coyote population. Canadian Journal of 

Zoology 73:942-954.
166

4.9 Small Mammals

Description

Twenty-four terrestrial mammals are known to inhabit 

PAIS (NPS 2012a, GULN 2010). With the exception 

of white-tailed deer, coyote, and the non-native nilgai 

(a rare occurrence in the park), all mammals in the 

park are considered small mammals. While the 

majority of native mammals in PAIS are rodents, the 

small mammal population also includes rabbits, 

moles, bats, opossums, and armadillos (NPS 2012a).

The primary habitats utilized by the small mammal 

population of the park include grasslands, vegetated 

dune communities, and wetlands/semi-permanent 

ponds (NPS 2012b).

While visitors do not often observe small mammals, 

several of these species (rodents in particular) make 

up important components of the park’s ecosystem. 

Rodents serve as the primary food source for avian 

predators, reptiles, and larger mammal species (e.g., 

coyote, gray fox [Urocyon cinereoargenteus], and 

badger (Sieg 1987). The black rat and the house 

mouse are two non-native small mammals introduced 

to the U.S. in the 18th century (Raun 1959) that have 

been identified in PAIS (NPS 2012a). These species, 

although uncommon in the park, compete with native 

rodents for food resources and burrows where they do 

occur. 

Measures

• Species diversity and abundance 

• Distribution

• Density

Reference Conditions/Values

The ideal reference condition for this component is species diversity and abundance and 

population densities and distribution prior to the intensive grazing and settlement of North Padre 

Island that occurred during the late 1800s through 1971. Unfortunately, little to no data exist for 

this time period. Future assessments of condition may need to use more contemporary data as a 

baseline for trend and condition comparisons.

Data and Methods

Bailey (1905) conducted a biological survey of Texas. This survey described the ranges and 

distributions of native species in relation to “life zones” located throughout Texas. The four main 

Photo 15. Spotted ground squirrel 

(Spermophilus spilosoma) (TPWD photo).

Photo 16. Hispid cotton rat (Sigmodon 

hispidus) (TPWD photo).
167

life zones identified were the Lower Austral Zone (including the Lower Sonoran Zone), Upper 

Austral Zone (including the Upper Sonoran Zone), Transition Zone, and Canadian Zone. 

Mammal species were only addressed for the Upper and Lower Austral Zones; PAIS was located 

in the Lower Austral Zone.

Raun (1959) compiled an annotated checklist of mammals for Mustang and Padre Islands. The 

checklist included terrestrial and marine mammals recorded on the islands and in adjacent 

waters. It was nearly 20 years before another mammalian checklist was compiled in the PAIS 

area, as Baker and Rabalais (1975) compiled a checklist of mammals found within PAIS,

including terrestrial and marine mammals, in 1975. Species abundance in PAIS was recorded as 

possible, rare, uncommon, fairly common, or common in the park.

Baccus (1977) investigated how 

recreational use of the beaches and 

dunes influence the flora and fauna of 

PAIS. Objectives included correlating 

the distribution and diversity of 

mammals with vegetation patterns to 

determine which rodent species may be 

indicators of habitat degradation. 

Sampling occurred at four main sites 

(Notraf, Vetraf, Pedtraf, and Shell) 

(Figure 18). The four sites were 

selected based on levels of disturbance, 

which include no vehicle traffic 

(Notraf), limited four-wheel traffic 

(Shell), mostly pedestrian traffic 

(Pedtraf), and unrestricted vehicle 

traffic (Vehtraf). Three stations were 

established at each site for sampling 

small mammals, each spaced 6 m (20 

ft) apart. Three lines were set, one at 

each station, with 25 Sherman live 

traps per line. Sampling data included 

species name, sex, station location, 

transect number, site, and date 

captured. There were a total of 1,800 

trap nights for all four study sites (450 

trap nights per site).

Segers (1984) studied the ecology of 

ground squirrels (Spermophilus sp.), including spotted ground squirrels, from 1976 through 

1977. The study area was approximately 3.2 km (1.9 mi) north of PAIS and was comprised of 

four vegetation zones: active dunes, deflation flat (trough where sand erosion is stabilized by 

moisture), herbaceous area, and saturated depression. One hundred live traps were laid in a 4.4 

hectare grid behind the island’s foredune ridge. The traps were checked twice a day for two 

consecutive days every week.

Figure 18. Locations of the four main sampling sites used 

during the Baccus (1977) study in PAIS.
168

Harris (1988) compiled a list of mammals found in PAIS during an inventory between June and 

December 1984. Traps were set in 18 locations within the northernmost 10 km (6.2 mi) of park, 

and were used to capture and identify small mammals; museum records were referenced for 

species not observed during the course of the inventory. Museum vouchers included species 

found in PAIS as well as on Mustang Island.

Frey and Jones (2008) conducted an 

extensive 2-year survey of mammals 

within PAIS, Mustang Island, and other 

small islands in the Laguna Madre. This 

study utilized literature, museum queries, 

and field surveys to determine the 

mammals found in PAIS. The field survey 

was conducted between May 2005 and 

March 2007. Trapping was done in the 

summer of 2005, 2006, and the fall of 

2006. Live trapping methods utilized 

Sherman traps, pitfall traps, mist nets, Hart 

traps, motion-sensor photography, and 

Tomahawk traps. Figure 19 shows all 

trapping locations in the study. The survey 

divided PAIS into three latitudinal sections 

(North Beach, Closed Beach, South 

Beach), and longitudinally divided the 

park into six zones: beach, foredune, 

interdune, primary dune, interior, interior 

dune, laguna matrix. 

Current Condition and Trend

Species Diversity and Abundance 

Small mammal diversity is a measure that takes into consideration both species richness and the 

relative abundance of different species. Often, the Shannon-Wiener species diversity index (H’) 

is used to represent this measure; when properly calculated, this index can “... determine the 

uncertainty that an individual picked at random will be of a given species” (UC 2012, p. L 5-2). 

The equation for the Shannon-Wiener diversity index is listed below.

H′ = −� (pi

s

i=1 )(lnpi)

pi = proportion of individuals of species (i) in a community (=ni/N; where ni is the number of 

individuals of a given species and N is the total number of individuals in a sample) (UC 2012).

The diversity index will result in an H' value that will typically be between 0 and 4; a value of 0 

indicates a community that displays low/no species complexity, while a value of 4 indicates a 

community of high species complexity. For this measure, only studies that surveyed small 

mammals and recorded the number of individuals in a survey were included.

Figure 19. Locations of mammal trap transects during 

the Frey and Jones (2008) survey in PAIS.
169

Bailey (1905) reported 15 species of small mammals (representing six orders) from Padre Island 

or nearby Mustang Island; nine of the 15 mammals were from the order Rodentia. Two of the 

species (the hispid cotton rat and the northern pygmy mouse [Baiomys taylori]) were not 

explicitly reported on the two islands, but their distribution area included PAIS and Mustang 

Island. Although abundance values were not provided for all of the mammals, a general 

description of abundance was provided for a few of the identified species. The Brazilian freetailed bat (Tadarida brasiliensis) was said to be common, the Gulf coast kangaroo rat was 

recorded as probably common, and the Tamaulipan hog-nosed skunk (Conepatus leuconotus) 

was said to occur occasionally. All the small mammal species documented by Bailey (1905) on 

the islands are displayed in Appendix M.

Raun (1959) recorded 17 small mammal species (representing five orders) on Padre and Mustang 

Islands; the order Rodentia represented the majority (10 spp.) of the small mammals observed. 

Raun (1959) recorded only the presence of species, and no surveys or formal estimates of 

abundance were completed. Appendix M displays the small mammals present on the Raun 

(1959) mammal checklist of Padre and Mustang Islands.

Baker and Rablais (1975) recorded 31 small mammal species (representing seven orders) in 

PAIS; no specific estimates of abundance were created, however. A number of the species (12) 

were only recorded as a “possible presence” on Padre Island. Seven species were considered 

uncommon, five species were fairly common, and six species were considered common in PAIS. 

The six species considered common in the park were the spotted ground squirrel, Mexican 

ground squirrel (Spermophilus mexicanus parvidens), black-tailed jackrabbit, Texas pocket 

gopher, Gulf coast kangaroo rat, and hispid cotton rat. The Tamaulipan hog-nosed skunk was 

stated to have a former range that extended into the area, but its occurrence in PAIS was thought 

to be highly improbable at the time of the Baker and Rablais (1975) study. Appendix M displays

the small mammals documented on the Baker and Rablais (1975) checklist and their status in 

PAIS.

Baccus (1977) documented 10 small mammals (representing four orders) during a Padre Island 

study in 1976. Five rodent species inhabited the beaches and dunes in PAIS, including the Gulf 

coast kangaroo rat, spotted ground squirrel, hispid cotton rat, fulvous harvest mouse 

(Reithrodontomys fulvescens), and northern pygmy mouse. The marsh rice rat (Oryzomys 

palustris texensis) was another rodent known to inhabit the islands, but it was not observed 

during the study. The other mammal species observed are presented in Appendix M. The site 

with vehicle traffic (Vetraf) showed lower species diversity than the other three study sites; it 

had a diversity value of 0.951. This may be due to reduced vegetation cover. The dominant 

species found at the Vetraf site was the spotted ground squirrel. The two sites with the highest 

diversity had no traffic (Notraf) or only pedestrian traffic (Pedtraf), and they had diversity values 

of 1.382 and 1.425, respectively (Table 34).
170

Table 34. Shannon-Wiener Diversity Indices for rodent species of the four main sampling sites in PAIS 

(Baccus 1977).

Sampling Sites

NOTRAF PEDTRAF VETRAF SHELL

Diversity (H) 1.382 1.425 0.951 1.359

Evenness (e) 2.295 2.039 1.9445 1.993

Richness (d) 1.791 2.449 1.661 2.532

Number of Species 4 5 3 5

Harris (1988) identified 27 small mammal species that were recorded during previous inventories 

or studies in the park; however, only nine species from seven orders were captured or observed 

during the survey in 1984. The most diverse order was Rodentia, and the rodent species captured 

included the spotted ground squirrel, Texas pocket gopher, Gulf coast kangaroo rat, marsh rice 

rat, and northern pygmy mouse. According to Harris (1988), the eastern pipistrelle (Sylvilagus 

floridanus) had only been documented by Zehner (1985), and was not observed during this 

study. A list of the other mammals captured during this study can be found in Appendix M.

Frey and Jones (2008) reported 21 small mammal species that are present or probably present in 

PAIS. The species represent seven orders, with the order Rodentia being the most diverse. A 

striped skunk was observed during the study, but the report did not specify if it was inside PAIS. 

Only three species were listed as probably present in the park. Those species were the whitenosed coati (Nasua narica), eastern pipestrelle, and Virginia opossum. These mammals were not 

documented in the park during the survey, but have been seen north of the park or recorded in 

previous studies. Abundance estimates were not created, but Frey and Jones (2008) indicated that 

the hispid cotton rat was the most common species trapped during the study. The Texas pocket 

gopher was commonly observed throughout the park’s terrestrial zones, while the northern 

pygmy mouse, fulvous harvest mouse, house mouse, and American badger were said to be in low 

abundance or uncommon. Appendix M displays all of the small mammals recorded during the 

inventory.

GULN (2010) summarized 20 small mammal species that are currently present or probably 

present in PAIS, based on the NPS Certified Species List. The list does not estimate abundance 

or diversity, however. The mammals identified in the summary represent seven orders, with the 

order Rodentia being the most diverse. Other orders include Carnivora, Cingulata, Chiroptera, 

Didelphidae, Lagomorpha, and Soricomorpha. Only three species were listed as probably present 

in the park. Those species were the white-nosed coati, eastern pipestrelle, and Virginia opossum.

Appendix M displays all of the small mammals recorded during the inventory. The NPS 

Certified Species List (NPS 2012) has remained unchanged since this summary, accounting for 

20 small mammals that are listed as probably present or present in PAIS. 

Distribution

Limited data exist for the distribution of small mammals across PAIS. Baccus (1977) reported

that all of the mammals observed inhabited the beaches and dunes in PAIS. The Gulf coast 

kangaroo rat and spotted ground squirrel seemed to prefer the open or sparsely vegetated areas, 

while the other species preferred areas with denser vegetative cover. Most of the species 

documented were found at the sampling site with no traffic (Notraf site). No marsh rice rats were 

collected during the study, but they are known to inhabit marshy areas with grasses and sedges. 
171

Table 35 displays the small mammals observed at the four main sites of the study (refer back to 

Figure 18 for site locations).

Table 35. Small mammal species collected in PAIS (Baccus 1977).

Common Name Notraf Pedtraf Vetraf Shell

eastern mole X

black-tailed jack rabbit X X

spotted ground squirrel X X X X

Padre Island pocket gopher X X X X

Gulf coast kangaroo rat X X X X

fulvous harvest mouse X X

northern pygmy mouse X X X

hispid cotton rat X X X X

badger X

Density

Currently, there are limited data regarding density of small mammals in PAIS. Segers (1984) 

calculated the density of the spotted ground squirrel population just north of PAIS on Padre 

Island between 1976 and 1977. The mean density of the population was 1.5 animals per hectare. 

The highest densities of spotted ground squirrels (3.2 and 2.3 animals per ha) occurred in August 

and September of 1976, respectively.

Threats and Stressors 

There are several threats to the park’s small mammal population, and PAIS staff identified eight 

stressors that have occurred or are presently occurring in the park. Those stressors include

invasive species, drought, erosion, predation, flooding and inundation, oil and gas development, 

marine debris (hazardous materials), and management actions.

The anthropogenic threats to small mammals include oil and gas development, marine debris, 

management actions, and invasive species. Oil and gas development can adversely affect small 

mammals in several ways. Small mammals could be killed during the drilling process; increased 

mortality could also result from vehicle collisions due to increased traffic (BLM 1981). Another 

anthropogenic threat to small mammals is hazardous marine debris. One of the largest 

environmental issues in the Gulf of Mexico is marine debris (EPA 1993). According to EPA 

(1993), injuries and deaths of animals as a result of debris have been increasing over the years. 

Small mammals could ingest or get entangled in beached debris (e.g., tar balls, plastic particles, 

fishing line), which may cause suffocation or predation due to entanglement. Invasive species 

also threaten the small mammals of PAIS through competition for resources. Invasive species do 

not have natural predators when introduced to a new environment. This allows them to outcompete native mammals for resources (e.g., food, shelter) and even prey on other native species 

(Pimentel et al. 2005). The Norway rat (Rattus norvegicus), for example, is an opportunistic 

feeder and feeds on the best quality food that is available (Major 2004). Invasive rat species have 

been known to eat native invertebrates and other vertebrates (e.g., birds, eggs) (Pimentel et al. 

2005).
172

The natural threats to the small mammals on PAIS are drought, flooding and inundation, erosion, 

and predation. Drought is a stressor to the island’s small mammals because precipitation is a 

significant source of freshwater input on barrier islands. There are only a few freshwater ponds 

in PAIS that serve as a source of fresh water; these ponds also double as a source of food for 

small mammals that prey on aquatic life found in those ponds (Gilbert et al. 2012). Flooding and 

inundation can cause significant stress to small mammals. Inundation usually occurs on the tidal 

flats as a result of wind action, but large storm events have caused nearly the entire island to be 

flooded in the past (Hice and Schmidly 2002). Flooding and inundation can destroy habitats, 

uproot vegetation, and cause mortality to many small mammals. According to Hice and 

Schmidly (2002), flooding and inundation can result in the need for mammals to recolonize after 

waters recede. Some may have had to swim to the Texas mainland; being in the open for long 

periods of time could increase the risk of predation. Erosion may cause stress to the burrowing 

small mammals in PAIS. Erosion is a common and necessary process on barrier islands due to 

wave action and the Gulf current. Increased erosion may put mammals such as the eastern mole, 

Texas pocket gopher, and black-tailed jackrabbit at risk by causing mounds to collapse into 

burrows. If erosion causes a burrow to collapse, these mammals may become more susceptible to 

predation while digging another burrow. Small mammals are preyed upon by several animals 

including coyotes, foxes, and raptors. Most rodents, however, have high productivity rates, 

which can aid the survival of the population even with predation (Sieg 1987). 

Data Needs/Gaps

Small mammal species richness has been well documented over the past 100 years. Species 

found in PAIS have been documented by Bailey (1905), Raun (1959), Baker and Rablais (1975), 

Baccus (1977), Harris (1988), Frey and Jones (2008), GULN (2010), and NPS (2012a); however, 

abundance, diversity, distribution, and density data are largely absent for PAIS. The 

establishment of a baseline small mammal survey that could be repeated routinely to document 

species richness and abundance would provide managers with meaningful information that can 

be used for future assessments of condition.

Overall Condition

Species Diversity and Abundance

The project team defined the Significance Level for species diversity and abundance as a 3. 

While several inventories and reports have documented the small mammal species that are 

present in PAIS, very few studies have documented species diversity or abundance. Without 

contemporary estimates of these measures, an assessment of condition is not possible. For this 

reason, a Condition Level was not assigned for this measure.

Distribution

The project team defined the Significance Level for distribution as a 3. There are historic reports 

of distribution by vegetation or habitat type, but some of these studies were conducted outside of 

the park. Baccus (1977) reported that all of the terrestrial mammals recorded were found in the 

beach or dune habitat, but this was not in PAIS nor is it current data. A current study on 

distribution within the park is necessary to make a proper assessment. As a result, a Condition 

Level was not assigned for this measure.
173

Density

The density measure was assigned a Significance Level of 3. There are limited data for density of 

small mammals in PAIS. Segers (1984) recorded densities of spotted ground squirrels north of 

PAIS; this study is 20 years old and needs to be replicated within park boundaries to determine 

the current density of the species. The density of other small mammals would be necessary to 

assess the current condition for this measure. Because of this data gap, a Condition Level for this 

measure was not assigned.

Weighted Condition Score

A Weighted Condition Score for small mammals in PAIS was not assigned because none of the 

measures had known Condition Levels.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
174

Literature Cited

Baccus, J. T., J. K. Horton, and P. D. Carangelo. 1977. A study of beach and dunes floral and 

faunal interrelations as influenced by recreational and user impact on Padre Island National 

Seashore. National Park Service, Padre Island National Seashore, Texas.

Bailey, V. 1905. Biological survey of Texas. North American Fauna 25:5-222.

Baker, K., and N. Rabalais. 1975. Checklist of mammals. National Park Service, Padre Island 

National Seashore, Texas.

Bureau of Land Management (BLM). 1981. Oil and gas environmental assessment of BLM 

leasing program, Lewistown District. U.S. Bureau of Land Management, Lewistown, 

Montana. 

Environmental Protection Agency (EPA).1993. Marine debris action agenda for the Gulf of 

Mexico. United States Environmental Protection Agency, Office of Water, Gulf of Mexico 

Program. EPA 800-K-93-002. Stennis Space Center, Mississippi.

Frey, J. K., and G. D. Jones. 2008. Mammal inventory of Padre Island National Seashore. Final 

report. Department of Fishery and Wildlife Sciences, New Mexico State University, Las 

Cruces, New Mexico.

Gilbert, S., K. Lackstrom, and D. Tufford. 2012. The impact of drought on coastal ecosystems in 

the Carolinas. Research Report CISA-2012-01. Carolinas Integrated Sciences and 

Assessments, Columbia, South Carolina. 

Gulf Coast Network (GULN). 2010. A summary of biological inventories conducted at Padre 

Island National Seashore: vertebrate, and vascular plant inventories. Natural Resource 

Technical Report NPS/GULN/NRTR—2010/402. National Park Service, Fort Collins, 

Colorado.

Harris, R. V. 1988. The mammals of Padre Island, Texas. Thesis. Corpus Christi State 

University, Corpus Christi, Texas.

Hice, C. L., and D. J. Schmidly. 2002. The mammals of coastal Texas: a comparison between 

mainland and barrier island faunas. The Southwestern Naturalist 47(2):244-256.

Major, H. L. 2004. Impacts of introduced Norway rats (Rattus norvegicus) on least auklets 

(Aethia pusilla) breeding at Kiska Island, Aleutian Islands, Alaska during 2001 – 2003. 

Thesis. Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, 

Canada.

National Park Service (NPS). 2012a. NPSpecies online 

database. https://irma.nps.gov/App/Species/Search (accessed 5 November 2012).

National Park Service (NPS). 2012b. Padre Island: 

mammals. http://www.nps.gov/pais/naturescience/mammals.htm (accessed 20 April 2012).
175

Pimentel, D., R. Zuniga, D. Morrison. 2005. Update on the environmental and economic costs 

associated with alien-invasive species in the United States. Ecological Economics 52:273-

288.

Raun, G. G. 1959. An annotated checklist of the mammals of Mustang and Padre Islands. Welder 

Wildlife Foundation, Sinton, Texas.

Segers, J. C., and B. R. Chapman. 1984. Ecology of the spotted ground squirrel, Spermophilus 

spilosoma (Merriam), on Padre Island, Texas. Special Publications - the Museum, Texas 

Tech University 22:105-112.

Sieg, C. H. 1987. Small mammals: pests or vital components of the ecosystem? Great Plains 

Wildlife Damage Control Workshop, Wildlife Damage Management, Internet Center for 

Proceedings. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1097&context=

gpwdcwp (accessed 28 February 2013).

Traweek, M., and R. Welch. 1992. Exotics in Texas. Texas Parks and Wildlife Department, 

Reproduced From PWD-BK-W7000-206-5/92. http://www.tpwd.state.tx.us/publications/

pwdpubs/media/pwd_bk_w7000_0206.pdf (accessed 20 April 2012).

University of Colorado Boulder (UC). 2012. Estimating species richness and diversity and 

assessing stream quality: a survey of the macro-invertebrates in Boulder 

Creek. http://www.colorado.edu/eeb/courses/2040/Lab_Manual/lab5.pdf (accessed 14 

November 2012).
176

4.10 Macroinvertebrates

Description

Macroinvertebrates are often used as biological 

indicators in assessing overall aquatic ecosystem 

health (EPA 2012). Their presence or absence may 

reflect ecosystem integrity or disturbances that can 

affect higher trophic levels (Rocha 1995, Segura et 

al. 2007). Along the Texas coast, 

macroinvertebrates are an important food source for 

shorebirds, fish, and other wildlife (Withers and 

Tunnell 1988, Rocha 1995). Species abundance and 

diversity, distribution, and density of 

macroinvertebrates in aquatic systems is one of the 

GULN Vital Signs, chosen to represent the overall 

health and condition of the park’s natural resources 

(Segura et al. 2007). Macroinvertebrate indicator 

species can range from sensitive species, such as 

caddisflies (Trichoptera) and Unionids (freshwater 

mussels of the family Unionidae, also known as 

naiads) to the much more tolerant midge (Diptera) 

and aquatic worm (Oligochaeta) species.

The aquatic and beach environments of PAIS are generally harsh (e.g., high salinity, pounding 

waves) and dynamic (e.g., tidal surges) (Parker 1959, Rocha 1995). The macroinvertebrates that 

inhabit these communities must be able to adapt to these extreme conditions. The Laguna Madre, 

due to its lack of water exchange with other water bodies, is typically hypersaline with high 

summer water temperatures (30-35 °C) (Parker 1959). The dominant aquatic macroinvertebrate 

taxa found in PAIS include crustaceans, polychaete worms, and shelled mollusks (Rocha 1995, 

Withers et al. 2004). Burrowing invertebrates, such as the ghost crab (Ocypode quadrata) (Photo 

17), are common on beaches, while bivalve mollusks (e.g., clams) are frequently found in 

intertidal areas and the Laguna Madre (Parker 1959, Rocha 1995). Additional macroinvertebrate 

species include echinoderms (e.g., sand dollars), cnidarians (e.g., jellyfish), and gastropods 

(Parker 1959, Rocha 1995, Withers et al. 2004). 

Measures

• Distribution

• Density

• Species diversity and abundance

Reference Conditions/Values

A park-wide reference condition has not been defined for the macroinvertebrate community. The 

condition of the community at the time of park establishment would be an appropriate reference 

condition, but no information is available from this time. The data and information presented 

here could be used as a baseline for future assessments.

Photo 17. A ghost crab at PAIS (photo by 

Shannon Amberg, SMUMN GSS).
177

Data and Methods

Macroinvertebrates have been sampled in several habitats in and around PAIS. The earliest work 

by Parker (1959) occurred in the Laguna Madre. Based on analyses of aquatic macroinvertebrate 

samples from 121 stations throughout the Laguna Madre, Parker (1959) identified five unique 

environments, three of which occur along PAIS: open hypersaline lagoon, enclosed hypersaline 

lagoon, and a small amount of “hypersaline lagoon influenced by adjacent lower-salinity bay” 

near the northern boundary (p. 2148). During the mid-1970s, Rabalais (1976) created a checklist 

of crab species found at PAIS and their general habitat. 

Throughout the 1990s, Withers (1993, 1996, 1998) studied the macroinvertebrates of several 

tidal flats on the Laguna Madre side of PAIS. From October 1991 to September 1992, Withers 

(1993) sampled benthic macroinvertebrates twice a month on an algal flat along the park’s 

northern boundary. A species list was generated, as well as density and distribution (damp, wet, 

or intertidal microhabitats) information. From November 1995 to March 1996, Withers (1996) 

studied the macroinvertebrate communities at two former drilling sites south of Malaquite Beach 

to explore the impact of drilling and restoration activities on these organisms. Withers (1996) 

looked at diversity and density at the restored sites and in nearby control areas. Finally, Withers 

(1998) sampled macroinvertebrates on three wind-tidal flats in PAIS from November 1997 

through March 1998. Density and diversity data from this study were compared to similar sites 

from Withers’ previous studies (1993 and 1996). 

Also in the early 1990s, Rocha (1995) studied benthic invertebrates on the Gulf of Mexico 

beaches of PAIS. Four transects, stretching from the backshore to subtidal areas (e.g., sandbars 

and troughs), were sampled monthly from October 1992 to September 1993 (Plate 27). In 

addition to generating a species list and characterizing species distribution along the transects, 

species density and diversity were also calculated (Rocha 1995).

Limited data regarding inland aquatic macroinvertebrates at PAIS were available from Sissom et 

al. (1990), who completed a biological inventory of three ponds in the park (Plate 27). 

Macroinvertebrate sampling included species richness and monthly density data from September 

1989 to August 1990.

Current Condition and Trend

Distribution

Several studies have explored the distribution of macroinvertebrate species within a particular 

environment (e.g., beaches, tidal flats). Parker (1959) identified three separate aquatic 

environments in the portion of the Laguna Madre that borders PAIS: open hypersaline lagoon, 

enclosed hypersaline lagoon, and saline lagoon influenced by a lower salinity bay. The 

distribution of macroinvertebrates across these three environments is shown in Table 36. Only 

three bivalve species were found in all three environments, while 10 species were found in just 

one environment (Parker 1959). 
178

Table 36. Macroinvertebrate species collected in three Laguna Madre environments that border PAIS 

(Parker 1959). Only species captured alive are included here; any species collected only as shells have 

been omitted.

Species Open 

hypersaline

Enclosed 

hypersaline

Low salinity, bay 

influenced

Gastropods (Snails/Slugs)

Crepidula glauca convexa x

Cerithidea variable x

Nassarius vibex x x

Bittium varium x

Mitrella lunata x

Odostomia bisuturalis x x

Bivalves (Mussels)

Brachidontes citrinus x x

Amygdalum papyri x

Laevicardium mortoni x x

Macoma brevifrons x

Mactra fragilis x x

Pseudocyrena floridana x

Anomalocardia cuneimeris x x x

Mulinia lateralis x x x

Tellina tampaensis x x x

Crassostrea virginica x

Lyonsia floridana hyaline x

Phacoides pectinatus x

Crustaceans (Crabs/Shrimp)

Callinectes sapidus x x

Total 14 6 11

Withers (1993) reported the distribution of benthic macroinvertebrates within damp, wet, and 

intertidal microhabitats on an algal flat. Microhabitats were divided based on water depth; 

intertidal areas contained 2-4 cm of water, wet areas up to 2 cm, and damp areas generally had 

no standing water but were wet just below the ground surface. Ten macroinvertebrate species 

were found in all three microhabitats (Withers 1993, Table 37). Twenty-two species were found 

in just one microhabitat, most often the intertidal areas. However, several insect families 

occurred only in the damp microhabitat (Withers 1993). Withers (1996) explored the distribution 

of macroinvertebrates at two former oil and gas development sites that had been restored (Plate 

27). At the Texaco site, six species were found within the restored area, six in an adjacent control 

area, and four within tire tracks (Withers 1996, Table 37). Only insects were found in tire tracks 

and no mollusks were found at the site. At Yarborough Pass, 26 macroinvertebrate species 

occurred in the restored area, 14 in the control area, and only six within tire tracks (Withers 

1996). Nearly all species found at the control and tire track locations were also present in the 

restored area. Withers (1998) sampled three tidal flats on the western side of the park: two algal 

flats (Dunn Ranch and Yarborough Pass) and one sandflat (Mile Marker 45) (Plate 27). Taxa 

found at all three sites included several polychaetes (segmented worms), one bivalve 

(Anomalocardia auberiana), a tanaid crustacean (Hargeria rapax), and two dipteran families 

(Dolichopodidae and Canaceidae) (Table 37). Amphipod crustaceans occurred at both algal flats, 

but not at the Mile Marker 45 sandflat. Nemerteans (ribbon worms) were recorded only at 

Yarborough Pass (Withers 1998).
179

Table 37. Distribution of benthic macroinvertebrates on tidal flats during three studies by Withers (1993, 

1996, 1998). For Withers (1993), D = damp, W = wet, and I = intertidal. For Withers (1996), Rs = restored, 

C = control (no known oil/gas disturbance), and TT = in tire tracks.

Withers 1993 Withers 1996 Withers 1998

Species Texaco 

site

Yarborough 

Pass

Yarborough 

Pass

Dunn

Ranch

Mile 

Marker 45

Phylum Nemertea W,I Rs,C x

Phylum Annelida, 

Class Polychaeta

Arenicola cristata W

Axiothella mucosa I x

Capitella capitata D,W,I x x x

Capitomastus aciculatus W,I

Chone duneri I

Demonax micropthalmus I Rs x x x

Dorvillea rubra I

Eteone heteropoda D,W,I Rs x x x

Exogone dispar I Rs

Haploscoloplos foliosus D,W,I Rs,C,TT x x

Laeonereis culveri x

Polydora ligni I

Polydora spp. C Rs,C x

Prionospio cristata I

Prionospio heterobranchia I Rs x

Prionospio pinnata Rs,C

Marphysa regalis x

Melinna maculata x x

Nainereis laevigata Rs x x

Nereis riisei x x

Sabella sp. A I Rs x

Spio pettibonniae Rs

Streblospio benedicti Rs

Syllis cornuta D,W,I Rs

Capitellidae Rs

Maldanidae TT

Phylum Mollusca, 

Class Bivalvia

Amygdalum papyrium W Rs,C

Anomalocardia auberiana W,I Rs,C x x x

Bulla striata D,I

Mulinia lateralis D,W,I

Tellina tampaensis I x x

Tellina sp. Rs

Phylum Arthropoda

Subphylum Chelicerata

Spiders W,I

Subphylum Crustacea

Corophium acherusicum Rs,C Rs,C x

Corophium louisianum D,W,I Rs,C x

Grandidierella bonnieroides x

Gammarus mucronatus Rs x

Hargeria rapax D,W,I Rs,C Rs,C,TT x x

Orchestia grillus x

Oxyurostylis smithii I

Sphaeroma 

quadridentatum

x

Subphylum Hexapoda, 

Class Insecta

Berosus sp. Rs,TT

Bledius sp. D,W x

Canaceidae D,W,I Rs,C,TT Rs x x x
180

Table 37. Distribution of benthic macroinvertebrates on tidal flats during three studies by Withers (1993, 

1996, 1998). For Withers (1993), D = damp, W = wet, and I = intertidal. For Withers (1996), Rs = restored, 

C = control (no known oil/gas disturbance), and TT = in tire tracks (continued).

Withers 

1993

Withers 1996 Withers 1998

Species Texaco 

site

Yarborough 

Pass

Yarborough 

Pass

Dunn

Ranch

Mile 

Marker 45

Carabidae D

Ceratopogonidae D,W,I Rs,C Rs,C,TT x x

Dolichopodidae D,W,I Rs,C,TT Rs,C,TT x x x

Empidiae I

Ephydridae C

Hemiptera (nymph) x

Homoptera I

Hydrophilidae (larvae) x x

Leuctridae sp. W

Melyridae D Rs,C

Nabidae TT TT

Pteromalidae I

Saldidae D

Scelionidae I

Staphylinidae (larvae) x x

Tipulidae x

Subphylum Hexapoda, 

Class Entognatha

Cyphoderus sp. I

Phylum Platyhelminthes

Turbellaria Rs,C

Rocha (1995) divided Gulf beaches into three regions (supra-, inter-, and subtidal) and nine 

zones within these regions. The distribution of benthic macroinvertebrates within these zones is 

presented in Table 38. Figure 20, created by Rocha (1995), shows the distribution of dominant 

macroinvertebrates among the three regions of the Gulf beaches. Rocha (1995) characterized the 

supratidal or backshore region as a rove beetle-ghost crab (Bledius-Ocypode) community. The 

intertidal region was divided among two communities: a haustoriid amphipod-spionid polychaete 

(Haustorius-Scolelepis) community in higher subzones and a bivalve-mole crab (Donax-Emerita) 

community in lower subzones (Rocha 1995). Due to high species diversity but low abundance, the 

subtidal community was difficult to classify. Previous researchers had identified the region as a ghost 

shrimp-bivalve- haustoriid amphipod (Callichirus-Donax- Haustoriidae) community (Hill and 

Hunter 1976, as cited in Rocha 1995). While ghost shrimp (Callichirus islagrande) were not detected 

by Rocha (1995) due to their deep burrowing behavior, observations of abundant burrow holes 

suggested that they were an important species in the subtidal community. Other common species 

included a polychaete (Lumbrineris impatiens), haustoriids (Haustorius sp.), bivalves (Donax sp.), 

and several crustaceans (Table 38). The subtidal zone also supported echinoderms, cnidarians, and 

gastropods (Rocha 1995).
181

Table 38. Distribution and abundance (number of individuals) of benthic macroinvertebrates by zone along Gulf beaches within PAIS (Rocha 

1995). UBS – upper backshore, MBS – mid backshore, HS – high intertidal, MS – mid intertidal, LS – low intertidal, T1 – first trough, B1 – first 

sandbar, T2 – second trough, B2 – second sandbar.

Supratidal Intertidal Subtidal

Taxa UBS MBS HS MS LS T1 B1 T2 B2

Phylum Cnidaria

Scyphozoan polyp 1 1 42 5 71

Phylum Nemertea 1 1

Phylum Annelida, Class 

Polychaeta

Scolelepis squamata 2 597 357 247 41 10 25 22

Dispio uncinata 2 1 1 7 6 8

Polydora ligni 2

Onuphis eremita oculata 1 4 2

Magelona riojai 1 3 1 1

Owenia fusiformis 1 2

Nereididae sp. 1

Nephtys picta 4 6 6 16 19 21

Lumbrineris impatiens 30 1 92 276 557 966 933 706 341

Phylum Arthropoda

Subphylum Crustacea

Callichirus islagrande 1

Isocheles wurdemanni 4 1 3

Emerita benedicti 2 168 169 116 38 34 32 30

Emerita portoricensis 17 54 28 2

Lepidopa websteri 1 1 1 1 3

Callinectes sapidus 1 1

Austinixa chacei 14 63 89 54 52 161 45

Pinnixa chaetopterana 2 6

Bowmaniella brasiliensis 12 1 7 7 14

Metamysidopsis swifti 1 1

Oxyurostylis smithi 48 10 10 4 2 4

Ancinus depressus 1 27 26 44 158 165 73 63

Lepidactylus triarticulatus 2

Acanthohaustorius sp. 2 2

Protohaustorius 

bousfieldi

2 2 2 0

Haustorius sp. 14 518 2,300 194 182 556 352 556 301

Corophium sp. 4 12 8 6 3

Talorchestia sp. 4 2

Tanaid (unidentified) 10 6 2

Subphylum Hexapoda

Bledius sp. 1 97

Dipteran larvae 34 1
182

Table 38. Distribution and abundance (number of individuals) of benthic macroinvertebrates by zone along Gulf beaches within PAIS (Rocha 

1995). UBS – upper backshore, MBS – mid backshore, HS – high intertidal, MS – mid intertidal, LS – low intertidal, T1 – first trough, B1 – first 

sandbar, T2 – second trough, B2 – second sandbar (continued).

Supratidal Intertidal Subtidal

Taxa UBS MBS HS MS LS T1 B1 T2 B2

Phylum Mollusca

Class Bivalvia

Anadara ovalis 2

Donax texasianus 2 52 117 46 43 80 133

Donax variabilis 6 181 1,138 482 128 158 153 126

Donax spat 731 89 321 77 30 41 41

Class Gastropoda

Hastula salleana 2 1 1 3

Oliva sayana 1

Polinices duplicatus 1

Phylum Echinodermata

Ophiophragmus moorei 1 15 13 35 17

Mellita 

quinquiesperforata

3
183

Figure 20. Distribution of dominant macroinvertebrate taxa along the Gulf beaches of PAIS. Name 

placement indicates the zone with highest abundance while lines show overall distribution (Rocha 1995). 

MLW = mean low water, MHW = mean high water.

Density

Macroinvertebrate density data are available for PAIS Gulf beaches, tidal flats, and inland ponds. 

On an algal flat along the park’s northern boundary, Withers (1993) found that densities were 

highly variable throughout the year, ranging from 0 to 70,000 organisms/m2

. Of the three 

microhabitats sampled, densities were lowest at the damp sites, peaking around 16,000 

organisms/m2 (Withers 1993). Densities reached 70,000 organisms/m2 at both the wet and 

intertidal areas. Tanaids showed the highest densities in all three microhabitats (Withers 1993).

Withers (1996) noted low densities of most macroinvertebrates at two tidal flats south of 

Malaquite Beach. On average, less than one polychaete, tanaid, and amphipod were detected per 

5.4 x 5 cm sediment core from the Texaco site. Insect larvae were slightly more common, 

reaching a density of 1.2 per core at control sites in November (Withers 1996). Densities were 

higher at Yarborough Pass. Peak mean monthly insect density was 15 organisms per core and 

peak tanaid density was around 100 organisms per core. Polychaete, amphipod, and mollusk 

densities never exceeded four organisms per core (Withers 1996).

Rocha (1995) calculated mean macroinvertebrate densities for each of the four Gulf study 

locations as well as by beach zone, averaging across all four locations (Table 39). Total 

macroinvertebrate density was highest at Closed Beach and decreased at each site heading south. 

This may indicate a sediment grain size influence, as Closed Beach has finer sands and the more 

southerly beaches contain more coarse sediments and shell fragments (Rocha 1995). Jansson 

(1967, as cited in Rocha 1995) previously suggested that grain size distribution influences water 

content, circulation, and oxygen availability, which then impact macroinvertebrate community 

structure. Grain size and shape may also simply influence the suitability of beach habitat for 
184

burrowing organisms (Stablein, written communication, 3 February 2013). Of the three beach 

zones, the intertidal area supported the highest densities, followed by the subtidal zone, and the 

backshore with much lower densities (Table 39). Crustacean and mollusk densities were also 

highest in the intertidal zone while polychaetes peaked in the subtidal zone. Rocha (1995) also 

noticed seasonal patterns in macroinvertebrate density. At all four sites, the highest densities 

were recorded primarily during the summer (June-August), with the subtidal zone showing the 

greatest seasonal variability. Crustacean densities were highest in January and June while 

polychaetes peaked during the warmer months, beginning in April (Rocha 1995).

Table 39. Mean benthic macroinvertebrate density (per m2) by study site and by major beach zone 

(across all study sites) (Rocha 1995).

Crustaceans Polychaetes Molluscs Total

Study site

Closed Beach 1,374 349 703 2,426

Mile 15 229 552 544 1,325

Mile 25 324 453 182 959

Mile 45 295 462 53 810

Beach zone

Backshore 451 41 32 524

Intertidal 796 482 713 1,991

Subtidal 482 520 177 1,179

Rocha (1995) also compared densities from this study to prior sampling efforts at Closed Beach 

(Shelton and Robertson 1981, Vega 1988). Rocha’s (1995) densities were lower than previous 

findings (Table 40); however, Rocha attributes this to differences in methodology (e.g., size of 

sampling equipment) rather than actual change over time.

Table 40. Comparison of mean macroinvertebrate densities (#/m2) between three sampling efforts at 

Closed Beach, PAIS (Rocha 1995).

Shelton and 

Robertson (1981) Vega (1988) Rocha (1995)

Overall mean 3,980 4,900 2,450

Highest month 6,616 (June) 11,660 (May) 7,560 (June)

Sissom et al. (1990) calculated crustacean densities based on monthly samples from three PAIS 

ponds (Table 41). Densities ranged from approximately 200 to over 20,000 organisms per liter of 

pond water. Pond A showed the highest mean and maximum densities; Pond B had the lowest 

mean density while Pond C produced the lowest minimum density (Sissom et al. 1990).

Table 41. Crustacean macroinvertebrate density (log of total number per liter of pond water) for three 

ponds within PAIS (Sissom et al. 1990). The months that maximum and minimum values occurred are 

given in parentheses.

Mean Minimum Maximum

Pond A 4.04 3.30 (Apr.) 4.33 (Sept.)

Pond B 3.45 3.04 (Oct.) 4.22 (Apr.)

Pond C 3.61 2.3 (Dec.) 4.21 (Apr.)
185

Species Diversity and Abundance 

In the earliest study of macroinvertebrates in or near PAIS, Parker (1959) reported 19 live 

species in the Laguna Madre waters bordering the park: 12 bivalves, six gastropods, and one 

crustacean (see Table 36). Of the three environments sampled within the Laguna, the open 

hypersaline lagoon showed the highest diversity with 14 species, but macroinvertebrates were 

most abundant in the enclosed hypersaline lagoon, where the number of living individuals per 

sample “outnumbered all of the other individuals in the rest of the Laguna Madre” (Parker 1959, 

p. 2147).

