U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2016–5078

An International Borderland of Concern: Conservation of 

Biodiversity in the Lower Rio Grande Valley

Prepared in cooperation with the U.S. Fish and Wildlife Service and Oklahoma State University
Front cover. Photographs of Amazilia yucatanensis (buff-bellied hummingbird), Falco femoralis septentrionalis (northern aplomado falcon), 

and Leopardus pardalis albescens (northern ocelot). Used with permission and modified from ©Larry Ditto Nature Photography.

Back cover. Photograph of Aythya americana (redheads). Used with permission and modified from ©Larry Ditto Nature Photography.

Lower banner artwork, Biological Report 88(36), November 1988, U.S. Fish and Wildlife Service.
An International Borderland of Concern: 

Conservation of Biodiversity in the

Lower Rio Grande Valley

By David M. Leslie, Jr.

Prepared in cooperation with the U.S. Fish and Wildlife Service and

Oklahoma State University

Scientific Investigations Report 2016–5078

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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Suggested citation:

Leslie, D.M., Jr., 2016, An international borderland of concern—Conservation of biodiversity in the Lower Rio 

Grande Valley: U.S. Geological Survey Scientific Investigations Report 2016–5078, 120 p., http://dx.doi.org/10.3133/

sir20165078.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-4060-2
iii

Acknowledgments

Funding for this project was provided by the U.S. Fish and Wildlife Service 

(USFWS) under Cooperative Agreements 20181J848 and F13AC00484 to 

Oklahoma State University. I am extremely grateful to Bryan R. Winton, 

Refuge Manager of the Lower Rio Grande Valley National Wildlife Refuge 

(NWR), for financial and logistical support of this project. Without a 

doubt, this report is richly supported by many superb photographs by Larry 

Ditto, former Project Leader of the Lower Rio Grande Valley and Santa 

Ana NWRs; his willingness to share these photographs that so perfectly 

capture the variety and color of the Lower Rio Grande Valley is greatly 

appreciated. Gwyn Carmean also permitted use of her seagrass images. 

My sincere thanks are extended to Professor Timothy Brush with the 

University of Texas-Pan American and Sonja Jahrsdoerfer, Project Leader 

at the Lower Green River NWR Complex and senior author of note on the 

original 1988 USFWS report, for their thorough critiques of this synthesis. 

I am also very grateful for input from the USFWS staff at the South 

Texas Refuge Complex, including Bob Barry, Gisela Chapa, Imer De La 

Garza, Chris Hathcock, Rob Jess, Marion Mason, Kelly McDowell, Chris 

Perez (particularly for his exceptional editing of multiple drafts of this 

report), Mitch Sternberg, Hilary Swarts, and Kim Wahl-Villarreal; Willy 

Cupit, Texas Parks and Wildlife Department; Oklahoma State University 

colleagues and support staff, including Shannon Brewer, Richard Dolman, 

Renee Flasch, Jim Long, Sheryl Lyon, and Cheryl McKnight; and Kevin 

Whalen, U.S. Geological Survey (USGS) Cooperative Research Units 

Program. Particular thanks to Vince Cavalieri (USFWS East Lansing 

Ecological Services Field Office) who assisted me early on with literature 

searches and preliminary organization of some sections of the report, 

Jesse Burton (National Park Service Natchez Trace Parkway) and Jim 

Long who drafted the maps in this report, Donna Shaver (National Park 

Service Padre Island National Seashore) who shared current information 

and her expertise on sea turtles in the Gulf of Mexico, and Erin Fernandez 

(USFWS Arizona Ecological Services Office) who commented on ocelot 

status in Arizona. The Oklahoma Cooperative Fish and Wildlife Research 

Unit (Oklahoma State University, Oklahoma Department of Wildlife 

Conservation, USGS, USFWS, and Wildlife Management Institute) 

provided technical support during the preparation of this synthesis. 