In a study of an algal flat on the northern edge of PAIS, Withers (1993) documented 38 total 

taxa. Diversity was highest in the intertidal habitat (31 taxa) and lowest in damp habitats (15 

taxa) (see Table 37). Withers (1996) recorded seven and 29 taxa on two tidal flats south of 

Malaquite Beach. The abundance of these organisms, ranging from rare (<3 per sample core) to 

abundant (>25 per sample core), were also reported (Table 42). The majority of taxa were 

classified as rare, while only one crustacean (Hargeria rapax) was considered abundant. Withers 

(1998) collected a total of 29 species at three tidal flats in the park. The algal flat at Yarborough 

Pass was most diverse with 25 species, while Mile Marker 45 and Dunn Ranch yielded 13 and 14 

species respectively (see Table 37). According to Withers (1998), this may be because northerly 

Laguna Madre flats are generally more diverse, with diversity typically decreasing as distance 

from Aransas Pass (near Corpus Christi) increases. Some research suggests that species richness 

on tidal flats may increase with proximity to seagrass meadows (Withers 1994). Regarding 

macroinvertebrate abundance, Withers (1998) noted that mollusks appear to be more abundant in 

locations with little to no algal cover.
186

Table 42. Abundance of macroinvertebrate taxa recorded by Withers (1996) at two sites within PAIS. A = 

abundant (average >25/core when present), C = common (11-25/core when present), U = uncommon (3-

10/core when present), R = rare (<3/core when present), * = found in only core during study.

Species Texaco Site Yarborough Pass

Phylum Nemertea R

Phylum Annelida, Class Polychaeta

Eteone heteropoda R

Exogone dispar R

Polydora spp. R* R

Prionospio heterobranchia R

Prionospio pinnata R

Nainereis laevigata R

Demonax micropthalmus R

Sabella sp. A R

Haploscoloplos foliosus U-R

Spio pettibonniae R*

Streblospio benedicti R*

Syllis cornuta R*

Capitellidae R*

Maldanidae R*

Phylum Mollusca, Class Bivalvia

Amygdalum papyrium R

Anomalocardia auberiana R

Tellina sp. R*

Phylum Arthropoda

Subphylum Crustacea

Corophium acherusicum R

Corophium louisanum U-R

Gammarus mucronatus R*

Hargeria rapax R A

Subphylum Hexapoda

Berosus sp. R

Canaceidae R R*

Ceratopogonidae R R-C

Dolichopodidae R R

Ephydridae R*

Melyridae R

Nabidae R* R*

Phylum Platyhelminthes

Turbellaria R
187

In a study of the park’s Gulf beaches, Rocha (1995) documented 40 total taxa from six phyla (see 

Table 38). Crustaceans were most diverse with 19 species, while polychaetes and mollusks 

contributed nine and six species respectively (Rocha 1995). Across all sample sites, subtidal 

zones supported the highest species diversity, as shown by Shannon’s diversity index (H’) (Table 

43). Crustaceans were also most abundant, comprising 41% of all individuals collected. 

Polychaetes made up 32% of individuals sampled and mollusks 25%. Cnidarians, nemerteans, 

and echinoderms were uncommon (Rocha 1995). The number of individuals of each species 

collected is presented in Table 38.

Table 43. Benthic macroinvertebrate species richness and diversity by beach zone (Rocha 1995). See 

Table 38 and Figure 20 for zone definitions and locations.

Supratidal Intertidal Subtidal

UBS MBS HS MS LS T1 B1 T2 B2

Number of species 7 5 15 18 21 28 27 26 24

Species diversity (H’) 0.96 0.73 1.99 2.55 2.81 2.52 2.42 2.75 3.01

Sissom et al. (1990) identified just six species in a study of three inland ponds within the park, 

primarily crustaceans (Table 44). Only one mollusk, the razor clam (Tagelus plebeius), was 

found in just one of the three ponds, although the population there was large (Sissom et al. 1990). 

Table 44. Macroinvertebrate species documented by Sissom et al. (1990) in three PAIS ponds.

Scientific name Common name Pond A Pond B Pond C

Mollusca

Tagelus plebeius razor clam x

Crustacea

Paleomonetes sp. grass shrimp x x x

Cladocera spp. water fleas x

Ostracoda spp. seed shrimp x x x

Copepoda spp. copepods x x x

Nematoda roundworms x

(Nov. & July)

x

(April only)

x

(April only)

Threats and Stressor Factors

Threats to the park’s macroinvertebrates include vehicle disturbance on beaches and tidal flats, 

habitat loss (e.g., flooding on tidal flats), soil compaction, harmful algal blooms (e.g., red tide), 

hazardous material spills, and alterations of water quality and/or hydrology. Vehicles can 

compact soil and cause other habitat destruction on the sensitive beaches and tidal flats of PAIS. 

This can impact macroinvertebrate communities, as shown in Withers (1996), where fewer 

species were found in tire tracks than in restored and control areas. Habitat loss is also caused by 

inundation of tidal flats, likely due to sea level rise (Withers and Tunnell 1998). Tidal flats 

decreased in area in the Laguna Madre/Corpus Christi Bay region between the 1950s and 1980; 

most of the lost tidal flats were replaced by open water or seagrass beds (Withers and Tunnell 

1998).

Macroinvertebrate populations are threatened by contaminants from oil and gas development and 

transport in and around PAIS (Rocha 1995, Withers 1996). For example, exposure to oil can 

cause mortality among bivalves, amphipods, and polychaetes (Withers and Tunnell 1998). 

Harmful algal blooms, such as red and brown tides, can also impact macroinvertebrates. The 

dinoflagellate in red tides produces a neurotoxin which can affect an animal’s nervous system, 
188

potentially causing paralysis (NOAA 2012). Drastic declines in mollusk, echinoderm, 

crustacean, and polychaete populations have been noted in the Gulf of Mexico following red 

tides (Dupont and Coy 2008). 

Data Needs/Gaps

According to Withers (1998, p. 32), short-term studies of benthic macroinvertebrate communities 

“tend to be inadequate for proper characterization.” The macroinvertebrate studies that have 

occurred in and near PAIS have typically provided single-year “snapshots” of the community at 

particular locations, which have not been revisited over time. Resampling the sites discussed in 

this assessment would provide insight into whether or not the macroinvertebrate community 

(diversity, density, etc.) has changed over time. Longer-term studies of several years would also 

provide a more detailed picture of the park’s macroinvertebrate populations. Additional data 

needs include further information on macroinvertebrate dispersal dynamics, colonization, and 

community succession (Withers and Tunnell 1998). Population recruitment should also be 

monitored over wider stretches of park shoreline (Stablein, written communication, 3 February 

2013). This information would provide not only a better understanding of the macroinvertebrate 

community, but also the wildlife populations that rely on them as a food source. 

Overall Condition

Distribution

The project team defined the Significance Level for distribution as a 3. While some 

macroinvertebrate distribution information is available for various locations within PAIS, studies 

have generally occurred for just one year and have not been repeated at these locations over time. 

In addition, all available data was collected over a decade ago. Therefore, it is not possible to 

determine if change is occurring or to assess current condition at this time. A Condition Level

was not assigned.

Density

The density measure was assigned a Significance Level of 3. As with distribution information, 

data for this measure are limited and do not allow for condition assessment. As a result, a 

Condition Level could not be assigned.

Species Diversity and Abundance 

The project team defined the Significance Level for this measure as a 3. While available data 

indicates some patterns in species diversity across various habitats within PAIS (Withers 1993, 

1998, Rocha 1995), the lack of comparable data over time prevents an assessment of current 

condition and trend (Condition Level = n/a). 

Weighted Condition Score

A Weighted Condition Score could not be calculated for macroinvertebrates at PAIS, as the 

current condition of all three measures is unknown.
189

Sources of Expertise

Wade Stablein, Biological Technician, PAIS

Martha Segura, I & M Program Coordinator, GULN
190

Literature Cited

Dupont, J. M., and C. Coy. 2008. Only the strong will survive: Red tides as communitystructuring forces in the eastern Gulf of Mexico. Pages 45-52 in Brueggeman, P., and N. W. 

Pollock (eds.). Diving for Science. Proceedings of the American Academy of Underwater 

Sciences, 27th Symposium, Dauphin Island, Alabama.

Environmental Protection Agency (EPA). 2012b. Macroinvertebrates and habitat. U.S. 

Environmental Protection Agency. http://water.epa.gov/type/rsl/monitoring/vms40.cfm

(accessed 13 December 2012).

Hill, G. W., and R. E. Hunter. 1976. Interaction of biological and geological processes in the 

beach and nearshore environments, northern Padre Island, Texas. Pages 169-187 in Davis, R.

A., and R. A. Ephington (eds.). Beach and nearshore sedimentation. Special Publication 

Number 24. Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma.

Jansson, B. 1967. The significance of grain size and pore water content for the interstitial fauna 

of sandy beaches. Oikos 18:311-322.

National Oceanic and Atmospheric Administration (NOAA). 2012. Ocean Facts: A “red tide” is 

a common term used for a harmful algal bloom. http://oceanservice.noaa.gov/facts/

redtide.html (accessed 4 January 2013).

Parker, R. H. 1959. Macro-invertebrate assemblages of central Texas coastal bays and Laguna 

Madre. Bulletin of the American Association of Petroleum Geologists 43(9):2100-2166.

Rabalais, N. 1976. Padre Island National Seashore checklist of crabs. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Rocha, D. D. 1995. Composition and distribution of the Gulf beach benthic invertebrate 

community at Padre Island National Seashore. National Park Service, Padre Island National 

Seashore, Corpus Christi, Texas.

Segura, M., R. Woodman, J. Meiman, W. Granger, and J. Bracewell. 2007. Gulf Coast Network 

Vital Signs monitoring plan. Natural Resource Report NPS/GULN/NRR-2007/015. National 

Park Service, Fort Collins, Colorado.

Shelton, C. R., and P. B. Robertson. 1981. Community structure of intertidal macrofauna on two 

surf-exposed Texas sandy beaches. Bulletin of Marine Science 31:833-842.

Sissom, S. L., R. D. Koehn, D. Lemke, R. R. Smith, C. Caudle, and F. Oxley. 1990. A baseline 

study of three ponds within the Padre Island National Park. National Park Service, Padre 

Island National Seashore, Corpus Christi, Texas.

Vega, M. E. 1988. The seasonal abundance and zonation of intertidal and subtidal infaunal 

macroinvertebrates on two Texas barrier island sandy beaches. Thesis. Corpus Christi State 

University, Corpus Christi, Texas.
191

Withers, K. 1993. Study to determine the abundance and distribution of benthic invertebrates and 

shorebirds on a north Padre Island blue-green algal flat. National Park Service, Padre Island 

National Seashore, Corpus Christi, Texas.

Withers, K. 1994. The relationship of macrobenthic prey availability to shorebird use of bluegreen algal flats in the upper Laguna Madre. Dissertation. Texas A&M University, College 

Station, Texas.

Withers, K. 1996. An evaluation of recovery of benthic invertebrate communities in vehicle 

tracks and restored oil and gas impacted areas on wind-tidal flats in the upper Laguna Madre, 

Padre Island National Seashore, Texas. National Park Service, Padre Island National 

Seashore, Corpus Christi, Texas.

Withers, K. 1998. Biological productivity of southerly wind-tidal flats within Padre Island 

National Seashore, Texas. National Park Service, Padre Island National Seashore, Corpus 

Christi, Texas.

Withers, K., and J. W. Tunnell. 1998. Identification of tidal flat alterations and determination of 

effects on the biological productivity of these habitats within the coastal bend. Texas Natural 

Resource Conservation Commission, Austin, Texas.

Withers, K., E. Smith, O. Gomez, and J. Wood. 2004. Assessment of coastal water resources and 

watershed conditions at Padre Island National Seashore, Texas. Technical report 

NPS/NRWRD/NRTR-2004/323. National Park Service, Water Resources Division, Fort 

Collins, Colorado.
192

Plate 27. Transect locations for Rocha’s (1995) Gulf beaches (Sites 1-4) and approximate locations of 

Withers’ (1996,1998) tidal flats sites along the Laguna Madre. The approximate locations of ponds 

sampled by Sissom et al. (1990) are also shown.
193

4.11 Reptiles

Description

PAIS provides habitat for a variety of reptiles, including snakes, lizards, turtles, and alligators. 

Snakes are the most diverse reptile group in PAIS, with 15 species documented in the park. 

There have been six lizard species documented in PAIS, two of which are the documented nonnative species in the park: the Mediterranean house gecko (Hemidactylus turcicus), and the sixlined racerunner (Aspidoscelis sexlineatus). 

The park is also home to the American alligator (NPS 2012). Four turtle species are found in 

PAIS, including the federally endangered Kemp’s Ridley sea turtle (Lepidochelys kempi). 

Chapter 4.14 provides a detailed discussion of the condition of sea turtles in PAIS. Reptiles play 

an important role in the food chain, as some species serve as both predator and prey species (ESI 

2011). Reptiles may also play a role in seed dispersal and act as pollinators for various plant 

species (TPWD 2012).

Measures

• Species abundance and diversity

• Species distribution

• Reproductive success

• Sex ratio

Reference Conditions/Values

The ideal reference condition for 

reptiles in PAIS would be the condition 

of reptile populations on Padre Island 

prior to the intensive cattle ranching and

grazing that began in the late 1880s and 

lasted until 1971. However, very little 

information is available from this 

historical period. The most 

comprehensive description of reptiles on 

the island comes from an inventory by 

Duran (2004). This description is used 

as the reference condition for this 

assessment.

Data and Methods

Duran (2004) conducted an inventory of reptiles and amphibians in PAIS between 2002 and 

2003. Major sampling efforts were focused on 17 different sites across grassland, wetland, 

wetland-grasslands, sparse vegetation, and dune habitats. Trapping methods included funnel-type 

minnow traps and drift-fence arrays; other inventory methods were visual encounters, auditory 

surveys, and road surveys. Many of the observations are linked with GPS points to get a general 

distribution in PAIS. During interpretation of Duran’s (2004) data, SMUMN GSS combined the 

Photo 18. Texas scarlet snake (Cemophora coccinea 

lineri) (Photo from Duran 2004).
194

western (Sistrurus catenatus tergeminus) and desert massasauga (S. c. edwardsii) observations. 

For this assessment, these species are identified as ‘massasauga.’

Current Condition and Trend

Species Abundance and Diversity

Duran (2004) observed 22 species in PAIS, three of which were non-native or introduced: the 

six-lined racerunner, Mediterranean house gecko, and the American Alligator. This inventory 

excluded sea turtles due to an ongoing sea turtle monitoring program within the park (Duran 

2004). NPS (2012b) listed a total of 25 species of reptiles present in PAIS, including snakes, 

lizards, turtles, tortoises, and alligators (Table 45). This list does not include one species 

mentioned by Duran (2004), the Texas indigo snake (Drymarchon corais erebennus); this 

species was noted as an unusual historical museum specimen and was not observed in the park 

during inventory. Duran (2004) noted the possible occurrence of two other species that were not 

observed during the study: the lined snake (Tropidoclonion lineatum) and the southern earless 

lizard (Holbrookia lacerata subcaudalis). These two species are documented by museum 

vouchers that indicate their presence in PAIS.

Table 45. Reptiles found in PAIS as documented by NPS (2012b). Bolded species were documented in 

the 2003 herpetological inventory (Duran 2004).

Scientific Name Common Name

Agkistrodon piscivorus leucostoma western cottonmouth

Alligator mississippiensis American alligator

Arizona elegans arenicola Texas glossy snake

Aspidoscelis sexlineatus six-lined racerunner

Caretta caretta loggerhead sea turtle

Cemophora coccinea lineri Texas scarlet snake

Chelydra serpentina serpentina common snapping turtle

Coluber constrictor oaxaca Mexican racer

Crotalus atrox western diamondback rattlesnake

Elaphe emoryi Great Plains rat snake

Eumeces obsoletus Great Plains skink

Hemidactylus turcicus Mediterranean house gecko

Heterodon platirhinos eastern hognose snake

Holbrookia propinqua propinqua keeled earless lizard

Lampropeltis triangulum annulata Mexican milksnake

Lepidochelys kempi Kemps Ridley sea turtle

Masticophis flagellum testaceus western coachwhip

Nerodia rhombifer diamondback water snake

Ophisaurus attenuatus western slender glass lizard

Scincella lateralis ground skink

Sistrurus catenatus massasauga

Tantilla gracilis flathead snake

Thamnophis marcianus marcianus checkered garter snake

Thamnophis proximus orarius Gulf Coast ribbon snake

Trachemys scripta elegans red-eared slider
195

Duran (2004) noted the keeled earless lizard (Holbrookia propinqua propinqua) as the most 

common reptile species in PAIS; this species has a Natural Heritage vulnerability status in the 

state (Cooper et al. 2005). Duran (2004) identified the western slender glass lizard (Ophisaurus 

attenuatus), western coachwhip (Masticophis flagellum testaceus), and the six-lined racerunner

as common species in the park. 

Several species observed during 

the survey are dependent on the 

permanent ponds on the northern 

side of PAIS, including the redeared slider (Trachemys scripta 

elegans), diamondback water 

snake (Nerodia rhombifer), 

checkered garter snake 

(Thamnophis marcianus), and the 

Gulf coast ribbon snake. The 

Texas scarlet snake was observed 

once during this survey, which 

was the first sighting of the 

species in PAIS; this snake is a 

state endangered species that has 

only been reported 10 times in its natural range (Duran 2004). Duran (2004) also identified the 

western diamondback rattlesnake as a potential conservation concern for PAIS due to few 

instances of live individuals observed and the number of dead individuals observed.

Species Distribution

Duran (2004) provides species distributions for 21 reptiles found in PAIS; a distribution was not 

provided for the Texas glossy snake. While the distribution was not defined for this species, 

Duran (2004) noted that grasslands are a documented habitat for the Texas glossy snake.

The identified distributions of the 21 species in Duran (2004) varied. Some reptile species were

found throughout the park, including the six-lined racerunner, western coachwhip, and keeled 

earless lizard. The western diamondback rattlesnake was uncommon during the inventory, but 

observations spanned both the northern and southern borders of the park. Two species were only 

trapped in trap 10, located in the middle of PAIS (see Plate 28). These species were the eastern 

hognose snake (Heterodon platirhinos) and the Texas scarlet snake. The Texas indigo snake, a 

species not observed during the inventory, was documented in 1985 near the Malaquite visitor 

center on the northern end of the seashore (Duran 2004). The Mexican racer (Coluber constrictor 

oaxaca) was only observed north of the park entrance in dune habitat. 

Duran (2004) found a majority of the reptile species were distributed throughout the northern 

half of the park. An American alligator was observed by the waste water treatment facility, 

which is located at the northern end of PAIS. Duran (2004) believed it must have been released 

near the area because PAIS is not native habitat for this species. Jim Lindsay (Pers. Comm.) 

clarified the alligator washed up on the beach in PAIS after a major storm and was placed in 

Pond C. It later moved to the sewage treatment pond and grew quite large. Thus, around 2007, it 

was captured and given to the zoo in San Antonio. Some reptiles are distributed in the northern 

Photo 19. Keeled earless lizard (Photo from Duran 2004).
196

half of the park due to dependence on permanent freshwater ponds, such as the red-eared slider, 

diamondback water snake, checkered garter snake, massasauga (Sistrurus catenatus), and gulf 

coast ribbon snake. The Mediterranean gecko was mostly observed on buildings including the 

ranger station and visitor center. The flathead snake (Tantilla gracilis), ground skink (Scincella 

lateralis), Mexican milk snake (Lampropeltis triangulum annulata), Great Plains rat snake 

(Pantherophis emoryi), Great Plains skink (Eumeces obsoletus), and western slender glass lizard 

were found in traps in the northern half of the park. The Great Plains skink was also found in trap 

17 at the southern end of the park.

Reproductive Success

Currently, no data are available that characterize the reproductive success of reptiles in PAIS. 

Sex Ratio

Currently, no data are available that characterize sex ratio in PAIS reptile populations. 

Threats and Stressor Factors

PAIS staff identified a number of threats to reptiles in the park, including flooding and salt-water 

inundation of wetlands and ponds, erosion, human disturbance, vehicle strikes, hazardous 

materials such as chemical spills (hazmat), extended drought, predation, habitat loss, disease, and 

oil and gas development.

Many threats and stressors to reptiles in the park can be categorized as anthropogenic. Park 

infrastructure and development increases the incidence of vehicle strikes on roads and leads to 

fragmentation or loss of habitat (NPS 2001). Reptiles may also be struck by vehicles traveling 

along the beaches or along the gravel roads that run through the grasslands and mudflat areas of 

the park. Oil and gas development within the park may result in additional habitat loss. 

Explosive charges detonated during gas exploration and drilling efforts may cause injury or 

death to burrowing reptiles (NPS 2001). The risk of chemical spills also increases in areas with 

oil and gas development.

The Gulf of Mexico currents continually carry and deposit marine debris and trash onto the PAIS 

beaches. Sources of this trash include the shrimping industry, offshore oil platforms, and debris 

from storm activity (Cappiello 2003). According to a nationwide survey in 1993, PAIS had more

trash than any other NPS unit along the U.S. coast (Cappiello 2003). Along with that distinction, 

PAIS is also the only NPS unit to have its own hazmat team (Cappiello 2003). The debris and 

pollution on land and in the water can result in suffocation, sickening, or strangulation of 

reptiles. 

Very few reptile species are salt tolerant. Most reptiles cannot survive in seawater, so saltwater 

inundation of wetlands and ponds in PAIS can be a substantial stressor (Karraker 2007). Reptiles

inhabiting the southern seashore may be at a higher risk than those on the northern seashore of 

PAIS due to the relative sea level rise and erosion rates occurring in the southern end of the park. 

According to White et al. (2007), the southern seashore has been eroding at a rate of 1.2 to 2.4 

m/year (3.0 to 7.9 ft/year). White et al. (2007) also stated that between 1949 and 1999, the 

relative sea level had risen at a rate of 3.38 mm/year.
197

Natural stressors for reptiles include drought, predation, and disease. Reptiles use ponds to 

remain cool when temperatures become too hot. Some species of reptiles are more dependent on 

ephemeral and permanent ponds, and may become stressed during drought periods. Reptiles that 

persist in PAIS seem better adapted to such conditions, yet are still susceptible during extended 

periods of drought.

Although disease in the reptile population has not been investigated in depth in PAIS, diseases 

could pose a significant threat to the park population of reptiles should they spread through the 

region. Ranavirus infections are known to impact amphibian populations by causing death in 

larvae or recently metamorphosed individuals in the park (Stablein, written communication, 22 

October 2012); diseases affecting reptiles have not been researched or documented in PAIS to 

date. 

Data Needs/Gaps

Very little information is available that characterizes reptile population parameters in PAIS. 

Duran (2004) describes only the abundance and diversity of species observed. Additionally, this 

survey is nearly a decade old. Current and long-term annual or biannual monitoring efforts 

would provide information necessary to determine trends in population sizes or diversity, 

reproductive success, and population structure (sex ratio). Prevalence of disease in reptiles in the 

park is not well understood. Periodic monitoring for detection of disease would help managers 

understand possible threats to populations in the park. Annual monitoring of the populations in 

the park will allow for a more accurate assessment of population condition.

Overall Condition

A number of species of snakes, lizards, and turtles have been documented in PAIS; however, 

there is no established annual herpetological monitoring in the park, and very little information 

characterizing reptile populations is currently available. While the NPS has a record of 

confirmed species in the park, this list does not allow for estimates of abundance, reproductive 

success, or other population parameters. Thus, it is not possible to assess the condition of reptiles 

in PAIS at this time. 

Species Abundance and Diversity

The project team defined the Significance Level for species abundance and diversity as a 3. 

Duran (2004) documented 22 species of reptiles in PAIS, some of which were more common or 

abundant than others (such as the keeled earless lizard and western coachwhip). Because a 

historical reptile inventory of PAIS does not exist, it is not possible to determine how diversity 

and abundance of reptile species may have changed over time. Several species present in the 

park are considered to be of conservation concern due to rarity or restriction of range. As a 

result, the measure was assigned a Condition Level of 1 or of low concern.

Species Distribution

The project team defined the Significance Level for species distribution as a 3. Limited data are 

available that characterize reptile species distributions in PAIS. Therefore, it is not possible to 

assign a Condition Level for this measure. 
198

Reproductive Success

The project team defined the Significance Level for reproductive success as a 3. There are no 

available data characterizing the reproductive success of reptiles in PAIS. Because of this data 

gap, a Condition Level for this measure was not assigned.

Sex Ratio

The project team defined the Significance Level for sex ratio as a 3. There are no available data 

characterizing the sex ratio of reptiles in PAIS. Because of this data gap, a Condition Level for 

this measure was not assigned.

Weighted Condition Score

A Weighted Condition Score for reptiles in PAIS was not assigned because >50% of the 

measures had unknown Condition Levels.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
199

Literature Cited

Cappiello, D. 2003. A trashed treasure/ inundated with garbage, Padre Island Seashore is the 

only national park with its own hazmat team. The Houston 

Chronicle. http://www.chron.com/CDA/archives/archive.mpl/2003_3712973/a-trashedtreasure-inundated-with-garbage-padre-is.html (accessed 7 June 2012).

Cooper, R. J., S. B. Cederbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore: Final report. Warnell School of Forest Resources, University of 

Georgia, Athens, Georgia. 

Duran, C. M. 2004. An inventory of reptiles and amphibians of Padre Island National Seashore, 

San Antonio Missions National Historical Park, and Palo Alto Battlefield National Historic 

Site. Texas Conservation Data Center, The Nature Conservancy, San Antonio, Texas.

Endangered Species International (ESI). 2011. Ecological role of reptiles. Endangered Species 

International. http://www.endangeredspeciesinternational.org/reptiles3.html (accessed 17 

May 2012).

Karraker, N. E. 2007. Are embryonic and larval green frogs (Rana clamitans) insensitive to road 

deicing salt? Herpetological Conservation and Biology 2(1):35-41.

National Park Service (NPS). 2001. Oil and gas management plan. National Park Service, Padre 

Island National Seashore, Texas.

National Park Service (NPS). 2012. NPSpecies search. IRMA Portal 

Online. https://irma.nps.gov/App/Species/Search (accessed 1 March 2012).

Texas Parks and Wildlife Department (TPWD). 2012. Wildlife fact 

sheet. http://www.tpwd.state.tx.us/huntwild/wild/species/ (accessed 17 May 2012).

White, W. A., T. A. Tremblay, R. L. Waldinger, and T. R. Calnan. 2007. Status and trends of 

wetland and aquatic habitats on Texas barriers: Upper Coast Strandpain-Chenier System and 

Southern Coast Padre Island National Seashore. Texas General Land Office, National 

Oceanic and Atmospheric Administration, Austin, Texas.
200

Plate 28. Trap locations used during 2002-2003 inventory of reptiles and amphibians in PAIS (Duran 

2004).
201

4.12 Sea Turtles

Description

Five species of sea turtles have been documented nesting at PAIS: the leatherback (Dermochelys 

coriacea), green (Chelonia mydas), Kemp’s ridley (Photo 20), hawksbill (Eretmochelys 

imbricata), and loggerhead (Caretta caretta). All of these species are federally listed as either 

threatened or endangered (NPS 2012). Additionally, PAIS waters provide important marine 

habitat for these species. Nearshore waters of the Gulf of Mexico in and adjacent to the park are 

used by various life stages and species for dispersal, foraging, and migration. Waters of the 

Mansfield Channel are used by various species to travel between the Gulf of Mexico and the 

Laguna Madre, and by juvenile green and hawksbill turtles for foraging and resting. Waters of 

the Laguna Madre are also used for foraging, primarily by juvenile green turtles. 

Sea turtles are considered 

keystone species, meaning the 

species plays a critical role in 

ecological community structure 

and its impact is greater than 

expected based on relative 

abundance or biomass (Mills et al. 

1993). Sea turtles also help 

maintain marine ecosystems. For 

instance, green sea turtles graze

on seagrass, which prevents 

overgrowth and increases seagrass 

productivity (Wilson et al. 2010). 

Hawksbill sea turtles feed on 

marine sponges, which normally 

out-compete coral species, and 

thus allow new coral species to 

colonize and grow (Wilson et al. 2010). Leatherback sea turtles consume large amounts of 

jellyfish daily, which helps regulate jellyfish populations. Loggerhead sea turtles prey on 

crustaceans; their feeding behavior of crushing and consuming the crustaceans’ shells helps 

increase nutrient recycling on the ocean bottom (Wilson et al. 2010).

The Kemp’s ridley sea turtle is the smallest species of sea turtle that inhabits the Gulf of Mexico, 

and is the most critically endangered sea turtle species in the world (NMFS, USFWS, and 

SEMARNAT 2011). Nearly all nesting is limited to beaches on the western coast of the Gulf of 

Mexico, with the majority of nests occurring in Tamaulipas and Veracruz, Mexico (NMFS, 

USFWS, and SEMARNAT 2011). Nesting also occurs regularly along coastal Texas, 

particularly along Padre Island and including PAIS (NMFS, USFWS, and SEMARNAT 2011). 

Kemp’s ridley nests comprise a majority of the documented sea turtle nests within the park, and 

PAIS supports more Kemp’s ridley nests than any other single location in the United States 

(Shaver 1999, 2005a). Populations sharply declined in the Gulf of Mexico in the mid-1900s, 

primarily due to overharvesting of adults for meat and leather by commercial fishers and 

harvesting of eggs believed to be an aphrodisiac; incidental capture in fishing and shrimping nets 

is also believed to have caused impacts to populations (Shaver 1987, Caillouet et al. 1996). 

Photo 20. Kemp’s ridley sea turtle (NPS photo).
202

Reintroduction and monitoring efforts along the Gulf beach of Padre Island National Seashore 

(Imprinting and Headstarting program) started in 1978 (Shaver 2005a). Each summer from 1978-

1988, a collection of Kemp’s ridley eggs from Rancho Nuevo, Tamaulipas, Mexico, where the 

majority of Kemp’s ridley turtles have historically nested, were transported to the incubation 

facility in PAIS in an effort to bolster population numbers and form a secondary breeding colony 

of this native species there (Shaver 1987, 1990, 1992a, 2005a, b).

Measures

• Number of nests per year

• Number of hatchlings released per year

• Diversity

Reference Conditions/Values

The reference conditions for the sea turtles in PAIS are the established management objectives 

for nesting success and population growth within park boundaries for each species of sea turtle 

present in the park. The objectives established by the U.S. National Marine Fisheries Service 

(NMFS) and USFWS recovery plans for each species provide context on the overall goals for 

species recovery and population growth along the Gulf coast. These plans are used to understand 

potential reference conditions or target management objectives for each species present and 

nesting within PAIS.

Kemp’s Ridley Sea Turtle

The NMFS, USFWS, and SEMARNAT (2011) bi-national recovery plan for the Kemp’s ridley 

sea turtle describes the beaches along the Texas coast as the second most vitally important 

nesting grounds for this species behind the beaches of the Tampaulipas and Veracruz region in 

Mexico. Kemp’s ridley nesting has increased in Mexico since the mid-1980s and in Texas since 

the mid-1990s. The Kemp’s Ridley Recovery Plan reported that the number of nests found was 

increasing at 12-18% per year and projected that this rate of increase would continue for the next 

several years (NMFS, USFWS, and SEMARNAT 2011). However, the numbers of nests found 

both in Mexico and Texas plateaued from 2009-2013 and fell short of those projections from 

2010-2013. About 55-65% of Kemp’s ridley nests found in Texas are located on North Padre 

Island, and about 50-55% of all Kemp’s ridley nests in the U.S. are located on beaches within 

PAIS (NMFS, USFWS, and SEMERNAT 2011). 

Park waters are also used by Kemp’s ridleys of various life stages. Since 1980, sea turtles found 

stranded (i.e., washed ashore or floating sick, weakened, or dead) at PAIS have been documented 

by PAIS staff and volunteers in conjunction with the national Sea Turtle Stranding and Salvage 

Network (STSSN) (Shaver 1998, 1999). Incidental capture in shrimp trawls has been a threat to 

sea turtles in Texas waters (Caillouet et al. 1996, Shaver 2005a) and the seasonal closure of 

nearshore Gulf of Mexico waters in and adjacent to the park (out to 5 nautical miles) established 

by TPWD has aided with conservation efforts in south Texas (Lewison et al. 2003). Nearshore 

Gulf of Mexico waters provide vital habitat for adult female and male Kemp’s ridley turtles 

during the nesting season and during migration between foraging areas in the northern Gulf of 

Mexico and nesting areas in Texas and Mexico (Shaver et al. 2005, 2013; Shaver and Rubio 
203

2008). The only observation of Kemp’s ridleys mating in Texas waters was in the Mansfield 

Channel (Shaver 1992b). 

Management of the Kemp’s ridley sea turtle population in PAIS is a high priority because of its 

secondary breeding colony status. Objectives for the population in the park are based on the 

number of nests located and protected each year and number of hatchlings successfully incubated 

and released each year. The target management action is to continue to protect nests and release 

hatchlings to contribute to the recovery of the species, so that hopefully someday it can be 

removed from the threatened and endangered species list. 

Green Sea Turtle

Green sea turtles are a global species with nesting and foraging grounds found all over the world. 

South Texas coastal waters provide important foraging habitat for this species, particularly for 

juveniles (Shaver 1994, 2000). In fact, the Laguna Madre and South Texas jetty systems 

(including the Mansfield Channel at PAIS) are among the most important developmental habitats 

for juvenile green turtles in the western Gulf of Mexico (Shaver 1994, 2000). Based on increases 

in observations of green turtles swimming in park waters and stranding on park beaches, it is 

apparent that the green turtle population has increased dramatically in south Texas in recent 

years (Shaver 2000, 2012, 2013). Although most nesting grounds for the Atlantic population of 

green sea turtles are located in Florida, Mexico, and Central and South America, PAIS and South 

Padre Island have documented several nests since the first recorded nest at PAIS in 1987 (Shaver 

1989, 2012, 2013). From 2002-2013, between one and 13 nests have been confirmed in PAIS 

each year, but some nests may have been missed due to nesting later in the season after main nest 

patrolling efforts were concluded (Shaver 2012, 2013; Donna Shaver, Chief, Division of Sea 

Turtle Science and Recovery, PAIS, email communication, 23 August 2013). The number of 

juveniles found in Texas waters far exceeds the production of hatchlings from Texas beaches; the 

likely origin for juveniles inhabiting Texas waters is Mexico, Florida, or the Caribbean 

(Anderson et al. 2013). The reference condition for green sea turtles in PAIS is a management 

target of maintaining each season the average number of nests confirmed in the park over the last 

decade.

Loggerhead Sea Turtle

Loggerhead sea turtles nest from Texas to as far north as Virginia on the east coast of the U.S., 

with the vast majority of nesting occurring on the Florida coast (NMFS and USFWS 2008).

Several loggerhead nests are found on PAIS beaches most years, and over the last 10 years the 

number of nests documented has ranged from 0-12 nests per year (Shaver 2012, 2013; Shaver, 

email communication, 23 August 2013). Some nests may go undetected due to nesting season 

extending past the main nest patrolling season. Various life stages of loggerheads have been 

documented stranded at PAIS and use nearshore Gulf of Mexico waters inside and adjacent to 

the park (Shaver 1998, 1999; Bjorndal et al. 2001, Bowen et al. 2004). The reference condition 

for loggerhead sea turtles in PAIS is a management target of maintaining each season the 

average number of nests confirmed in the park over the last decade. 

Leatherback Sea Turtle

Leatherback sea turtles are found worldwide, but their primary nesting grounds in the Atlantic 

Ocean are located mainly in the Caribbean and northern coast of South America, although some 

nests have been documented in Florida and the Gulf of Mexico coastline in Mexico (NMFS and 
204

USFWS 1992). Leatherback nests have only been confirmed in Texas at PAIS. In 2008, a 

leatherback nest was documented at PAIS (Shaver 2009); this nest represents the only time the 

species has been recorded nesting in the state since the 1930s (NPS 2013b; Shaver, email 

communication, 23 August 2013). Thus, the reference condition for leatherback turtles in PAIS 

is occasional sightings of adults and infrequent or rare incidence of nesting in the park.

Hawksbill Sea Turtle

Hawksbill sea turtles are found primarily in the Gulf of Mexico and the Caribbean. Primary 

nesting locations in the U.S. include Puerto Rico, the Virgin Islands, and the coast of Florida 

(NMFS and USFWS 1993). The first hawksbill turtle nest recorded on the Texas coast was 

documented at PAIS in 1998 (Shaver 1999b) and it remains the only documented hawksbill nest 

on Texas coastline (NPS 2013a). Their occurrence at PAIS is infrequent with only one 

documented case of nesting. However, park waters of the Gulf of Mexico and Mansfield Channel 

are used by juvenile hawksbill turtles and observations of stranded or captured individuals are 

noted when they occur (Shaver 1999b, 1999c, 2000; Bowen et al. 2007). Thus, the reference 

condition for hawksbill turtles in PAIS is occasional sightings of individuals and rare incidence 

of nesting in the park.

Data and Methods

Shaver (2013) summarizes data on recorded Kemp’s ridley sea turtle nests located along the 

Texas Coast from 1978 to 2012, which is based on the monitoring efforts of Dr. Donna Shaver at 

the Sea Turtle Recovery Project at PAIS. Eggs from most confirmed nests from North Padre 

Island northward on the Texas coast are transported to an incubation facility in PAIS, where they 

are hatched. Eggs from some nests found at the southern end of PAIS are transported to large 

screen enclosures called corrals. Hatchlings from the incubation facility and corrals are released 

on the beach at PAIS and allowed to crawl into the Gulf of Mexico and go free. The nesting 

records include year observed, number of nests, number of eggs, and number of hatchlings 

released. 

Annual turtle nesting and stranding reports are produced as part of an on-going incubating and 

nesting monitoring program at PAIS (Shaver 2005a, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 

2013). These reports detail the efforts of several organizations and participants, including NPS, 

USFWS, Animal Rehabilitation Keep (ARK) in Port Aransas, TX, Sea Turtle, Inc., Texas A&M 

University at Galveston, TPWD, STSSN, and others. During the main sea turtle nesting season 

(April to mid-July), these groups patrol Gulf of Mexico beaches in search of nesting Kemp’s 

ridley turtles and sea turtle nests. Several thousand kilometers of beach along the Texas coast are 

patrolled each year as part of this effort. Nests are also found as a result of reports from others 

working on the beach or the public. Patrollers identify species, locate and examine nests, relocate 

eggs from a majority of nests to an incubation facility or corral, and document any turtle 

strandings during each season. Hatchlings are also released from the incubation facility and 

corrals and their releases are documented. Sea turtle strandings occur year-round. Stranded sea 

turtles are located during nesting patrols, while PAIS staff members and volunteers conduct other 

functions on the beach, and in response to reports from visitors. 
205

Current Condition and Trend

Number of Nests

Shaver (2013) reports a total of 1,384 Kemp’s ridley sea turtle nests were documented between 

1996 and 2012. Patrolling for nests and protection efforts began at PAIS in 1986, but were 

limited for the first decade (NMFS, USFWS, and SEMARNAT 2011). Patrolling and protection 

efforts began on other Texas beaches south of PAIS in 1999, and began north of North Padre 

Island in following years. This limited patrol coverage may be a reason for the small number of 

nests that were found and reported before patrols became comprehensive. The number of nests 

increased significantly between 1996 and 2012. Table 46 displays the number of Kemp’s ridley

nests found on the Texas coast; most of these nests were taken to the incubation facility or 

corrals in Texas for protection from predation and other threats.

Table 46. Number of Kemp’s ridley sea turtle nests located along the Texas coast, 1996-2012 (Shaver 

2013).

Year Number of Nests

1996 6

1997 9

1998 13

1999 16

2000 12

2001 8

2002 38

2003 19

2004 42

2005 50

2006 102

2007 128

2008 195

2009 197

2010 141

2011 199

2012 208

Annual reports by Shaver (2005-2013) summarize data on the number of confirmed turtle nests

recorded across several years of patrolling along the Texas coast. Four sea turtle species were 

observed throughout the last nine years (2004-2012) of patrolling efforts, including Kemp’s 

ridley, green, loggerhead, and leatherback sea turtles. Table 47 displays the reported nest 

numbers for three of the four turtle species documented along the Texas coast and within PAIS. 

The total number of Kemp’s ridley nests has increased from 2004-2012, with the exception of 

2010. NMFS, USFWS, and SEMARNAT (2011), the bi-national recovery plan for the Kemp’s 

ridley sea turtle, reports a total of 911 Kemp’s ridley nests documented along the Texas coast 

from 2002 and 2010. There were several green and loggerhead sea turtle nests during the years 

of reports. NMFS and USFWS (2008) reports an average of 906 loggerhead nests documented in 

the northern Gulf of Mexico recovery unit between 1995 and 2004, an area that includes the 

Texas coast and PAIS beaches; however, very few of these are confirmed in Texas or PAIS 

annually.
206

Table 47. The total number of Kemp’s ridley, green, and loggerhead sea turtle nests confirmed in PAIS 

and along the Texas coast, 2004-2012 (Shaver 2012, 2013). 

Kemp’s ridley Green Loggerhead

Report Year In PAIS Total Texas In PAIS Total Texas In PAIS Total Texas

2004 22 42 1 1 1 1

2005 27 50 4 4 2 3

2006 64 102 2 2 1 2

2007 73 128 2 3 4 6

2008 93 195 4 5 1 3

2009 117 197 1 1 0 0

2010 74 141 5 5 9 9

2011 117 199 6 3 0 0

2012 106 209 6 8 1 5

NPS (2013a) also summarizes hawksbill nesting activity along the Texas coast based on a report 

by Shaver (1999b). The first hawksbill nest to be reported and confirmed in PAIS occurred in 

1998 (Shaver 1999b); since then, no hawksbill nests have been confirmed in Texas (NPS 2013a).

Only one confirmed leatherback turtle nest has been confirmed along the Texas coast in recent 

years, which was documented in PAIS in 2008 (Shaver 2009).

Number of Hatchlings Released

Annual reports by Shaver (2005-2013) summarize data on the total number of turtle eggs 

collected along the Texas coast and hatchlings reared and released from the incubation facility at 

PAIS and corrals in Texas from 2004-2012 (Note that reports are authored the year following the 

calendar year of survey efforts). Table 48 displays the total number of eggs collected and 

hatchlings released for three turtle species observed during the 2004-2012 monitoring seasons. A 

total of 118,916 Kemp’s ridley eggs were recorded in nests along the Texas coast and 99,746

hatchlings were released from the PAIS incubation facility and corrals in Texas between 2004 

and 2012. A few eggs are found broken each year during egg collection and are not transported 

for incubation; the broken eggs are included in the annual totals. Likewise, a total of 3,985 green 

sea turtle eggs were recorded, from which 3,076 hatchlings were successfully released. Finally, a 

total of 3,211 loggerhead eggs were recorded, with 2,525 hatchlings successfully released into 

the Gulf of Mexico. In 2008, one leatherback nest was recorded; however, none of the six eggs 

hatched. 
207

Table 48. The total number of Kemp’s ridley, green, and loggerhead sea turtle eggs recorded in nests 

that were collected and taken to the PAIS incubation facility or Texas corrals and total number of 

hatchlings released to the Gulf of Mexico, 2004-2012 (Shaver 2005-2013). This includes the few eggs 

found broken upon collection of nests.

Kemp’s ridley Green Loggerhead

Report Year Total Number 

of Eggs

Total 

Released

Total Number 

of Eggs

Total 

Released

Total Number 

of Eggs

Total 

Released

2004 3,928 3,298 113 105 90 88

2005 4,700 3,402 428 380 221 117

2006 9,717 7,475 300 189 263 190

2007 12,565 10,594 341 298 638 579

2008 17,933 15,819 547 496 367 333

2009 17,518 14,506 137 130 0 0

2010 13,584 11,983 642 529 1,070 881

2011 18,904 16,092 613 499 0 0

2012 20,067 16,577 864 450 562 337

Based on Shaver (1999b), NPS (2013a) summarized data on the number of hawksbill turtle eggs 

collected and hatchlings released from PAIS. The one hawksbill nest recorded at PAIS in 1998 

contained 140 eggs, of which 132 hatchlings were released into the Gulf of Mexico. 

Diversity and Distribution

Nests and stranded individuals of all five species have been confirmed at PAIS since 1998 

(Shaver 1989, 1994, 1998, 1999b, 1999c, 2000, 2005a, 2005b, 2006-2013; Shaver and Rubio 

2008; Shaver et al. 2013). Park beaches are used for nesting and park waters are used for 

dispersal, foraging, and migration.

Shaver serves as Texas Coordinator of the STSSN and her annual reports summarize stranded 

sea turtles and sea turtle nests recorded in Texas. In these reports, distribution is not specifically 

stated for PAIS, apart from whether the nests were observed within PAIS or at some other 

location along the Texas Coast. However, distribution of stranded sea turtles has been recorded 

in conjunction with the STSSN since 1980; data are aggregated by Shrimp Statistical Zone and 

posted on the NOAA website (http://www.sefsc.noaa.gov/species/turtles/strandings.htm). 

Locations of sea turtle nests found at PAIS have been recorded since the mid-1980s. Distribution 

data for green and Kemp’s ridley turtles have also been recorded through satellite tracking 

(Shaver 2000, Shaver and Rubio 2008, Shaver et al. 2013).

Kemp’s ridley and green turtles are the most numerous and widespread species at the park, 

whereas the leatherback is the least numerous and widespread. Nesting by Kemp’s ridley and 

loggerhead turtles occurs along the entire Gulf of Mexico beachfront. In contrast, nesting by 

green and leatherback turtles is concentrated on Big Shell and Little Shell beaches. Strandings of 

all five species occur along the entire Gulf of Mexico beachfront of PAIS. The green turtle is the 

most numerous species in the Laguna Madre and Mansfield Channel, but occasionally the other 

four species are found there too.

According to NMFS and USFWS (1992), there were 77 hawksbill turtle observations along the 

Texas coast between 1972 and 1984; however, it is not indicated whether any of these 

observations were specific to PAIS. These were likely stranded individuals. Hawksbills are 

found stranded at PAIS annually, and a large portion of those 77 were likely from PAIS. Most of 
208

the hawksbills stranded at PAIS are juveniles. Juvenile hawksbills were also captured during 

directed capture netting conducted at the Mansfield Channel during the 1990s (Shaver 2000). 

Only one hawksbill has been recorded nesting in PAIS, which occurred in 1998 (Shaver 1999b, 

as summarized by NPS 2013a). There have been no documented observations of this species 

nesting before or after this time. 

Threats and Stressor Factors

There are several threats to sea turtles in the park. PAIS park staff members have identified 

seven anthropogenic and four natural threats that have occurred or are presently occurring in the 

park: beach driving, boat traffic, oil spills, dredging, poaching, commercial and recreational 

fishing, marine debris, extreme weather events, harmful algal blooms, predation, and disease.

A majority of the threats to sea 

turtles in the park are 

anthropogenic. Marine debris is 

a serious threat to sea turtles in 

PAIS. Sea turtles can become 

entangled, ingest debris, and be 

smothered by floating and nonfloating debris (Plotkin and 

Amos 1990, NOAA 2006). 

Entanglements have been 

reported for all five species 

found in or near PAIS (Plotkin 

and Amos 1990). Photo 21

illustrates how entanglement 

could decrease turtle mobility. 

Some sea turtle species (e.g., loggerhead, leatherback) may be more vulnerable to debris 

ingestion because marine debris may resemble their natural prey (Tomas et al. 2002). For 

example, floating plastic bags may be mistaken for jellyfish (a prey species), which results in 

many mortalities caused by the ingestion of or suffocation with plastic. However, Kemp’s ridley, 

green, and hawksbill turtles, especially small individuals, also consume marine debris (Shaver 

1991, Shaver and Plotkin 1998). Commercial and recreational fishermen are largely responsible 

for discarding marine debris and accidentally snagging and drowning turtles in shrimp nets 

(Plotkin and Amos 1990). Beach driving is another anthropogenic threat to sea turtles. In Texas 

outside of PAIS, adult loggerhead and Kemp’s ridley turtles have been crushed and killed by 

passing vehicles, as have hatchling Kemp’s ridleys that were on their way to the sea (Shaver, 

email communication, 23 August 2013). Oil spills are also a threat to sea turtles. These spills can 

affect turtles at every stage of life, but tend to be more damaging at earlier life stages (Milton et 

al. 2003). According to Milton et al. (2003), petroleum hydrocarbons bind to lipids, which occur 

in high concentrations in young turtles. If oil contamination reaches a nest and alters the gas 

exchange, nest temperature, and sand moisture, it will cause an increase in mortality of the 

embryos (Milton et al. 2003). If oil reaches foraging grounds turtles can ingest the oil 

incidentally when aiming to consume flora or fauna there. Oil spills can also suffocate seagrass 

and other benthic organisms, which are important food sources for sea turtles (NMFS and 

USFWS 1991). According to NMFS and USFWS (1991), a 3-month hopper dredging operation 

Photo 21. Hawksbill sea turtle entangled in rope (NPS photo).
209

is to blame for the mortality of about 100 green sea turtles. Water recreation, in particular 

boating, is a common threat to sea turtles, and coastal development may increase boat traffic near 

PAIS. Injuries caused by propellers or colliding with boats may result in the stranding and death 

of sea turtles. According to NMFS and USFWS (1991), there were 111, 175, and 179 stranded 

turtles with propeller or collision injuries each year from 1986 to 1988 in the Gulf of Mexico. 

Those numbers accounted for less than 10% of the total number of stranded turtles documented 

in the Gulf of Mexico and Atlantic (NMFS and USFWS 1991). Poaching is a potential threat to 

sea turtles in PAIS. It is illegal to harvest eggs as well as juvenile and adult sea turtles from U.S. 

beaches and waters, but harvesting still occurs (NMFS and USFWS 1991, 1992, 1993, 2008;

NMFS, USFWS, and SEMARNAT 2011). In the past, some sea turtle species were popular 

items in culinary dishes (NMFS and USFWS 1991). Between 1983 and 1989, 28 arrests were 

made for possession of whole turtles and turtle parts (NMFS and USFWS 1991).

The natural threats to sea turtles include extreme weather events, harmful algal blooms, 

predation, and disease. Every few years, severe cold fronts affect south Texas and cause 

hypothermic stunning of green turtles, primarily in south Texas bays and passes (Shaver 2000). 

Hypothermic stunning during these periods of freezing air temperatures is the most significant 

threat to green turtles in Texas waters (Shaver 2000). If green turtles are not quickly found 

during these events and transported to rehabilitation facilities, they will succumb due to exposure 

or predation. These hypothermic stunning events can involve hundreds of green turtles found at 

the park, many of which are located alive. Pike and Stiner (2007) suggest that seasonal tropical 

storms can adversely affect the reproductive season of sea turtles, as well as threaten turtles at 

various life stages. For instance, eggs and hatchlings seem to be most vulnerable to exposure due

to limited or no mobility. Seasonal storms can cause high tides and salt-water inundation of turtle 

nests, disrupting the oxygen exchange among the turtle eggs (Pike and Stiner 2007). High tides 

can also occur at other times that are not associated with storm events. High tides can erode the 

beach, removing sand on top of the nest and sometimes washing the eggs out to sea. The saltwater can also cause the loss of water within the egg, which could result in decreased hatching 

success. Some sea turtle species may be affected more than others by seasonal weather events, in 

that some species nest during times with higher incidence of extreme events (Pike and Stiner 

2007). For example, leatherback sea turtles lay their eggs earlier in the nesting season before the 

peak of the storm season and, thus, the nests may be less affected by weather events; conversely, 

green sea turtle reproductive success may be more affected due to nesting later in the summer, 

during peak tropical storm and hurricane season (Pike and Stiner 2007). However, efforts to 

protect eggs in the PAIS incubation facility and south Texas corrals have mitigated substantial, 

potential recruitment losses due to extreme weather events. 

Harmful algal blooms (HAB) may pose a threat to sea turtles, although blooms have not been 

documented as being a substantial problem in PAIS. Algal blooms become harmful when they 

release toxins, causing such issues as respiratory and neurological problems (EPA 2012). HABs 

can also cause low-oxygen zones and block out sunlight, resulting in fishkills and dying seagrass, 

both of which are important food sources for some sea turtle species (EPA 2012). The longer 

HABs persist, the more these events can adversely affect the surrounding habitat and organisms. 

Predation is a threat to sea turtles in PAIS; however, predation rates decrease as turtles grow and 

mature. Nest depredation seems to be the most common form of predation on sea turtles. The 

main predators of eggs and hatchlings on the beach at PAIS include raccoons, coyotes, skunks, 
210

badgers, ghost crabs, and ants (NMFS and 

USFWS 1991; NMFS, USFWS, and 

SEMARNAT 2011). Seabirds and a 

variety of fish typically prey on hatchlings 

and young turtles. Larger, mature turtles 

experience less predation, but according to 

NMFS and USFWS (1991), tiger sharks

have been observed preying upon adult 

green sea turtles. 

Disease is the last acknowledged natural 

threat to sea turtles in PAIS. One disease 

that has been observed on green sea turtles 

in the park is fibropapilloma; a condition 

causing growths or tumors on the turtles’ 

soft tissues (Tristan et al. 2010). Photo 22

illustrates the growths that occur on green turtles affected by fibropapillomatosis. This disease 

can occur both externally and internally (Arthur et al. 2007). Normally this condition is not fatal,

but if the growths occur on the eyes, mouth, and flippers they can negatively affect feeding and 

mobility (Tristan et al. 2011).

Data Needs/Gaps

Current and long-term data are available for the number of nests, number of hatchlings released, 

and species diversity. Sea turtle distribution in PAIS is summarized in publications (Shaver 

1994, 2000; Shaver and Rubio 2008; Shaver et al. 2013) and the NOAA website reporting 

locations of stranded turtles documented by the STSSN 

(http://www.sefsc.noaa.gov/species/turtles/strandings.htm). Monitoring efforts focus on the 

Kemp’s ridley sea turtle, with less attention focused on the other four species of sea turtle 

documented in the park. This is because of the long-term, bi-national, multi-agency program to 

form a secondary nesting colony at PAIS, the critically endangered status of the species over the 

years of this program, and importance of PAIS beaches to nesting Kemp’s ridley turtles (i.e.,

more Kemp’s ridley nests are located at PAIS than at any other location in the U.S.). Less 

nesting conservation efforts have been focused on the other species because far fewer of their 

nests have been confirmed each year versus those confirmed for the Kemp’s ridley turtle. 

Continued annual monitoring of the populations in the park will allow for accurate assessment of 

population conditions in the future. Regarding work with sea turtles in the marine environment 

and work with stranded sea turtles, more effort has been undertaken with green turtles than any 

other species due to their numerical dominance and propensity for stranding alive in large events. 

Overall Condition

Number of Nests

The project team defined the Significance Level for number of nests as a 3. Shaver (2013) 

documented several years of nesting data collected along the Texas coast, including nests 

documented in PAIS. The number of confirmed nests for Kemp’s ridley sea turtles each season 

increased at approximately 12-18% per year from the mid-1990s through 2009, but plateaued 

from 2009 through 2013. The increasing trend in Kemp’s ridley nests was mostly likely the 

Photo 22. Green turtle with fibropapillomatosis (Photo 

by Keuper-Bennett and Bennett 2011).
211

result of the incubating, release, and monitoring research program on-going since 1978; 

protection efforts on the nesting beaches in Mexico; and protections in the marine environment 

including mandatory use of Turtle Excluder Devices (TEDs) and the seasonal area closure for 

shrimp trawling off south Texas established by TPWD in 2000. Monitoring and study must be 

continued to identify long-term trends and examine potential impacts to nest numbers from the 

Deepwater Horizon Oil Spill. Additionally, protection efforts must be continued to help restore 

the species. There is a slight increase in the number of loggerhead and green turtle nests found at 

PAIS in recent years. Very few nests of the leatherback and hawksbill turtles have been 

documented at PAIS. Thus, a Condition Level of 2 was assigned to this measure.

Number of Hatchlings Released

The Significance Level for number of hatchlings released is a 3. Shaver (2013) documented 

several years of nesting data collected along the Texas coast. Most eggs from confirmed nests are 

transported to an incubation facility in PAIS or south Texas corrals. Hatchlings are released to 

the wild and guarded during release. Hatchling success is higher for eggs protected in the 

incubation facility and corrals than for eggs left at nesting sites to hatch. The number of Kemp’s 

ridley turtle eggs recorded and resulting hatchlings released into the wild has been increasing for 

over a decade. For loggerhead and green sea turtles, the number of eggs brought for protected 

care and hatchlings released is more variable from year to year, which is likely influenced to

some degree by the low number of nests (from 0-13) documented each season at PAIS and the 

nesting habits and season of the species. These species tend to nest every three years whereas 

Kemp’s ridleys tend to nest every other year. Also, these species nest later in the year when 

detection efforts are more limited. Thus, the measure was assigned a Condition Level of 2.

Diversity and Distribution

The project team defined the Significance Level for diversity and distribution as a 3. Shaver 

(1989, 1999b, 2000, 2009, 2012, 2013) has documented five species of sea turtle in PAIS, some 

of which were more common or abundant than others (e.g., Kemp’s ridley and green sea turtles). 

Several species present in the park are of conservation concern due to rarity or restriction of 

range. Thus, a Condition Level of 2 was assigned to this measure.

Weighted Condition Score

A Weighted Condition Score of 0.667 was calculated for sea turtles in PAIS, meaning the 

component is of moderate concern with a stable trend. This is due primarily to the rarity and 

conservation concern of sea turtle populations along the Texas Gulf coastline. 
212

Sources of Expertise

Dr. Donna Shaver, Chief, Division of Sea Turtle Science and Recovery, Texas Coordinator, Sea 

Turtle Stranding and Salvage Network, PAIS
213

Literature Cited

Anderson, J. D., D. J. Shaver, and W. J. Karel. 2013. Genetic diversity and natal origins of green 

turtles (Chelonia mydas) in the Western Gulf of Mexico. Journal of Herpetology 47(2):251-

257.

Arthur, K., C. Limpus, G. Balazs, A. Capper, J. Udy, G. Shaw, U. Keuper-Bennett, and P. 

Bennett. 2007. The exposure of green turtles (Chelonia mydas) to tumor promoting 

compounds produced by the cyanobacterium Lyngbya majuscula and their potential role in 

the aetiology of fibropapillomatosis. Harmful Algae 7:114-125.

Bjorndal, K. A., A. B. Bolten, B. Koike, B. A. Schroeder, D. J. Shaver, W. G. Teas, and W. N. 

Witzell. 2001. Somatic growth function for immature loggerhead sea turtles in southeastern 

U.S. waters. Fishery Bulletin 99:240-246.

Bowen, B. W., A. L. Bass, S.-M. Chow, M. Bostrom, K. A. Bjorndal, A. B. Bolten, T. Okuyama, 

B. M. Bolker, S. Epperly, E. LaCasella, D. Shaver, and others. 2004. Natal homing in 

juvenile loggerhead turtles (Caretta caretta). Molecular Ecology 13(12):3797-3808.

Bowen, B. W., W. S. Grant, Z. Hillis-Starr, D. J. Shaver, K. A. Bjorndal, A. B. Bolten, and A. L. 

Bass. 2007. Mixed-stock analysis reveals migration of juvenile hawksbill turtles 

(Eretmochelys imbricata) in the Caribbean Sea. Molecular Ecology 16:49-60.

Caillouet, C. W., Jr., D. J. Shaver, W. G. Teas, J. N. Nance, D. B. Revera, and A. C. Cannon.

1996. Relationship between sea turtle strandings and shrimp fishing effort in the 

northwestern Gulf of Mexico: 1986-1989 versus 1990-1993. Fishery Bulletin 94(2):237-249.

Environmental Protection Agency (EPA). 2012. The facts about harmful algal blooms. United 

States Environmental Protection Agency 

Online http://water.epa.gov/polwaste/nps/outreach/upload/HABs_final_12-6-12.pdf

(accessed 9 January 2013).

Keuper-Bennett, U., and P. Bennett. 2011. Green turtle with fibropapillomatosis photo. NOAA 

Fisheries Office of Protected 

Resources. http://www.nmfs.noaa.gov/pr/species/turtles/photos.htm (accessed 9 January 

2013).

Lewison, B., L. Crowder, and D. J. Shaver. 2003. The impact of Turtle Excluder Devices and 

fisheries closures on loggerhead and Kemp’s ridley strandings in the western Gulf of Mexico. 

Conservation Biology 17(4):1089-1097.

McDaniel, C. J., L. B. Crowder, and J. A. Priddy. 2000. Spatial dynamics of sea turtle abundance 

and shrimping intensity in the U.S. Gulf of Mexico. Conservation Ecology 4(1):15.

Mills, L. S., M. E. Soulé, and D. F. Doak. 1993. The keystone-species concept in ecology and 

conservation. BioScience 43(4):219-224.
214

Milton, S., P. Lutz, and G. Shigenaka. 2003. Oil toxicity and impacts on sea turtles. Pages 35-47 

in Oil and Sea Turtles: Biology, Planning, and Response. NOAA National Ocean Service, 

Office of Response and Restoration, Silver Spring, MAryland.

National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). 1991. 

Recovery plan for the U.S. population of Atlantic green turtle. National Marine Fisheries 

Service, Washington, D.C.

National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). 1992. 

Recovery plan for leatherback turtles in the U.S. Caribbean, Atlantic and Gulf of Mexico. 

National Marine Fisheries Service, Washington, D.C.

National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). 1993. 

Recovery plan for hawksbill turtles in the U.S. Caribbean Sea, Atlantic Ocean, and Gulf of 

Mexico. National Marine Fisheries Service, St. Petersburg, Florida.

National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). 2008. 

Recovery plan for the northwest Atlantic population of the loggerhead sea turtle (Caretta

caretta), Second Revision. National Marine Fisheries Service, Silver Spring, Maryland.

National Marine Fisheries Service (NMFS), U.S. Fish and Wildlife Service (USFWS) and 

Secretary of Environment and National Resources (SEMARNAT). 2011. Bi-national 

recovery plan for the Kemp’s ridley sea turtle (Lepidochelys kempii), Second Revision. 

National Marine Fisheries Service, Silver Spring, Maryland.

National Oceanic and Atmospheric Administration (NOAA). 2006. Marine debris: Impacts in the 

Gulf of Mexico. NOAA Fisheries Service, Southeast Regional Office, Protected Resource 

Division, St. Petersburg, Florida.

National Park Services (NPS). 2012. Certified species list for reptiles in Padre Island National 

Seashore. NPSpecies Search. https://irma.nps.gov/App/Species/Search (accessed 4 January 

2013).

National Park Service (NPS). 2013a. The hawksbill sea turtle. Padre Island National 

Seashore. http://www.nps.gov/pais/naturescience/hawksbill.htm (accessed 8 January 2013).

National Park Service (NPS). 2013b. The leatherback sea

turtle. http://www.nps.gov/pais/naturescience/leatherback.htm (accessed 8 January 2013).

Pike, D. A., and J. C. Stiner. 2007. Sea turtle species vary in their susceptibility to tropical 

cyclones. Oecologia 153:471-478.

Plotkin, P., and A. F. Amos. 1990. Effects of anthropogenic debris on sea turtles in the 

northwestern Gulf of Mexico. The University of Texas at Austin Marine Science Institute, 

Port Aransas, Texas. In Shomura, R. S., and H. L. Codfrey (editors). Proceedings of the 

Second International Conference on Marine Debris, 2-7 April 1989, Honolulu, Hawaii. NOM 

Technical Memo NOM-TM-NMFS-SWFSC-154. National Marine Fisheries Service, 

Washington, D.C.
215

Shaver, D. J. 1987. Padre Island Kemp’s ridley sea turtle project update. Park Science 7(4):8-9.

Shaver, D. J. 1989. Green sea turtle geographic distribution. Herpetological Review 20(1):14.

Shaver, D. J. 1990. Kemp’s ridley project at Padre Island enters a new phase. Park Science 

10(1):12-13.

Shaver, D. J. 1991. Feeding ecology of Kemp's ridley in south Texas waters. Journal of 

Herpetology 25(3):327-334. 

Shaver, D. J. 1992a. Kemp’s ridley research continues. Park Science 12(4):26-27.

Shaver, D. J. 1992b. Kemp's ridley sea turtle reproduction. Herpetological Review 23(2):59. 

Shaver, D. J. 1994. Relative abundance, temporal patterns, and growth of sea turtles at the 

Mansfield Channel, Texas. Journal of Herpetology 28(4):491-497.

Shaver, D. J. 1998. Sea turtle strandings along the Texas coast, 1980-94. Pages 57-72 in

Characteristics and causes of Texas marine strandings. R. Zimmerman (ed.). NOAA 

Technical Reports NMFS 143. National Marine Fisheries Service, Washington, D.C.

Shaver, D. J. 1999a. Kemp’s ridley sea turtle project at Padre Island National Seashore, Texas. 

Pages 342-347 in Proceedings from the 17th annual Gulf of Mexico information transfer 

meeting. M. McKay and J. Nides, eds. MMS 99-0042. Minerals Management Service, Gulf 

of Mexico OCS Region, New Orleans, Louisiana.

Shaver, D. J. 1999b. Padre Island National Seashore Kemp's ridley sea turtle project and sea 

turtle strandings 1998 report. U.S. Geological Survey, Reston, Virginia. 

Shaver, D. J. 1999c. Sea turtle strandings in the Gulf of Mexico. Pages 31-33 in Sharing our Gulf 

conference proceedings, June 10-12, 1998, College Station, Texas. D. Owens (compiler) and 

M. Evans (editor). TAMU-SG-99-104. Texas Sea Grant Program, Texas A& University, 

College Station, Texas.

Shaver, D. J. 2000. Distribution, residency, and seasonal movements of the green sea turtle, 

Chelonia mydas (Linnaeus, 1758), in Texas. Dissertation, Texas A&M University, College 

Station, Texas. 

Shaver, D. J. 2005a. Analysis of the Kemp’s ridley imprinting and headstart project at Padre 

Island National Seashore, Texas, 1978-88, with subsequent Kemp’s ridley nesting and 

stranding records on the Texas coast. Chelonian Conservation and Biology 4(4):846-859. 

Shaver, D. J. 2005b. Kemp’s ridley sea turtle project at Padre Island National Seashore and 

Texas sea turtle nesting and stranding 2004 report. National Park Service, Padre Island 

National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2006. Kemp’s ridley sea turtle project at Padre Island National Seashore and Texas 

sea turtle nesting and stranding 2005 report. National Park Service, Padre Island National 
216

Seashore, Corpus Christi, Texas.

Shaver, D. J. 2007. Texas sea turtle nesting and stranding 2006 report. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2008. Texas sea turtle nesting and stranding 2007 report. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2009. Texas sea turtle nesting and stranding 2008 report. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2010. Texas sea turtle nesting and stranding 2009 report. National Park Service,

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2011. Texas sea turtle nesting and stranding 2010 report. National Park Service,

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2012. Texas sea turtle nesting and stranding 2011 report. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J. 2013. Texas sea turtle nesting and stranding 2012 report. National Park Service, 

Padre Island National Seashore, Corpus Christi, Texas.

Shaver, D. J., K. Hart, I. Fujisaki, C. Rubio, A. R. Sartain, J. Pena, P. M. Burchfield, D. Gomez 

Gamez, and J. Ortiz. 2013. Foraging area fidelity for Kemp’s ridleys in the Gulf of Mexico. 

Journal of Ecology and Evolution doi: 10.1002/ece3.594.

Shaver, D. J., and P. T. Plotkin. 1998. Marine debris ingestion by sea turtles in south Texas: preand post-MARPOL Annex V. Page 124 in Proceedings of the 16th annual symposium on sea 

turtle biology and conservation, February 28 - March 1, 1996, Hilton Head, South Carolina. 

R. Byles and Y. Fernandez (compilers). NOAA Technical Memorandum NMFS-SEFSC-412. 

National Marine Fisheries Service, Washington, D.C.

Shaver, D. J., and C. Rubio. 2008. Post-nesting movement of wild and head-started Kemp’s 

ridley sea turtles (Lepidochelys kempii) in the Gulf of Mexico. Endangered Species Research 

4:43-55.

Shaver, D. J., B. A. Schroeder, R. A. Byles, P. M. Burchfield, J. Pena, R. Marquez, and H. J. 

Martinez. 2005. Movements and home ranges of adult male Kemp’s ridley sea turtles 

(Lepidochelys kempii) in the Gulf of Mexico investigated by satellite telemetry. Chelonian 

Conservation and Biology 4(4):817-827.

Tomas, J., R. Guitart, R. Mateo, and J. A. Raga. 2002. Marine debris ingestion in loggerhead sea 

turtles, Caretta caretta, from the western Mediterranean. Marine Pollution Bulletin 44:211-

216.

Tristan, T., D. J. Shaver, J. Kimbro, T. deMaar, T. Metz, J. George, and A. Amos. 2010. 

Identification of Fibropapillomatosis in green sea turtles (Chelonia mydas) on the Texas 
217

coast. Journal of Herpetological Medicine and Surgery 20(4):109-112.

Wilson, E. G., K. L. Miller, D. Allison, and M. Magliocca. 2010. Why healthy oceans need sea 

turtles: the importance of sea turtles to marine ecosystems. OCEANA 

Online. http://oceana.org/sites/default/files/reports/Why_Healthy_Oceans_Need_Sea_Turtles

.pdf (accessed 2 January 2013).
218

4.13 Amphibians

Description

PAIS is suitable habitat for a small variety of amphibians, which may be due to the Laguna 

Madre acting as a barrier to the mainland. Amphibians act as key indicator species as they are 

especially susceptible to ecological changes due to their permeable skin (Smith and Keinath 

2007). Any toxins that have been absorbed by amphibians can be spread throughout the 

ecosystems if infected amphibians are consumed by other animals (Smith and Keinath 2007). 

Amphibians require access to freshwater at all life stages, and wet seasons are essential for the 

survival of many species in PAIS (Schmidly et al. 1996); most of the amphibian species in PAIS 

have been found near ephemeral ponds (Cooper et al. 2005). Adequate water sources are 

necessary for successful reproduction; without water sources, amphibians can go years without a 

successful breeding event (Dayton 2005). 

Measures

• Species abundance and diversity

• Species distribution

• Reproductive success

• Age class structure

Reference Conditions/Values

The ideal reference condition for 

amphibians in PAIS would be the condition 

of amphibian populations on Padre Island 

prior to the intensive cattle ranching and 

grazing that began in the late 1880s and 

lasted until 1971. However, very little 

information is available from this historical 

period. The most comprehensive 

description of amphibians on the island comes from an inventory by Duran (2004). This 

description is used as the reference condition for this assessment.

Data and Methods

Duran (2004) conducted an inventory of reptiles and amphibians in PAIS between 2002 and 

2003. Major sampling efforts were focused on 17 different sites across grassland, wetland, 

wetland-grasslands, sparse vegetation, and dune habitats (see Plate 29 for trapping locations in 

the park). Trapping methods included funnel-type minnow traps and drift-fence arrays; other 

inventory methods were visual encounters, auditory surveys, and road surveys. Other than a few 

isolated reports, little research has been devoted to amphibians in PAIS (Cooper et al. 2005).

Photo 23. Green tree frog (Hyla cinerea) (Photo by 

Duran, taken in 2003 herpetological inventory for PAIS).
219

Current Condition and Trend

Species Abundance and Diversity

NPS (2012) lists a total of five species of amphibians in PAIS, all of which are anurans and 

native to PAIS. Duran (2004) documented seven anuran species in PAIS: Rio Grande leopard 

frog (Lithobates berlandieri), green tree frog, Hurter’s spadefoot toad, spotted chorus frog, 

Woodhouse’s toad (Anaxyrus woodhousii), and a species of narrow-mouthed toad (Gastrophryne 

sp.) that was not able to be identified with certainty. According to Duran (2004), the tadpoles 

found seemed to be integrated between the eastern and Great Plains narrow-mouthed toad. The 

green tree frog was the most abundant amphibian species encountered during the survey, as 

several green tree frogs were spotted at every sample site with permanent or semi-permanent 

water (Duran 2004). According to Duran (2004), the Woodhouse’s toad is an unusual 

observation in PAIS due to the species’ distribution being located several hundred kilometers

north. Table 49 displays the amphibian species currently confirmed in PAIS and encountered 

during the Duran (2004) survey.

Table 49. Amphibians that have been confirmed in PAIS (NPS 2012, Duran 2004). Species scientific 

names were updated if necessary using Integrated Taxonomic Information System (ITIS 2012).

Scientific Name Common Name

Frequency of Occurrence 

Duran study 2002-2003

Anaxyrus woodhousii Woodhouse's toad Common*

Hyla cinerea green tree frog Common

Pseudacris clarkii spotted chorus frog Rare*

Lithobates berlandieri Rio Grande leopard frog Common

Scaphiopus hurterii Hurter's spadefoot Common

Gastrophryne sp.+ Species of narrow-mouthed toad Rare*

*denotes species that were first accounts within the park during the Duran 2004 study.

+ species unable to be identified with certainty by Duran 2004; does not occur in the certified species list 

for PAIS.

Species Distribution

The majority of anuran species in the PAIS ecosystem depend on ephemeral pools created by 

large rain events for breeding (Dayton et al. 2004). The distribution of anurans found in PAIS 

shows very little variation. 

All anuran species are heavily concentrated in the northern half of the park (Duran 2004; see 

Plate 30), likely due to the greater likelihood of ephemeral ponding in this part of the park rather 

than higher sampling effort during inventory (Stablein, written communication, 19 October 

2012). The Duran (2004) inventory documented several observations for all amphibian species 

known to occur in PAIS. The Woodhouse’s toad and Rio Grande leopard frog were the only two 

amphibian species to be observed on the southern end of the park. Both species were found near 

or at trap 14, which was located in a grassland habitat (see Plate 30). Presence of the narrowmouthed toad species is limited to specimens recorded near Park Road 22 on Mustang Island 

(observation location does not show up on Plate 30because it is north of the northern park 

boundary).
220

Reproductive Success

Currently, there are no data regarding reproductive success of amphibians in PAIS. Without 

established annual surveys and monitoring programs, this measure cannot be assessed.

Age Class Structure

There are also no data regarding age class structure in PAIS. Without established annual surveys 

and monitoring programs, this measure cannot be assessed.

Threats and Stressor Factors

According to Stuart et al. (2004), amphibians are generally more threatened than birds and 

mammal species. There are several threats to amphibians in the park. PAIS park staff have

identified 10 stressors that have occurred or are presently occurring in the park: saltwater 

inundation, human disturbance, vehicle strikes, hazardous materials such as chemical spells

(hazmat), drought, predation, habitat loss, disease, and oil and gas development.

Many threats can be categorized as anthropogenic. Humans negatively impact amphibians and 

their habitat by constructing roads and by drilling wells for oil and gas development. Amphibians 

cannot quickly escape construction areas and can be struck by vehicles when on the roads and 

beaches (PAIS 2001). The risk of chemical spills also increases in areas with oil and gas 

development.

Pollution in the air, on land, and in the water can result in compromised health, suffocation, and 

strangulation of amphibians. The gulf currents that historically brought in natural items and 

wood debris from ship wrecks now bring in trash items that are tossed into the ocean by the 

shrimping industry and off shore oil industries; trash tossed into rivers that flow into the Laguna

and Gulf also accumulates in PAIS (Cappiello 2003). According to a nationwide survey in 1993,

PAIS had the most trash of any NPS unit along the U.S. coast; as a result, it is the only NPS unit 

to have its own hazmat team (Cappiello 2003).

Many amphibians are salt intolerant and cannot survive in sea water; salt-water inundation of 

PAIS may therefore prove to be another threat (Karraker 2007). The amphibians inhabiting the

southern seashore may be at a higher risk than those on the northern seashore of PAIS due to the 

relative sea level rise and erosion rates. According to White et al. (2007), the southern seashore 

has been eroding at a rate of 1.2 to 2.4 m/year (3.9 to 7.9 ft/year). White et al. (2007) stated that 

between 1949 and 1999, relative sea level had risen at a rate of 3.38 mm/year on the southern 

shore.

Natural stressors for amphibians are drought, predation, and disease. Amphibians require a 

freshwater source throughout each stage of life. If drought were to occur, the small number of 

freshwater pools in PAIS could dry up, causing the amphibians on the island to become more 

vulnerable. According to Duran (2004), amphibians found on the mainland of Texas near PAIS 

(but not on the island) may lack drought survival mechanisms.

Chytrid fungus, specifically Batrachochytrium dendrobatidis, is a chytrid pathogen of 

amphibians that could potentially affect amphibian populations in PAIS. The pathogen has been 

identified as the cause of severe population declines on several continents, including North 

America (Piotrowski et al. 2004). Amphibians infected by B. dendrobatidis develop 
221

chytridiomycosis, an infectious non-hyphal zooporic fungus that causes roughening and 

reddening of the skin, convulsions, ulcers and hemorrhages, and sporadic death. Not all 

amphibians infected with B. dendrobatidis develop chytridiomycosis or die; environmental 

factors, such as pH of the environment, drought, and temperature at time of infection, may affect 

mortality rates. Some research indicates that the fungus growth is inhibited by high temperatures 

(28 °C or 82 °F) and exposure of infected individuals to high temperatures may kill the fungus 

(Woodhams et al. 2003). Also, ranaviruses have been associated with die-offs of amphibians and 

reptiles in over 25 states and more than 20 species of amphibians and turtles in the U.S. (USGS 

2012). Mortality due to ranaviruses occurs mostly in larval amphibians, true frogs, and chorus 

frogs. Infected individuals may exhibit subtle or severe hemorrhages in ventral skin, often 

appearing as an irregular rash; onset of illness is sudden and often affects most individuals in a 

wetland (up to or exceeding 90%) (USGS 2012). Neither disease has been detected in PAIS to 

date, but may greatly impact amphibian populations if diseases are transmitted to the island.

Data Needs/Gaps

There are limited data on amphibians in PAIS. The 2004 Duran inventory only describes the 

abundance, diversity, and possible distribution of most of the species observed, and is now 

outdated. A more current study and long-term monitoring of amphibians are needed to assess any 

trends within the park. The Gulf Coast Network has plans to initiate a long-term monitoring 

program of amphibians in the park in the near future.

Overall Condition

PAIS is home to a small variety of amphibians in the area due to the Laguna Madre acting as a 

barrier to the mainland. However, because there have been no annual herpetological surveys in 

the park, a quantitative condition evaluation of amphibians in PAIS cannot be completed at this 

time. While the NPS has a record of confirmed species in the park, this list does not include

estimates of current species richness or diversity, which were the NPS-specified measures for 

amphibians in PAIS. Annual monitoring of the populations in the park will allow for a more 

accurate assessment of these parameters.

Species Abundance and Diversity

The project team defined the Significance Level for species abundance and diversity as a 3. There 

is a lack of diversity on the island, but this measure does not appear to be of particular concern 

for those species that do occur in PAIS. However, long-term monitoring data may prove 

beneficial to managers. This measure was assigned a Condition Level of 0 or no concern.

Species Distribution

The project team defined the Significance Level for species distribution as a 3. There are limited 

distribution data for PAIS, but this measure does not appear to be of particular concern at this 

time. Long-term monitoring of amphibian distribution may prove beneficial to managers. This 

measure was also assigned a Condition Level of 0 or of no concern.

Reproductive Success

The project team defined the Significance Level for reproductive success as a 3. There are no 

data for reproductive success of amphibians in PAIS. Because of this data gap, a Condition Level

for this measure was not assigned.
222

Age Class Structure

The project team defined the Significance Level for age class structure as a 3. There are no data 

for age class structure of amphibians in PAIS. Because of this data gap, a Condition Level for 

this measure was not assigned. 

Weighted Condition Score

A Weighted Condition Score for amphibians in PAIS was not assigned because 50% of the 

measures had unknown Condition Levels.

Sources of Expertise

Wade Stablein, Biological Technician, PAIS
223

Literature Cited

Cappiello, D. 2003. A trashed treasure/ inundated with garbage, Padre Island Seashore is the 

only national park with its own hazmat team. The Houston 

Chronicle. http://www.chron.com/CDA/archives/archive.mpl/2003_3712973/a-trashedtreasure-inundated-with-garbage-padre-is.html (accessed 7 June 2012).

Cooper, R. J., S. B. Cederbaum, and J. J. Gannon. 2005. Natural resource summary for Padre 

Island National Seashore: Final Report. Warnell School of Forest Resources, University of 

Georgia, Athens, Georgia. 

Dayton, G. H. 2005. Community assembly of xeric-adapted anurans at multiple spatial scales. 

Dissertation. Texas A&M University, College Station, Texas.

Duran, C. M. 2004. An inventory of reptiles and amphibians of Padre Island National Seashore, 

San Antonio Missions National Historical Park, and Palo Alto Battlefield National Historic 

Site. Texas Conservation Data Center, The Nature Conservancy, San Antonio, Texas. 

Karraker, N. E. 2007. Are embryonic and larval green frogs (Rana clamitans) insensitive to road 

deicing salt? Herpetological Conservation and Biology 2(1):35–41.

National Park Service (NPS). 2012. Certified species list for amphibians in Padre Island National 

Seashore. NPSpecies Search, IRMA Portal. https://irma.nps.gov/App/Species/Search

(accessed 4 May 2012).

National Park Service (NPS). 2001. Oil and gas management plan. National Park Service, Padre 

Island National Seashore, Texas. 

Piotrowski, J. S., S. L. Annis, and J. E. Longcore. 2004. Physiology of Batrachochytrium 

dendrobatidis, a chytrid pathogen of amphibians. Mycologia January/February 96(1):9-15.

Schmidly, D. J., R. Wharton, J. Woolley, K. Arnold, J. K. Jones Jr., C. Jones, A. M. Powell, J. F. 

Scudday, and R. A. Manning. 1996. Biodiversity survey of the North Rosillos Area, Big 

Bend National Park. National Park Service, Southwest Region, Santa Fe, New Mexico.

Smith, B. E., and D. A. Keinath. 2007. Northern leopard frog (Rana pipiens): a technical 

conservation assessment. U.S. Forest Service, Rocky Mountain Region, Golden, Colorado.

Stuart, S. N., J. S. Chanson, N. A. Cox, B. E. Young, A. S. L. Rodriques, D. L. Fischman, and R. 

W. Waller. 2004. Status and trends of amphibian declines and extinctions worldwide. 

Sciencexpress:1-5.

U.S. Geological Survey (USGS). 2012. 

Ranavirus. http://www.nwhc.usgs.gov/disease_information/other_diseases/ranavirus.jsp

(accessed 20 November 2012).
224

White, W. A., T. A. Tremblay, R. L. Waldinger, and T. R. Calnan. 2007. Status and trends of 

wetland and aquatic habitats on Texas barriers: Upper Coast Strandpain-Chenier System and 

southern coast Padre Island National Seashore. Texas General Land Office, National Oceanic 

and Atmospheric Administration, Texas. 

Woodhams, D. C., R. A. Alford, and G. Marantelli. 2003. Emerging disease of amphibians cured 

by elevated body temperature. Diseases of aquatic organisms 55(1):65-67.
225

Plate 29. Trapping locations used during the 2002-2003 PAIS inventory (Duran 2004). The locator map 

displays the extents that the traps occurred.
226

Plate 30. Amphibian species found during the 2002-2003 PAIS inventory (Duran 2004). Location of 

narrow-mouthed toad is not included in the map because no GPS locations were recorded.
227

4.14 Water Quality

Description

Good water quality is essential to a healthy aquatic ecosystem. Many organisms (e.g., plant 

communities, animals) rely on balanced and stable water quality measures to survive and thrive 

(NPS 2012a). Water quality is a Vital Sign for parks in the GULN. Dissolved oxygen, 

temperature, pH, salinity, turbidity, and nutrients are core water quality measures identified by 

PAIS staff as important for understanding condition of water quality in the park. 

Photo 24. The Gulf of Mexico shore along PAIS (NPS photo).