©Larry Ditto Nature Photography
iv
v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Original Report and Its Impact............................................................................................................1

Need for Updated Compilation ...........................................................................................................2

Survey of LRGV-centric Literature since 1988 .................................................................................4

Report Objectives..................................................................................................................................4

Unique Aspects of the Lower Rio Grande Valley......................................................................................6

General Description..............................................................................................................................6

Floral Characteristics...........................................................................................................................6

IVC Ecological Systems in the LRGV ......................................................................................14

Tamaulipan Floodplain .....................................................................................................14

Tamaulipan Mixed Deciduous Thornscrub...................................................................15

Texas Coastal Salt and Brackish Tidal Marsh..............................................................15

Tamaulipan Closed Depression Wetland......................................................................15

Tamaulipan Calcareous Thornscrub..............................................................................17

Tamaulipan Ramadero .....................................................................................................17

Tamaulipan Savanna Grassland.....................................................................................17

Texas Coast Dune and Coastal Grassland....................................................................17

Tamaulipan Loma Shrubland and Grassland................................................................18

Texas Saline Coastal Prairies .........................................................................................18

Tamaulipan Palm Grove Riparian Forest.......................................................................18

Tamaulipan Saline Lake...................................................................................................18

Seagrass Meadows of Laguna Madre...................................................................................19

Restoration of Threatened Plant Communities.....................................................................20

Threatened and Endangered Plants ......................................................................................22

Overview.............................................................................................................................22

Ashy Dogweed ..................................................................................................................25

South Texas Ambrosia .....................................................................................................25

Star Cactus ........................................................................................................................27

Tamaulipan Kidneypetal...................................................................................................27

Walker’s Manioc ...............................................................................................................27

Zapata Bladderpod...........................................................................................................27

Faunal Characteristics.......................................................................................................................28

Invertebrates ..............................................................................................................................28

Amphibians and Reptiles..........................................................................................................34

Fishes...........................................................................................................................................35

Birds.............................................................................................................................................36

Northern Aplomado Falcon.............................................................................................36

Orioles.................................................................................................................................38

Other Riparian Dependents.............................................................................................38

Doves and Pigeons...........................................................................................................40

Thornscrub Species .........................................................................................................40

Waterfowl, Colonial Waterbirds, and Shorebirds/Seabirds ......................................40
vi

Unique Aspects of the Lower Rio Grande Valley—Continued

Faunal Characteristics—Continued

Mammals.....................................................................................................................................42

Bats .....................................................................................................................................42

Small Mammals.................................................................................................................42

Northern Ocelot in the LRGV...........................................................................................43

Ongoing Challenges Facing the LRGV......................................................................................................46

Human Population Growth and Associated Challenges ..............................................................46

Urbanization, Industrialization, and Economic Development ............................................47

Energy Development .................................................................................................................49

Oil and Gas.........................................................................................................................49

Wind Power.......................................................................................................................49

Pollution and Waste Management .........................................................................................51

Human Health and Associated Benefits of Land Conservation.........................................51

Agriculture ...........................................................................................................................................52

Changes in Crop Production ....................................................................................................52

Pesticides, Herbicides, and Associated Contamination .....................................................53

Continued Agricultural Pressure on Native Habitats and Water Resources ..................55

Flood Control and Water Development...........................................................................................56

Flood-control Activities.............................................................................................................56

Irrigation and Water Rights......................................................................................................56

Detrimental Effects of Water Development on LRGV Biota................................................58

Exotic and Invasive Species .............................................................................................................60

Plants ...........................................................................................................................................60

Water Hyacinth .................................................................................................................60

Hydrilla................................................................................................................................60

Giant Reed..........................................................................................................................60

Buffelgrass.........................................................................................................................62

Guineagrass.......................................................................................................................62

Saltcedar............................................................................................................................62

Animals........................................................................................................................................62

Red Imported Fire Ant ......................................................................................................62

Africanized Honey Bee ....................................................................................................62

Exotic or Naturalized Birds .............................................................................................63

Exotic Mammals................................................................................................................63

Climate Change ...................................................................................................................................63

Border Issues and Homeland Security ...........................................................................................64

Conservation Opportunities for the LRGV in the 21st Century..............................................................67

Productive Partnerships....................................................................................................................67

Ecotourism and Conservation Activities................................................................................67

Agency and Nongovernmental Organization Cooperation.................................................68

Cooperation with Mexico .........................................................................................................71

Conservation and Land Acquisition.................................................................................................72

Future Management Directions and Needs............................................................................................73

Wildlife Corridors................................................................................................................................78
vii

Conclusion.....................................................................................................................................................79

References....................................................................................................................................................80

Appendixes A–C

A. Research conducted on seagrass of the Lower and Upper Laguna

Madre since the early 1990s...................................................................................................115