PAIS is bordered by the Gulf of Mexico on the east (Photo 24) and the Laguna Madre on the 

west, which separates the barrier islands from mainland Texas. The Laguna Madre is a 

hypersaline lagoon that can be one and a half to three times as salty as the ocean, and is a highly 

sensitive and important ecosystem (NPS 2012b). The Laguna Madre is divided into two halves 

(upper and lower) and covers approximately 665,936 ha (1,645,563 ac) (Chapman and Wallace 

2009). PAIS also supports several freshwater ponds in the northern part of the park, which 

typically hold water year-round with the exception of particularly dry years.

The Laguna Madre, the Gulf of Mexico, the freshwater ponds, wetlands, and the vast number of 

acres that become ephemeral ponds after significant rain events on the island provide critical 

habitat for a variety of plants and animals important to PAIS ecology, such as shoal grass, sea 

turtles, fishes, various avian species, and amphibians.

Measures

• Dissolved oxygen

• Temperature

• pH

• Salinity

• Turbidity

• Nutrients (including total phosphorus, nitrate-nitrite, orthophosphate, and ammonia)
228

Dissolved Oxygen

Dissolved oxygen (DO) is critical for organisms that live in water. Fish and zooplankton filter 

out or “breathe” dissolved oxygen from the water to survive (USGS 2010). Generally, oxygen 

enters water from the atmosphere, through transpiration by aquatic plants, or through ground 

water discharge. In the Laguna Madre, the largest source of DO is due to transpiration by vast 

seagrass meadows that readily produce oxygen, especially with increased levels of sunlight. DO 

transpiration by seagrasses slows down at night (diurnal fluctuation in levels) and especially in 

winter months, as grasses go into senescence from late October to March (seasonal fluctuation in 

levels), when water temperatures and daylight hours decrease (Joe Meiman, GULN and CUPN 

Hydrologist, written communication, 14 March 2013). As the amount of DO drops, it becomes 

more difficult for water-based organisms to survive (USGS 2010). The concentration of DO in a 

water body is closely related to water temperature; cold water holds more DO than does warm 

water (USGS 2010). Thus, DO concentrations are subject to seasonal fluctuations as low 

temperatures in the winter and spring allow water to hold more oxygen, and warmer 

temperatures in the summer and fall cause water to hold less oxygen (USGS 2010).

Temperature

Water temperature greatly influences water chemistry and the organisms that live in aquatic 

systems. Not only can it affect the ability of water to hold oxygen, water temperature also affects 

biological activity and growth within water systems (USGS 2010). All aquatic organisms, from 

fish to insects to zoo- and phytoplankton, have a preferred or ideal temperature range for 

existence (USGS 2010). As temperature increases or decreases too far past this range, the 

number of individuals and species able to live there eventually decreases. In addition, higher 

temperatures allow some compounds or pollutants to dissolve more easily in water and they can 

be more toxic to aquatic life (USGS 2010).

pH

pH is a measure of the level of acidity or alkalinity of water and is measured on a scale from 0 to 

14, with 7 being neutral (USGS 2010). Water with a pH of less than 7.0 indicates acidity, 

whereas water with a pH greater than 7.0 indicates alkalinity. Aquatic organisms have a 

preferred pH range that is ideal for growth and survival (USGS 2010). Chemicals in water can 

change the pH and harm animals and plants living in the water; thus, monitoring pH can be 

useful for detecting natural and human-caused changes in water chemistry (USGS 2010).

Salinity

Salinity is the measure of dissolved salts in water; it is usually stated in parts per thousand (ppt) 

(EPA 2006). Salinity levels can affect other water quality measures, including dissolved oxygen. 

The higher the salinity in a body of water, the lower the amount of oxygen it can hold (EPA 

2006). The level of salinity also controls the types of organisms (plants and animals) that can 

survive in the body of water. Some species, such as shoal grass, can withstand higher levels of 

salinity, but other species only tolerate lower salinity levels (EPA 2006). 

Turbidity

Turbidity assesses the amount of fine particle matter (e.g., clay, silt, plankton, microscopic 

organisms, or finely divided organic or inorganic matter) that is suspended in water by 

measuring the scattering effect that solids have on light passing through water (USGS 2010). For 

instance, the more light that is scattered, the higher the turbidity measurement will be. The 
229

suspended materials that make water turbid can absorb heat from sunlight, increasing the water 

temperature in waterways and reducing the concentration of dissolved oxygen in the water 

(USGS 2010). The scattering of sunlight by suspended particles decreases photosynthesis by 

plants and algae, which contributes to decreased DO concentrations in the water (USGS 2010). 

Suspended particles also irritate and clog the gill structures of many fish and amphibians, making 

it difficult to thrive (USGS 2010).

Nutrients

Nutrients are essential for plant and animal growth. Nitrogen and phosphorus are particularly

important for aquatic life, but a surplus can cause negative effects. Organisms can also be 

affected by chemical and biological processes that change the form of the nutrient and transfer it

to or from water, soil, biological organisms, and the atmosphere. Nitrogen has different forms 

including ammonia and nitrate. Ammonia is a compound that occurs when dissolved nitrogen 

combines with hydrogen in natural water. The number of hydrogen atoms determines if the water 

will be ionic or unionized; the former is more toxic to fish than the latter. Nitrate is a compound 

that occurs when dissolved nitrogen combines with oxygen in natural water. Both ammonia and 

nitrate are very water-soluble and easily transferred though groundwater and streams. Phosphates 

are compounds of dissolved phosphorus in combination with oxygen and hydrogen. Phosphate 

compounds are moderately soluble and less mobile then nitrates; however, they tend to bond 

well with soil particles, and can be transported though erosion (Mueller and Helsel 2009).

Reference Conditions/Values

The reference condition for PAIS’s water quality is the Texas Commission on Environmental 

Quality (TCEQ) water quality criterion for surface waters in Texas. Table 50 displays water 

quality parameter standards set by the TCEQ. The TCEQ (2002) water quality criteria for 

saltwater and estuary systems serves as the reference condition for nutrient conditions in the 

Laguna Madre and Gulf of Mexico (Table 51). 

Table 50. Texas Commission on Environmental Quality standards for surface-water quality (TCEQ 2010).

Water Quality Measure TCEQ Standards

Dissolved Oxygen > 4.5 mg/L (Laguna Madre marine waters); 5.0 mg/L (freshwater)

Temperature < 95°F (35°C) Laguna Madre marine waters; N/A for freshwater on barrier islands

pH 6.5 – 8.5 (marine waters) or 9.0 (freshwater)

Salinity N/A

Turbidity N/A

Nutrients See Table 2

Table 51. TCEQ 2002 water quality criteria for nutrients in saltwater estuaries in PAIS (TCEQ 2002, as 

cited in Withers et al. 2004).

Nutrients TCEQ Saltwater (Estuaries) Standards

Ammonia-N (mg/L) 0.1

Nitrate+Nitrite-N (mg/L) 0.26

Orthophosphate (mg/L) 0.16

Total Phosphorous (mg/L) 0.22

Chlorophyll a (μg/L) 11.5
230

Data and Methods

General Baseline Inventory

In 2003, the NPS published results of surface-water quality data retrievals for PAIS using six

EPA national databases: Storage and Retrieval (STORET) water quality database management 

system, River Reach File (RF3), Industrial Facilities Discharge (IFD), Drinking Water Supplies 

(DRINKS), Water Gages (GAGES) and Water Impoundments (DAM) (NPS 2003). The retrieval 

resulted in 210,665 observations for various parameters at 257 monitoring stations operated by 

the NPS, USGS, EPA, and Texas Natural Resource Conservation Commission from 1941 to 

1998. A number of stations (73) were found within the park; however, only three of these 

stations (PAIS 0119, PAIS 0016, PAIS 0080) yielded long-term records (NPS 2003) (Plate 31). 

PAIS 0080 (named Laguna Madre Estuary Line 125) is located in the Laguna Madre at 

Yarborough Pass. PAIS 0016 (named Laguna Madre Estuary Line 194 Site 01) is located in 

PAIS along the GIWW in the Laguna Madre just north of Baffin Bay. PAIS 0119 (named 

Laguna Madre Estuary Line 053 Site 1) is located in the Laguna Madre directly west of Dagger 

Hill. The water quality parameters measured at these sites include DO, temperature, pH, 

turbidity, and various nutrients. Data were collected from these stations only several times per 

year or intermittently over several years. As a result, these data points represent a snapshot in 

time and are difficult to use in 

determining trends over time or current 

condition of water quality parameters.

This inventory used EPA standards for 

water quality to determine exceedances, 

which are noted as results from this 

inventory are presented.

Freshwater ponds and groundwater

Sissom et al. (1990) conducted a 

baseline study of three ponds in PAIS 

from September 1989 through August 

1990. The three freshwater ponds were 

surveyed on or near the 15th of each 

month, at which time samples were 

taken with the intention of detecting 

possible contaminants, such as inorganic 

nutrients and heavy metals. Water 

quality parameters, including 

temperature, turbidity, oxygen, salinity, 

and pH were also measured. Because 

only one sample was taken in each pond 

per month, the data points represent a 

snapshot view of water quality in the 

ponds at one point in time. Figure 21

shows the locations of the three 

freshwater ponds in PAIS. Photo 25

shows Pond B in early winter.

Figure 21. Location of the three freshwater ponds in PAIS.
231

Photo 25. Pond B located near PAIS Park Headquarters (seen in the background). The freshwater ponds 

in PAIS provide crucial habitat for many species of waterfowl that migrate through the region (Photo by 

Shannon Amberg, SMUMN GSS).

Based on historical data, NPS (1996) describes the near surface groundwater in the park as a lens 

of freshwater approximately 9 m (30 ft) thick that overlays brackish saltwater. This lens is 

approximately 3 km (2 mi) in width and is recharged mostly by precipitation events and less 

commonly by saltwater wave surges. It is believed the groundwater flows easterly from the 

Laguna Madre toward the Gulf of Mexico (NPS 1996), although the hydrology of the area has 

not been assessed completely. 

Gulf of Mexico and Laguna Madre

An inventory of water quality for the Gulf of Mexico’s surface waters at PAIS has not been 

conducted to date (NPS 1996, Wade Stablein, Pers. Comm.).

Nicolau (2005) collected monthly grab samples from six sites in the Laguna Madre between 

2003 and 2005. A total of six sampling locations were used to collect data: three sites in the 

upper Laguna Madre and three sites in the lower Laguna Madre. Water quality parameters 

examined in this study included water temperature, DO, and salinity measures. A suite of 

nutrients (ammonia, nitrate+nitrite, orthophosphate, total phosphate, chlorophyll-a) were also 

recorded at each sampling site. 

Wilson and Dunton (2011) surveyed areas near PAIS as part of a monitoring program for 

seagrass in Texas; water quality measures were recorded, including DO, pH, and salinity. Leaf 

tissues were also collected and analyzed from each station with a vegetated bottom. Only one 

species of seagrass (shoal grass) was analyzed for nutrient levels. Sampling station locations 

were distributed among three estuary systems, including the Laguna Madre, which was separated 

into two segments, ULM and LLM. The LLM sampling sites were all located just outside the 

park boundary. A 2012 summary report of repeat survey efforts is anticipated to be available 
232

soon, but was not available at the time this assessment was developed; however, various maps 

and figures featuring 2012 summary data were available and are included in this document where 

appropriate.

The NPS (2012c) NPSTORET database was accessed to query data for two stations located in 

the Laguna Madre near PAIS. The stations, located at Baffin Bay and Bird Island, report on 

observations for various nutrients, including nitrogen, nitrite, nitrate, ammonia, phosphorus, and 

orthophosphate. Many of the nutrient observations were recorded as “present” but less than the 

quantification limit; some were recorded as a specific numeric value. Samples are collected on a 

monthly basis.

NPS (2013) provides real-time data collected via data loggers installed at two TCEQ water 

quality monitoring stations in the Laguna Madre: one located at Baffin Bay (TCEQ Surface 

Water Quality Management Station 13444) and another located at Bird Island (TCEQ Station 

13445). Water quality samples drawn at these stations are collected approximately monthly.

GULN initiated an effort to monitor water quality conditions continuously (in real-time) in the 

Laguna Madre to better understand the dynamics of water quality in the hypersaline lagoon. This 

called for the installation of data loggers that would capture observations every 15 minutes, 

rather than occasionally. The intent of the effort is to gather data that may be used to model the 

dynamics of the system and which environmental factors affect various water quality conditions 

(e.g., weather events, diurnal or seasonal patterns, isolated disturbance events such as dredge 

activity). Water quality parameters observed in this effort include DO, salinity, turbidity, water 

temperature, and pH. Data collection began in 2008 and continues through the present; however, 

most of the data are in raw form and have yet to be analyzed in great detail. Data presented here 

represent very preliminary analyses to provide a snapshot of the interactions and complexities of 

water quality conditions in the system.

Plate 31 displays the locations for the long-term monitoring stations featured in NPS (2003), 

NPS (2012c) NPSTORET database, and the NPS (2013) GULN monitoring effort on the Laguna 

Madre. 

Current Condition and Trend

Dissolved Oxygen

Sissom et al. (1990) documented dissolved oxygen in the three freshwater ponds in PAIS. DO 

observations in Pond A ranged from 3.60 to 12.0 mg/L; one exceedance (3.60 mg/L) occurred in 

August 1990. Pond B had dissolved oxygen values ranging from 5.40 to 10.60 mg/L; no 

exceedances were recorded during sampling. DO observations in Pond C ranged from 5.40 to 

10.0 mg/L; no exceedances were recorded in Pond C during sampling. 
233

Table 52 displays the dissolved oxygen values (mg/L) observed in the three ponds in PAIS 

during sampling from September 1989 through August 1990.

Table 52. Dissolved oxygen values (mg/L) for three freshwater ponds located in PAIS, September 1989 

through August 1990 (Sissom et al. 1990). Exceedances are denoted with bold red text.

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Pond A 9.60 9.30 7.40 8.40 5.40 7.80 5.60 12.0 8.60 7.65 7.0 3.60

Pond B 10.40 6.60 9.0 8.40 5.40 9.30 10.0 7.20 6.20 10.6 7.30 5.60

Pond C 9.30 7.40 6.0 6.0 5.40 6.20 6.40 5.80 8.0 7.45 8.0 10.0

NPS (2003) reported 91 dissolved oxygen observations collected at three monitoring stations

within the park. Of these, eight observations (all from one station location) exceeded the EPA

threshold of 4.5 mg/L for marine systems. Table 53 displays the DO values from each station.

Table 53. Summary of dissolved oxygen observations (mg/L) from three water quality stations in the 

Laguna Madre in PAIS, including minimum, maximum, and mean values, and number of exceedances 

(NPS 2003).

Station Time Period Number of Observations Minimum Maximum Mean Exceedances

PAIS0016 1968-1974 30 4.9 9.3 6.6 0

PAIS0080 1968-1978 32 0.0 10.9 5.1 8

PAIS0119 1968-1982 29 4.6 14.4 7.1 0

Wilson and Dunton (2011) recorded 282 DO measurements from the lower Laguna Madre and 

142 observations from the upper Laguna Madre. Of these observations, a total of 10 in the lower 

Laguna Madre and 58 in the upper Laguna Madre were found to be below the TCEQ threshold 

considered to be protective of marine aquatic life (>4.5 mg/L). Dissolved oxygen levels varied 

greatly in both sections of the Laguna Madre across observations, which may be due to 

interactions with salinity, water temperature, and respiration of marine plants. Water temperature 

and respiration also vary diurnally, which would create a diurnal affect in DO saturation as well. 

Table 54 displays the DO values for the upper and lower Laguna Madre collected by Wilson and 

Dunton (2011). Plate 32 graphically represents DO values in the Laguna Madre based on 2011 

and 2012 survey results.

Table 54. DO observations collected in the lower and upper Laguna Madre in 2011, including minimum, 

maximum, and mean values (mg/L) (Wilson and Dunton 2011).

Laguna Madre Section Number of Observations Minimum Maximum Mean

Lower Laguna Madre 282 2.37 10.98 7.17

Upper Laguna Madre 142 1.34 10.64 5.33

Nicolau (2005) reported the Laguna Madre as an impaired water body for depressed dissolved 

oxygen levels, as is documented in the 2002 Texas Clean Water Act 303(d) listing. However, it 

is very difficult to assess trends in dissolved oxygen levels due to the complex interactions of 

many factors that affect DO in a hypersaline environment, some of which result in large DO 

fluctuations during the diurnal cycle.

Preliminary analysis of one year (August 2008 to August 2009) of NPS (2013) dissolved oxygen 

data from the Baffin Bay and Bird Island loggers is presented in Figure 22. DO levels appear to 

fluctuate diurnally, seasonally, and possibly as a result of isolated events, such as storms or 
234

disturbances. The daily fluctuation in DO concentration appears larger during the summer 

months and is less pronounced during winter months; this is largely a function of transpiration 

from seagrass meadows, which transpire large amounts of oxygen during summer months when 

daylight hours are long and water temperatures are warmer, but produce much less DO as 

seagrass meadows go into senescence during winter months when water temperatures are cooler 

and daylight hours are decreased. Further analysis is needed to understand which factors affect 

fluctuations in DO levels in the Laguna Madre. The greatest daily DO fluctuation recorded in 

2008 occurred at the Bird Island data logger, in which DO ranged from 1.1 mg/L to 11.8 mg/L. 

Figure 22 clearly shows that DO levels frequently drop below the TCEQ threshold considered 

protective of marine aquatic life in the Laguna Madre (4.5 mg/L). This appears to occur more 

often in the warmer summer months than in the cooler winter months.

Figure 22. Fluctuations in daily DO observations at Baffin Bay and Bird Island data loggers in the Laguna 

Madre in PAIS, August 2008 through August 2009 (NPS 2013).

Temperature

Sissom et al. (1990) reported temperature measurements from the three freshwater ponds in 

PAIS from September 1989 through August 1990. Temperatures in Pond A during sampling 

ranged from 13.0 to 33.0 °C. Temperature observations in Pond B ranged from 12.0 to 35.0 °C, 

while temperatures in Pond C ranged from 12.0 to 33.0°C. For all three ponds, the lowest 

temperatures were observed during winter months and the highest temperatures were observed in 

summer and early fall. Table 55 displays monthly temperatures (°C) across the three ponds in 

PAIS from September 1989 through August 1990.

Table 55. Water temperature values (°C) for three freshwater ponds located in PAIS, September 1989 

through August 1990 (Sissom et al. 1990).

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Pond A 33.0 30.0 19.0 13.0 16.0 13.0 26.0 29.0 30.0 29.5 30.0 33.0

Pond B 32.0 30.0 20.0 12.0 18.0 15.0 24.5 29.0 31.0 32.0 28.0 35.0

Pond C 31.0 30.0 20.0 12.0 17.0 15.0 24.0 31.0 31.0 32.5 29.0 33.0

0

2

4

6

8

10

12

14

16

18

20

8/1/2008

8/15/20088/29/20089/12/20089/26/2008

10/10/200810/24/2008

11/7/2008

11/21/2008

12/5/2008

12/19/2008

1/2/2009

1/16/20091/30/20092/13/20092/27/20093/13/20093/27/20094/10/20094/24/2009

5/8/2009

5/22/2009

6/5/2009

6/19/2009

7/3/2009

7/17/20097/31/20098/14/20098/28/2009

DO (mg/L)

Baffin Bay Bird Island

TCEQ Standard = 4.5 mg/L
235

NPS (2003) documented a total of 93 water temperature observations at monitoring stations in 

the Laguna Madre in PAIS between 1968 and 1982. No exceedances were recorded at these 

stations during the period of data collection. Because the data are not continuous, it is difficult to 

determine any trends in water temperature that may have occurred during this time. Table 56

displays the summary characteristics of water temperature observations from the three 

monitoring stations in the Laguna Madre in PAIS. 

Table 56. Summary of water temperature observations (°C) from three water quality stations in the 

Laguna Madre in PAIS, including minimum, maximum, mean values, and exceedances (NPS 2003).

Station Time Period Number of Observations Minimum Maximum Mean Exceedances

PAIS0016 1968-1974 32 23.4 29.2 26.4 0

PAIS0080 1968-1978 32 17.7 29.8 24.7 0

PAIS0119 1968-1982 29 12.5 31.1 25.1 0

Nicolau (2005) calculated monthly water temperature averages for 24 consecutive months for 

three stations in the upper Laguna Madre and three stations in the lower Laguna Madre between 

August 2003 and July 2005. Water temperature at the upper Laguna Madre stations ranged from

13.9 to 31.7 °C. Figure 23 displays monthly average water temperatures at three sampling 

locations in the upper Laguna Madre in PAIS between 2003 and 2005. Water temperatures at the 

lower Laguna Madre sampling locations ranged from 13.5 to 30.7 °C. Figure 24 displays 

monthly average water temperatures at the three sampling locations in the lower Laguna Madre 

in PAIS between 2003 and 2005. For both upper and lower Laguna Madre, the warmest water 

temperatures occurred in late summer and early fall months and the coolest water temperatures 

occurred in winter months.

Figure 23. Average monthly water temperatures (°C) at three sampling locations in the upper Laguna 

Madre in PAIS between 2003 and 2005 (Nicolau 2005).

10

15

20

25

30

35

August-03

September-03

October-03

November-03December-03

January-04

February-04

March-04

April-04

May-04

June-04

July-04

August-04

September-04

October-04

November-04December-04

January-05

February-05

March-05

April-05

May-05

June-05

July-05

Temperature (°C)

Upper Laguna Madre

Station 17121 Station 13445 Station 18188
236

Figure 24. Average monthly water temperatures (°C) at three sampling locations in the lower Laguna 

Madre in PAIS between 2003 and 2005 (Nicolau 2005).

Preliminary analysis of one year (August 2008 to August 2009) of NPS (2013) data on water 

temperatures at the Baffin Bay and Bird Island loggers in Laguna Madre is presented in Figure 

25. Water temperatures fluctuate daily and seasonally with rising and falling ambient 

temperatures. There is a clear, but not unexpected, annual pattern of lower water temperatures in 

the cooler winter months and higher water temperatures in the warmer summer months. It is not 

clear how weather events affect daily or periodic water temperature fluctuations and more 

analysis is needed to understand how such changes in water temperature alters other water 

quality parameters, such as dissolved oxygen. During summer months, water temperatures can 

reach as high as 35 °C (95 °F), but fall as low as 10 °C (50 °F) during the winter months. In the 

analysis of temperatures from August 2008 through August 2009, only two records exceeded the 

TCEQ standard with a measurement of 35.15 °C (95.3 °F) and 35.1 °C (95.2 °F), occurring on 4 

August and 8 August 2008, respectively.

10

15

20

25

30

35

August-03

September-03

October-03

November-03December-03

January-04

February-04

March-04

April-04

May-04

June-04

July-04

August-04

September-04

October-04

November-04December-04

January-05

February-05

March-05

April-05

May-05

June-05

July-05

Temperature (°C)

Lower Laguna Madre

Station 13449 Station 13447 Station 13446
237

Figure 25. Fluctuations in daily water temperature (°C) observations from Baffin Bay and Bird Island data 

loggers in the Laguna Madre in PAIS, August 2008 through August 2009 (NPS 2013).

pH

Sissom et al. (1990) documented seasonal averages for pH in the three main freshwater ponds in 

PAIS. Pond A had a pH range from 8.13 to 8.46; there were no exceedances of the TCEQ 

standard considered protective of freshwater aquatic life (9.0 for freshwater) recorded at this 

pong during sampling. pH in Pond B ranged from 7.7 to 9.8; one exceedance (9.8) occurred in 

the month of September during sampling. Pond C had a pH range from 8.2 to 9.2; one 

exceedance (9.2) occurred in the month of September during sampling. Table 57 shows pH 

values for each month during the study across the three freshwater ponds in PAIS.

Table 57. pH values for three freshwater ponds located in PAIS, September 1989 through August 1990 

(Sissom et al. 1990). Exceedances are denoted by bolded red text.

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Pond A 8.8 8.5 8.1 8.2 8.0 8.2 8.5 8.5 8.3 8.5 8.4 8.2

Pond B 9.8 8.6 8.5 7.7 8.0 8.5 8.5 8.6 8.4 8.5 8.6 8.3

Pond C 9.2 8.3 8.5 8.5 8.4 8.2 8.4 8.4 8.5 8.5 8.3 8.3

NPS (2003) documented a total of 92 pH observations collected at monitoring locations in the 

Laguna Madre in PAIS between 1968 and 1982. A total of 15 observations exceeded the upper 

limits of the EPA threshold considered protective of marine aquatic life (6.5 to 8.5 standard 

units): nine observations at PAIS0080 and six observations at PAIS0119. Across the three 

monitoring locations, pH ranged from 7.5 to 8.7. Due to the intermittent nature of these data, it is 

not possible to determine a trend in pH values over time. Table 58 shows the summary 

characteristics of pH observations from the three PAIS stations in the Laguna Madre. 

0

5

10

15

20

25

30

35

40

8/1/2008

8/15/20088/29/20089/12/20089/26/2008

10/10/200810/24/2008

11/7/2008

11/21/2008

12/5/2008

12/19/2008

1/2/2009

1/16/20091/30/20092/13/20092/27/20093/13/20093/27/20094/10/20094/24/2009

5/8/2009

5/22/2009

6/5/2009

6/19/2009

7/3/2009

7/17/20097/31/20098/14/20098/28/2009

Temperature (°C)

Baffin Bay Bird Island

TCEQ Standard < 95°F (35°C)
238

Table 58. Summary of pH observations from three water quality stations in the Laguna Madre in PAIS, 

including minimum, maximum, and mean values, and number of exceedances (NPS 2003).

Station Time Period Number of Observations Minimum Maximum Mean Exceedances

PAIS0016 1968-1974 33 7.5 8.4 8.1 0

PAIS0080 1968-1978 30 7.8 8.7 8.3 9

PAIS0119 1968-1982 29 7.8 8.7 8.3 6

Wilson and Dunton (2011) recorded 285 pH observations in the lower Laguna Madre and 143 

observations in the upper Laguna Madre during sampling in 2011. A total of 27 observations 

exceeded the upper limit of the TCEQ standard considered protective of marine aquatic life: 17 

observations in the lower Laguna Madre and 10 observations in the upper Laguna Madre. Table 

59 displays a summary of pH observations collected from the upper and lower Laguna Madre 

during sampling by Wilson and Dunton (2011). Plate 33 graphically represents pH values in the 

Laguna Madre as a result of recent monitoring efforts (2011 and 2012).

Table 59. Summary of pH observations from the upper and lower Laguna Madre, including minimum, 

maximum, and mean values collected in 2011 (Wilson and Dunton 2011).

Laguna Madre Section Number of 

Observations

Minimum Maximum Mean Exceedances

Lower Laguna Madre 285 7.50 9.04 8.09 17

Upper Laguna Madre 143 6.99 8.84 7.96 10

Preliminary analysis of one year (August 2008 to August 2009) of NPS (2013) data on pH from 

the Baffin Bay and Bird Island loggers in the Laguna Madre is presented in Figure 26. None of 

the pH measurements exceeded the TCEQ standard range. Initial analysis of data suggests that 

pH varies widely both diurnally and seasonally. For instance, in the snapshot of data from 2008 

shown in Figure 26, pH levels appear to be lower and fluctuate less during the cooler winter 

months, but variability increases substantially during the warmer summer months. Further 

analysis is needed to understand the effect of bio-chemical changes in the Laguna Madre as 

temperature changes diurnally and seasonally. 
239

Figure 26. Fluctuations in daily pH observations from Baffin Bay and Bird Island data loggers in the 

Laguna Madre in PAIS, August 2008 through August 2009 (NPS 2013).

Salinity

Sissom et al. (1990) documented salinity values from three ponds in PAIS during a sampling 

effort in 1989 and 1990. Salinity in Pond A ranged from 0.12 to 0.64 ppt. Salinity in Pond B 

ranged from 0.12 to 2.5 ppt; all but one observation (2.5 ppt in August 1990) fell well below 1.0 

ppt. It is not clear what occurred in August 1990 to substantially increase the salinity level in 

Pond B over all other months of sampling. Salinity observations in Pond C were substantially 

higher overall than the other two ponds, with observations ranging from 10.5 to 19.8 ppt. Of the 

three ponds, Pond C is located the closest to the Gulf of Mexico. Sissom et al. (1990) speculated 

several theories for increased salinity in Pond C, including increased levels are 1) due to higher 

incidence of wash-over of saltwater from the Gulf of Mexico during storm events, 2) leaching 

from salt deposits in the substrate of the island, or 3) salt entering the pond from groundwater 

coming in through the substrate from the bay. Regardless of how the salt entered the pond, 

Sissom et al. (1990) refers to it as a brackish environment rather than freshwater. Table 60

displays salinity observations (ppt) for the three ponds in PAIS from September 1989 through 

August 1990.

Table 60. Salinity values (ppt) for three freshwater ponds located in PAIS, September 1989 through 

August 1990 (Sissom et al. 1990).

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Pond A 0.48 0.64 0.64 0.32 0.32 0.36 0.24 0.24 0.2 0.2 0.12 0.3

Pond B 0.80 0.40 0.20 0.22 0.20 0.12 0.18 0.20 0.18 0.20 0.15 2.5

Pond C 14.60 12.0 14.0 16.5 18.0 19.80 10.5 12.0 11.80 14.0 15.30 19.0

Nicolau (2005) calculated monthly salinity averages for 24 consecutive months for three stations

in the upper Laguna Madre and three stations in the lower Laguna Madre between August 2003 

and July 2005. Salinity levels at the upper Laguna Madre stations ranged from 25 to 50 ppt, with 

most observations measured between 25 and 40 ppt. Figure 27 displays monthly average salinity 

7.5

7.7

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8/15/20088/29/20089/12/20089/26/2008

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1/2/2009

1/16/20091/30/20092/13/20092/27/20093/13/20093/27/20094/10/20094/24/2009

5/8/2009

5/22/2009

6/5/2009

6/19/2009

7/3/2009

7/17/20097/31/20098/14/20098/28/2009

pH

Baffin Bay Bird Island
240

values at three sampling locations in the upper Laguna Madre in PAIS from 2003 to 2005. 

Salinity values at the lower Laguna Madre sampling locations were found to be overall more 

variable than those values of the upper Laguna Madre, with some salinity values recorded as 

being quite low (range across sampling locations was 6.09 to 44.71 ppt). Figure 28 displays 

monthly average salinity values at the three sampling locations in the lower Laguna Madre 

between 2003 and 2005.

Figure 27. Average monthly salinity values (ppt) from three water quality stations in the Laguna Madre, 

August 2003 through July 2005 (Nicolau 2005).

Figure 28. Average monthly salinity values (ppt) from three stations in the lower Laguna Madre in PAIS, 

August 2003 through July 2005 (Nicolau 2005).

Wilson and Dunton (2011) recorded 284 salinity observations in the lower Laguna Madre and 

142 observations in the upper Laguna Madre during sampling in 2011. The salinity in the lower 

0

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Upper Laguna Madre

Station 17121 Station 13445 Station 18188

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Salinity (ppt)

Lower Laguna Madre

Station 13449 Station 13447 Station 13446
241

Laguna Madre ranged from 30.82 to 55.64 ppt; salinity levels were much more varied in the 

upper Laguna Madre, ranging from 7.43 to 74.20 ppt. Table 61 displays a summary of salinity 

observations from the upper and lower Laguna Madre in 2011. Plate 34 graphically represents 

salinity values in the Laguna Madre as a result of recent monitoring efforts (2011 and 2012).

Table 61. Summary of salinity observations from the upper and lower Laguna Madre, including minimum, 

maximum, and mean values collected in 2011 (Wilson and Dunton 2011).

Laguna Madre Section Number of Observations Minimum Maximum Mean

Lower Laguna Madre 284 30.82 55.64 41.64

Upper Laguna Madre 142 7.43 74.20 49.32

Preliminary analysis of one year (August 2008 to August 2009) of NPS (2013) data on salinity

from the Baffin Bay and Bird Island loggers in the Laguna Madre is presented in Figure 29. 

Salinity in the Laguna Madre appears to have increased steadily from mid-summer 2008 through 

mid-summer 2009, showing a numeric increase of around 15 to 20 ppt across the year at both 

sampling locations. Further analysis is necessary to understand what other factors may be 

contributing to the overall increase in salinity, as well as which factors have a dynamic 

relationship with salinity in the lagoon that influence changes daily, seasonally, or across several 

years. For instance, drought may contribute substantially to increased salinity over time, as 

evaporation rates exceed rates of precipitation, thus concentrating salts in the lagoon system. 

Precipitation records were not considered in conjunction with these data during this preliminary 

analysis, but would be pertinent in understanding trends in salinity during more in-depth analysis 

of the dataset.

Figure 29. Fluctuations in daily salinity values (ppt) from Baffin Bay and Bird Island data loggers in the 

Laguna Madre in PAIS, August 2008 through August 2009 (NPS 2013).

Turbidity

Sissom et al. (1990) documented turbidity levels from the three main freshwater ponds in PAIS. 

Turbidity in Pond A ranged from 10.0 to 70.0 NTU; highest turbidity measurements in Pond A 

appeared to occur in spring to early summer months, perhaps due to the effects of increased 

20

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55

8/1/2008

8/15/20088/29/20089/12/20089/26/2008

10/10/200810/24/2008

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11/21/2008

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12/19/2008

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5/8/2009

5/22/2009

6/5/2009

6/19/2009

7/3/2009

7/17/20097/31/20098/14/20098/28/2009

Salinity (ppt)

Baffin Bay Bird Island
242

precipitation. Due to blue green algae present in Pond B nearly all year, turbidity levels 

consistently measured higher than the other ponds. During sampling, Pond B turbidity ranged 

from 25 to 190 NTU; turbidity decreases when water temperatures drop or the pond freezes 

during winter months, but increases again after it thaws in the spring. Turbidity observations in 

Pond C ranged from 0 to 20 NTU during sampling. Table 62 displays turbidity observations at 

the three ponds in PAIS from September 1989 through August 1990.

Table 62. Turbidity observations (NTU) for three freshwater ponds located in PAIS, September 1989 

through August 1990 (Sissom et al.1990).

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Pond A 12.0 18.0 29.0 10.0 15.0 25.0 42.5 70.0 65.0 33.0 20.0 20.0

Pond B 110.0 90.0 180.0 190.0 25.0 90.0 93.0 138.0 101.0 144.0 140.0 155.0

Pond C 18.0 14.0 10.0 5.0 2.0 18.0 20.0 30.0 3.0 20.0 0.0 0.0

NPS (2003) reported 21 turbidity observations collected at three monitoring stations in the 

Laguna Madre in PAIS from 1974 to 1976. No exceedances were observed among the PAIS 

stations (standard used is 50 NTU). Table 63 displays the turbidity records from the three PAIS 

stations.

Table 63. Summary of turbidity observations (NTU) from three water quality stations in the Laguna Madre 

in PAIS, including minimum, maximum, and mean values, and number of exceedances (NPS 2003).

Station Time Period Number of Observations Minimum Maximum Mean Exceedances

PAIS0016 1974-1974 6 8 16 10 0

PAIS0080 1974-1976 13 4 28 12.4 0

PAIS0119 1974-1974 2 4 4 4 0

NPS (2013) has documented continuous turbidity levels as well as a number of turbidity events 

from 2008 through 2012 (data are still recorded currently). Turbidity can be affected by a 

number of factors including weather events, recreation activities, shipping or marine vehicle 

travel through the lagoon, etc. Waters with increased turbidity can become difficult for plant and 

animal life to thrive in. Trends in turbidity are not presented here as more analysis is needed. 

However, based on preliminary analysis of data from August 2008 through August 2009, data 

suggest that spikes in turbidity can be short- (a few hours) or long-lived (several days or weeks) 

in the Laguna Madre. Three large spikes in turbidity were recorded at the Bird Island station in 

the lagoon between August 2008 and August 2009. Figure 30 shows an example of a relatively 

long-lasting turbidity spike that occurred near the Bird Island logger between 30 August 2008 

and 5 September 2008. Figure 31 shows the largest of three short-lived turbidity spikes captured 

at the Bird Island station on 25 June 2009. The largest of the three spikes measured at 804 NTU,

while subsequent spikes were much lower; this series of spikes lasted approximately seven 

hours. Further analysis is needed to understand which factors contribute to spikes in turbidity in 

addition to overall consistent turbidity conditions.
243

Figure 30. Turbidity event lasting several days captured at the Bird Island data logger in the Laguna 

Madre in PAIS between 30 August and 5 September 2008 (NPS 2013). Data points represent 

observations taken throughout each day.

Figure 31. Short-term spike in turbidity (approximately 7 hours) as captured at the Bird Island data logger 

in the Laguna Madre in PAIS on 25 June 2009 (NPS 2013).

Nutrients

Nicolau (2005) studied levels of six different nutrients at six monitoring stations in the upper and 

lower Laguna Madre from August 2003 through August 2005. A total of 48 observations were 

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Bird Island

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10:00:4510:30:4511:00:4511:30:4512:00:4512:30:4513:00:4513:30:4514:00:4514:30:4515:00:4015:30:4516:00:4416:30:5117:00:4517:30:5218:00:5918:30:4419:00:4519:30:4520:00:4520:30:4521:00:4521:30:4522:00:4522:30:4523:00:4523:30:45

Turbidity (NTU)

Bird Island (25 June 2009)
244

recorded at three monitoring stations in the upper Laguna Madre, in which 16 water samples 

were collected at each location and analyzed for nutrients. Likewise, 48 observations were 

recorded at three monitoring stations in the lower Laguna Madre, with 16 water samples 

collected at each site and analyzed for nutrients over the two year sampling period. Table 64

displays summary characteristics of water samples analyzed for five nutrients in the upper and 

lower Laguna Madre. Concentrations of chlorophyll-a were elevated at all three monitoring 

stations in the upper Laguna Madre in 50% (Station 17121) to 75% (Stations 13445 and 18188) 

of samples collected, totaling 31 exceedances of the TCEQ threshold (11.5 μg/L) out of 48 

samples collected. All other nutrients in the upper Laguna Madre were within the range deemed 

acceptable for protection of marine aquatic life, with the exception of total phosphate which 

exceeded the upper limits in two samples (one at Station 13445 and one at Station 17121). 

Samples from the lower Laguna Madre reflect similar results, in that chlorophyll-a exceeded the 

upper limits of the TCEQ threshold in 19 of 48 samples (approximately 40% of samples); 10 of 

the exceedances occurred at Station 17121 and 9 occurred at station 18188. Samples collected at 

Station 17121 revealed elevated levels of all nutrients under investigation at least once and up to 

four times during the two-year period when samples were collected.

Table 64. Nutrient ranges and exceedances from six water sampling locations in the upper and lower 

Laguna Madre in PAIS, August 2003 and August 2005 (Nicolau 2005). Note: samples were not collected 

from November 2004 through February 2005.

Upper Laguna Madre

Station 13445 Station 17121 Station 18188

Nutrient Range Exceeded Range Exceeded Range Exceeded

Ammonia (mg/L) <0.02-0.03 0 <0.02-0.05 0 <0.02-0.06 0

Nitrate+Nitrite (mg/L) <0.02-<0.04 0 <0.02-<0.04 0 <0.02-<0.04 0

Orthophosphate(mg/L) <0.04-0.08 0 <0.04 0 <0.04 0

Total Phosphate(mg/L) 0.03-0.24 1 0.04-0.24 1 0.04-0.13 0

Chlorophyll-a (μg/L) 1.8-30.5 12 0.08-40.5 7 8.1-58.6 12

Lower Laguna Madre

Station 13445 Station 17121 Station 18188

Nutrient Range Exceeded Range Exceeded Range Exceeded

Ammonia (mg/L) <0.02-0.06 0 <0.02-0.15 2 <0.02-0.06 0

Nitrate+Nitrite (mg/L) <0.04-0.04 0 <0.04-0.88 4 0.02-0.06 0

Orthophosphate(mg/L) 0.02-<0.04 0 0.02-0.18 1 0.02-0.07 0

Total Phosphate (mg/L) 0.03-0.08 0 <0.06-0.27 1 0.04-0.17 0

Chlorophyll-a (μg/L) 0.8-5.3 0 4.4-87.5 10 2.0-36.4 9

NPS (2012c) NPSTORET provided monthly nutrient data collected from October 2010 to 

November 2012 at two monitoring locations in the Laguna Madre, one at Baffin Bay and the 

other at Bird Island. The database includes observations for ammonia, nitrate+nitrite, 

orthophosphate, total phosphorus, and chlorophyll-a. Nearly all records included in the query 

report nutrients present and at levels less than the quantification limit, which means they are 

within the acceptable threshold considered protective of marine aquatic life. A few observations 

provide actual numeric values, of which nearly all were within the acceptable limits for nutrients 

in a marine ecosystem. The exception to this are observations of chlorophyll-a in October and 
245

November 2012 at both Bird Island and Baffin Bay data loggers; observations at Baffin Bay in 

October and November 2012 were 27.2 and 24.0 μg/L, while observations at Bird Island were 

12.0 and 17.5 μg/L, respectively. These observations all exceed the TCEQ threshold considered 

protective of marine aquatic life (11.5 μg/L). 

Threats and Stressor Factors

Oil and gas exploration and development are a major threat to water quality in PAIS. There are

six active oil pads in the park, and a total of 11 wells that are active. Two more pads are

scheduled for restoration in the near future, and no new wells are planned for the near future 

(Stablein, written communication, 14 May 2013). If any leaks or spills occurred near the 

shoreline or inundation zone, it could result in contaminated water in the Laguna Madre. Boat 

traffic and dredging activity can result in re-suspension of material, which increases turbidity;

high turbidity levels can negatively affect aquatic organisms such as fish and seagrass species. 

Another threat to the park’s water quality, from a public health standpoint, is tied in with high 

levels of E. coli and fecal coliform bacteria that are found in runoff after high rain events. 

Potential sources of fecal contaminants include the numerous state-approved fishing cabins in the 

Laguna Madre (most of the cabins are floating in the Laguna) (Stablein, written communication, 

14 May 2013). These cabins are mandated to contain their human waste and have it pumped out; 

however, it is certainly possible that some of these cabins will dump their waste directly into the 

Laguna. To check for fecal contamination, PAIS tests the water near areas of high visitor use six 

times a week; three samples are taken from the Malaquite Visitor Center area, and three samples 

are taken from the Bird Island windsurfing area.

Natural events such as hurricanes and tropical storms can cause major inundations, which can 

adversely affect water quality in PAIS. Storms can cause changes in turbidity and salinity. 

According to Steward et al. (2006), waves and wind can cause erosion and re-suspend sediment, 

causing increased turbidity. Strong wind and wave action from storms will only worsen the 

erosion in the park that has been caused by the practice of brush clearing. Storms can also cause 

decreased salinity levels that can remain low for months, depending on the extent of the storm 

(Steward et al. 2006). Furthermore, rivers that once reached the Laguna Madre no longer do, 

primarily due to high water demands in the area. During periods of drought, there may be times 

when the salinity becomes very high, as the cycle of tides coming in and water evaporating out 

of the Laguna will elevate the salinity of the remaining water.

Hurricanes and storms can also adversely affect fish and seagrass species. The loss or depletion 

of seagrass can be a stressor on water quality. Seagrass communities help create a stable lagoon 

bottom (Moore 2004), which may lead to overall lower turbidity levels. Seagrass also absorbs 

nutrients and oxygenates water, resulting in improved water quality (TPWD 2011); loss of 

seagrass beds could cause an increase in nutrients in the water column.

Although algae are natural components of marine and fresh water systems, excessive growth of 

algae can become a significant threat to users of water bodies and the ecological balance and 

health of a system. Algae that have grown excessively dense, some species of which can be toxic 

in high biomass blooms (i.e., HABs), can impact water quality by altering the quantity and 

quality of light in the water column. This, in turn, can affect benthic flora and fauna that depend 

on light to thrive (EPA 2013, USGS 2013). Excessive blooms can accumulate as a layer of thick 
246

scum or a mat on the water surface, which upon decomposing, can stimulate hypoxia (high rates 

of oxygen consumption that result in very low dissolved oxygen levels) and lead to degradation 

or mortality of fish, shellfish, and invertebrate communities, as well as degrade aquatic plant 

habitats (EPA 2013). Algal toxins in HABs can cause mortality in terrestrial animals or 

waterbirds that use affected water sources (USGS 2013). Likewise, recreational users of water 

bodies affected by HABs can become ill from exposure to the cyanobacteria and toxins they 

create. The red tide that occurs in the Gulf of Mexico is of primary concern to PAIS managers. 

This brevetoxin has caused deaths among the resident coyotes in the park, and has contributed to 

the death of visitor’s dogs as well (Stablein, written communication, 14 May 2013).

Brown tides (Aureoumbra lagunensis) also occur in the park, although more commonly in the 

Laguna Madre. Nutrient loading caused by winter freezes (i.e., fish kills from freezing weather) 

and heavy rain events result in runoff from agricultural fields on the mainland reaching the 

Laguna. Brown tides typically occur after a winter freeze and the associated fish kill from that 

freeze (Stablein, written communication, 14 May 2013). Population growth in the lower Laguna 

Madre may result in higher levels of runoff, and this runoff may also contribute to brown tide 

events. Brown tide events in high concentrations and densities may block out sunlight in the 

water and prevent seagrass colonies from receiving the necessary sunlight to survive (TPWD 

2013).

Data Needs/Gaps

A number of data sources are available that speak to water quality in PAIS (either for the 

freshwater ponds or the Laguna Madre), but many of these datasets are outdated, short-term in 

nature, or intermittent in nature of collection. These circumstances make it difficult to understand 

the current conditions of water quality in the park-managed waters and how conditions may have 

changed over time. Data presented in NPS (2003) also used EPA standards for comparisons to 

determine exceedances, which are less conservative than TCEQ water quality standards 

commonly used for water quality assessment in Texas. Observations reported in NPS (2003) 

include only the date of observation and minimum, maximum, mean, and median values; 

individual records of observation are not included. This makes it impossible to know with 

certainty the total number of observations that met or exceeded TCEQ standards in addition to 

exceeding EPA standards. Wilson and Dunton (2011) provide data that are much more current 

for several water quality measures, but each sampling location provides only one data record per 

measure. 

The NPS (2013) dataset is continuous and contains almost four full years of observations for DO, 

temperature, pH, salinity, and turbidity. Observations are collected every 15 minutes, 365 days a 

year (with the exception of some small data gaps), beginning in August 2008 to current. 

However, analysis of these data is in the very preliminary stages as of current and offers little 

insight into current condition for this assessment. Upcoming analysis of this dataset should prove 

to be highly valuable to understanding recent trends in water quality in the Laguna Madre, 

especially the complexities of the relationships among various water quality parameters and the 

other factors that have an influence on water quality. 

Overall, consistent water quality sampling would provide PAIS managers with better insight into 

current water quality conditions (freshwater and marine) in the park. GULN has undertaken a 

consistent monitoring effort in the Laguna Madre, but there is nothing in place for Gulf of 
247

Mexico waters or the freshwater ponds in the park. Consistent monitoring for these water 

features in addition to the Laguna Madre would, in the future, begin to illustrate trends in water 

quality that may be occurring over time.

Overall Condition

Dissolved Oxygen

The project team defined the Significance Level for dissolved oxygen as a 3. Nicolau (2005) 

restated that the Laguna Madre was considered impaired under the 2002 Texas Clean Water Act 

303(d) list because of the depressed dissolved oxygen levels; however, this listing is outdated 

and NPS (2013) data suggest that Laguna Madre DO levels fluctuate diurnally and seasonally.

Wilson and Dunton (2011) found average DO levels to be within the acceptable limits for 

protection of marine organisms. For the small segment of NPS (2013) data that were analyzed 

preliminarily, DO levels in the Laguna Madre overall are well within the range considered 

acceptable for marine aquatic life. There is not enough recent, long-term data analyzed to date 

that would help determine the current condition of DO levels in the freshwater ponds in PAIS. A

Condition Level of 1 was assigned for dissolved oxygen, meaning it is of low concern in the 

park.

Temperature

The project team defined the Significance Level for temperature as a 3. Sissom et al. (1990) 

recorded one measurement from Pond B that exceeded the TCEQ temperature standard; 

however, these data are more than 20 years old. NPS (2003) and Nicolau (2005) did not record 

any exceedances in Laguna Madre water temperatures. Preliminary analysis of NPS (2013) data 

from August 2008 to August 2009 showed just two temperature measurements in the Laguna 

Madre in August 2008 that exceeded TCEQ standards for marine water temperature; however, 

more recent data have yet to be analyzed. Therefore, a Condition Level was not assigned at this 

time. 

pH

The project team defined the Significance Level for pH as a 3. Sissom et al. (1990) recorded two 

pH exceedances from the freshwater Ponds B and C. NPS (2003) did not record any pH 

exceedances in the Laguna Madre. Wilson and Dunton (2011) recorded only two exceedances 

from two upper Laguna Madre sampling stations. Preliminary analysis of NPS (2013) data 

revealed no pH exceedances in the Laguna Madre PAIS from August 2008 to August 2009;

however, more recent data have yet to be analyzed. A Condition Level of 0, meaning pH is of no 

concern, was assigned at this time.

Salinity

The project team defined the Significance Level for salinity as a 3. Sissom et al. (1990) recorded 

salinity levels for the three permanent ponds in PAIS; only Pond C had salinity measurements 

over 1 ppt (10.5 – 19.8 ppt). Nicolau (2005) observed salinity levels that ranged between 25 and 

50 ppt in the Laguna Madre, which is considered a moderate salinity level. Saltwater fish and 

seagrass can still thrive under these conditions. Wilson and Dunton (2011) recorded several 

salinity measurements above 50 ppt in the upper Laguna Madre, with a mean salinity of 49.3 ppt. 

The highest salinity observation recorded was 74.2 ppt. The NPS (2013) data displayed a trend in 

increasing water salinity from August 2008 to August 2009; however, observations during that 
248

time did not exceed 50 ppt. Further analysis of the NPS (2013) data is needed to understand 

recent and current trends in salinity of the Laguna Madre. Thus, a Condition Level of 2 was 

assigned for salinity, meaning salinity is of moderate concern.

Turbidity

The project team defined the Significance Level for turbidity as a 3. Turbidity in the freshwater 

ponds was found to be within TCEQ standards protective of freshwater aquatic life with the 

exception of Pond B, which consistently sustains high turbidity due to blue-green algae colonies. 

Turbidity in the Laguna Madre can vary diurnally and seasonally due to weather conditions and 

temperature, and sporadically due to isolated disturbances. Preliminary analysis of the NPS 

(2013) dataset from August 2008 to August 2009 revealed several instances of turbidity spikes at 

Baffin Bay and Bird Island in the Laguna Madre, some that persisted for several days and some 

that lasted for just a few hours. There are more consistent turbidity spikes at the Baffin Bay 

logger, which may be the result of more boat traffic. Further analysis of the NPS (2013) data is 

needed to understand recent and current trends in turbidity of the Laguna Madre, and the factors 

that influence it. A Condition Level of 1 was assigned at this time, meaning turbidity is of low 

concern.

Nutrients

The project team defined the Significance Level for nutrients as a 3. Nicolau (2005) found that 

four of five nutrients studied in the Laguna Madre fell within the acceptable limits for protection 

of marine aquatic life; the exception was Chlorophyll-a, which exceeded TCEQ limits in 65% of 

samples collected in the upper Laguna Madre and 40% of samples collected in the lower Laguna 

Madre from August 2003 to August 2005. NPS (2012c) NPSTORET data report most 

observations with nutrient levels below the quantifiable limit, with the exception of chlorophylla, which were found to exceed TCEQ standards in October and November 2012. Thus, a 

Condition Level of 1 was assigned, meaning nutrients are of low concern.

Weighted Condition Score

The Weighted Condition Score for water quality in PAIS was calculated as 0.333, meaning the 

current condition is of moderate concern. A trend in water quality could not be determined at this 

time.
249

Sources of Expertise

Joe Meiman, Hydrologist, GULN

Wade Stablein, Biological Technician, PAIS
250

Literature Cited

Chapman, K., and J. Wallace. 2009. Laguna Madre. Western Hemisphere Shorebirds Reserve 

Network. http://www.whsrn.org/site-profile/laguna-madre (accessed 16 April 2012).

Environmental Protection Agency (EPA). 2006. Voluntary estuary monitoring manual. Chapter 

14: Salinity. U.S. Environmental Protection Agency, Washington, D.C.

Environmental Protection Agency (EPA). 2013. Cyanobacterial harmful algal blooms 

(CyanoHABs). http://water.epa.gov/scitech/swguidance/standards/criteria/nutrients/cyanohab

s.cfm (accessed 6 March 2013).

Gulf Coast Network (GULN). 2012. PAIS time series data. Excel Spreadsheet. Received from J. 

Meiman, December 2012.

Moore, K. A. 2004. Influence of seagrasses on water quality in shallow regions of the lower 

Chesapeake Bay. Journal of Coastal Research 45:162-178.

Mueller, D. K., and D. R. Helsel. 2009. Nutrients in the nation’s waters -too much of a good 

thing? http://pubs.usgs.gov/circ/circ1136/circ1136.html#INTRO (accessed 16 April 2012). 

National Park Service (NPS). 1996. Padre Island National Seashore resources management plan 

1996. National Park Service, Padre Island National Seashore, Texas.

National Park Service (NPS). 2003. Baseline water quality data inventory and analysis, Padre 

Island National Seashore. Technical Report NPS/NRWRD/NRTR-2001/287. National Park 

Service, Washington, D.C. 

National Park Service (NPS). 2012a. Water quality. National Park Service, Southern Plains 

Network. http://science.nature.nps.gov/im/units/sopn/vs_waterquality.cfm (accessed 14 

December 2012).

National Park Service (NPS). 2012b. Padre Island National Seashore coasts/shorelines 

website. http://www.nps.gov/pais/naturescience/coasts.htm (accessed 16 January 2013).

National Park Service (NPS). 2012c. GULN NPSTORET web query tool data. Excel 

Spreadsheet. Retrieved from http://science.nature.nps.gov/im/units/guln/npstoret/Default.cfm

(accessed 7 November 2012).

National Park Service (NPS). 2013. Laguna Madre continuous water quality data set collected at 

Baffin Bay and Bird Island monitors, 2008-present. Excel spreadsheet.

Nicolau, B. A. 2005. Oso Bay and Laguna Madre total maximum daily load project – phase III 

and IV data report. Center for Coastal Studies, Texas A&M University – Corpus Christi,

Corpus Christi, Texas. 

Sissom, S. L., R. D. Koehn, D. Lemke, R. R. Smith, C. Caudle, and F. Oxley. 1990. A baseline 

study of three ponds within Padre Island National Park. National Park Service, Padre Island 

National Seashore, Corpus Christi, Texas.
251

Steward, J. S., R. W. Virnstein, M. A. Lasi, L. J. Morris, J. D. Miller, L. M. Hall, and W. A. 

Tweedale. 2006. The impacts of the 2004 hurricanes on hydrology, water quality, and 

seagrass in the central Indian River Lagoon, Florida. Estuaries and Coasts 29(6A):954-965.

Texas Commission on Environmental Quality (TCEQ). 2002. Guidance for assessing Texas 

surface and finished drinking water quality data, 2002. Texas Commission on Environmental 

Quality, Austin, Texas.

Texas Commission on Environmental Quality (TCEQ). 2010. Texas surface water quality 

standards. http://www.tceq.texas.gov/assets/public/permitting/waterquality/standards/docs/T

SWQS2010/TSWQS2010_rule.pdf (accessed 16 December 2012).

Texas Parks and Wildlife Department (TPWD). 2011. FAQ: seagrass protection 

expansion. http://www.tpwd.state.tx.us/business/feedback/public_comment/proposals/jfk_sta

te_scientific_area/ (accessed 14 December 2012).

U.S. Geological Survey (USGS). 2010. Common water measurements: USGS water science for 

schools. U.S. Geological Survey. Information from “A Primer on Water Quality” by 

Swanson, H. A. and H. L. Baldwin, U.S. Geological Survey, 

1965. http://ga.water.usgs.gov/edu/characteristics.html (accessed 16 April 2012).

U.S. Geological Survey (USGS). 2013. Harmful algal blooms –

HABs. http://geology.com/usgs/harmful-algal-blooms/ (accessed 6 March 2013).

Wilson, C. J., and K. H. Dunton. 2011. Raw data from 2011 tier 2 sampling effort. Excel 

Spreadsheet. Retreived from http://www.texasseagrass.org/Data.html (accessed 14 December 

2012).

Wilson, C. J., and K. H. Dunton. 2012. Results maps for 2011 and 2012: Seagrass monitoring 

program for Texas coastal waters. http://www.texasseagrass.org/Maps.html (accessed 6 

March 2013).

Withers, K., E. Smith, O. Gomez, and J. Wood. 2004. Assessment of coastal water resources and 

watershed conditions at Padre Island National Seashore, Texas. Technical Report 

NPS/NRWRD/NRTR-2004/323. National Park Service, Padre Island National Seashore, 

Texas.

.
252

Plate 31. Locations of the long-term water quality sampling stations featured in NPS (2003), NPS (2012c) 

NPSTORET database, and the NPS (2013) Gulf Network monitoring effort on the Laguna Madre in PAIS. 

Purple markers indicate stations used in the NPS (2012c) data query; Yellow markers indicate TCEQ 

SWQM stations where NPS (2013) data are collected; Red markers indicate stations queried for the NPS 

(2003) baseline water quality inventory.
253

Plate 32. Dissolved oxygen values in the Laguna Madre from 2011 (left) and 2012 (right) based on 

monitoring surveys conducted by Wilson and Dunton (2012).
254

Plate 33. pH values in the Laguna Madre from 2011 (left) and 2012 (right) based on monitoring surveys 

conducted by Wilson and Dunton (2012).
255

Plate 34. Values in the Laguna Madre from 2011 (left) and 2012 (right) based on monitoring surveys 

conducted by Wilson and Dunton (2012).
256

4.15 Air Quality

Description

Air pollution can significantly affect natural resources and their associated ecological processes.

Consequently, air quality in parks and wilderness areas is protected and regulated through the 

1916 Organic Act and the Clean Air Act of 1977 (CAA) and the CAA’s subsequent 

amendments. The CAA defines two distinct categories of protection for natural areas, Class I and 

Class II airsheds. Class I airsheds receive the highest level of air quality protection as offered 

through the CAA; only a small amount of additional air pollution is permitted in the airshed 

above baseline levels. For Class II airsheds, the increment ceilings for additional air pollution 

above baseline levels are slightly greater than for Class I areas and allow for moderate 

development (EPA 2008a). PAIS is designated as a Class II airshed. 

Measures

• Nitrogen deposition

• Sulfate deposition

• Mercury deposition/concentration

• Ozone concentration

• Particulate matter (PM2.5)

• Visibility

Atmospheric Deposition of Nitrogen and Sulfur

Nitrogen and sulfur oxides are emitted into the atmosphere primarily through the burning of 

fossil fuels, industrial processes, and agricultural activities (EPA 2008b). While in the 

atmosphere, these emissions form compounds that may be transported long distances and settle 

out of the atmosphere in the form of pollutants such as particulate matter (e.g., sulfates, nitrates, 

ammonium) or gases (e.g., nitrogen dioxide, sulfur dioxide, nitric acid, ammonia) (EPA 2008b, 

NPS 2008). Atmospheric deposition can be in wet (i.e., pollutants dissolved in atmospheric 

moisture and deposited in rain, snow, low clouds, or fog) or dry (i.e., particles or gases that settle 

on dry surfaces as with windblown dusts) form (EPA 2008b). Deposition of sulfur and nitrogen 

can have significant effects on ecosystems, including acidification of water and soils, excess 

fertilization or increased eutrophication, changes in the chemical and physical characteristics of 

water and soils, and accumulation of toxins in soils, water, and vegetation (NPS 2008, reviewed 

in Sullivan et al. 2011a and 2011b). The native vegetation in the semi-arid and grassland plant 

communities in PAIS is considered sensitive to excess nitrogen and acidic deposition (Sullivan et 

al. 2011c, 2011d).

Mercury

Sources of atmospheric mercury include fuel combustion and evaporation (especially coal-fired 

power plants), waste disposal, mining, industrial sources, and natural sources such as volcanoes 

and evaporation from mercury-enriched soils, wetlands, and oceans (EPA 2008b). Mercury 

deposited into rivers, lakes, and oceans can accumulate in various aquatic species, resulting in 
257

exposure to wildlife and humans (EPA 2008b). PAIS supports several freshwater ponds that are 

important habitat for a variety of wildlife, especially migrating waterfowl and waterbirds.

Ozone

Ozone occurs naturally in the earth’s atmosphere where, in the upper atmosphere, it protects the 

earth’s surface against ultraviolet radiation (EPA 2008b). However, it also occurs at the ground 

level (i.e., ground-level ozone) where it is created by a chemical reaction between nitrogen 

oxides and volatile organic compounds (VOCs) in the presence of heat and sunlight (NPS 2008). 

Ozone is also one of the most widespread pollutants affecting vegetation and human health in the 

U.S. (NPS 2008). Considered phytotoxic, ozone can cause significant foliar injury and growth 

effects for sensitive plants in natural ecosystems (EPA 2008a, NPS 2008). Specific effects 

include reduced photosynthesis, premature leaf loss, and reduced biomass, and prolonged 

exposure can increase vulnerability to insects and diseases or other environmental stressors (NPS 

2008). At high concentrations, ozone can aggravate respiratory and cardiovascular diseases in 

humans, reduce lung function, cause acute respiratory problems, and increase susceptibility to 

respiratory infections (EPA 2008b, EPA 2010a); this could be a concern for visitors and staff 

engaging in aerobic activities in the park, such as hiking.

Particulate Matter (PM) and Visibility

Particulate matter (PM) is a complex mixture of extremely small particles and liquid droplets 

suspended in the atmosphere. Fine particles (PM2.5) are those smaller than 2.5 micrometers in 

diameter (EPA 2009). Particulate matter largely consists of acids (such as nitrates and sulfates), 

organic chemicals, metals, and soil or dust particles (EPA 2008a, EPA 2009). Fine particles are a 

major cause of reduced visibility (haze) in many national parks and wildernesses (EPA 2010b). 

PM2.5 can be directly emitted from sources such as forest fires, or they can form when gases 

emitted from power plants, industries, and/or vehicles react with air (EPA 2009, EPA 2010b).

Particulate matter either absorbs or scatters light. As a result, the clarity, color, and distance seen 

by humans decreases. Water in the atmosphere causes particles like nitrates and sulfates to 

expand, increasing their light-scattering efficiency (EPA 2010b). PM2.5 is also a concern for

human health as these particles can easily pass through the throat and nose and enter the lungs 

(EPA 2008b, EPA 2009, EPA 2010b). Short-term exposure to these particles can cause shortness 

of breath, fatigue, and lung irritation (EPA 2008b, EPA 2009).

Reference Conditions/Values

The NPS Air Resources Division (ARD) developed an approach for rating air quality conditions 

in national parks, based on the current NAAQS, ecosystem thresholds, and visibility 

improvement goals (Table 65) (NPS 2010a). Assessment of current condition of nitrogen and 

sulfur atmospheric deposition is based on wet (rain and snow) deposition. Ozone condition is 

based on the NAAQS standard of 75 parts per billion (ppb) (an annual fourth-highest daily 

maximum 8-hr concentration, averaged over 3 years). Visibility conditions are assessed in terms 

of a Haze Index, a measure of visibility (termed deciviews) that is derived from calculated light 

extinction and represents the minimal perceptible change in visibility to the human eye (NPS 

2010a). Finally, NPS ARD recommends the following values for determining air quality 

condition (Table 65). The “good condition” metrics may be considered the reference condition 

for PAIS.
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Table 65. National Park Service Air Resources Division air quality index values (NPS 2010a).

Condition

Ozone 

concentration

(ppb)

Wet Deposition 

of N or S 

(kg/ha/yr)

Visibility

(dv*)

Significant Concern ≥76 >3 >8

Moderate Condition 61-75 1-3 2-8

Good Condition ≤60 <1 <2

*a unit of visibility proportional to the logarithm of the atmospheric extinction (TCEQ 2012); one deciview 

represents the minimal perceptible change in visibility to the human eye.

Data and Methods

Monitoring in the Park

There is no active on-site monitoring of air quality parameters at PAIS.

NPS Data Resources

Although data on air quality parameters are not actively collected within park boundaries, data 

collected at several regional monitoring stations for various parameters can be used to estimate 

air quality conditions in PAIS. NPS ARD provides estimates of ozone, wet deposition of 

nitrogen and sulfur, and visibility that are based on data interpolations from all air quality 

monitoring stations operated by NPS, EPA, various states, and other entities, averaged over the 

most recent five years (e.g., 2006-2010). These estimates are available from the Explore Air 

website (NPS 2012) and are used to evaluate air quality conditions. On-site or nearby data are 

needed for a statistically valid trends analysis, while a five-year average interpolated estimate is 

preferred for the condition assessment. 

Other Air Quality Data Resources

The National Atmospheric Deposition Program–National Trends Network (NADP) database 

provided annual average summary data for nitrogen and sulfur concentration and deposition in 

the southern coastal Texas region. Monitoring site TX39, located in Corpus Christi, Texas, is 

approximately 24 km (15 mi) north of PAIS. Although no longer operational, this site provided 

deposition data for the region from 2002 through 2006. There are no active NADP mercury 

monitors near PAIS.

The EPA Air Trends database provides annual average summary data for ozone concentrations 

near PAIS. Ozone concentrations are collected at monitoring site number 48-355-0025, located 

in Corpus Christi, Texas approximately 32 km (20 mi) northwest of PAIS. The site is operated 

by the Texas Commission on Environmental Quality and has actively collected data from 

January 1990 through April 2012 (EPA 2012). The Air Trends database also provided data for 

particulate matter concentrations (PM2.5) from a monitoring site near PAIS. The Corpus Christi 

Huisache monitoring site (ID 48-355-0032), located approximately 32 km (20 mi) northwest of 

PAIS, has actively collected data from January 2000 through March 2012. Results from monitors 

located within 16 km (10 mi) from parks are generally considered to be representative of park 

conditions (Ellen Porter, NPS Air Resources Division Air Quality Specialist, phone 

communication, 25 October 2012). Data recorded at monitors beyond this distance from parks 

may represent regional conditions, but may not be representative of actual park conditions.
259

The Clean Air Status and Trends Network (CASTNet) provides summaries of the composition of 

nitrogen and sulfur deposition in various regions around the U.S. Similarly, the Interagency 

Monitoring of Protected Visual Environments Program (IMPROVE) actively monitors visibility 

conditions in Class I airsheds across the U.S. However, the nearest CASTNet and IMPROVE 

monitoring sites are located in Big Bend National Park, approximately 675 km (420 mi) west of 

PAIS. This distance and the variations among terrain make it difficult to extrapolate data 

accurately; thus, data from these monitoring stations were not considered in this assessment.

Special Air Quality Studies

Sullivan et al. (2011a) assessed the relative sensitivity of national parks to the potential effects of 

acidification caused by acidic atmospheric deposition from nitrogen and sulfur compounds. The 

relative risk for each park was assessed by examining three variables: the level of exposure to 

emissions and deposition of nitrogen and sulfur; inherent sensitivity of park ecosystems to 

acidifying compounds (N and/or S) from deposition; and level of mandated park protection 

against air pollution degradation (i.e., Wilderness and Class I). The outcome was an overall risk 

assessment that estimates the relative risk of acidification impacts to park resources from 

atmospheric deposition of nitrogen and sulfur (Sullivan et al. 2011a). Using the same approach, 

Sullivan et al. (2011b) assessed the sensitivity of national parks to the effects of nutrient 

enrichment by atmospheric deposition of nitrogen. The outcome was an overall risk assessment 

that estimates the relative risk to park resources of nutrient enrichment from increased nitrogen 

deposition.

Current Condition and Trend

Atmospheric Deposition of Nitrogen and Sulfur

Five-year interpolated averages of total nitrogen (from nitrate and ammonium) wet deposition 

and total sulfur (from sulfate) wet deposition are used to estimate condition for deposition; using 

a five-year average smoothes out annual variations in precipitation, such as heavy precipitation 

one year versus drought conditions in another. The current 5-year average (2006-2010) estimates 

total wet deposition of nitrogen in PAIS at 2.5 kg/ha/yr, while total wet deposition of sulfur is 2.3 

kg/ha/yr (NPS 2012). Relative to the NPS ratings for air quality conditions (see Table 65 for 

ratings values), atmospheric deposition of both nitrogen and sulfur falls into the Moderate 

Concern category. However, several factors are considered when rating the condition of 

atmospheric deposition, including effects of deposition on different ecosystems (NPS 2010a). 

Based on the NPS process for rating air quality conditions, ratings for parks with ecosystems 

considered potentially sensitive to nitrogen or sulfur deposition typically are adjusted up one 

condition category. In general, semi-arid ecosystems are considered to be sensitive to increased 

levels of nitrogen and sulfur, as acidification and nutrient enrichment can cause shifts in native 

species composition and allow encroachment of exotic species and grasses (reviewed in Sullivan 

et al. 2011a and 2011b). PAIS comprises semi-arid vegetation communities, which may be at 

risk from increased deposition, particularly nitrogen. Thus, the condition for deposition of 

nitrogen and sulfur in PAIS are considered to be of Significant Concern.

Figure 32 shows the annual average concentrations of sulfate, nitrate, and ammonium recorded 

in Corpus Christi, Texas, the NADP monitor nearest to PAIS (approximately 24 km north of the 

park).
260

Figure 32. Annual average concentrations of sulfate (SO4), nitrate (NO3), and ammonium (NH4) in 

precipitation (mg/L) in Corpus Christi, Texas, 2002-2006 (NADP monitoring site TX39, located 

approximately 24 km north of PAIS) (Source: NADP 2012). Note: Ammonium (NH4) is included because it 

adds significantly to total nitrogen deposition.

Relative risk of acidification and nutrient enrichment of ecosystems was assessed by examining 

exposure to nitrogen deposition and acidification, inherent sensitivity of park ecosystems, and 

mandates for park protection. Sullivan et al. (2011c) ranked PAIS as having moderate exposure 

to acidifying (nitrogen and sulfur) pollutants, very low ecosystem sensitivity to acidification, and 

moderate park protection due to its Class II airshed status. The ranking of overall risk from 

acidification due to acid deposition was low relative to other parks (Sullivan et al. 2011c). In a 

separate examination, Sullivan et al. (2011b) used the same approach to assess the sensitivity of 

national parks to nutrient enrichment effects from atmospheric nitrogen deposition relative to 

other parks. PAIS was ranked as having moderate risk for nitrogen pollutant exposure, moderate 

ecosystem sensitivity, and moderate park protection mandates (Class II airshed). The ranking of 

overall risk of effects from nutrient enrichment from atmospheric nitrogen deposition was 

moderate relative to other parks (Sullivan et al. 2011d).

Atmospheric Deposition of Mercury

To date, no monitoring data are available for mercury deposition or concentration in PAIS. The 

nearest monitoring station is located in Gregg County, TX, approximately 731 km (455 mi) 

northeast of the park. It is not appropriate to interpolate from stations that far from the park.

Ozone Concentration

The NAAQS standard for ground-level ozone is the benchmark for rating current ozone 

conditions within park units. In 2008, the standard was strengthened from 80 ppb to 75 ppb, 

based on the annual 4th highest daily maximum 8-hour concentration, averaged over 3 years.

The condition of ozone in NPS park units is determined by calculating the 5-year average of the 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2002 2003 2004 2005 2006

Concentration (mg/L)

NO3

SO4

NH4
261

fourth-highest daily maximum of 8-hour average ozone concentrations measured at each monitor 

within an area over each year (NPS 2010a). The current 5-year average (from 2006-2010) for 

PAIS indicates an average ground-level ozone concentration of 67.1 ppb (NPS 2012), which falls 

under the Moderate Concern category based on NPS guidelines. 

A portable ozone monitor was also used to collect ozone concentration data within PAIS during 

2007 and 2008. The fourth-highest daily maximum 8-hour concentrations for those years were 

65.0 and 75.0 ppb, respectively. Long-term data that characterize ozone concentrations within 

the park do not exist. However, ozone concentrations are monitored daily by TCEQ in Corpus 

Christi, TX, approximately 32 km (20 mi) northwest of PAIS. Although results from this monitor 

may not be representative of ozone concentrations within PAIS, they can be considered to 

represent concentrations in the region of the park. Figure 33 illustrates the trend in annual fourthhighest daily maximum 8-hour values from 1995 to 2012; these are presented with both the old 

and revised national standards to provide perspective on acceptable versus potentially harmful 

ozone conditions in the region. Measurements within the last seven years appear to be well 

within the updated NAAQS standard considered to be protective of human health.

Figure 33. Annual 4th highest 8-hour maximum ozone (O3) concentrations (ppb) in the PAIS region, 

1995-2012 (Source: EPA 2012). Note: Site 48-355-0026 is the monitor located in Corpus Christi, Texas, 

approximately 32 km (20 mi) northwest of PAIS. Prior to 2008, the NAAQS ozone standard was 0.08 ppm 

(80 ppb) (shown in red); in March 2008, the standard was amended to 0.075 ppm (75 ppb) (shown in 

blue).

Kohut (2004) assessed ozone concentrations in the GULN and the risk of injury to plant species 

that are sensitive to sustained ozone exposure. Data from 1995-1999 indicate ozone 

concentrations in PAIS during this time frequently exceeded 60 ppb each year and occasionally 

40

50

60

70

80

90

100

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Concentration (ppb)

New National Standard

Old National Standard
262

exceeded 80 ppb each year. Concentrations exceeded 100 ppb rarely, although one year 

catalogued 16 hours above this threshold; at these levels, it is possible for vegetation to sustain 

injury. Sensitive plant species begin to experience foliar injury when exposed to ozone 

concentrations of 80-120 ppb/hour for extended periods of time (8 hours or more); however, the 

levels of exposure experienced in PAIS are not likely to cause foliar damage (Kohut 2004). 

Overall, the risk of foliar injury from ozone is low (Kohut 2004). Smooth cordgrass (Spartina 

alterniflora) is identified in PAIS as a plant species that is sensitive to elevated ozone levels

(Kohut 2004).

Particulate Matter (PM2.5)

The NAAQS standard for PM2.5 is a weighted annual mean of 15.0 μg/m3 or 35 μg/m3 in a 24-

hour period over an average of 3 years (EPA 2010b). Particulate matter concentrations, 

monitored at two different stations in the PAIS region, are available from 2000 through 2012. 

Weighted annual average PM2.5 concentrations in the PAIS region have been relatively stable 

since 2000, fluctuating between 9 and 11 μg/m3 (Figure 34). All measurements are well within 

the EPA standards for levels that are protective of human health; concentrations on the haziest 

days contribute to occasional impaired visibility in the park.

Figure 34. Annual particulate matter (PM2.5) concentrations (weighted annual mean) near PAIS, 2000-

2012 (EPA 2012). Note: Corpus Christi Huisache monitoring site (ID 48-355-0032) is located 

approximately 32 km (20 mi) northwest of PAIS.

Visibility

Visibility impairment occurs when airborne particles and gases scatter and absorb light; the net 

effect is called “light extinction,” which is a reduction in the amount of light from a view that is 

0

5

10

15

20

25

30

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Concentration (μg/m3)

National Standard 

weighted annual mean
263

returned to an observer (EPA 2003). In response to the mandates of the CAA of 1977, federal 

and regional organizations established IMPROVE in 1985 to aid in monitoring of visibility 

conditions in Class I airsheds. The goals of the program are to 1) establish current visibility 

conditions in Class I airsheds; 2) identify pollutants and emission sources causing the existing 

visibility problems; and 3) document long-term trends in visibility (NPS 2010b). Visibility in 

PAIS is often naturally impaired by water vapor fog from the Gulf of Mexico; however, it may at 

times be impaired by atmospheric particulate matter.

The most current 5-year average (2006-2010) estimates average visibility in PAIS to be 9.1 dv 

above average natural visibility conditions (NPS 2012). This falls into the Moderate Concern

category for NPS air quality condition assessment.

The clearest and haziest 20% of days each year are also examined for parks (NPS 2012), as these 

are the measures used by States and EPA to assess progress towards meeting the national 

visibility goal. Conditions measured near 0 dv are clear and provide excellent visibility, and as 

dv measurements increase, visibility conditions become hazier. The most current 5-year average 

(2006-2010) estimates visibility at PAIS at 8.9 dv on the 20% clearest days and 20.5 dv on the 

20% haziest days (NPS 2012).

Threats and Stressor Factors

Park managers have identified a number of threats and stressors to air quality in PAIS. These 

include the aerosolized toxins of harmful algal blooms, land use activities within and adjacent to 

the park (particularly oil and gas exploration, emissions from development), vehicle emissions 

from nearby highway/roads and traffic along the beaches where driving is permitted, emissions 

from nearby urbanized areas, and smoke from grassland fires in and around the park.

Outbreaks of HABs, specifically red tides, have been identified as a threat to air quality and 

visitor respiration, particularly when events are sustained. HABs arise when annual blooms of 

the naturally occurring microscopic algae Karenia brevis grow out of control (NOAA 2012b). K. 

brevis produces a potentially fatal neurotoxin, known as brevetoxin, which at high concentrations

can affect the central nervous systems of fish, shellfish, birds, and marine and terrestrial 

mammals, as well as cause respiratory problems in people exposed to the aerosols or sicken 

people who eat contaminated molluscan shellfish (NOAA 2012a, TPWD 2012b). Inhaled 

aerosolized red tide toxins can cause eye and nasal irritation, wheezing, difficulty breathing

(similar to asthma), and a persistent nonproductive cough (Fleming et al. 2007).

Nitrogen deposition results from nitrogen oxides in vehicle emissions, power plants, and other 

combustion sources, and ammonia from agricultural activities and fires. In ecosystems adapted to 

naturally low amounts of nitrogen (such as semi-arid systems and grasslands), increased nitrogen 

deposition can alter plant communities and reduce diversity (Sullivan 2011b). Higher nitrogen 

levels favor certain plant species, like fast-growing invasive species, at the expense of native 

forbs and shrubs (Sullivan 2011b). Sulfur emissions and particulate matter often originate from 

such sources as coal-fired power plants, petroleum refining, and chemical processing operations, 

many of which are located in central, southern and coastal Texas, as well as northern Mexico. 

Prevailing seasonal winds may carry these emissions into PAIS.
264

Data Needs/Gaps

There are monitors in nearby Corpus Christi, TX that provide particulate matter and ozone 

concentration data as both daily and annual average summaries for the region. However, for data 

to be considered representative of park conditions, monitors should be within 16 km (10 mi) of 

the park; these active monitors are approximately 30 km (20 mi) from the northern part of PAIS.

Additionally, the nearest active NADP monitor that provides annual averages for nitrogen and 

sulfur deposition is locate in Beeville, Texas, approximately 136 km (84 miles) northwest of 

PAIS headquarters; an inactive NADP monitor in Corpus Christi provided data from 2002-2006. 

Finally, the nearest CASTNet and IMPROVE sites, which monitor acid deposition and visibility 

respectively, are located in Big Bend National Park, over 800 km northwest of PAIS. Periodic or 

consistent monitoring of nitrogen and sulfur deposition and visibility would help managers better 

understand the local air quality conditions in and around PAIS.

Overall Condition

Nitrogen Deposition

The Significance Level for atmospheric deposition of nitrogen was defined as a 3. Minimal 

annual data have been collected intermittently (2002-2006) at a monitor approximately 30 km 

(20 mi) north of the park and is not enough to determine local or regional trends in deposition

rates and concentrations over time. However, Sullivan et al. (2011b, 2011d) and NPS (2010a) 

rate the semi-arid and grassland ecosystems in PAIS as moderately sensitive to nutrient 

enrichment by nitrogen deposition and at moderate risk of exposure to the pollutant. Overall risk 

is ranked as moderate relative to other parks. Likewise, current interpolated estimates for 

nitrogen deposition are considered to be of moderate to significant concern based on NPS criteria 

for rating air quality when factoring in the sensitivity of the ecosystem. Deposition of nitrogen is 

of significant concern (Condition Level = 3).

Sulfate Deposition 

The Significance Level for atmospheric deposition of sulfate was defined as a 3. Minimal annual 

data have been collected intermittently (2002-2006) at a monitor approximately 30 km (20 mi)

north of the park and is not enough to determine local or regional trends in deposition rates and 

concentrations over time. Sullivan et al. (2011a, 2011c) rate the sensitivity of semi-arid and 

grassland ecosystems in PAIS to acidification by sulfur deposition and other acids as very low 

and at moderate risk of pollutant exposure. Overall risk is ranked low relative to other parks. 

Current interpolated estimates for sulfate deposition fall into the moderate to significant concern 

category based on NPS criteria for rating air quality when factoring in sensitivity of the 

ecosystem. Deposition of sulfate is of moderate concern (Condition Level = 2).

Deposition/concentration of Mercury 

The project team defined the Significance Level for mercury concentration as a 3. No data are 

available to summarize mercury deposition/concentration rates in or near PAIS. Because there is 

no record of mercury deposition/concentration for much of south and western Texas, it is not 

possible to determine a Condition Level for this measure.

Ozone Concentration

The Significance Level for ozone concentration was defined as a 3. Current average ground-level 

ozone concentrations fall into the moderate concern category based on NPS criteria for rating air 
265

quality condition. Annual 4th highest 8-hour maximum concentrations (1993 through 2012) 

indicate a declining trend since 1995, but with fluctuations between 65 and 75 ppb in the last six 

years. All measurements, including those taken in the park in 2007 and 2008, are within EPA 

standards protective of human health. GULN (2004) suggests the risk of foliar injury from ozone 

is low for the park. Therefore, the Condition Level for ozone concentration is a 1, of low 

concern.

Particulate Matter Concentration (PM2.5)

The Significance Level for concentration of fine particulate matter (PM2.5) was defined as a 3. 

Trends in PM2.5 concentrations near PAIS have been relatively stable over the last 12 years and 

are well within the EPA standards for levels that are protective of human health. The Condition 

Level for PM2.5 is a 1, of low concern.

Visibility

The Significance Level for visibility was defined as a 3. Current interpolated average visibility 

estimates for PAIS fall into the moderate concern category based on NPS criteria. However, no 

data are collected at the park, and the nearest visibility monitor is over 676 km (420 mi) west of 

the park; this makes it difficult to determine average conditions or trends in visibility conditions 

in PAIS. The Condition Level for visibility could not be determined.

Weighted Condition Score

The Weighted Condition Score (WCS) for the air quality component is 0.583, indicating the 

condition is of moderate concern. The trend was determined to be stable.

Sources of Expertise

Ellen Porter, Biologist, NPS Air Resources Division
266

Literature Cited

Environmental Protection Agency (EPA). 2003. Guidance for estimating natural visibility 

conditions under the Regional Haze Rule. EPA-454/B-03-005. Office of Air Quality 

Planning and Standards. Emissions, Monitoring and Analysis Division, Research Triangle 

Park, North Carolina.

Environmental Protection Agency (EPA). 2008a. Air and radiation: Clean Air Act Title 

I. http://epa.gov/oar/caa/title1.html#ic (accessed 16 October 2012).

Environmental Protection Agency (EPA). 2008b. National air quality: Status and trends through 

2007. EPA-454/R-08-006. Office of Air Quality Planning and Standards, Air Quality 

Assessment Division, Research Triangle Park, North Carolina.

Environmental Protection Agency (EPA). 2009. Air quality index: A guide to air quality and 

your health. EPA-456/F-09-002. Office of Air Quality Planning and Standards, Outreach and 

Information Division, Research Triangle Park, North Carolina.

Environmental Protection Agency (EPA). 2010a. Ground-level ozone standards 

designations. http://www.epa.gov/ozonedesignations/ (accessed 19 October 2012).

Environmental Protection Agency (EPA). 2010b. Air & radiation: particulate 

matter. http://www.epa.gov/air/ (accessed 24 October 2012).

Environmental Protection Agency (EPA). 2012. AirData: Access to monitored air quality 

data. http://www.epa.gov/airdata/ad_maps.html (accessed 25 October 2012). 