B. Peer-reviewed publications focused on the Lower Rio Grande Valley

on 58 bird species since the late 1980s.................................................................................116

C. Peer-reviewed publications focused on the Lower Rio Grande Valley

on northern ocelots by subject categories since the late 1980s ......................................119

Figures

1.
viii

9. Photographs showing diversity of butterflies in the Lower Rio Grande Valley, 
ix

21. Photographs showing the border fence along the Lower Rio Grande 

Tables

 1. Numbers of publications focused on the Lower Rio Grande Valley 
x

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

meter per second (m/s) 3.281 foot per second (ft/s) 

meter per year (m/yr) 3.281 foot per year (ft/yr) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second per square 

kilometer ([m3/s]/km2)

91.49 cubic foot per second per square 

mile ([ft3/s]/mi2)

cubic meter per second (m3/s) 22.83 million gallons per day (Mgal/d) 

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

kilometer per hour (km/h) 0.6214 mile per hour (mi/h)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as

°C = (°F – 32) / 1.8.
xi

Abbreviations

< less than

> greater than

≥ greater than or equal to

CBP U.S. Customs and Border Protection

CONANP Comisión Nacional de Áreas Naturales Protegidas

DDE dichlorodiphenyldichloroethylene

DDT dichlorodiphenyltrichloroethane

EPA U.S. Environmental Protection Agency

GIS geographic information system

GPS Global Positioning System

Hg mercury

IBWC International Boundary and Water Commission

IPM Integrated Pest Management

IUCN International Union for the Conservation of Nature and Natural Resources

IVC International Vegetation Classification

LRGV Lower Rio Grande Valley

MOU Memorandum of Understanding

NGO nongovernmental organization

NWR national wildlife refuge

PHI potential hazard index

ppt parts per thousand

STRC South Texas Refuge Complex

TPWD Texas Parks and Wildlife Department

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

vs. versus

WMA Wildlife Management Area

Lasaia sula (blue metalmark) ©Larry Ditto Nature Photography
xii
An International Borderland of Concern: Conservation of 

Biodiversity in the Lower Rio Grande Valley

By David M. Leslie, Jr.

Abstract

The Lower Rio Grande Valley (LRGV) of southern 

Texas is located on the United States-Mexico borderland 

and represents a 240-kilometer (150-mile) linear stretch that 

ends at the Gulf of Mexico. The LRGV represents a unique 

transition between temperate and tropical conditions and, as 

such, sustains an exceptionally high diversity of plants and 

animals—some of them found in few, or no other, places in the 

United States. Examples include Leopardus pardalis albescens

(northern ocelot) and Falco femoralis septentrionalis (northern 

aplomado falcon)—both endangered in the United States and 

emblematic of the LRGV. The U.S. Fish and Wildlife Service 

(USFWS) manages three national wildlife refuges (Santa Ana, 

Lower Rio Grande Valley, and Laguna Atascosa) that together 

make up the South Texas Refuge Complex, which actively 

conserves biodiversity in about 76,006 hectares (187,815.5 

acres) of native riparian and upland habitats in the LRGV. 

These diminished habitats harbor many rare, threatened, and 

endangered species. This report updates the widely used 1988 

USFWS biological report titled “Tamaulipan Brushland of 

the Lower Rio Grande Valley of South Texas: Description, 

Human Impacts, and Management Options” by synthesizing 

nearly 400 peer-reviewed scientific publications that have 

resulted from biological and sociological research conducted 

specifically in the four Texas counties of the LRGV in the past 

nearly 30 years. This report has three goals: (1) synthesize 

scientific insights gained since 1988 related to the biology 

and management of the LRGV and its unique biota, focusing 

on flora and fauna of greatest conservation concern; (2) 

update ongoing challenges facing Federal and State agencies 

and organizations that focus on conservation or key natural 

resources in the LRGV; and (3) redefine conservation 

opportunities and land-acquisition strategies that are feasible 

and appropriate today, given the many new and expanding 

constraints that challenge conservation activities in the LRGV. 