Fleming, L. E., B. Kirkpatrick, L. C. Backer, J. A. Bean, A. Wanner, A. Reich, J. Zaias, Y. S. 

Cheng, R. Pierce, J. Naar, and others. 2007. Aerosolized red-tide toxins (Brevetoxins) and 

asthma. Chest131(1):187-194.

Interagency Monitoring of Protected Visual Environments (IMPROVE). 2011. Regional Haze 

Rule summary data through 1988-2010: Means for best, middle, and worst 20% visibility 

days. http://vista.cira.colostate.edu/improve/Data/IMPROVE/summary_data.htm (accessed 

25 October 2012).

Kohut, R. J. 2004. Ozone risk assessment for Gulf Coast Network. NPS/NRPC/ARD/NRTR—

2004. National Park Service, Fort Collins, 

Colorado. http://www.nature.nps.gov/air/Pubs/pdf/03Risk/gulnO3RiskOct04.pdf (accessed 

13 November 2012).

National Atmospheric Deposition Program (NADP). 2012. National Atmospheric Deposition 

Program – National Trends Network Monitoring location 

TX39. http://nadp.sws.uiuc.edu/sites/siteinfo.asp?net=NTN&id=TX39 (accessed 12 October

2012).

National Park Service (NPS). 2008. Air Atlas summary tables for I & M 

Parks. http://www.nature.nps.gov/air/permits/aris/networks/docs/SummariesAirAtlasRevised

11072003.pdf (accessed 24 October 2012). 
267

National Park Service (NPS). 2010a. Rating air quality conditions. Air Resources Division, 

Natural Resources Program Center. National Park Service, Denver, 

Colorado. http://www.nature.nps.gov/air/Planning/docs/20100112_Rating-AQConditions.pdf (accessed 26 October 2012).

National Park Service (NPS). 2010b. Air quality in national parks: 2009 annual performance and 

progress report. Natural Resource Report NPS/NRPC/ARD/NRR—2010/266. National Park 

Service, Denver, Colorado. 

National Park Service (NPS). 2012. NPS Air quality estimates: ozone, wet deposition, and 

visibility. http://www.nature.nps.gov/air/Maps/AirAtlas/IM_materials.cfm (accessed 24 

October 2012).

National Oceanic and Atmospheric Administration (NOAA). 2012a. Ocean Facts: A “red tide” is 

a common term used for a harmful algal 

bloom. http://oceanservice.noaa.gov/facts/redtide.html (accessed 20 August 2012).

National Oceanic and Atmospheric Administration (NOAA). 2012b. Brevetoxin and Florida red 

tides. http://www.nmfs.noaa.gov/pr/pdfs/health/brevetoxin.pdf (accessed 20 October 2012).

Sullivan, T. J., G. T. McPherson, T. C. McDonnell, S. D. Mackey, and D. Moore. 2011a. 

Evaluation of the sensitivity of inventory and monitoring national parks to acidification 

effects from atmospheric sulfur and nitrogen deposition: main report. Natural Resource 

Report NPS/NRPC/ARD/NRR—2011/349. National Park Service, Denver, Colorado.

Sullivan, T. J., T. C. McDonnell, G. T. McPherson, S. D. Mackey, and D. Moore. 2011b. 

Evaluation of the sensitivity of inventory and monitoring national parks to nutrient 

enrichment effects from atmospheric nitrogen deposition: main report. Natural Resource 

Report NPS/NRPC/ARD/NRR—2011/313. National Park Service, Denver, Colorado.

Sullivan, T. J., G. T. McPherson, T. C. McDonnell, S. D. Mackey, and D. Moore. 2011c. 

Evaluation of the sensitivity of inventory and monitoring national parks to acidification 

effects from atmospheric sulfur and nitrogen deposition: Gulf Coast Network (GULN). 

Natural Resource Report NPS/NRPC/ARD/NRR—2011/358. National Park Service, Denver, 

Colorado.

Sullivan, T. J., G. T. McPherson, T. C. McDonnell, S. D. Mackey, and D. Moore. 2011d. 

Evaluation of the sensitivity of inventory and monitoring national parks to nutrient 

enrichment effects from atmospheric nitrogen deposition: Gulf Coast Network (GULN). 

Natural Resource Report NPS/NRPC/ARD/NRR—2011/310. National Park Service, Denver, 

Colorado.

Texas Commission on Environmental Quality (TCEQ). 2012. Overview: Regional 

haze. http://www.tceq.texas.gov/airquality/sip/bart/haze_overview.html (accessed 16 October 

2012).
268

Texas Parks and Wildlife Department (TPWD). 2012. Red tide in Texas: Frequently asked 

questions. http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/ 

redtide/faq.phtml (accessed 20 October 2012).
269

4.16 Dark Night Skies

Description

A lightscape is a place or environment characterized by the natural rhythm of the sun and moon 

cycles, clean air, and of dark nights unperturbed by artificial light (NPS 2007). The NPS directs 

each of its units to preserve, to the greatest extent possible, these natural lightscapes (NPS 2006). 

Natural cycles of dark and light periods during the course of a day affect the evolution of species 

and other natural resource processes such as plant phenology (NPS 2006, 2012a). Several species 

require darkness to hunt, hide their location, navigate, or reproduce (NPS 2012a). In addition to 

the ecological importance of dark night skies, park visitors expect skies to be free of light 

pollution and allow for star observation. 

PAIS is located on the southern Texas coast and is affected by light pollution from nearby cities, 

most notably Corpus Christi, Texas (primarily in northern PAIS); and Brownsville, Texas (the 

southern reach of PAIS) (NPS 2012b). Figure 35 illustrates the approximate levels of artificial 

sky brightness against the natural sky brightness in southeastern Texas (NPS 2012b).

Figure 35. Artificial sky brightness in southeastern Texas (NPS 2012b). Red indicates higher incidence of 

artificial brightness while blue indicates less artificial brightness.
270

Measures

During site visits, the NPS Night Sky Team (NST) collects data for a suite of measures in order 

to define the current condition of dark night skies in a park unit. The suite of measures that the 

NST typically uses on a visit includes:

• Sky luminance over the hemisphere in high resolution (thousands of measurements 

comprise a data set), reported in photometric luminance units (V magnitudes per square 

arc second or milli-candela per square meter) or relative to natural conditions, often 

shown as a sky brightness contour map of the entire sky. V magnitude is a broadband 

photometric term in astronomy, meaning the total flux from a source striking a detector 

after passing through a “Johnson-Cousins V” filter. It is similar to the “CIE photopic” 

broadband function for wavelengths of light to which the human eye is sensitive (Bessell 

1990);

• Integrated measures of anthropogenic sky glow from selected areas of sky that may be 

attributed to individual cities or towns (known as city light domes), reported in milli-Lux 

of hemispheric illuminance or vertical illuminance;

• Integration of the entire sky illuminance measures, reported either in milli-Lux of total 

hemispheric (or horizontal) illuminance, milli-Lux of anthropogenic hemispheric (or 

horizontal) illuminance, V-magnitudes of the integrated hemisphere, or ratio of 

anthropogenic illuminance to natural illuminance;

• Vertical illuminance from individual (or groups of) outdoor lighting fixtures at a given 

observing location (such as the Wilderness boundary), in milli-Lux;

• Visual observations by a human observer, such as Bortle Class and Zenithal limiting 

magnitude;

• Integrated synthesized measure of the luminance of the sky within 50 degrees of the 

Zenith, as reported by the Unihedron Sky Quality Meter, in V magnitudes per square arc 

second;

Reference Conditions/Values

The reference condition for this resource is defined in terms of sky luminance and illuminance at 

the observer’s location from anthropogenic sources as follows:

No portion of the sky background brightness exceeds natural levels by more than 200 

percent, and the sky brightness at the Zenith does not exceed natural Zenith sky 

brightness by more than 10 percent. The ratio of anthropogenic hemispheric illuminance 

to natural hemispheric illuminance from the entire night sky does not exceed 20 percent. 

The observed light from a single visible anthropogenic source (light trespass) is not 

observed as brighter than the planet Venus (0.1 milli-Lux) when viewed from within any 

area of the park designated the naturally dark zone (Dan Duriscoe, NPS Night Sky Team, 

pers. comm., 2011).
271

Achieving this reference condition for preserving natural night skies is well summarized in the 

NPS Management Policies (2006) as follows in section 4.10: “The Service will preserve, to the 

greatest extent possible, the natural lightscapes of parks, which are natural resources and values 

that exist in the absence of human-caused light.”

Implementing this directive in PAIS requires that facilities within the park and local 

communities that utilize outdoor lighting meet outdoor lighting standards that provide for the 

maximum amount of environmental protection while meeting human needs for safety, security, 

and convenience. This means that outdoor lights within the park produce zero light trespass 

beyond the boundary of their intended use, be of an intensity that meets the minimum 

requirement for the task (but does not excessively exceed that requirement), be of a color that is 

toward the yellow or orange end of the spectrum to minimize sky glow, and be controlled 

intelligently (preventing unnecessary dusk to dawn bright illumination of areas).

Data and Methods

Data were collected in PAIS at two locations in 2009 by Wade Stablein and Teresa Jiles, NPS 

Night Sky Team Research Associate. One location was near Big Shell Beach, while the other 

was near the Bird Island Boat Ramp (Plate 35). Data were collected for a suite of measures 

during this visit, and a description of some of these measures is provided below.

Anthropogenic light in the night environment can be very significant, especially on moonless 

nights. Unshielded lamps mounted on tall poles have the greatest potential to cause light 

pollution, since light directly emitted by the lamp has the potential to follow an unobstructed 

path into the sky or the distant landscape. This type of light spill has been called glare, intrusive 

light, or light trespass (Narisada and Schreuder 2004). The dark-adapted human eye will see 

these individual light sources as extremely bright points in a natural environment. These sources 

also have the potential to illuminate the landscape, especially vertical surfaces aligned 

perpendicular to them, often to a level that approaches or surpasses moonlight. The brightness of 

such objects may be measured as the amount of light per unit area striking a “detector” or a 

measuring device, or entering the observer’s pupil. This type of measure is called illuminance

(Ryer 1997).

Illuminance is measured in lux (metric) or foot-candles (English), and is usually defined as 

luminous flux per unit area of a flat surface (1 lux = 1 lumen / m2

). However, different surface 

geometries may be employed, such as a cylindrical surface or a hemispheric surface. Integrated 

illuminance of a hemisphere (summed flux per unit area from all angles above the horizon) is a 

useful, unbiased metric for determining the brightness of the entire night sky. Horizontal and 

vertical illuminance are also used; horizontal illuminance weights areas near the Zenith much 

greater than areas near the horizon, while vertical illuminance preferentially weights areas near 

the horizon, and an azimuth of orientation must be specified.

Direct vertical illuminance from a nearby anthropogenic source will vary considerably with the 

location of the observer, since this value varies as the inverse of the square of the distance from 

light source to observer (Ryer 1997). Therefore, measures of light trespass are usually made in 

sensitive areas (such as public campgrounds).
272

Anthropogenic light which results in an upward component will be visible to an observer as “sky 

glow.” This is because the atmosphere effectively scatters light passing through it. The sky is 

blue in daytime because of Rayleigh scattering by air molecules, which is more effective for light 

of shorter wavelengths. For this reason, bluish light from outdoor fixtures will produce more sky 

glow than reddish light. Larger particles in the atmosphere (aerosols and water vapor droplets) 

cause Mie scattering and absorption of light, which is not as wavelength-dependent and is more 

directional. When the air is full of larger particles, this process gives clouds their white 

appearance and produces a whitish glow around bright objects (e.g., the sun and moon). The 

pattern of sky glow as seen by a distant observer will appear as a dome of light of decreasing 

intensity from the center of the city on the horizon. As the observer moves closer to the source,

the dome gets larger until the entire sky appears to be luminous (Garstang 1989).

Light propagated at an angle near the horizon will be effectively scattered, and the sky glow 

produced will be highly visible to an observer located in the direction of propagation. Predictions 

of the apparent light dome produced by a sky glow model demonstrate this (Luginbuhl et al. 

2009). Light reflected off surfaces (e.g., a concrete road or parking area) becomes visible light 

pollution when it is scattered by the atmosphere above it, even if the light fixture has a “full 

cutoff” design and is not visible as glare or light trespass to a distant observer. For this reason, 

the intensity and color of outdoor lights must be carefully considered, especially if light-colored 

surfaces are present near the light source.

Light domes from many cities, as they appear from a location within Joshua Tree National Park, 

are shown in Figure 36 and Figure 37, as a grayscale and in false color. This graphic 

demonstrates that the core of the light dome may be tens or hundreds of times brighter than the 

extremities. A logarithmic scale for sky luminance and false color are commonly used to display 

monochromatic images or data with a very large dynamic range, and are used extensively in 

reports of sky brightness by the NST.

Figure 36. Grayscale representation of sky luminance from a location in Joshua Tree National Park 

(Figure provided by Dan Duriscoe, NPS Night Sky Team).
273

Figure 37. False color representation of Figure 2 after a logarithmic stretch of pixel values (Figure 

provided by Dan Duriscoe, NPS Night Sky Team).

The brightness (or luminance) of the sky in the region of the light domes may be measured as the 

number of photons per second reaching the observer for a given viewing angle, or area of the sky

(such as a square degree, square arc minute, or square arc second). The NST utilizes a digital 

camera with a large, dynamic range, monochromatic charge-coupled device (CCD) detector and 

an extensive system of data collection, calibration, and analysis procedures (Duriscoe et al. 

2007). This system allows for the accurate measurement of both luminance and illuminance, 

since it is calibrated on standard stars that appear in the same images as the data and the image 

scale in arc seconds per pixel is accurately known. Sky luminance is reported in astronomical 

units of V-magnitudes per square arc second, and in engineering units of milli-candela per square 

meter. High resolution imagery of the entire night sky reveals details of individual light domes 

that may be attributed to anthropogenic light from distant cities or nearby individual sources. 

These data sets may be used for both resource condition assessment and long-term monitoring.

Figure 36 and Figure 37 contain information on natural sources of light in the night sky as well 

as anthropogenic sources. The appearance of the natural night sky may be modeled and predicted 

in terms of sky luminance and illuminance over the hemisphere, given the location, date, time, 

and the relative brightness of the natural airglow (the so-called “permanent aurora” which varies 

in intensity over time) (Roach and Gordon 1973). The NST has constructed such a model, and 

uses it in analysis of data sets to remove the natural components. This results in a more accurate 

measure of anthropogenic sky glow (Figure 38). Figure 37 represents “total sky brightness” 

while Figure 38 displays “anthropogenic sky glow” or “net light pollution.” This is an important 

distinction, especially in areas where anthropogenic sky glow is of relatively low intensity.
274

Figure 38. Contour map of anthropogenic sky glow at a location in Joshua Tree National Park, analogous 

to Figure 37 with natural sources of light subtracted (Figure provided by Dan Duriscoe, NPS Night Sky 

Team).

The accurate measurement of both anthropogenic light in the night sky and the accurate 

prediction of the brightness and distribution of natural sources of light allows for the use of a 

very intuitive metric of the resource condition - a ratio of anthropogenic to natural light. Both 

luminance and illuminance for the entire sky or a given area of the sky may be described in this 

manner (Hollan 2008). This so-called “light pollution ratio” is unitless and is always referenced 

to the brightness of a natural moonless sky under average atmospheric conditions, or, in the case 

of the NST data, the atmospheric conditions determined from each individual data set.

The reference conditions for anthropogenic sky luminance were identified as no more than 200 

percent brighter than natural conditions in any area of the sky and no more than 10 percent 

brighter at the Zenith. These values correspond to light pollution ratios of 2.0 and 0.1, 

respectively. The NST has obtained values of 50-100 for this measure at the core of city light 

domes seen from several areas administered by the NPS, including Lake Mead National 

Recreation Area, Saguaro National Park, and Colorado National Monument (NPS Night Sky 

Team, unpublished data). This is because these NPS areas are very close to the cities of Las 

Vegas, Nevada; Tucson, Arizona; and Grand Junction, Colorado, respectively. 

A quick and accurate method of quantifying sky brightness near the Zenith is the use of a 

Unihedron Sky Quality Meter. The Unihedron Sky Quality Meter is a single-channeled handheld photometric device. A single number in magnitudes per square arc second is read from the 

front of the device after its photodiode and associated electronics are pointed at the Zenith and 

the processor completes its integration of photon detection. Because the meter is relatively 

inexpensive and easy to use, a database of measures has grown since its introduction 

(see http://unihedron.com/projects/darksky/database/index.php). The NST produces values from 

each data set as both a synthesized value derived from the high-resolution images and by handheld measures with a Unihedron Sky Quality Meter. The performance of the Sky Quality Meter 

has been tested and reviewed by Cinzano (2005). While fairly accurate and easy to use, the value 

it produces is biased toward the Zenith. Therefore, the robustness of data collected in this manner 

is limited to areas with relatively bright sky glow near the Zenith, corresponding to severely light 
275

polluted areas. While not included in the reference condition, a value of about 21.85 would be 

considered “pristine”, providing the Milky Way is not overhead and/or the natural airglow is not 

unusually bright when the reading is taken. 

Visual observations are important in defining sky quality, especially in defining the aesthetic 

character of night sky features. A published attempt at a semi-quantitative method of visual 

observations is described in the Bortle Dark Sky Scale (Bortle 2001). Observations of several 

features of the night sky and anthropogenic sky glow are synthesized into a 1-9 integer interval 

scale, where class 1 represents a “pristine sky” filled with easily observable features and class 9 

represents an “inner city sky” where anthropogenic sky glow obliterates all the features except a 

few bright stars. Bortle Class 1 and 2 skies possess virtually no observable anthropogenic sky 

glow (Bortle 2001).

Another visual method for assessing sky quality is Zenithal Limiting Magnitude (ZLM), which is 

the apparent brightness or magnitude of the faintest star observable to the unaided human eye, 

which usually occurs near the Zenith. This method involves many factors, the most important of 

which is variability from observer to observer. A ZLM of 7.0-7.2 is usually considered “pristine” 

or representing what should be observed under natural conditions; observation of ZLM is one of 

the factors included in the Bortle Dark Sky Scale. Zenith Limiting Magnitude is often referenced 

in literature on the quality of the night sky, and is the basis for the international “Globe at Night” 

citizen-scientist program (see http://www.globeatnight.org/index.html). The NST has 

experimented with the use of this observation in predicting sky quality, and has found that it is a 

much coarser measure and prone to much greater error than accurate photometric measures over 

the entire sky. For these reasons, it is not included in the reference conditions section.

Current Condition and Trend

The NPS Night Sky Team (NST) visited PAIS in March 2009, and measured the quality of the 

park’s night skies at two locations: Big Shell Beach and the Bird Island Boat Ramp (Plate 35).

Big Shell Beach 

Data were collected from Big Shell Beach on the night of 30 March 2009; monitoring began at 

4:27 AM, and lasted until 6:37 AM. Important statistics from these data are presented in Table 

66.
276

Table 66. Sky quality photometric report for Big Shell Beach, 30 March 2009 (NPS 2012c).

PARK: Padre Island NS EQUIPMENT: IMG3, 50mm f/2, 6084

SITE NAME: Big Shell OBSERVERS: T Jiles, W Stablein

LONGITUDE: -97.38006 AIR TEMP (°F): 63

LATITUDE: 27.04985 REL HUMID (%): 56

ELEVATION (m): 7 WIND SP (mph): 1.5

DATE (UT): March 29, 2009 CCD TEMP (°C): -20

TIME START (UT): 4:27:58 EXP (seconds): 14

DATA QUALITY: excellent BORTLE CLASS: 3 ZLM: 6.47

SKY BRIGHTNESS DATA

Time 

(UT)

Extinction 

coefficient 

(mag/airmass)

Std Err Y 

Extinction 

Stars 

(mags)

Zenith 

(mag/sq 

arc-sec)

Whole 

Sky 

(mags)

Sky 

Above 

20° 

Altitude 

(mags)

Brightest 

(mag/sq arc-sec)

Darkest 

(mag/sq arcsec)

1st Start 4:27:58 21.60

 End 4:51:04 0.238 0.033 21.64 -7.69 -6.73 17.91 21.81

2nd Start 5:21:04 21.66

 End 5:44:09 0.233 0.026 21.66 -7.64 -6.70 17.96 21.84

3rd Start 6:14:07 21.66

 End 6:37:12 0.226 0.030 21.67 -7.61 -6.67 17.99 21.87
277

According to Teresa Jiles (NPS, NST), the night of 30 March 2013 was

Clear, calm and dry, especially for this coastal site. This site [Big Shell] has a view due 

east where sea turtle nesting occurs every mid to late spring. The view to the west, 

southwest is King Ranch where there are flashing red lights from wind turbines and a 

grass fire that lessened with intensity as the night waned, smoke drifted south. A bright 

green light near the naval base is unshielded and very intense, but does not make it to the 

east beach.

A contour map of night sky brightness in fisheye projection (Figure 39), and a panoramic 

projection of the anthropogenic sky glow (Figure 40) were created from the data gathered during 

the NST visit.

Figure 39. Contour map of sky brightness in fisheye projection, Big Shell Beach, 30 March 2009. The sky 

dome to the north is Corpus Christi, TX, while King Ranch’s sky glow is visible in the southwest.

King 

Ranch 

grass fire 

– burned 

down 

early
27

8

Figure 40. Anthropogenic light measured at Big Shell Beach, 30 March 2009, in panoramic projection.

King Ranch Corpus Christi light dome Sea turtle nesting beach
279

As is seen in Figure 39 and Figure 40, light glow is visible at the Big Shell site from both the 

King Ranch area and from Corpus Christi to the north. This site was the darkest of the two 

visited by the NST, partially due to the fact that this location is 32 km (20 mi) further south than 

Bird Island and consequently further away from the bright lights of Corpus Christi. Big Shell had 

a ZLM of 6.47, and was classified as a Bortle Class 3 sky, which is indicative of a ‘rural sky’ 

(some light pollution visible on the horizon, clouds are illuminated at the horizon, but are dark 

overhead). 

Bird Island Boat Ramp

Data were collected from the Bird Island Boat Ramp on the night of 30 March 2009; monitoring 

began at 5:32 AM, and lasted until 6:29 AM. Important statistics from these data are presented in 

Table 67.
280

Table 67. Sky quality photometric report for Bird Island Boat Ramp, 30 March 2009 (NPS 2012d).

PARK: Padre Island EQUIPMENT: IMG3, 50mm f/2, 6084

SITE NAME: Bird Island Boat Ramp OBSERVERS: T Jiles, W Stablein

LONGITUDE: -97.30969 AIR TEMP (°F): 68

LATITUDE: 27.47332 REL HUMID (%): 66

ELEVATION (m): 1 WIND SP (mph): 8

DATE (UT): March 30, 2009 CCD TEMP (°C): -20

TIME START (UT): 5:32:04 EXP (seconds): 12

DATA QUALITY: fair BORTLE CLASS: 4 ZLM: 5.8

SKY BRIGHTNESS DATA

Time (UT)

Extinction 

coefficient 

(mag/airmass)

Std Err Y 

Extinction 

Stars 

(mags)

Zenith 

(mag/sq 

arc-sec)

Whole 

Sky 

(mags)

Sky 

Above 

20° 

Altitude 

(mags)

Brightest (mag/sq 

arc-sec)

Darkest 

(mag/sq arcsec)

1st Start 5:32:04 20.76

 End 5:53:29 0.438 0.049 20.86 -8.91 -7.81 16.04 20.98

2nd Start 6:08:17 20.98

 End 6:29:43 0.34 0.042 20.84 -8.89 -7.75 16.09 21.04
281

According to Teresa Jiles, on the night of 30 March this site had low clouds near the horizon that 

distorted the Corpus Christi light dome that was located due north. The light from Corpus Christi 

spills onto the west coastal waters at the Bird Island Boat Launch site, and the coastal waters 

reflect the light up, making the sky appear brighter.

As was done at the Big Shell site, a contour map of night sky brightness in fisheye projection 

(Figure 41), and a panoramic projection of the anthropogenic sky glow (Figure 42) were created 

from the data gathered during the NST visit. The NST also compared Figure 41 to two other 

parks: the North Rim of Grand Canyon National Park (which has some of the darkest night skies 

in the NPS; Figure 43), and the Santa Monica Mountains National Recreation Area (which has 

some of the brightest night skies in the NPS; Figure 44).

Figure 41. Contour map of sky brightness in fisheye projection, Bird Island Boat Ramp, 30 March 2009. 

The sky dome to the north is Corpus Christi, TX.
282

Figure 42. Anthropogenic light measured at Bird Island Boat Launch, 30 March 2009, in panoramic projection.
283

Figure 43. Contour map of sky brightness in fisheye projection; Bird Island Boat Ramp on the left, and the North Rim of Grand Canyon National 

Park on the right. 
284

Figure 44. Contour map of sky brightness in fisheye projection; Bird Island Boat Ramp on the left, and Santa Monica Mountains National 

Recreation Area on the right. 
285

Compared to the Big Shell NST sampling site, the Bird Island Boat Ramp is significantly 

brighter (Figure 45). Bird Island had a ZLM of 5.8 (compared to 6.47 at Big Shell), and was 

classified as a Bortle Class 4 sky (indicative of a ‘suburban transition’ area, with light pollution 

domes visible in various directions over the horizon; Bortle 2001). Bird Island is closest to the 

north end of the park’s boundary, and is also closer to North Padre Island and Corpus Christi 

than Big Shell. The water at the Bird Island Boat Ramp site reflects much of the light from the 

Corpus Christi area into the thin cloud layers that are typically seen at night, making the sky 

seem very bright (NPS 2012c, d).

Figure 45. Contour map of sky brightness in fisheye projection; Big Shell Beach on the left, and Bird 

Island Boat Launch on the right.

Threats and Stressor Factors

As a barrier island situated between two major U.S. cities (Brownsville, TX; and Corpus Christi, 

TX), PAIS is threatened by the light pollution from theses cities. However, PAIS is in a unique 

region of the U.S. and faces some non-typical threats to the quality of the dark night skies. Oil 

and gas extraction has taken place on Padre Island since before the park’s establishment in 1961 

(Lawson 2009), and over 70 oil and gas operations operated in PAIS between 1951-1981 (NPS 

2005). Currently, only six active pads and 11 active wells occur within the park (Stablein, written 

communication, 13 May 2013). 

These oil and gas operations use heavy machinery and result in an increase of vehicular traffic in 

the park. The lights that are used with these operations will likely detrimentally affect the quality 

of the dark night skies in the park. Furthermore, the new light bulbs that are used on several of 

the automobiles travelling through the park (specifically the bright blue bulbs) are very bright 

and have an effect on the overall quality of night skies to observers in their vicinity (Lindsay, 

interview, 31 December 2011).

Another unique threat to the dark night skies in PAIS are recreational lights from fishing vessels 

in the Laguna Madre and the Gulf of Mexico. The lights attached to both recreational and 

commercial fishing boats, although not excessively bright, do contribute a small amount of light 

pollution at the horizon.
286

Air transparency affects night sky quality. Estimates of optical depth or b_ext from PM10 air 

monitoring stations correlate well with this effect. Haze and smoke will reduce contrast and dim 

light from the night sky, resulting in a loss of detail and character in the Milky Way, and a 

reduction in the number of stars seen by the observer, particularly near the horizon. These effects 

are reduced by higher altitude observations. However, pristine night sky quality may only be 

truly observed when pristine air quality exists above the observer.

Data Needs/Gaps

A draft plan for natural lightscape management in PAIS, which includes zoning the park area to 

indicate where outdoor lighting is required and where the naturally dark zones occur, would 

greatly benefit park managers and researchers. Light trespass measurements should be taken at 

the boundary of the naturally dark zones close to park developments and close to Corpus Christi. 

In this manner, it may be determined if standards for environmental protection are being met. 

While monochromatic and color digital photographs provide qualitative information, calibrated 

photometry is required to make a definitive judgment of the resource condition.

Continued measurement of the entire sky brightness condition should occur on a periodic basis, 

about once every 5 years, with Big Shell Beach and Bird Island Boat Ramp as the preferred 

observing sites, in order to track external threats.

Overall Condition

The northern region of PAIS (Bird Island Boat Ramp) has noticeable light pollution originating

from nearby Corpus Christi; this region also has elevated levels of light pollution reflecting off 

of the coasts and low-lying clouds. Big Shell Beach is located further south of the Bird Island 

Boat Ramp, and it has lower levels of light pollution, although the Corpus Christi and King 

Ranch light domes arre still visible (Figure 45). No survey of the night sky has been completed 

in the southern portion of the park; Figure 35 suggests that this region may be at the northern 

boundary of the artificial sky brightness that originates from Brownsville, TX, although this is 

only speculation based on a simulated map. 

While the number of oil and gas developments has decreased in recent years (and no new 

developments are currently being planned), the nearby cities of Brownsville and Corpus Christi 

continue to grow and contribute anthropogenic light trespass to the PAIS night sky. The data 

used in this assessment were collected in 2009, and significant change in the levels of light 

trespass and the size of the light domes in the night sky may have occurred. Due to the bright 

night skies observed at the Bird Island Boat Ramp, and the presence of noticeable light domes at 

Big Shell Beach, a Condition Level of 2 was assigned for the NPS suite of night sky measures. 
287

Weighted Condition Score

The Weighted Condition Score for dark night skies at PAIS is 0.666, indicating this component is 

of moderate concern.

Sources of Expertise

National Park Service Night Sky Team: Dan Duriscoe, Chad Moore, Teresa Jiles, Jeremy White, 

and Robert Meadows
288

Literature Cited

Bessell, M. S. 1990. UBVRI Passbands. Publications of the Astronomical Society of the Pacific 

102:1181.

Bortle, J. 2001. Introducing the Bortle dark-sky scale. Sky & Telescope 101(2):126-129. 

Cinzano, P. 2005. Night sky photometry with sky quality meter. ISTIL Internal Report n. 9, v. 

1.4. http://www.unihedron.com/projects/darksky/sqmreport_v1p4.pdf (accessed 19 June 

2012).

Cinzano, P., F. Falchi, and C. D. Elvidge. 2001. The first world atlas of artificial sky brightness. 

Monthly Notices of the Royal Astronomical Society 328:689-707.

Duriscoe, D. M., C. B. Luginbuhl, and C. A. Moore. 2007. Measuring night-sky brightness with 

a wide-field CCD camera. Publications of the Astronomical Society of the Pacific 119:192-

213.

Garstang, R. H. 1989. Night-sky brightness at observatories and sites. Publications of the 

Astronomical Society of the Pacific 101:306-329.

Hollan, J. 2008. What is light pollution and how do we quantify it? Workshop paper at Darksky 

2007 conference, Brno, Czech 

Republic. http://amper.ped.muni.cz/hollan/light/lp_what_is.pdf (accessed 19 June 2012).

Lawson, A. L. 2009. Impacts of oil and gas development on wintering grassland birds at Padre 

Island National Seashore, Texas. Thesis. Texas A&M University, College Station, Texas.

Luginbuhl, C. B., D. M. Duriscoe, C. W. Moore, A. Richman, G. W. Lockwood, and D. R. 

Davis. 2009. From the ground up II: sky glow and near-ground artificial light propagation in 

Flagstaff, Arizona. Publications of the Astronomical Society of the Pacific 121:204-212. 

Narisada, K., and D. Schrueder . 2004. Light pollution handbook. Astrophysics and space 

sciences library volume 322. Springer Publishing, New York, New York.

National Park Service (NPS). 2005. Padre Island National Seashore: administrative 

history. http://www.cr.nps.gov/history/online_books/pais/adhi8.htm (accessed 6 March 

2013).

National Park Service (NPS). 2006. Management policies 2006. ISBM 0-16-076874-8. U.S. 

Department of the Interior. National Park Service, Washington, D.C.

National Park Service (NPS). 2012a. Air resources division – natural 

lightscapes. http://nature.nps.gov/night/index.cfm (accessed 19 June 2012).

National Park Service (NPS). 2012b. Lightscape/night sky: Padre Island National 

Seashore. http://www.nps.gov/pais/naturescience/lightscape.htm (accessed 25 April 2012).
289

National Park Service (NPS). 2012c. PI090329_data_contours.ppt. Microsoft PowerPoint. 

Received from W. Stablein, June 2012.

National Park Service (NPS). 2012d. PI090330_data_contours.ppt. Microsoft PowerPoint. 

Received from W. Stablein, June 2012.

Roach, F. E., and J. L Gordon. 1973. The light of the night sky. D. Reidel Publishing, Dordrecht, 

Holland. 

Ryer, A. 1997. The light measurement handbook. International Light, Inc., Technical 

Publications Division, Newburyport, Massachusetts.
290

Plate 35. Dark night sky sampling locations within PAIS; both sites were surveyed on 30 March 2009.
291

4.17 Coastal Dunes and Beaches

Description

Barrier islands consist of a complexity of geological formations (KellerLynn 2010). Back 

beaches form the wet or swash zone along the shoreline, while the fore beach defines the area of 

coppice or emergent dunes. Adjacent to the fore beach is the fore-island dune ridge, which is the 

first significant rise in elevation from the beach zone. Behind the fore-island dune ridge are 

stabilized and active blowout dunes (KellerLynn 2010). These are dynamic features, in that they 

change frequently under the powerful influence of consistent offshore winds, tides, wave action, 

weather patterns, and storm events (Weise and White 1980, KellerLynn 2010, USGS 2013). In 

PAIS, these geologic features are a primary aspect of what makes the park unique; the beach and 

dune zones create important habitat for a variety of animal species, as well as recreational 

opportunities for visitors. 

Weise and White (1980) detail the geology of PAIS. They describe a shore zone, which consists 

of the beaches adjacent to the water’s edge, and emergent dunes, which are located immediately 

behind the shore zone (Photo 26). The beaches along the seashore are the source of sand that 

form and build the emergent and fore-island dunes (Weise and White 1980). Wave action 

deposits sand and sediment on the beaches, which are dried, picked up, and carried inland by 

steady offshore winds. The sands are deposited and trapped by vegetation, creating fore-island 

and back-island dunes (Weise and White 1980, USGS 2013).

Photo 26. Emergent dunes in PAIS (left); beach zone adjacent to the water’s edge in PAIS (right) (Photos 

by Shannon Amberg, SMUMN GSS).

Established fore-island dunes, created mostly through wind-blown sand deposited behind the 

shore, typically form the first and highest dune ridge behind the shore zone (Weise and White 

1980, USGS 2013) (Photo 27). These dunes run parallel to the beach and shoreline and often are 

covered at least partially with vegetation, which acts to catch more sand as it is carried inland, as 

well as stabilize the dunes from erosion that may result from light or moderate winds (Weise and 

White 1980, USGS 2013).
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Back-island dunes are located behind the fore-island dune ridge, and typically tend to be lower in 

height than the fore-island dunes and support a significantly higher density of vegetation cover 

(USGS 2013). However, what used to be the tallest dunes in PAIS (until recently eroded by 

recreational impact, including Green Hill) are all located behind the fore-island dune ridge 

(Lindsay, pers. communication, 12 July 2013). Active dunes, in contrast, have little to no 

vegetation, allowing the dune sands to move freely across the island with predominant winds 

(USGS 2013). Photo 28 shows examples of vegetated back-island dunes (stabilized dunes) and 

active blowout dunes in PAIS. Figure 46 shows a cross-section of the geomorphologic features 

of PAIS, depicting beach and dune formations moving from the Gulf side (east) toward the 

Laguna Madre (west).

Photo 27. View of the fore-island dune ridge as seen from the Gulf beach in PAIS (left); atop the foreisland dune ridge looking south down the length of the fore-island ridge parallel to the beach (right) 

(Photos by Shannon Amberg, SMUMN GSS).

Photo 28. An active blowout dune in the northern part of PAIS (left); vegetated back-island dunes, or 

stabilized dunes, as viewed from the grasslands in the northern part of PAIS (right) (Photos by Shannon 

Amberg, SMUMN GSS).
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Figure 46. Geomorphologic formations of Padre Island National Seashore, showing the major zones in 

land formation across a barrier island (KellerLynn 2010).

Wind is the dominant process in the creation of barrier islands and shaping the shoreline and land 

features at PAIS (KellerLynn 2010). In turn, predominant 

winds drive ocean currents and create waves that 

transport sediments and deposit or erode materials onto 

the beaches along the shoreline (KellerLynn 2010). 

Longshore Gulf currents, converging at 27° latitude also 

influence island development by carrying sediment 

toward the island, where it is easily deposited onto 

beaches by wind generated wave and tidal action 

(KellerLynn 2010). 

The sediment that makes up a majority of the dunes and 

beaches in PAIS is fine sand, which becomes coarser in 

composition from the north to the south boundary of the 

seashore (KellerLynn 2010). Shell and shell fragments

(shell hash) are also a common component in beach 

composition in PAIS. Specifically, as much as 80% of 

beach composition consists of shells, occurring primarily 

at the convergence zone of the longshore (the Yucatan 

and Texas longshore) currents at Little Shell and Big 

Shell beaches, where the collision of these currents 

suspends materials and deposits them on shore (Weise 

and White 1980, KellerLynn 2010). Photo 29 shows the 

predominantly shell composition of Big Shell Beach in 

PAIS.

Persistent blowing sands are ubiquitous in PAIS and the rate of sand and sediment migration 

across the island is such that maintenance of permanent roads and walkways (moving piled sand

off the roads) is quite frequent (KellerLynn 2010). 

Photo 29. Big Shell Beach at PAIS 

(Photo taken by Shannon Amberg, 

SMUMN GSS).
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Measures 

• Annual change in shoreline position

• Rate of change in dune elevation (emergent, fore-island dune and back-island dunes)

• Rate of dune migration

• Rate of sand deposition/erosion (for beaches, emergent, fore-island dunes, and backisland dunes)

Reference Conditions/Values

Barrier islands and eolian dune systems are highly dynamic geomorphological features that 

undergo near constant change due to the influence of winds, waves, ocean currents, tidal action, 

storm events, and erosion and accretion of materials (KellerLynn 2010). The constant state of 

fluctuation and change for these geomorphic features is a natural process, periodically migrating 

landward or accreting along the shoreline. PAIS was primarily established to protect much of 

Padre Island from further development, and to ensure the natural barrier island features, and the 

dynamic processes that create them, could be left uninhibited. Because of the dynamic nature of 

the environment and constant change the coastal beaches and dunes undergo as a natural process, 

it was decided that a reference condition was unnecessary for this component. However, 

measures have been designated to track the rate of change and movement over time for the 

barrier island features. 

Data and Methods

Nearly all studies at PAIS or along the Gulf coast of Texas focus on the dynamics and mechanics 

of the eolian dune (built by wind and water) and barrier island systems versus documenting 

change in dune structure and migration or shoreline position over time. Morton (1977) described 

the causes and mechanics of shoreline change on the Texas Gulf coast, including those factors 

that contribute most to beach erosion. Weiner (1982) documented the impact of predominant 

winds and weather patterns on oblique dune elevations and migration across the barrier island 

and how these changes vary seasonally. Hummel and Kocurek (1984) discussed the rate of dune 

migration during 1 year of mapping, and how migration direction is affected seasonally by 

predominant winds and weather patterns. Kocurek et al. (1992) documented various phases 

(constructive and destructive) of dune development, and found that deposition and accumulation 

of sands for dune building is dependent upon airflow patterns, sand supply, and moisture levels

occurring seasonally. Dahl and Goen (1974) determined the importance of native vegetation in 

capturing blowing sands and stabilizing and building the foredune ridge.

The NPS completed a Geologic Resources Inventory (GRI) for PAIS in 2010 (compiled by 

KellerLynn 2010). The purpose of the GRI was to provide park managers with baseline 

information on geologic resources and increase understanding of the processes that influence 

geomorphology in the park. The goal of this GRI was to provide PAIS managers with sound 

geologic information that may be used in resource management decision-making, as ecosystems 

within PAIS are fundamentally shaped by the geology of the eolian beach and dune systems. The 

PAIS GRI describes the geomorphology of the area of North Padre Island within park boundaries 

and discusses its importance in creating unique ecosystems along the Texas coast. The GRI also 
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discusses in depth various geologic issues or threats and stresses to geomorphological processes 

in the park that may require management attention (KellerLynn 2010). 

Some studies have documented rates of shoreline change, dune migration, and materials 

deposition or erosion. Gibeaut et al. (2001) provides data regarding more recent shoreline 

position. Gibeaut et al. (2001) is a summary report of long-term rates of shoreline change along 

the Texas Gulf Coast shoreline. The area of study spans the coastline from Aransas Pass to the 

north boundary of PAIS, including Mustang and North Padre Islands. Gibeaut et al. (2001) did 

not study shoreline change within PAIS. This study used airborne LiDAR (Light Detection and 

Ranging) surveys to map the shoreline from Aransas Pass to the north boundary of PAIS; 

mapping occurred on 21 September 2000. Four passes were made by an aircraft at an altitude of 

750 m (2,460 ft) and imagery data were captured extending approximately 500 m (1,640 ft) 

inland from the seashore. The data identified shoreline, foredunes, secondary dunes, and 

oceanfront structures. From the LiDAR data points, a DEM with a 1.5 m by 1.5 m grid was 

constructed and used to determine the current shoreline position; this shoreline was deemed an 

order of magnitude more accurate in position than earlier mapping using aerial photography. 

Historical shorelines were mapped on vertical aerial photographs (scale of 1:24,000 or larger) in 

1937, 1956/58/59, 1965/69, 1974, and 1990/95. These were scanned and digitized in ArcView 

GIS software, then exported and analyzed by the Shoreline Shape and Projection Program 

(SSAP) developed by the Bureau of Economic Geology at The University of Texas at Austin. 

Historical shoreline position was compared to shoreline position as of September 2000. Linear 

regression modeling was used to calculate the average annual rate of shoreline change over time. 

Results are presented in the Current Condition and Trend section below.