The LRGV faces every contemporary conservation challenge 

of the 21st century, but ongoing human population growth and 

its associated demands, international border issues, and oil, 

gas, and alternative energy development dominate impacts 

that affect conservation in the LRGV. Continued careful 

syntheses of existing and future information collected in 

the LRGV are needed on many biological and sociological 

topics to guide conservation activities. Quick response will 

no doubt be necessary to face contemporary and difficultto-predict challenges such as climate change, diminished 

water availability and quality, spread of invasive species, and 

habitat loss and fragmentation. Complexities of a guarded 

international borderland add pressure to small patches of 

native habitat that remain in many places of the LRGV, 

particularly along the Rio Grande. Large connected corridors 

of restored native habitat could be the best option to maintain, 

and even enhance, the exceptional biodiversity of the LRGV 

in the face of exceptional human demand.

Introduction

Original Report and Its Impact

In 1988, the U.S. Fish and Wildlife Service (USFWS) 

published a biological report titled “Tamaulipan Brushland 

of the Lower Rio Grande Valley of South Texas: Description, 

Human Impacts, and Management Options” (Jahrsdoerfer and 

Leslie, 1988). The report synthesized available scientific and 

managerial information from the Lower Rio Grande Valley 

(LRGV) as comprehensively as possible given the knowledge 

base and search capabilities at the time. The report has been 

widely used and cited more than 120 times in a wide variety 

of mostly scientific literature. The cumulative number of 

citations of Jahrsdoerfer and Leslie (1988) through time 

reflects both continued interest in the LRGV and the value of 

such syntheses to science and management. The report is out 

of print, outdated, and in need of revision, but it is available 

online (http://www.dtic.mil/dtic/tr/fulltext/u2/a322826.pdf, 

accessed April 28, 2016).

The LRGV was defined as the southernmost counties of 

Texas (Cameron, Hidalgo, Starr, and Willacy)—a traditional 

definition used in this update. Jahrsdoerfer and Leslie (1988) 

provided descriptions and assessments of (1) physiographic, 

floral, and faunal characteristics of the LRGV; (2) changing 

human impacts and historical and recent changes in the biota 

and management of the LRGV; and (3) land-use patterns 
2 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

and managerial approaches to brushland habitats and plant 

and animal resources. The report included summaries from 

scientific journals, agency documents, and communications 

with area experts on pertinent aspects of the LRGV. The stated 

goal of the report was “to provide a single-source reference 

of historical review, land-use planning, and management of 

brushland habitats and wildlife populations in the Lower Rio 

Grande Valley” (Jahrsdoerfer and Leslie, 1988, p. iii). 

Need for Updated Compilation

The USFWS has invested considerable time and resources 

in the LRGV. It is the only Federal agency with the primary 

mission of “working with others, to conserve, protect, and 

enhance fish, wildlife, and plants and their habitats for the 

continuing benefit of the American people” (USFWS, n.d.). 

In extreme southern Texas, this mission is shepherded by the 

multifaceted activities of the South Texas Refuge Complex 

(STRC), a collection of three national wildlife refuges 

(NWRs): Laguna Atascosa, Lower Rio Grande Valley, and 

Santa Ana (fig. 1). Collectively, STRC encompasses 76,006 

hectares (ha) (187,815.5 acres) in Cameron County (44,462 

ha), Willacy County (15,455 ha), Hidalgo County (10,590 

ha), and Starr County (5,499 ha), representing fee-owned 

land, conservation easements, and donated land. The STRC 

protects, conserves, and manages many unique characteristics 

of the ecologically diverse LRGV, including the federally 

endangered Leopardus pardalis albescens (northern ocelot), 

Puma yagouaroundi cacomitli (Gulf Coast jaguarundi), and 

Falco femoralis septentrionalis (northern aplomado falcon). 

Nineteen federally threatened or endangered plant and animal 

species and nearly 60 State-protected species occur in habitats 

of or surrounding the three NWRs in the STRC.

Santa Ana NWR, established in 1943, is the smallest 

of the three NWRs at 845 ha (2,087.5 acres). It is located in 

the Rio Grande flood plain in southern Hidalgo County. The 

ongoing goal (and challenge) is to conserve dwindling oldgrowth subtropical riparian forests, resaca (oxbow) wetlands, 

and associated native brushland for the unique bird community 

in the LRGV—many species of which are not found anywhere 

else in the United States. Santa Ana NWR also boasts a high 

diversity of butterflies that, along with the nearly 400 species 

of birds recorded there, attracts thousands of nature enthusiasts 

each year, greatly enhancing the educational mission of the 

USFWS. At the time of purchase, Santa Ana NWR was an 

island of native habitat surrounded by thousands of acres of 

farmland. Since then, and particularly with the establishment 

of the Lower Rio Grande Valley NWR and subsequent land 

purchases and restoration, some of the original character of the 

LRGV has returned.