In a report from the Bureau of Economic Geology at the University of Texas at Austin, Paine et 

al. (2011) characterize long-term rates of shoreline change along the Gulf Coast of Texas from 

1930 through 2007. The Texas coastline was divided into three segments (Upper Texas Coast, 

Central Texas Coast, and Lower Coast/Padre Island). Rates of shoreline change were calculated 

through 2007 using shoreline positions depicted in topographic charts from the 1800s, aerial 

photography (1930-2007), ground surveys using GPS (mid-1990s), and an airborne LiDAR 

survey (conducted in 2000). Net rates of change were calculated for 11,731 sites, spaced 50 m 

apart, along the Texas Gulf of Mexico shoreline (535 km [332 mi]). Net shoreline change is 

discussed in the Current Conditions and Trend section below. 

The Harte Research Institute at Texas A&M University – Corpus Christi has collected updated 

shoreline positions along the Texas coast (since Paine et al. 2011 and Gibeaut et al. 2001); 

however, new shoreline rates of change have not been calculated yet (Diana Del Angel, Coastal 

Geoscientist, pers. comm., 29 July 2013).

Pendleton et al. (2004) provide a coastal vulnerability assessment of PAIS to rising sea levels. 

Using a coastal vulnerability index, the authors characterized relative vulnerability to future sealevel rise for the PAIS Gulf of Mexico shoreline. The coastal vulnerability index ranks 

geomorphology, regional coastal slope, rate of relative sea-level rise, rate of change along 

shorelines, mean tidal range, and mean significant wave height in terms of their physical 

contribution to coastal change related to sea-level rise. Taken into consideration in this approach 

is the coastal system’s natural ability to adapt to changing environmental conditions in 

conjunction with its susceptibility to such change. The index provides a quantitative measure of 
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vulnerability to sea-level rise, although this is a relative measure. The assessment highlights the 

regions in PAIS where the physical effects of future sea-level rise may be greatest; these are 

discussed in the threats and stressors section below. 

The GULN is working to establish a monitoring program to assess rates of change in materials 

deposition and erosion along the shoreline and in the dunes of PAIS using LiDAR technology 

and volumetric change analyses.

Current Condition and Trend

Annual Change in Shoreline Position

Early studies show that shoreline change along the Texas coast is a highly dynamic and complex 

process. According to Morton (1977), shoreline change along the Texas Gulf coast is caused by a 

complex interaction of multiple factors, including climate, available sediments, coastal 

processes, conditions of relative sea level rise, and various human activities. Although the retreat 

pattern of barrier islands toward the mainland is a natural occurrence, the author identified jettied 

inlets and navigation channels as the greatest detractors of available sediment (sediment sink), 

depriving natural processes of the sediment necessary for beach deposition and building; this 

may speed up the natural movement of the island toward the mainland. Morton (1977) suggested 

that rates of erosion and total length of eroding shoreline on the Texas coast have increased since 

historical documentation; data collected by Morton suggest this has been due to human impacts 

and sea level rise. It is further suggested that most of the Texas coastline would continue a 

natural retreat pattern toward the mainland.

Gibeaut et al. (2001) determined that the shoreline position between Aransas Pass and the north 

boundary of PAIS has been retreating since the 1930s, when the first aerial photos of the 

shoreline were taken. Plate 36 shows the location of Aransas Pass relative to the north boundary 

of PAIS; this stretch of shoreline between Aransas Pass and PAIS was the focus of Gibeaut et 

al.’s (2001) shoreline position study. Figure 47 shows the trend in shoreline position change 

along the stretch of shoreline studied from the 1930s through 2000. There are some areas of 

variability in change, which are likely caused by human alterations such as jetties, piers and 

seawalls, dredging, or other development activities. 
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Figure 47. Average annual change in shoreline position (m/yr) from Aransas Pass to the north boundary 

of Padre Island National Seashore (reproduced from Gibeaut et al. 2001).

Paine et al. (2011) calculated the long-term average rate of change for Texas Gulf of Mexico 

shoreline position between the 1930s (some images dated back to the late 1800s) and 2007 to be 

1.24 m of net retreat per year. Of 11,731 sites evaluated along the entire Texas coastline, net 

retreat was found at 9,830 (more than 80%) sites. The Lower Coast segment, comprised

primarily of Padre Island, is largely undeveloped except for concentrated development at the 

northern and southern ends of the island. This segment encompasses 183 km (114 mi) of 

shoreline, which is classified primarily as a sandy barrier island with a well-developed eolian 

dune system along most of its length (Paine et al. 2011). Rates of change for shoreline position 

were calculated across 3,663 sites along the lower coast segment. Figure 48 shows the calculated 

net change in shoreline position for the Lower Texas Coast segment including PAIS. Despite 

Padre Island’s location in a longshore drift convergent zone, net shoreline retreat (toward the 

mainland) was found at 84% of sites. Shoreline change at individual sites ranged from an 

advancement of 3.2 m/yr to retreat at 7.5 m/yr (Paine et al. 2011). Locations where advancing 

shoreline was detected included the segments (approximately 5 km in length) adjacent to the 

north and south jetties at Brazos Santiago Pass (south of PAIS), a segment near Little Shell 

Beach within PAIS (approximately 15 km [9 mi] long), and a stretch (approximately 5 km [3 mi] 

long) in the northern part of PAIS in central Kleberg County (Paine et al. 2011). The authors 

report that many locations with the highest rates of net shoreline retreat on Padre Island occur 

south of PAIS toward the southern end of the barrier island. Overall, PAIS is one of the areas of 
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lower Texas coastline where the least amount of shoreline retreat is occurring; however, within 

PAIS, where retreat is occurring, the highest rates were measured along an 8 km (5 mi) stretch 

north of the Mansfield Channel jetties (>3 m/yr). Paine et al. (2011) conclude that engineering 

structures have affected shoreline position significantly in certain locations, namely jetties that 

are associated with shipping and boater access channels. These calculations will be updated 

using new LiDAR survey results captured in spring 2012.

Figure 48. Net rates of long-term change in shoreline position for the lower Texas Gulf of Mexico, 

including PAIS, calculated from shoreline positions from 1930 through 2007 (Paine et al. 2011).

PAIS managers have observed accretion and erosion throughout the seashore over the years. In 

particular, the shoreline in the northern one-third of the park has been accreting over time. 

During the time when cattle ranching occurred on the island, sediments were carried out into the 

Laguna Madre, extending the island toward the mainland. When cattle were removed from the 

island, and as vegetation recovered, sand transport declined substantially, causing the Laguna 

Madre shoreline to erode quickly. Additionally, waves generated by boating and shipping traffic 
299

on the GIWW may be adding to the incidence of erosion along the Laguna Madre shoreline 

(Lindsay, personal communication, 29 July 2013).

Rate of Change in Dune Elevation

Weiner (1982) described that, depending on predominant wind patterns and season, oblique 

dunes on South Padre Island can change in elevation significantly. During early spring (March) 

and early fall (October), variable wind patterns and direction tend to flatten the dunes, while 

predominant winds that persist in a general direction during the summer and winter months build 

oblique dunes throughout the season.

LiDAR data as recent as 2012 do exist for PAIS, but dune elevations have not been extracted and 

analyzed yet to estimate elevation for the features within PAIS. These data could be compared to 

older or future LiDAR surveys to understand how features change over time.

Houser and Mathew (2011) studied the variability of dune height and extent on South Padre 

Island and the influence of sediment supply and foreshore and shoreline morphology. Using 

LiDAR data collected in 2000 and 2005, the authors characterized the alongshore variation in 

dune morphology and identified how dune height and extent are related to shoreline shape, 

potential for sediments to be transported, and sediment availability. The authors concluded that 

dune morphology (height and extent) is a result of both transport potential of sediments and 

sediment availability. For instance, the authors note that the largest dunes on South Padre Island 

are found where the beach is intermediate width but with a large supply of sediment available for 

transport on the backshore and foreshore areas. They argue that dune elevation and extent are a 

function of both transport potential and volume of sediment available for transport.

Rate of Dune Migration

Rates of dune migration for active dunes can vary yearly, but average rates can be determined by 

examining the position of specific dunes on aerial photographs or satellite imagery captured 

across several years (Weise and White 1980, USGS 2013). No recent studies have focused on 

rates of dune migration across the barrier island. LiDAR data as recent as 2012 do exist for 

PAIS, but current dune positions and degree of migration over time have not been extracted and 

calculated yet for the features within the park. 

It appears that dune feature migration varies, depending on annual or seasonal wind patterns. 

Weiner (1982) found that, from April through September, during prevailing onshore winds, 

oblique dunes migrate northwestward. During times of high velocity wind events in the winter, 

dunes can migrate rapidly southward, while frequent changes in wind pattern and direction 

during the early spring and early fall months can cause dunes to flatten (Weiner 1982). Active 

(transgressive) dunes on South Padre Island were found to migrate up to 5-15 m/year (16-49 

ft/year) during mapping completed in 2005 through 2009 (Diana Del Angel, Coastal Scientist, 

Harte Research Institute, pers. comm. 15 July 2013). Catastrophic weather events, such as 

hurricanes or tropical storms, have less long-term effect on dune migration than do the 

predominant winds and long-term weather patterns (Weiner 1982). Dahl and Goen (1974) found 

that foredunes with well-established vegetation prevented sand migration into the backdune areas 

of the island.
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Hummel and Kocurek (1984) mapped interdune areas of the back-island dunes on North Padre 

Island over the course of 1 year. They described the back-island dune areas as young (about 100 

years old) and consisting of persistent oblique dunes up to 6 m (19.7 ft) high and areas of sheet 

sand. Hummel and Kocurek (1984) found that, over 1 year of mapping, sand transport on the 

island was shown to be highly complex and cyclical. The dune field and individual dunes 

showed a net migration of approximately 15 m/year (49 ft/year) to the northwest overall, but are 

seasonally affected by varying weather frontal systems and wind direction. In winter, sands are 

shifted to expand the interdune area, while in summer, sands shift from the interdune area and 

build back-island dunes in size (Hummel and Kocurek 1984).

Rate of Sand Deposition/Erosion

Kocurek et al. (1992) documented that back-island dune fields on Padre Island go through 

seasonal changes, both destructive and constructive, in which they are reduced to nearly a flat 

surface during the winter months and reform again in the spring and summer months. Deposition 

of dry materials initially depends on weight and roughness of the materials, lowering the 

capacity of transport as elements become heavier and rougher. The building and survival of 

dunes depends on predominant and secondary airflow patterns, sand supply, advanced stage of 

development, and overall size (Kocurek et al. 1992). Dunes ultimately grow at the expense of 

transporting sand away from the interdune flats; however, moisture and damp surfaces allow 

increased and sustained accumulation on interdune flats. Kocurek et al. (1992) describe the Padre 

Island dune fields as representative of an eolian system where interdune flats expand and 

accumulate sands during periods of water table rise and constrict when the water table is static or 

falls.

Dahl and Goen (1974) conducted a 5-year study examining an experimental fore-island dune 

stabilization with native grasses. The authors found that native vegetation, particularly grasses, 

helped to stabilize and build the fore-island dune ridge through capture of blowing sands. Plots 

planted with native grasses captured 2-2.5 times more sand than dune areas with limited or no 

vegetation. Total sand accumulation in the unplanted areas was well-below the accumulation in 

the experimental planting areas. 

Threats and Stressor Factors

Although erosion and sand transport is a natural dynamic in the barrier island and eolian dune 

systems, a number of factors can impact dune and beach integrity in PAIS. These include 

drought, vegetation loss, hurricanes, storm surges, high tide events, various forms of human 

disturbance, restriction of available sediments, and relative sea-level rise.

Drought and Loss of Vegetation

Vegetation plays a key role in trapping windblown sands to build and stabilize dunes along the 

fore-island dune ridge and preventing sand migration into back-island areas (Weise and White 

1980). Sparsely vegetated dunes are more susceptible to breaching by heavy winds and strong 

storm surges. Sand and sediment not stabilized by vegetation is easily transported across the 

island, toward and into the mud flats, bays, and lagoon. Barrier islands are the first land feature 

in the path of storm surges (from hurricanes or tropical storms) and a well-developed and 

stabilized vegetated fore-island dune ridge is an important defense against inundation and 

destruction for the mainland. Loss of vegetation would compromise the stability and integrity of 
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the fore-island dune ridge, reducing the barrier island’s ability to dissipate large amounts of wave 

and water current energy (Weise and White 1980, USGS 2013). 

Hurricanes, Storm Surges, and High Tides

Tropical storms and hurricanes have a long history of impacting the Texas Gulf of Mexico 

coastline (Paine et al. 2011). Barrier islands along the Texas Gulf coast are the first land feature 

in the path of approaching hurricanes and the accompanying storm surges (KellerLynn 2010). 

NOAA historical accounts indicate a total of 64 hurricanes and 56 tropical storms have made 

landfall along the Texas coastline between 1850 and 2010; an average of four hurricanes and 

four tropical storms hit the Texas coast every decade (Roth 2010). 

Scientists have determined two primary influences in the erosion potential of hurricanes and 

tropical storms: storm surge height and duration. The longer the seas are elevated above normal 

along the coastlines during a storm event, the more likely it is that sediment is eroded from the 

beach and redistributed (Paine et al. 2011). Heavy winds and high tides, which accompany 

severe storm events, can cut into fore-island dunes, overwashing into and inundating low-lying 

areas or carrying sands and sediment back to the beaches and out into the Gulf (Weise and White 

1980, KellerLynn 2010). During particularly strong storm events, winds and storm surges can 

carry sands to the back side of the barrier island and into the Laguna Madre, redistributing the 

sands across the island. Where vegetation is sparse, the fore-island dune ridge is not well 

developed and the force of wind and waves from strong storms and surges can create gaps or 

breaks in the ridge. Subsequently, sand is blown or washed into the back island area, forming 

blowout dunes and back-island active dunes that migrate toward the lagoon and deposit sediment 

into the lagoon, unless stabilized by vegetation (Weise and White 1980). Big storm events 

accelerate the natural erosion and deposition processes on a barrier island, changing beaches and 

dune features in ways that would typically take months or even years under normal conditions 

(Weise and White 1980, KellerLynn 2010, Paine et al. 2011).

In an assessment of the effect of Hurricane Allen on South Padre barrier island vegetation, Judd 

and Sides (1983) found that the percent cover of backshore zone vegetation following the 

hurricane was only 11% of what it was along line transects prior to the hurricane. Foredunes 

were found to be completely flattened by the storm. Low-growing plant species with shallow 

root systems were most vulnerable to dislodge and transportation by storm surge waters, whereas 

taller, grass-like species persisted. Loss of vegetative cover makes dunes susceptible to erosion 

and sands vulnerable to wind transport lagoon-ward.

The Harte Research Institute at Texas A&M University at Corpus Christi recently developed a 

Storm Susceptibility index for the Texas coast. This index identifies the theoretical level of 

protection that beaches and fore-island dune features provide against storm surge and erosion 

that would result from a tropical storm or hurricane event. This index may be able to provide 

managers with information on which areas are more likely to overwash during storm events of 

varying magnitudes (Del Angel, pers. communication, 29 July 2013).

Human Disturbance (including vehicle traffic, recreation in dunes, and oil and gas drilling) 

PAIS is unique in that most of the seashore’s beaches (approximately 101 km [63 mi]) are open 

to motor vehicle traffic; only just over 7 km (4.5 mi) of beach are off limits to beach driving 

(between the North Beach and South Beach access roads, including Malaquite Beach) 
302

(KellerLynn 2010). Over the years, exploring the seashore by driving along the beaches has 

become a common and popular occurrence (KellerLynn 2010). Various studies in the 1970s and 

1980s investigated the impacts of recreational driving on the dune and beach environments. 

KellerLynn (2010) summarizes many observed impacts, including: inhibiting shoreline 

accretion, increasing shoreline erosion and instability during storms, lowering dune elevation and 

changing dune form, reducing vegetation height, destroying vegetation and destabilizing dunes, 

changing distribution of beach and coppice dune fauna and invertebrates (e.g., ghost crabs, sea 

turtles, and birds), impacting benthic invertebrate populations, and negatively affecting the 

natural rebuilding capacity of fore-island dunes and coppice dunes between major storm events. 

McAtee and Drawe (1981) documented damage to beach and fore-island dune vegetation on 

North Padre Island and PAIS caused by vehicle and pedestrian traffic. Damage was also 

compounded by weather (i.e., drought or heavy rains), salinity, and evaporation. Blum and Jones 

(1985) found that the density and complexity of vegetation on fore-island dunes on North Padre 

Island varied based on the amount of vehicular and pedestrian traffic experienced at different 

study sites. Sites sustaining greater amounts of traffic exhibited decreased density and 

complexity of vegetation. 

Baccus et al. (1977) found a significant difference in diversity of floral composition between

dune areas experiencing heavy versus light recreational traffic. Similarly, beaches experiencing 

heavier traffic exhibited higher incidence of impact (i.e., erosion, sand displacement, damage to 

vegetation) than those with less visitation. Baccus et al. (1977) goes on to suggest the dune and 

beach capacity to entrap blowing sands is substantially compromised when vegetation is 

damaged or destroyed.

Houser et al. (2013) used LiDAR data to determine if beach driving contributed to differences in 

beach and dune morphology between areas of open access and areas where driving is restricted. 

The authors found that beach driving in PAIS does not affect fore-island dune height or volume, 

but does significantly decrease the elevation of the crest and base of dunes compared to sections 

of beach where driving is restricted. Beach driving was not found to contribute to net loss of 

sediment, but driving on the beach does make fore-island dunes more susceptible to scarping and 

overwash during tropical storms and hurricanes (Houser et al. 2013). 

Oil and gas development occurs within PAIS under special permits for drilling. Despite many 

precautions taken and mitigative measures in place to prevent potential spills and accidental 

leaks, the presence of such activity presents a threat to the geology, groundwater, and sensitive 

habitats of the park. Daily traffic to and from oil and gas drilling sites is limited to a maximum 

number of trucks per day and driving at or below 40 km/hr (25 mi/hr) to reduce potential for 

erosion or other damage to features, vegetation, or fauna (KellerLynn 2010).

Activities Restricting the Flow of Sediments

Activities such as off-shore sand mining, dredging in the Laguna Madre, channels and jetties, 

and dams on rivers can deprive the Gulf shores of sediment and sands that are crucial for beach 

and dune building. Two artificial channels, the Mansfield Channel and Brazos Santiago Pass, 

were created through Padre Island to connect the Gulf of Mexico to the Laguna Madre 

(KellerLynn 2010). Mansfield Channel marks the southern boundary of PAIS; it is a maintained 

pass that is used for shipping traffic and recreational access from the lagoon to the Gulf. 
303

Mansfield Channel is lined with rock jetties 

that extend approximately 500 m (1,640 ft) 

out into the Gulf. These jetties cause 

inshore sediments to bypass the near shore 

or littoral zone and disrupt the transport 

ofsediments along the long shore (Jeffrey 

Bracewell, GIS specialist, pers. Comm., 29 

July 2013). Sediment becomes trapped on 

the south side of the Mansfield Channel and 

jetty, building the beach out to the end of 

the jetty. The beach on the north side of the 

jetty erodes at a rate that requires 

renourishment every few years (Lindsay, 

pers. Comm., 29 July 2013). Any sediment 

that moves past the south jetty is 550 m

(1,800 ft) out in the Gulf. Flow from 

Mansfield Channel pushes the sediment 

even further out into the Gulf; it may take 

as much as 40 km (25 mi) of northerly 

travel in the Gulf for the sediment to move 

back to shore (Lindsay, pers. Comm., 29 July 2013). Photo 30 shows the influence of the 

Mansfield Channel jetties on the transport of sediment into the Gulf and along the shoreline.

Sediment is periodically dredged from the Mansfield and Brazos Santiago passes to keep the 

channels deep enough for boating and barge traffic to pass safely. Dredging removes materials 

that would typically flow through the channels and potentially deposit onto beaches by way of 

currents or storm events. Dredging also occurs on the GIWW.

Relative Sea Level Rise

Climate models predict that by 2100, global sea level will rise approximately 0.35 m (0.23 to 

0.47 m) (IPCC 2007). Impacts to shorelines can include erosion, saltwater inundation of 

wetlands and groundwater aquifers, threats to cultural, historic and natural resources, and threats 

to infrastructure. Rising sea levels also contribute to shoreline retreat via submergence (through 

inundation of low-lying areas) and increasing the incidence of retreat by physical erosion (from 

currents and wave action) (Paine et al. 2011). Paine et al. (2011) summarize the most recent 

relative sea-level rise rates for coastal Texas; the highest rates of rise (>5 mm/year) are occurring 

at gauge sites along the upper and central Texas coast (Galveston, Sabine Pass, and Rockport), 

while the lowest rates (1.93 mm/year) are occurring near Port Mansfield. Gauges at Port Isabel, 

north Padre Island, and Freeport have calculated rates of sea-level rise between 3.48 and 4.35 

mm/year (Paine et al. 2011).

Pendleton et al. (2004) estimate the overall vulnerability of PAIS to sea level rise as very high 

for the entire shoreline, specifically the tidal range. However, the actual vulnerability varies 

along the shoreline. For instance, 17% of the Gulf shoreline at PAIS is determined to have very 

high vulnerability, 28% has high vulnerability, 29% has moderate vulnerability, and 26% has 

what is categorized as low vulnerability (Pendleton et al. 2004). The authors determined areas 

Photo 30. Imagery of Mansfield Channel jetties at the 

southern boundary of PAIS shows the flow of 

sediments through the channel and away from the 

littoral nearshore zone (Source: Google maps 

imagery https://maps.google.com, 2013).
304

most highly vulnerable are those that currently have the highest occurrence of overwash and the 

highest rates of shoreline change. Those areas considered vulnerable are more susceptible to 

shoreline erosion, saltwater intrusion into groundwater, inundation of wetlands, and damage to 

infrastructure (Pendleton et al. 2004).

NOAA’s National Water Level Observation Network has been monitoring sea level rise with 

tide stations for over 150 years. Changes in mean sea level (MSL) have been computed at 128 

long-term water level stations using a minimum span of 30 years of observations at each 

location. Measurements are averaged by month to remove the effects of phenomena such as 

storm surges and compute an accurate linear sea level trend. Based on data from 1948 through 

2012, sea level rise in the Padre Island region (both north and south islands) is recorded at 

approximately 0-6 mm/year or 0-2 ft/century (NOAA 2013). Three monitoring stations in the 

North/South Padre Island region have recorded data on sea level rise over the last century: 

Station 8779750 at South Padre Island, Station 8778490 at Port Mansfield, and Station 8774770 

at Rockport, Texas. Trends in mean annual sea level rise are presented below.

The mean sea level trend calculated for conditions at the South Padre Island, TX station is an 

increase of 3.48 mm/year (+/- 0.75 mm/year), based on monthly mean sea level data from 1958 

to 2006 (Figure 49). This is equivalent to a change of 1.14 ft in 100 years (NOAA 2013). The 

mean sea level trend calculated for conditions at the Port Mansfield, Texas station is an increase 

of 1.93 mm/year (+/- 0.97 mm/year), based on monthly mean sea level data from 1963 to 2006

(Figure 50). This is equivalent to a rise of 0.63 ft in 100 years (NOAA 2013). The mean sea level 

trend calculated for conditions at the Rockport, Texas station (north of Padre Island) is an 

increase of 5.16 mm/year (+/- 0.67 mm/year), based on monthly mean sea level data from 1948 

to 2006 (Figure 51). This is equivalent to a rise of 1.69 ft in 100 years (NOAA 2013). It is noted 

that a portion of apparent sea level rise may be due to subsidence of land due to unconsolidated 

deltaic sediments that make up the barrier island system (Lindsay, pers. Comm., 29 July 2013).
305

Figure 49. Trend in mean sea level rise recorded at Padre Island, TX, 1958 to 2006 (NOAA 2013).

Figure 50. Trend in mean sea level rise recorded at Port Mansfield, TX, 1963 to 2006 (NOAA 2013).
306

Figure 51. Trend in mean sea level rise recorded at Rockport, TX, 1948-2006 (NOAA 2013).

Data Needs/Gaps

Recent LiDAR (2012) imagery, available from the Bureau of Economic Geology, captures data 

for the shoreline and dunes (water’s edge to approximately 200-300 m in from the shoreline) 

along Padre Island but does not capture data for the entire barrier island (center parts of the 

island). It is possible these data may be used to estimate dune migration within PAIS, but may 

not capture the patterns of those features greater than 300 meters from the shoreline. Information 

on dune elevation and shoreline position could be extracted and compared with older shoreline 

position analyses and dune elevation data to provide managers with a more complete picture of 

change over time. Current analyses for shoreline position change include calculations through 

2007 for PAIS and 2011 for north of the park. 

Consistent annual monitoring of changes in shoreline position and dune migration across the 

island would help park managers understand the rate of change occurring within the park on a 

seasonal and yearly basis, as well as what effects storm events and tidal surges may have on 

these features. Available imagery or LiDAR data and GIS analysis and comparison to earlier 

images can be quite useful in estimating change in eolian features and shoreline position in the 

park over time. A longer-term study of sand deposition/erosion would help managers understand 

the average rate at which materials are moved, deposited, and eroded by wind and wave actions, 

ultimately providing understanding of the implications for long-term integrity of the beaches and 

dune system. Benchmarks have been installed along the PAIS shoreline in 2012 and will be used 

to determine erosion and accretion rates in the near future and long-term.
307

Overall Condition

Annual Change in Shoreline Position

The project team defined the Significance Level for annual change in shoreline position as a 3, as 

an indication of loss or gain of material. Analysis of recent LiDAR data compared to historical 

imagery and aerial photos indicates shoreline position change is variable along the PAIS 

shoreline (both the Gulf of Mexico and the Laguna Madre). Paine et al. (2011) found a few 

locations in the northern part of PAIS and at the southern boundary have experienced significant 

advancement in shoreline position, while a few locations have also experienced significant 

retreat. Overall, with the exception of several sites experiencing high net retreat or advancement, 

net shoreline position change seems relatively stable (Paine et al. 2011). While shoreline position 

change is a natural process in barrier islands, engineered structures, such as jetties and channels, 

are documented to influence the transport of sediments that build beaches and dunes (Paine et al. 

2011). Thus, a Condition Level of 2 was assigned, indicating moderate concern.

Rate of Change in Dune Elevation

The project team defined the Significance Level for rate of change in dune elevation as a 3.

Recent LiDAR data are available for 2012, but dune elevation information has not yet been 

extracted or calculated for PAIS. Thus, a Condition Level was not assigned.

Rate of Dune Migration

The project team defined the Significance Level for rate of dune migration as a 3. Recent LiDAR 

data are available for dune migration, but have not yet been extracted or calculated for PAIS. 

Thus, a Condition Level was not assigned.

Rate of Sand Deposition/Erosion

The project team defined the Significance Level for rate of sand deposition/erosion as a 3. 

Limited information exists that focuses sand deposition/erosion on Texas barrier islands and in 

PAIS; however, this information is outdated. Thus, a Condition Level was not assigned.
308

Weighted Condition Score

Although LiDAR data do exist that could characterize the current condition of the measures, 

information has not yet been extracted from the dataset or analyzed for these measures. Thus, the 

Condition Level for several measures could not be determined. A Weighted Condition Score

could not be calculated with Condition Levels undetermined for a majority of the measures. 

Sources of Expertise

James Lindsay, Chief of Science and Resource Management, PAIS

Diana Del Angel, Coastal Geoscientist, Harte Research Institute, Texas A&M University at 

Corpus Christ

Jeffrey Bracewell, GIS specialist, NPS Inventory & Monitoring, GULN
309

Literature Cited

Baccus, J. T. 1977. A study of beach and dunes floral and faunal interrelationships as influenced 

by recreational and user impact on Padre Island National Seashore. Southwest Texas State 

University unpublished report, San Marcos, Texas.

Blume, M, and J. R. Jones. 1985. Variation in vegetation density and foredune complexity at 

North Padre Island, Texas. Texas Journal of Science 37:63-73.

Dahl, B. E. and J. P. Goen. 1977. Monitoring of foredunes on Padre Island, Texas. Report No. 

77-8. U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, 

Virginia.

Gibeaut, J. C., T. Hepner, R. Waldinger, J. Andrews, R. Gutierrez, T. A. Tremblay, R. Smyth, 

and L. Xu. 2001. Changes in Gulf shoreline position, Mustang and North Padre Islands, TX. 

Bureau of Economic Geology, University of Texas at Austin, Austin, Texas.

Houser, C., and S. Mathew. 2011. Alongshore variation in foredune height in response to 

transport potential and sediment supply: South Padre Island, Texas. Geomorphology 125:62-

72.

Houser, C., B. Labude, L. Haider, B. Weymer. 2013. Impact of driving on the beach: Case 

studies from Assateague and Padre Island National Seashores. Ocean and Coastal 

Management 71:33-45.

Hummel, G., and G. Kocurek. 1984. Interdune areas of the back-island dune field, North Padre 

Island, Texas. Sedimentary Geology 39(1-2):1-26.

Intergovernmental Panel on Climate Change (IPCC). 2007. Climate change 2007: The physical 

science basis. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. 

Tignor, and H. L. Miller (eds). Contribution of Working Group 1 to the Fourth Assessment 

Report of the Intergovernmental Panel on Climate Change. IPCC. Geneva, Switzerland. 

Judd, F. W., and S. L. Sides. 1983. The effect of Hurricane Allen on the near-shore vegetation of 

South Padre Island. The Southwestern Naturalist 28(3):365-369.

KellerLynn, K. 2010. Padre Island National Seashore: Geologic resources inventory report. 

Natural Resources Report NPS/NRPC/GRD/NRR—2010/246. National Park Service, Fort

Collins, Colorado.

Kocurek, G., M. Townsley, E. Yeh, K. Havholm, and M. L. Sweet. 1992. Dune and dune-field 

development on Padre Island, Texas, with implications for interdune deposition and watertable-controlled accumulation. Journal of Sedimentary Petrology 62(4):622-635.

McAtee, J. W., and D. L. Drawe. 1981. Human impact on beach and foredune microclimate on 

North Padre Island, Texas. Environmental Management 5:121-134.

Morton, R. A. 1977. Historical shoreline changes and their causes, Texas Gulf Coast. The 

University of Texas at Austin, Bureau of Economic Geology. Geological Circular 77-6:352-
310

364.

National Oceanic and Atmospheric Administration (NOAA). 2013. Tides and currents: Sea 

levels online. http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml (accessed 11 June 

2013).

Paine, J., S. Mathew, and T. Caudle. 2011. Texas Gulf shoreline change rates through 2007. 

Bureau of Economic Geology unpublished report, The University of Texas at Austin, Austin, 

Texas.

Pendleton, E. A., E. R. Thieler, S. J. Williams, and R. L. Beavers. 2004. Coastal vulnerability 

assessment of Padre Island National Seashore (PAIS) to sea-level rise. Open-File Report 

2004-1090. U.S. Geological Survey, Reston, Virginia.

Roth, D. 2010. Texas hurricane history: National Weather Service. Camp Springs, 

Maryland. http://rigin.hpc.ncep.noaa.gov/research/txhur.pdf (accessed 19 July 2013).

United States Geological Survey (USGS). 2013. National Park research: Coastal processes, 

barrier islands. http://geomaps.wr.usgs.gov/parks/coast/barrierislands/index.html (accessed 

11 June 2013). 

Weiner, S. P. 1982. Deposition and stratification of oblique dunes on South Padre Island, TX. 

Gulf Coast Association of Geological Societies Transactions 32:521-525.

Weise, B. R., and W. A. White. 1980. Padre Island National Seashore: A guide to the geology, 

natural environments, and history of a Texas barrier island. Texas Bureau of Economic 

Geology, Austin, Texas.
311

Plate 36. Location of NOAA sea level rise monitoring stations relative to PAIS.
312

Chapter 5 Discussion

Chapter 5 provides an opportunity to summarize assessment findings and discuss the overarching 

themes or common threads that have emerged for the featured components. The data gaps and 

needs identified for each component are summarized and the role these play in the designation of 

current condition is discussed. Also addressed is how condition analysis relates to the overall 

natural resource management issues of the park.

5.1 Component Data Gaps

The identification of key data and information gaps is an important objective of NRCAs. Data 

gaps or needs are those pieces of information that are currently unavailable, but are needed to 

help inform the status or overall condition of a key resource component in the park. Data gaps 

exist for most key resource components assessed in this NRCA. Table 68 provides a detailed list 

of the key data gaps by component. Each data gap or need is discussed in further detail in the 

individual component assessments (Chapter 4). 

Table 68. Identified data gaps or needs for the featured components in PAIS.

Component Data Gaps/Needs

Terrestrial Vegetative 

Communities

 Comprehensive species inventory

 Assessment of current and historic coverage of grassland and dune 

vegetation

 Assessment of non-native plant species extent and distribution, and how 

non-native species are impacting native communities

 Assessment of prevalence of bare ground coverage in the park

 Study of oak motte extent, genetic status, factors affecting existence

Algal Mats on Mud Flats (Wind 

tidal flats)

 Current mapping of extent of algal mats within park boundaries

 Long-term surveys of benthic invertebrates or resampling of locations that 

have been sampled in previous study efforts

Seagrass Community  Continuation of long-term monitoring effort to assess trends in species 

composition and distribution of seagrass beds within park boundaries

Emergent Wetland and Pond 

Communities

 Assessment of how wetlands have changed over time (in size, quality) and 

contributing factors to such change

 Comprehensive native plant survey specific to wetlands and ponds

 Repeated analysis of wetland/pond elevation to monitor trends in 

sedimentation or filling over time

Migratory Bird Species  Establishment of an annual monitoring program to build a long-term data set 

on populations

 Establish formal survey for migratory raptors

Resident Bird Species  Establishment of annual monitoring program or breeding bird survey

 Resuming Christmas Bird Count (CBC) efforts

Colonial Waterbirds  Continued monitoring to determine trends in breeding populations

 A survey or estimate of nesting success

 Monitoring rate of erosion on islands used as primary nesting habitat
313

Table 68. Identified data gaps or needs for the featured components. (continued)

Component Data Gaps/Needs

Coyotes  Study of current coyote population density and distribution 

 Estimation of reproductive success on the barrier island

 Assessment of disease prevalence

Small Mammals  Assessment of species abundance, diversity, distribution and density 

throughout the park

 Establishment of annual small mammal survey

Macroinvertebrates  Resampling of previous study sites to build longer-term data sets

 Long-term monitoring of species composition and population recruitment

 Assessment of dispersal dynamics, colonization, community succession

Reptiles  Current and long-term monitoring to assess population sizes and diversity, 

reproductive success, and sex ratio

 Assessment of disease prevalence

Sea Turtles  Continued annual monitoring of populations in the park

Amphibians  Current survey to identify species within the park and long-term monitoring 

to determine trends in populations

Water Quality  Establish monitoring program for freshwater ponds in the park

 Continued water quality sampling for the standard parameters in the Laguna 

Madre 

Air Quality  Consistent monitoring of nitrogen and sulfur deposition, ozone 

concentration, mercury deposition, particulate matter concentration and 

visibility within park boundaries

Dark Night Skies  Draft plan for natural lightscape management showing areas where outdoor 

lighting are required and where naturally dark zones occur

 Periodic light trespass measurements into naturally dark zones to see how 

development outside park boundaries may be affecting conditions within the 

park and near park edges

 Calibrated photometry data to define dark night skies condition

Coastal Dunes and Beaches  Annual or seasonal monitoring of changes in shoreline position and dune 

migration lagoonward

 Monitoring of impacts to shoreline and fore-island dune ridge from storm 

surges and hurricanes

 Long-term study of sand deposition/erosion from wave action and wind 

patterns

Many of the park’s data needs involve establishing monitoring programs, as a number of the 

park’s components have either outdated data or lack data all together. Many of the components 

analyzed in this report lacked enough data to facilitate long-term trend analyses. 

Annual or consistent monitoring programs of some manner would establish a history of data 

records for most of the featured components or would repeat previous survey efforts in which 

current information could be compared to historic data and observations. This would help 

managers determine any changes in resource condition over time. 
314

Sensitive terrestrial communities, such as the park’s grassland and wetland vegetation and algal 

mats, are in need of up-to-date, comprehensive surveys of species and extent, as well as an 

investigation on how these communities have changed under the influence of non-native species 

advance, human impacts and disturbance, and weather changes. Several populations of terrestrial 

faunal species, such as reptiles, amphibians, and small mammals, are in need of contemporary 

data that describe population characteristics, diversity, and abundance. Since PAIS is an 

important stopover for many species of migratory birds, as well as habitat for numerous species 

of resident birds, the establishment of monitoring efforts for assessing population trends seems 

necessary. The components that describe environmental quality (i.e., air quality, water quality, 

dark night skies) are in need of expanded and more consistent monitoring efforts. In particular,

for air and water quality, managers would benefit from more consistent sampling efforts (both in 

timing and methodology) within the park, as these initiatives would provide valuable insights 

into trends occurring seasonally and annually. Some of these needs are now being addressed 

through recently implemented GULN monitoring efforts with water quality in the Laguna 

Madre; however, data analyses are in beginning stages. 

5.2 Component Condition Designations

The conditions assigned to each resource component featured in Chapter 4 are presented in Table 

69 (definitions of condition graphics are located in Figure 52 following Table 69). It is important 

to remember that the graphics represented are simple symbols for the overall condition and trend 

assigned to each component. Because the assigned condition of a component (as represented by 

the symbols in Figure 52) is based on a number of factors and an assessment of multiple 

literature and data sources, it is strongly recommended that the reader refer back to each specific 

component assessment in Chapter 4 for a detailed explanation and justification of the assigned 

condition. Condition designations for some components are supported by existing datasets and 

monitoring information and/or the expertise of NPS staff, while other components lack historic 

data, a clear understanding of reference conditions (i.e., what is considered desirable or natural), 

or even current information. 
315

Table 69. Summary of current condition and condition trend for featured PAIS NRCA components. Higher 

Weighted Condition Scores indicate higher concern.

Component WCS Condition

Biological Composition

Ecological communities

Terrestrial Vegetative Communities N/A

Algal Mats on Mud Flats N/A

Seagrass Communities N/A

Emergent Wetland and Pond Communities N/A

Wildlife and other Biotics

Birds

Migratory Birds N/A

Resident Birds N/A

Colonial Waterbirds 0.667

Mammals

Coyotes N/A

Small Mammals N/A

Macroinvertebrates

Macroinvertebrates N/A

Herpitiles

Reptiles N/A

Sea Turtles 0.667

Amphibians N/A

Environmental Quality

Water Quality 0.333

Air Quality 0.583
316

Table 70. Summary of current condition and condition trend for featured PAIS NRCA components. Higher 

Weighted Condition Scores indicate higher concern.

Component WCS Condition

Dark Night Skies 0.667

Physical Characteristics

Geomorphology

Coastal Dunes and Beaches N/A

Figure 52. Symbols used for individual component assessments with condition or concern designations 

along the vertical axis and trend designations along the horizontal.

Of the 17 components featured in this assessment, 12 did not have sufficient data available to 

designate current condition. For featured components with available data and limited knowledge 

gaps, the resource conditions assigned varied. In total, five components had sufficient available 

data to determine current condition, all of which were assessed as being of moderate concern. 

These were colonial waterbirds, sea turtles, water quality, air quality, and dark night skies. 

Four of the five components had enough available data or information to determine a trend in 

resource condition. Sea turtles, air quality, and dark night skies all had stable trends, while 

colonial waterbirds was determined to have a declining trend in resource condition.

5.3 Park-wide Condition Observations 

Despite the great variety in vegetation and physical features within PAIS’s boundaries, many of 

the resources discussed in this report are interrelated and share similar management concerns 

(e.g., data gaps, threats from outside the park).
317

Ecological Communities

PAIS comprises several unique ecological communities that are vital resources for the park, 

providing habitat for wildlife and performing critical ecological functions. These communities 

include the terrestrial vegetation (grasslands and dune vegetation) that covers much of the island, 

emergent wetland and pond communities, seagrass beds in the Laguna Madre, and the algal 

mats/mud flats occurring along the leeward side of PAIS. These unique communities are also 

among the most visible to park visitors exploring the different parts of the island. Limited 

research has been conducted on many of the ecological communities within the park and, due to 

a lack of available data and information (both recent and historic for comparison), current 

condition could not be assessed for the featured ecological communities. 

Wildlife and other Biotics

A diversity of wildlife species or groups were featured in nine distinct components within this 

assessment, including both migratory and resident birds, colonial waterbirds, reptiles and 

amphibians, coyotes, small mammals, and sea turtles. Due to a lack of available recent and 

historical data and information, current condition and trend could not be determined for seven of 

the nine faunal components. Many of these components have had limited or sporadic monitoring 

or survey efforts completed in the park over the last several decades. The notable exceptions are 

colonial waterbirds and sea turtles. A local conservation group collects annual data on colonial 

waterbird populations in the Laguna Madre, which is then made available to PAIS managers for 

their records. Sea turtle nests and hatchling emergence is monitored annually for the five species 

that occur in PAIS; specifically, much data exists regarding the endangered Kemp’s ridley, as 

species nesting and hatchling success is closely monitored in PAIS and carefully handled by 

biologists because of its status as a high conservation priority. Establishing regular monitoring 

efforts for the remaining featured components would fill many data gaps in understanding the 

condition of other wildlife and biotic populations in the park and the interrelationships that exist 

among species and habitats.

Environmental Quality

Environmental quality is important in maintaining healthy functioning ecosystems. The quality 

of air and water in an ecosystem can substantially affect the health of terrestrial and aquatic 

organisms that occur within parks. The park’s air and water quality are currently of moderate 

concern; air quality has a stable trend, while trends for water quality could not be determined due 

to insufficient history of data. Nitrogen and sulfate deposition are at least a moderate concern

regarding PAIS air quality, primarily due to the sensitivity of semi-arid and grassland ecosystems 

to acidification and/or nutrient enrichment from these pollutants. Water quality is on the low end 

of the moderate concern category, due primarily to concern about salinity levels in the Laguna 

Madre. Preliminary analysis of data for most measures of water quality are within established 

thresholds and elicit low concern at this time; however, more analyses are needed to understand 

trends over time and interrelationships of water quality parameters. 

The park’s dark night skies resource is also currently of moderate concern, with a stable trend. 

Light pollution from the mainland and developed areas of North Padre Island impact the dark 

night skies quality. However, rate of development outside park boundaries is currently slow or 

limited, which is helping to preserve dark sky quality presently. 

319

Park-wide Threats and Stressors

Several threats and stressors influence the condition of multiple resources throughout PAIS. 

These include energy development (e.g., oil, gas, and wind), extreme weather events (e.g., 

hurricanes, tropical storms, and drought), relative sea level rise, and human impacts (e.g., 

recreation and development outside of the park).

Energy development within PAIS and in the area around the park would potentially affect the 

park’s dark night skies, air quality, ecological communities, and certain biotics such as bird 

populations or small mammals. Oil drilling currently occurs within the park and, although these 

operations are carefully permitted, transportation traffic to and from the oil pads can damage 

sensitive grassland species or impact wildlife along beaches and dunes. Likewise, accidental 

spills of oil or other chemicals at drilling sites could damage important grassland habitats and 

associated wildlife, as well as contaminate groundwater. Oil platforms in the Gulf of Mexico are 

also visible from PAIS beaches. Spills at these development sites could adversely affect the 

beach environment and terrestrial and marine organisms that use the shoreline along PAIS. Wind 

energy development is occurring in the Laguna Madre and on the mainland of Texas and is 

directly visible from PAIS. These sites could pose a risk to migrating or resident bird populations 

as well as detract from the park’s viewscape. 

Hurricanes and tropical storms strike the Gulf coast of Texas at an average rate of 0.67 storms 

per year, or approximately two storms every 3 years; most of these weather events occur between 

late spring and early fall, with prime storm activity occurring from August through October 

(Weise and White 1980). When wind velocities reach 119 km/hr (74 mi/hr), tropical storms 

become categorized as hurricanes; the most severe hurricanes have sustained winds in excess of 

322 km/hr (200 mi/hr) (Weise and White 1980). Heavy winds and high tides that accompany 

severe storm events can cut into fore-island dunes, overwashing into and inundating low-lying 

areas with saltwater or carrying sands and sediment back to the beaches and out into the Gulf 

(Weise and White 1980, KellerLynne 2010). Big storm events accelerate the natural erosion and 

deposition processes on a barrier island, changing beaches and dune features in ways that would 

typically take months or even years under normal conditions (Weise and White 1980, KellyLynn 

2010).

Sea-level rise is a significant concern for PAIS. Pendleton et al. (2004) estimate the overall 

vulnerability of PAIS to sea level rise as very high for the entire shoreline, specifically the tidal 

range. However, the actual vulnerability varies along the shoreline, with approximately 35% of 

the PAIS shoreline rated as having very high or high vulnerability. Impacts to shorelines can 

include erosion, saltwater inundation of wetlands and groundwater aquifers, threats to cultural, 

historic and natural resources, and threats to infrastructure. Sea-level rise poses a particular threat 

to the algal mats on the wind tidal flats as it would cause permanent inundation if it were to 

exceed the rates at which sediment is deposited on the the flat (Morton and Holmes 2009). Thus, 

the extent of wind-tidal flats would decrease, limiting the extent of algal mats and leading to a 

loss of suitable habitat for macroinvertebrate populations and many species of birds. 

Most of PAIS’s beaches are open to motor vehicle traffic (approximately 101 km) with very few 

restrictions (approximately 7 km of beach is off limits to vehicles). Research over the last several 

decades has determined a number of observed impacts from such recreational driving, including 

encouraging shoreline erosion and inhibiting accretion of sediments onto shore, damaging beach 
320

and dune vegetation, impacting wildlife and benthic communities, destabilizing dunes, and 

negatively affecting the rebuilding capacity of coppice and fore-island dunes (KellerLynn 2010), 

and increasing susceptibility to overwash (Houser et al. 2013). Pedestrian traffic on the dunes 

can be impacting as well, destroying dune vegetation and impacting soils. In addition to catching 

new blowing sediments picked up from the beaches, vegetation stabilizes the dunes in the middle 

of the island and prevents sands and sediments from blowing away and migrating lagoonward. 

Pedestrian and motor vehicle traffic can damage above ground plant structure, as well as root 

systems, leading to sparser coverage and higher incidence of bare ground and wind erosion. By 

far, vehicle and pedestrian traffic is most damaging to the extremely sensitive algal mats and 

mud flat ecosystem. Scars and trenches made from passing vehicles can take months, years, or 

longer to heal, disrupting water flow across the flats and jeopardizing sensitive 

macroinvertebrate communities that populate the system and serve as a main food source for 

many wading bird species.

Overall Conclusions

PAIS is a very unique and diverse park that supports a variety of unique ecological communities 

and wildlife populations. The diversity in PAIS’s communities ranges from grassland, to beaches 

and sand dunes, to wetlands and tidal flats, to hypersaline lagoon. Visitors to PAIS have the 

opportunity to view unique landscapes and eolian dune geology, as well as wildlife ranging from 

very small native rodents to bands of migrating, resident or colonial nesting birds, to larger 

mammals such as deer and coyote. 

This assessment serves as a review and summary of available data and literature for featured 

natural resources in the park. The information presented here may serve as a baseline against 

which any changes in condition of components in the future may be compared. Unfortunately, 

many of the components analyzed here were not assigned an overall condition due to data gaps; 

some of these needs are being addressed by recently implemented GULN monitoring programs, 

which will provide valuable information for condition assessment in the near future. In order for 

PAIS managers to better understand the health, integrity, and trends of these valuable resources, 

additional research is needed for the specified components identified in this document.
321

Literature Cited

Houser, C., B. Labude, L. Haider, and B. Weymer. 2013. Impact of driving on the beach: Case 

studies from Assateague and Padre Island National Seashores. Ocean and Coastal 

Management 71:33-45.

KellerLynn, K. 2010. Padre Island National Seashore: Geologic resources inventory report. 

Natural Resources Report NPS/NRPC/GRD/NRR—2010/246. National Park Service, Fort

Collins, Colorado.

Morton, R. A., and C. W. Holmes. 2009. Geological processes and sedimentation rates of windtidal flats, Laguna Madre, Texas. U.S. Geological Survey, Austin, Texas.

Pendleton, E. A., E. R. Thieler, S. J. Williams, and R. L. Beavers. 2004. Coastal vulnerability 

assessment of Padre Island National Seashore (PAIS) to sea-level rise. Open-File Report 

2004-1090. U.S. Geological Survey, Reston, Virginia.

Weise, B. R., and W. A. White. 1980. Padre Island National Seashore: A guide to the geology, 

natural environments, and history of a Texas barrier island. Texas Bureau of Economic 

Geology, Austin, Texas.
322

Appendices

Appendix A. Native plant species documented in PAIS.

Scientific Name Common Name Baccus 

1977

Drawe 

et al. 

1981

Carls et 

al. 1991

Nelson 

et al. 

2000

Lonard 

et al. 

2004

Amaranthus greggii Gregg’s amaranth x x

Ambrosia psilostachya Cuman ragweed x x

Andropogon glomeratus bushy bluestem x

Asclepias oenotheroides zizotes milkweed x x

Baptisia bracteata longbract wild indigo x

Cenchrus spinifex coastal sandbur x x x x

Chamaecrista fasciculata

 var. fasciculata

partridge pea x x x x x

Chamaesyce bombensis Dixie sandmat x x

Chamaesyce cordifolia heartleaf sandmat x x

Chamaesyce serpens matted sandmat x

Commelina erecta whitemouth dayflower x x x

Croptilon divaricatum slender scratchdaisy x

Croptilon rigidifolium stiffleaf scratchdaisy x

Croton capitatus hogwort x

Croton glandulosus sand croton x x

Croton punctatus gulf croton x x x x

Cyperus echinatus globe flatsedge x

Cyperus esculentus yellow nutsedge x

Cyperus retroflexus oneflower flatsedge x

Digitaria arenicola sand crabgrass x

Digitaria cognata Carolina crabgrass x x

Eleocharis sp. spikerush x

Eragrostis secundiflora red lovegrass x x

Eragrostis spectabilis purple lovegrass x x

Erigeron procumbens Corpus Christi fleabane x

Euthamia leptocephala bushy goldentop x

Fimbristylis castanea marsh fimbry x

Galactia canescens hoary milkpea x x

Helianthus debilis cucumberleaf sunflower x

Heliotropium racemosum coastal plain heliotrope x

Heterotheca subaxillaris camphorweed x x x x

Houstonia subviscosa nodding bluet x

Hydrocotyle bonariensis water pennywort x x x x

Indigofera miniata coastal indigo x x x

Ipomoea imperati beach morning-glory x x x x x

Ipomoea pes-caprae bayhops x x

Oenothera drummondii beach evening primrose x x x x

Panicum amarum bitter panicgrass x x x x

Panicum capillarioides slender panicgrass x x
323

Scientific Name Common Name Baccus 

1977

Drawe 

et al. 

1981

Carls et 

al. 1991

Nelson 

et al. 

2000

Lonard 

et al. 

2004

Paspalum monostachyum gulfdune paspalum x x x x x

Paspalum setaceum thin paspalum x x x x x

Paspalum vaginatum seashore paspalum x

Pennisetum ciliare* buffelgrass x

Phlox drummondii annual phlox x

Phlox glabrifolia Rio Grande phlox x

Phyla nodiflora frog fruit; turkey tangle 

fogfruit

x x

Physalis cinerascens var.

 spathulifolia

smallflower groundcherry x x x

Physalis viscosa starhair groundcherry x x

Rayjacksonia 

 phyllocephala

camphor daisy x

Rhynchosia americana American snoutbean x x x

Rhynchosia minima least snoutbean x

Sabatia arenicola sand rose gentian x

Schizachyrium scoparium 

 var. littorale

shore little bluestem x x x x

Schoenoplectus pungens 

 var. longispicatus

common threesquare x

Senecio riddellii Riddell’s ragwort x x x

Senecio spartioides broom-like ragwort x

Sesuvium portulacastrum shoreline seapurslane x x

Spartina patens saltmeadow cordgrass x x x

Spiranthes vernalis spring lady’s tresses x

Sporobolus pyramidatus Madagascar or whorled 

dropseed

x

Sporobolus tharpii Tharp’s dropseed x

Sporobolus virginicus seashore dropseed x x x

Stillingia sylvatica queen’s-delight x

Trichoneura elegans Silveus’ grass x

Triplasis purpurea purple sandgrass x

Uniola paniculata seaoats x x x x x

Urochloa ciliatissima fringed signalgrass x x

Vaseyochloa multinervosa Texasgrass x x

Total 41 23 21 31 24

* indicates a non-native species
324

Appendix B. Plant species documented in interior/grassland habitats (Drawe et al. 1981 [low coastal 

sands and shoregrass flats]; Carls et al. 1991 [secondary dunes & vegetated flats]; Nelson et al. 2000 

[barrier flat]; Lonard et al. 2004 [vegetated flats]). Some scientific and common names were updated to 

match those accepted by the Integrated Taxonomic Information System (ITIS).

Scientific Name Common Name Drawe et 

al. 1981

Carls et 

al. 1991

Nelson et 

al. 2000

Lonard et 

al. 2004

Acacia farnesiana sweet acacia x

Agalinis maritima saltmarsh false foxglove x x

Ambrosia psilostachya Cuman ragweed x x x

Andropogon glomeratus bushy bluestem x x x x

Aristida longespica var. 

geniculata

slimspike threeawn x x

Asclepias oenotheroides zizotes milkweed x x

Baptisia bracteata longbract wild indigo x x x x

Batis maritima turtleweed x

Blutaparon vermiculare silverhead x x x

Borrichia frutescens bushy seaoxeye x

Buchnera americana American bluehearts x

Calylophus serrulatus yellow sundrops x x

Cenchrus echinatus southern sandbur x

Cenchrus spinifex coastal sandbur x x

Chamaecrista fasciculata partridge pea x x x

Chamaesyce bombensis Dixie sandmat x

Chamaesyce cordifolia heartleaf sandmat x

Chamaesyce serpens matted sandmat x

Cirsium horridulum yellow thistle x

Commelina erecta var. 

 angustifolia

whitemouth dayflower x x x

Conoclinium betonicifolium betonyleaf thoroughwort x x x x

Coreopsis tinctoria golden tickseed

Croptilon divaricatum slender scratchdaisy x

Croton capitatus var.

 lindheimeri

Lindheimer’s hogwort x

Croton glandulosus sand croton x

Croton punctatus gulf croton x

Cyperus rotundus nutgrass x

Cyperus echinatus globe flatsedge x

Dalea emarginata wedgeleaf prairie clover x

Dichanthelium sp. rosette grass x

Dichanthelium acuminatum tapered rosette grass x

Dichanthelium sabulorum var.

 thinium

hemlock rosette grass x x

Dichanthelium 

 sphaerocarpon var. 

 sphaerocarpon

roundseed panicgrass x

Digitaria cognata Carolina crabgrass x

Eleocharis sp. spikerush x

Eleocharis geniculata Canada spikesedge x
325

Scientific Name Common Name Drawe et 

al. 1981

Carls et 

al. 1991

Nelson et 

al. 2000

Lonard et 

al. 2004

Eleocharis montevidensis spike sedge; sand spikerush x

Eragrostis secundiflora red lovegrass x x

Eragrostis secundiflora ssp.

 oxylepis

red lovegrass x

Eragrostis spectabilis purple lovegrass x

Erigeron procumbens Corpus Christi fleabane x x x

Eupatorium compositifolium yankeeweed x

Eustachys petraea pinewoods fingergrass x x

Eustoma exaltatum catchfly prairie gentian x

Fimbristylis caroliniana Carolina fimbry x

Fimbristylis castanea marsh fimbry x x x x

Flaveria brownii Brown’s yellowtops x

Gaillardia pulchella Indian blanket x x

Galactia canescens hoary milkpea x

Heliotropium curassavicum 

 var. curassavicum

salt heliotrope x x

Heterotheca subaxillaris camphorweed x x

Hydrocotyle bonariensis water pennywort x x x x

Indigofera miniata coastal indigo x x x

Ipomoea imperati beach morning-glory x

Ipomoea pes-caprae bayhops x

Iva angustifolia narrowleaf marsh elder x x

Juncus sp. rushes x x

Limonium carolinianum lavender thrift x x x

Linum alatum winged flax x

Lythrum alatum var. 

 lanceolatum

winged lythrum x

Lythrum californicum California loosestrife x

Mimosa latidens Kairn’s sensitive-briar x x

Monanthochloe littoralis shoregrass x

Oenothera drummondii beach evening primrose x x

Opuntia macrorhiza twistspine pricklypear x x

Panicum amarum bitter panicgrass x

Panicum amarum var.

 amarulum

coastal or bitter panicgrass x

Panicum capillarioides slender panicgrass x

Paspalum monostachyum gulfdune paspalum x x x x

Paspalum plicatulum brownseed paspalum x

Paspalum setaceum thin paspalum x x x

Paspalum vaginatum seashore paspalum x

Phlox drummondii annual phlox x

Phyla nodiflora frog fruit; turkey tangle fogfruit x x x

Physalis cinerascens var.

 spathulifolia

smallflower groundcherry x x

Physalis viscosa starhair groundcherry x

Polygala alba white milkwort x x x
326

Scientific Name Common Name Drawe et 

al. 1981

Carls et 

al. 1991

Nelson et 

al. 2000

Lonard et 

al. 2004

Polygala incarnata procession flower x

Polypogon monspeliensis* annual rabbitsfoot grass x

Quercus fusiformis Texas live oak x

Rayjacksonia phyllocephala camphor daisy x x x

Rhynchosia americana American snoutbean x x x x

Rhynchosia minima least snoutbean x x

Rhynchospora colorata white-top sedge x

Sabatia arenicola sand rose gentian x x

Salicornia bigelovii dwarf saltwort x

Salix nigra black willow x

Samolus ebracteatus limewater brookweed x x

Schizachyrium scoparium

 var. littorale

shore little bluestem x x x

Schoenoplectus pungens 

 var. longispicatus

common threesquare x x x

Sesuvium portulacastrum shoreline seapurslane x x

Sisyrinchium biforme wiry blue-eyed grass x x

Sisyrinchium sagittiferum spearbract blue-eyed grass x

Spartina patens saltmeadow cordgrass x x x

Spiranthes vernalis spring lady’s tresses x x x

Sporobolus pyramidatus Madagascar or whorled 

dropseed

x

Sporobolus tharpii Tharp’s dropseed x

Sporobolus virginicus seashore dropseed x x x

Stemodia lanata gray-woolly twintip x x x

Stillingia sylvatica queen’s-delight x

Tidestromia lanuginosa wooly tidestromia x

Tragia sp. noseburn x

Uniola paniculata seaoats x x x

Yucca treculeana Don Quixote’s lace x

Total 61 52 52 17

* indicates a non-native species
327

Appendix C. List of macroinvertebrate species and respective microhabitat types observed during a 

study of PAIS between 1991 and 1992 (Withers 1993).

Species Damp Wet Intertidal

Phylum Nemertea X X

Phylum Annelida

 Class Polychaeta

Arenicola cristata X

Axiothella mucosa X

Capitella capitata X X X

Capitomastus aciculatus X X

Chone duneri X

Demonax micropthalmus X

Dorvillea rubra X

Eteone heteropoda X X X

Exogone dispar X

Haploscoloplos foliosus X X X

Polydora ligni X

Prionospio cristata X

Prionospio heterobranchia X

Sabella sp. A X

Syllis cornuta X X X

Phylum Mollusca

 Class Bivalvia

Amygdalum papyrium X

Anomalocardia auberiana X X

Bulla striata X X

Mulinia lateralis X X X

Tellina tampaensis X

Phylum Arthropoda

Subphylum Chelicerata

Spiders X X

Subphylum Crustacea

Corophium louisianum X X X

Hargeria rapax X X X

Oxyurostylis smithii X

Subphylum Hexapoda, 

 Class Insecta

Bledius sp. X X

Canaceidae X X X

Carabidae X

Ceratopogonidae X X X
328

Species Damp Wet Intertidal

Dolichopodidae X X X

Empidiae X

Homoptera X

Leuctridae sp. X

Melyridae X

Pteromalidae X

Saldidae X

Scelionidae X

Subphylum Hexapoda, 

 Class Entognatha

Cyphoderus sp. X

Total Number of Species 15 18 31
329

Appendix D. List of macroinvertebrate species observed at two former oil and gas development sites that 

have been restored in PAIS (Withers 1996).

Texaco site Yarborough Pass

Species Restored Control Tire Tracks Restored Control Tire Tracks

Phylum Nemertea X X

Phylum Annelida

 Class Polychaeta

Demonax micropthalmus X

Eteone heteropoda X

Exogone dispar X

Haploscoloplos foliosus X X X

Polydora spp. X X X

Prionospio heterobranchia X

Prionospio pinnata X X

Nainereis laevigata X

Sabella sp. A X

Spio pettibonniae X

Streblospio benedicti X

Syllis cornuta X

Capitellidae X

Maldanidae X

Phylum Mollusca

 Class Bivalvia

Amygdalum papyrium X X

Anomalocardia auberiana X X

Tellina sp. X

Phylum Arthropoda

Subphylum Crustacea

Corophium acherusicum X X X X

Corophium louisianum X X

Gammarus mucronatus X

Hargeria rapax X X X X X

Subphylum Hexapoda, 

Class Insecta

Berosus sp. X X

Canaceidae X X X X

Ceratopogonidae X X X X X

Dolichopodidae X X X X X X

Ephydridae X

Melyridae X X

Nabidae X X
330

Texaco site Yarborough Pass

Species Restored Control Tire Tracks Restored Control Tire Tracks

Phylum Platyhelminthes

Turbellaria X X

Total Number of Species 6 6 4 27 13 6
331

Appendix E. List of macroinvertebrate species observed at three southerly wind-tidal flats in PAIS 

between 1997 and 1998 (Withers 1998).

Species Yarborough Pass Dunn Ranch Mile Marker 45

Phylum Nemertea x

Phylum Annelida

 Class Polychaeta

Axiothella mucosa x

Capitella capitata x x x

Demonax micropthalmus x x x

Eteone heteropoda x x x

Haploscoloplos foliosus x x

Laeonereis culveri x

Polydora spp. x

Prionospio heterobranchia x

Marphysa regalis x

Melinna maculata x x

Nainereis laevigata x x

Nereis riisei x x

Sabella sp. A x

Phylum Mollusca

 Class Bivalvia

Anomalocardia auberiana x x x

Tellina tampaensis x x

Phylum Arthropoda

Subphylum Crustacea

Corophium acherusicum x

Corophium louisianum x

Grandidierella bonnieroides x

Gammarus mucronatus x

Hargeria rapax x x

Orchestia grillus x

Sphaeroma quadridentatum x

Subphylum Hexapoda

 Class Insecta

Bledius sp. x

Canaceidae x x x

Ceratopogonidae x x

Dolichopodidae x x x

Hemiptera (nymph) x

Hydrophilidae (larvae) x x

Staphylinidae (larvae) x x

Tipulidae x

Total Number of Species 25 14 13
332

Appendix F. Vascular plants within 60 m (200 ft) of the three ponds sampled at PAIS (Sissom et al. 

1990).

Scientific name Common name Pond A Pond B Pond C

Hydrocotyle bonariensis water pennywort x

Asclepias oenotheroides zizotes milkweed x

Cirsium horridulum yellow thistle x

Croptilon divaricatum slender scratchdaisy x

Erigeron procumbens Corpus Christi fleabane x

Gaillardia pulchella Indian blanket x

Heterotheca pilosa golden aster x

Heterotheca subaxillaris camphorweed x x

Iva angustifolia narrowleaf marsh elder x x

Pluchea odorata var. odorata sweetscent x

Cyperus rotundus* flatsedge; nutgrass x

Rhynchospora colorata white-top sedge x

Eleocharis montevidensis spike sedge x

Fimbristylis castanea marsh fimbry x

Schoenoplectus americanus American bulrush x

Croton capitatus hogwort x

Croton glandulosus sand croton x

Croton punctatus gulf croton x

Chamaesyce cordifolia heart-leaf sandmat x

Chamaesyce maculata spotted sandmat x

Baptisia bracteata longbract wild indigo x

Chamaecrista fasciculata partridge pea x

Galactia canescens hoary milkpea x

Indigofera miniata coastal indigo x

Sesbania vesicaria bladder pod x

Eustoma exaltatum catchfly prairie gentian x

Triadenum virginicum St. John’s wort x x

Sisyrinchium biforme wiry blue-eyed grass x

Plantago rhodosperma redseed plantain x

Andropogon glomeratus bushy bluestem x

Cenchrus spinifex coastal sandbur x

Dactyloctenium aegyptium* Egyptian grass x

Eragrostis secundiflora red lovegrass x

Eustachys petraea finger grass x

Leptochloa fusca ssp. 

 fascicularis

bearded sprangletop x

Paspalum setaceum thin paspalum x x

Pennisetum ciliare* buffelgrass x

Schizachyrium scoparium little bluestem x

Setaria parviflora yellow bristlegrass x

Sporobolus pyramidatus Madagascar dropseed x

Polygala incarnata pink milkwort x

Eriogonum multiflorum wild buckwheat x
333

Scientific name Common name Pond A Pond B Pond C

Samolus ebracteatus limewater brookweed x x

Agalinis maritima saltmarsh false foxglove x

Bacopa monnieri coastal water hyssop x

Buchnera americana American bluehearts x

Physalis viscosa ground cherry x

Typha domingensis narrow-leaf cattail x

Phyla nodiflora frog fruit x

Total 19 20 15

* indicates non-native species
334

Appendix G. Algae documented in three PAIS ponds by Sissom et al. (1990).

Genus Pond A Pond B Pond C

Cyanophyta (blue-green algae)

 Polycystis x x x

 Oscillatoria x x x

 Coelosphaerium x x x

 Rhaphidiopsis x x x

 Merismopedia x x

 Chroococcus x x x

 Spirulina x x

Chlorophyta (green algae)

 Dictyosphaerium x x

 Pediastrum x x x

 Protococcus x x x

 Crucigenia x x

 Botryococcus x x x

 Spirogyra x x

 Microspora x

 Zygnema x

 Tribonema x

 Selenastrum x x

 Scenedesmus x

 Desmidium x

 Hydrocera x

 Staurastrum x x x

 Cosamarium x x

 Closterium x x

 Tetemorus x

 Penium x

 Ankistrodesmus x

 Evastrum x

Total 25 17 11
335
336

Appendix H. Fungal species documented in three PAIS ponds by Florence Oxley (Sissom et al. 1990).

Taxa Pond A Pond B Pond C

Cladosporium x x x

Altenaria x x x

Cephalosporium x x x

Curvularia x x

Fusarium x x x

Drechslera x x x

Nigrospora x x x

Aspergillus x x x

Phoma x x x

Penicillium x x x

Ascochyta x x

Macrophoma x x

Didymosphaenia maritima x

Didymosphaenia enalia x

Leptosphaeria discors x

Leptosphaeria oraemaris x

Leptosphaeria australiensis x

Leptosphaeria marina x

Epicoccum x x x

Pithomyces x x x

Eurotiales x x

Pestalotia x x x

Rhizopus x

Tetraploa

Pleospora x

Stagnospora x

Myrothecium x

Trichoderma x

Candelabrella x

Humicola x

Stachybotrys x

Bipolaris x

Total 19 23 17
337

Appendix I. Neotropical migrant species that are confirmed in PAIS (NPS 2013a, TPWD 2013).

Scientific Name Common Name

Anas cyanoptera cinnamon teal

Anas discors blue-winged teal

Chaetura pelagica chimney swift

Amazilia yucatanensis buff-bellied hummingbird

Archilochus alexandri black-chinned hummingbird

Archilochus colubris ruby-throated hummingbird

Buteo platypterus broad-winged hawk

Buteo swainsoni Swainson's hawk

Elanoides forficatus swallow-tailed kite

Ictinia mississippiensis Mississippi kite

Pandion haliaetus osprey

Charadrius semipalmatus semipalmated plover

Charadrius wilsonia Wilson's plover

Himantopus mexicanus black-necked stilt

Pluvialis dominica American golden plover

Pluvialis squatarola black-bellied plover

Mycteria americana wood stork

Falco columbarius merlin

Falco peregrinus peregrine falcon

Actitis macularia spotted sandpiper

Arenaria interpres ruddy turnstone

Bartramia longicauda upland sandpiper

Calidris alba sanderling

Calidris bairdii Baird's sandpiper

Calidris canutus red knot

Calidris fuscicollis white-rumped sandpiper

Calidris himantopus stilt sandpiper

Calidris mauri western sandpiper

Calidris melanotos pectoral sandpiper

Calidris minutilla least sandpiper

Calidris pusilla semipalmated sandpiper

Limnodromus griseus short-billed dowitcher

Limosa haemastica Hudsonian godwit

Numenius phaeopus whimbrel

Phalaropus tricolor Wilson's phalarope

Tringa flavipes lesser yellowlegs

Tringa melanoleuca greater yellowlegs

Tringa solitaria solitary sandpiper

Tryngites subruficollis buff-breasted sandpiper

Coccyzus americanus yellow-billed cuckoo
338

Scientific Name Common Name

Coccyzus erythropthalmus black-billed cuckoo

Crotophaga sulcirostris groove-billed ani

Gallinula galeata common gallinule

Porphyrula martinica purple gallinule

Guiraca caerulea blue grosbeak

Passerina ciris painted bunting

Passerina cyanea indigo bunting

Pheucticus melanocephalus black-headed grosbeak

Spiza americana dickcissel

Polioptila caerulea blue-gray gnatcatcher

Ammodramus savannarum grasshopper sparrow

Melospiza lincolnii Lincoln's sparrow

Spizella passerina chipping sparrow

Hirundo pyrrhonota cliff swallow

Hirundo rustica barn swallow

Progne subis purple martin

Riparia riparia bank swallow

Stelgidopteryx serripennis northern rough-winged swallow

Icterus bullockii Bullock's oriole

Icterus cucullatus hooded oriole

Icterus galbula Baltimore oriole

Icterus spurius orchard oriole

Dumetella carolinensis gray catbird

Dendroica castanea bay-breasted warbler

Dendroica cerulea cerulean warbler

Dendroica dominica yellow-throated warbler

Dendroica fusca blackburnian warbler

Dendroica magnolia magnolia warbler

Dendroica nigrescens black-throated gray warbler

Dendroica palmarum palm warbler

Dendroica pensylvanica chestnut-sided warbler

Dendroica petechia yellow warbler

Dendroica striata blackpoll warbler

Dendroica tigrina Cape May warbler

Dendroica virens black-throated green warbler

Geothlypis trichas common yellowthroat

Helmitheros vermivorus worm-eating warbler

Icteria virens yellow-breasted chat

Limnothlypis swainsonii Swainson's warbler

Mniotilta varia black-and-white warbler

Oporornis formosus Kentucky warbler

Oporornis philadelphia mourning warbler
339

Scientific Name Common Name

Parula americana northern parula

Protonotaria citrea prothonotary warbler

Seiurus aurocapillus ovenbird

Seiurus motacilla Louisiana waterthrush

Seiurus noveboracensis northern waterthrush

Setophaga ruticilla American redstart

Vermivora celata orange-crowned warbler

Vermivora chrysoptera golden-winged warbler

Vermivora peregrina Tennessee warbler

Vermivora pinus blue-winged warbler

Vermivora ruficapilla Nashville warbler

Wilsonia canadensis Canada warbler

Wilsonia citrina hooded warbler

Wilsonia pusilla Wilson's warbler

Piranga ludoviciana western tanager

Piranga olivacea scarlet tanager

Piranga rubra summer tanager

Troglodytes aedon house wren

Catharus fuscescens veery

Catharus minimus gray-cheeked thrush

Catharus ustulatus Swainson's thrush

Hylocichla mustelina wood thrush

Contopus borealis olive-sided flycatcher

Contopus virens eastern wood-pewee

Empidonax alnorum alder flycatcher

Empidonax flaviventris yellow-bellied flycatcher

Empidonax minimus least flycatcher

Empidonax traillii willow flycatcher

Empidonax virescens Acadian flycatcher

Myiarchus cinerascens ash-throated flycatcher

Myiarchus crinitus great crested flycatcher

Tyrannus forficatus scissor-tailed flycatcher

Tyrannus tyrannus eastern kingbird

Tyrannus verticalis western kingbird

Vireo bellii Bell's vireo

Vireo flavifrons yellow-throated vireo

Vireo flavoviridis yellow-green vireo

Vireo gilvus warbling vireo

Vireo olivaceus red-eyed vireo

Vireo philadelphicus Philadelphia vireo

Vireo griseus white-eyed vireo

Caprimulgus carolinensis Chuck-will's-widow
340

Scientific Name Common Name

Caprimulgus vociferus whip-poor-will

Chordeiles acutipennis lesser nighthawk

Chordeiles minor common nighthawk
341

Appendix J. Resident bird species identified on the PAIS Certified Species List (NPS 2013).

Scientific Name Common Name

Aix sponsa wood duck

Dendrocygna autumnalis black-bellied whistling-duck

Dendrocygna bicolor fulvous whistling-duck

Amazilia yucatanensis buff-bellied hummingbird

Buteo albicaudatus white-tailed hawk

Buteo jamaicensis red-tailed hawk

Buteo lineatus red-shouldered hawk

Elanus leucurus white-tailed kite

Parabuteo unicinctus Harris' hawk

Ixobrychus exilis least bittern

Charadrius alexandrinus snowy plover

Charadrius vociferus killdeer

Haematopus palliatus American oystercatcher

Cathartes aura turkey vulture

Coragyps atratus black vulture

Falco femoralis aplomado falcon

Falco sparverius American kestrel

Polyborus plancus crested caracara

Podilymbus podiceps pied-billed grebe

Tachybaptus dominicus least grebe

Columba livia rock dove

Columbina inca Inca dove

Columbina passerina common ground-dove

Zenaida macroura mourning dove

Chloroceryle americana green kingfisher

Geococcyx californianus greater roadrunner

Colinus virginianus northern bobwhite

Meleagris gallopavo wild turkey

Fulica americana American coot

Rallus elegans king rail

Rallus longirostris clapper rail

Eremophila alpestris horned lark

Cardinalis cardinalis northern cardinal

Cardinalis sinuatus pyrrhuloxia

Corvus cryptoleucus Chihuahuan raven

Aimophila cassinii Cassin's sparrow

Ammodramus maritimus seaside sparrow

Arremonops rufivirgatus olive sparrow

Chondestes grammacus lark sparrow

Agelaius phoeniceus red-winged blackbird
342

Scientific Name Common Name

Icterus gularis Altamira oriole

Molothrus aeneus bronzed cowbird

Molothrus ater brown-headed cowbird

Quiscalus mexicanus great-tailed grackle

Sturnella magna eastern meadowlark

Lanius ludovicianus loggerhead shrike

Mimus polyglottos northern mockingbird

Toxostoma curvirostre curve-billed thrasher

Toxostoma longirostre long-billed thrasher

Baeolophus atricristatus black-crested titmouse

Passer domesticus house sparrow

Auriparus flaviceps verdin

Sturnus vulgaris European starling

Campylorhynchus brunneicapillus cactus wren

Thryomanes bewickii Bewick's wren

Thryothorus ludovicianus Carolina wren

Turdus migratorius American robin

Pitangus sulphuratus great kiskadee

Pyrocephalus rubinus Vermilion flycatcher

Tyrannus couchii Couch's kingbird

Melanerpes aurifrons golden-fronted woodpecker

Picoides scalaris ladder-backed woodpecker

Nyctidromus albicollis common pauraque

Phalaenoptilus nuttallii common poorwill

Athene cunicularia burrowing owl

Bubo virginianus great horned owl

Tyto alba barn owl
343

Appendix K. Resident bird species observed on the PAIS CBC from 1974-1990.

Species

1974-

75

1975-

76

1976-

77

1977-

78

1978-

79

1979-

80

1980-

81

1981-

82

1982-

83

1983-

84

1984-

85

1985-

86

1986-

87

1987-

88

1988-

89

1989-

90

wood duck 2 4

wild turkey 6 71 36 53

northern bobwhite 50 101 175 240 79 17 54 40 21 18 7 26 12 8 34

least grebe 3 4 21 3 4 1

pied-billed grebe 5 42 46 16 35 87 7 45 16 2 3 8 3

black vulture 14 2 4 12 2 2 5 4 2

turkey vulture 149 40 50 107 5 242 261 14 29 14 29 73 42

Harris's hawk 4 11 3 5

white-tailed hawk 11 6 10 13 4 14 20 6 2 1 8 5 6 5 7 4

red-tailed hawk 11 5 1 15 1 16 12 2 2 1 5 1 4 1

crested caracara 3 7 4 10 12 20 1 2 2 2

American kestrel 68 31 42 68 11 82 91 30 30 11 6 25 16 6 22

clapper rail 1 1

king rail 3

American coot 3 264 365 900 355 1,459 33 26 179 1 268 200

snowy plover 31 34 10 12 14 19 7 49 32 1 30 17 15 1

killdeer 373 131 86 141 93 73 176 80 41 20 21 39 53 8 11 9

mourning dove 745 280 348 615 15 278 444 42 2 29 117 4 213 1 24 8

Inca dove 6 7

common ground-dove 6 9 14 4 2 7

greater roadrunner 9 5 5 5 5 3 4

great horned owl 2 1 3 5 2 1 2 1

burrowing owl 2 1 1

common pauraque 2

golden-fronted woodpecker 3 1 11 3 12

ladder-backed woodpecker 2 3 3 2 1 2

Vermillion flycatcher 3 4 1 3 2
344

Species

1974-

75

1975-

76

1976-

77

1977-

78

1978-

79

1979-

80

1980-

81

1981-

82

1982-

83

1983-

84

1984-

85

1985-

86

1986-

87

1987-

88

1988-

89

1989-

90

Couch's kingbird 1

loggerhead shrike 68 50 158 58 2 60 79 8 12 6 6 25 4 1 5

horned lark 109 85 30 36 62 87 67 71 18 18 16 45 14 15

black-crested titmouse 3 3

verdin 1 2

cactus wren 1 1 2 1

Carolina wren 2 1

Bewick's wren 2 2 7 1 1 7

American robin 449 82 209 2 179 8 4 1 137 1

northern mockingbird 195 54 56 37 31 19 1 2 48

long-billed thrasher 7 30 40 12 2 2 7

curve-billed thrasher 9 19 3 5 4 6 1

European starling 13 15 9 9 20

olive sparrow 2 3 2

Cassin's sparrow 9 7 2 8 1 2 1 1

lark sparrow 35 81 82 38 1 4 2 9

seaside sparrow 6 2 3 20

northern cardinal 106 41 94 133 54 49 71

pyrrhuloxia 31 66 166 99 46 20 22

red-winged blackbird 606 1,184 495 1,513 796 1,636 965 387 721 452 16 714 552 13 22 253

eastern meadowlark 471 454 660 742 591 734 455 574 269 267 331 430 192 86 150

great-tailed grackle 1 29 27 43 9 54 66 38 4 7 2 25 362 5 15

brown-headed cowbird 520 1120 281 835 2 184 89 1 1 40 424

house sparrow 15 46 39 75 9 31

Total 4149 4338 3525 5835 2090 5354 3176 1415 1201 1083 564 1157 2844 292 255 503
345

Appendix L. Colonial waterbird species that have been identified in PAIS, and appear on the NPS 

Certified Species List (NPS 2013).

Family Scientific Name Common Name

Anhingidae Anhinga anhinga anhinga

Ardeidae Ardea alba great egret

Ardeidae Ardea herodias great blue heron

Ardeidae Botaurus lentiginosus American bittern

Ardeidae Bubulcus ibis cattle egret

Ardeidae Egretta caerulea little blue heron

Ardeidae Egretta rufescens reddish egret

Ardeidae Egretta thula snowy egret

Ardeidae Egretta tricolor tricolored heron

Ardeidae Nyctanassa violacea yellow-crowned night-heron

Ardeidae Nycticorax nycticorax black-crowned night-heron

Fregatidae Fregata magnificens magnificent frigatebird

Laridae Chlidonias niger black tern

Laridae Larus argentatus herring gull

Laridae Larus atricilla laughing gull

Laridae Larus californicus California gull

Laridae Larus delawarensis ring-billed gull

Laridae Larus fuscus lesser black-backed gull

Laridae Larus glaucoides Iceland gull

Laridae Larus hyperboreus glaucous gull

Laridae Larus marinus great black-backed gull

Laridae Larus occidentalis western gull

Laridae Larus pipixcan Franklin's gull

Laridae Larus thayeri Thayer's gull

Laridae Rissa tridactyla black-legged kittiwake

Laridae Rynchops niger black skimmer

Laridae Sterna anaethetus bridled tern

Laridae Sterna antillarum least tern

Laridae Sterna caspia Caspian tern

Laridae Sterna forsteri Forster's tern

Laridae Sterna fuscata sooty tern

Laridae Sterna hirundo common tern

Laridae Sterna maxima royal tern

Laridae Sterna nilotica gull-billed tern

Laridae Sterna sandvicensis sandwich tern

Pelecanidae Pelecanus erythrorhynchos American white pelican

Pelecanidae Pelecanus occidentalis brown pelican

Phalacrocoracidae Phalacrocorax auritus double-crested cormorant
346

Family Scientific Name Common Name

Phalacrocoracidae Phalacrocorax brasilianus neotropic cormorant

Threskiornithidae Ajaia ajaja roseate spoonbill

Threskiornithidae Eudocimus albus white ibis

Threskiornithidae Plegadis chihi white-faced ibis

Threskiornithidae Plegadis falcinellus glossy ibis
347

Appendix M. Summary of small mammal species present or probably present within PAIS.

Scientific Name Common Name Bailey

(1905)

Raun 

(1959)

Baker and 

Rabalais (1975)

Baccus 

(1977)

Harris 

(1988)

Frey and 

Jones (2008)

GULN 

(2010)

NPS

(2012a)

Baiomys taylori northern pygmy mouse X XUC X X X X X

Chaetodipus hispidus hispid pocket mouse XP

Conepatus leuconotus 

texensis

Tamaulipan hog-nosed 

skunk X X XHI

Cryptotis parva berlanieri least shrew XP

Dasypus novemcinctus nine-banded armadillo X XUC X X X

Didelphis virginiana Virginia opossum XP X X X X

Dipodomys compactus Gulf coast kangaroo rat X XC X X X X X

Dipodomys ordi compactus Ord kangaroo rat X

Geomys personatus Texas pocket gopher X X XC X X X X X

Lasiurus borealis borealis red bat XP

Lasiurus intermedius northern yellow bat X X XP

Lasiurus seminolus seminole bat XP

Lepus californicus black-tailed jack rabbit X X XC X X X X X

Mephitis mephitis varians striped skunk XUC X

Mus musculus house mouse* X XFC X X X

Myocastor coypus nutria

Nasua narica white-nosed coati X X X

Nycticeius humeralis evening bat XP

Onychomys leucogaster northern grasshopper mouse X XFC X X X

Oryzomys palustris marsh rice rat X X XFC X X X X X

Perognathus flavus Baird’s pocket mouse

Perognathus leucopus white-footed mouse X X XP

Perognathus merriami Merriam’s pocket mouse X X XUC

Pipeistrellus subflavus eastern pipistrelle XP X X X

Procyon lotor common raccoon X XFC X X X

Rattus norvegicus Norway rat*

Rattus rattus black rat* XUC X X X

Reithrodontomys fulvescens fulvous harvest mouse X X XUC X X X X

Scalopus aquaticus eastern mole X X X X X X X

Sigmodon hispidus hispid cotton rat X X XC X X X X

Spermophilus mexicanus Mexican ground squirrel XC

Spermophilus spilosoma spotted ground squirrel X X XC X X X X X

Spilogale putorius indianola eastern spotted skunk XP

Sylvilagus floridanus eastern cottontail X XP
348

Scientific Name Common Name Bailey

(1905)

Raun 

(1959)

Baker and 

Rabalais (1975)

Baccus 

(1977)

Harris 

(1988)

Frey and 

Jones (2008)

GULN 

(2010)

NPS

(2012a)

Tadarida brasiliensis Brazilian free-tailed bat X XFC X X X X

Taxidea taxus badger X XUC X X X X

Urocyon cinereoargenteus common gray fox XP X X X

*exotic/non-native species

Relative abundance from Baker and Rabalais (1975) P= possible, UC = uncommon, FC = fairly common, C = common, HI = highly improbable.
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