Lower Rio Grande Valley NWR was established in 1979 

to conserve and restore diminishing native habitats in the 

LRGV. In 1988, it contained 50 tracts totaling 9,817 ha (24,258 

acres) in Cameron, Hidalgo, Starr, and Willacy Counties; 

as of May 2016, it contained 147 tracts totaling 39,035 ha 

(96,458.4 acres), many of them along the Rio Grande. Its 

original mission of establishing a continuous riparian corridor 

along the Rio Grande through land purchase and conservation 

easements with private landowners has had to adapt to growing 

pressures from the expanding human population in the LRGV 

and particularly the recent construction of a segmented and 

highly patrolled “secure fence” along the United States-Mexico 

border—segments of which bisect some tracts. Border security 

issues have caused some traditional USFWS partners, such 

as Texas Parks and Wildlife Department (TPWD), the Nature 

Conservancy, and the National Audubon Society, to cease 

activities on some of their properties near the border along 

the Rio Grande. These and other challenges have forced the 

USFWS to rethink its original management strategies for all 

three NWRs in the STRC. Two inland tracts of the Lower Rio 

Grande Valley NWR containing saline lakes are designated as 

a Western Hemisphere Shorebird Reserve Network site and 

protect about 10 percent of the global population of wintering 

Numenius americanus (long-billed curlews). 

Laguna Atascosa NWR is the southernmost waterfowl 

refuge in the Central Flyway of North America and the second 

largest of the southern Texas NWRs at about 36,359 ha 

(89,845.3 acres), having grown from about 19,640 ha (48,532 

acres) in the late 1980s (Jahrsdoerfer and Leslie, 1988). It 

conserves dense tracts of thorn brushland (critical to the 

northern ocelot and Gulf Coast jaguarundi), coastal prairies, 

freshwater wetlands, sand dunes, tidal flats, and seagrass 

meadows associated with the Laguna Madre. Diversity of these 

habitats results in a commonly heard claim that nearly one-half 

of all bird species in the United States can be found in Laguna 

Atascosa NWR at one time or another throughout the year. 

Wetlands and adjoining uplands are used by migrating and 

wintering waterfowl, notably Aythya americana (redheads), 

Grus canadensis (sandhill cranes), and numerous shorebirds. 

Laguna Atascosa NWR is designated as part of the Western 

Hemisphere Shorebird Reserve Network’s Laguna Madre site. 

Federally designated critical habitat of the threatened (in this 

part of its distribution) Charadrius melodus (piping plover) 

occurs in Laguna Atascosa and Lower Rio Grande Valley 

NWRs. Laguna Atascosa NWR also is classified as a category 

V protected landscape/seascape by the International Union for 

the Conservation of Nature and Natural Resources (IUCN). 

Like few other places in the world, the USFWS’s 

STRC and associated areas throughout the LRGV face every 

contemporary conservation challenge of the 21st century. Since 

the publication of Jahrsdoerfer and Leslie (1988), significant 

research has been conducted in the LRGV, in response to the 

unprecedented changes that have occurred in the area. In some 

areas, scientific research and resulting publications have been 

replete, but in many others, they are lacking. There has been 

no synthesis of the breadth of research that has resulted from 

these efforts. This report could serve as a road map for Federal 

and State agencies responsible for conservation of the LRGV 

to direct focused managerial activities and scientific research 

related to conservation in areas with the greatest need.
laf16-0737_fig01

STARR

COUNTY

HIDALGO

COUNTY

CAMERON

COUNTY

WILLACY

COUNTY

TEXAS

Laguna Atascosa National Wildlife RefugeLower Rio Grande Valley National Wildlife RefugeSanta Ana National Wildlife Refuge

EXPLANATION

281

83

281

77

2

186

10

0 10 20 40 MILES

N 0 10 20 40 KILOMETERS

Base map image is the intellectual property of Esriand is used herein under license. Copyright © 2014 Esriand its licensors. All rights reserved.Refuge boundaries from U.S. Fish and Wildlife Service.County borders and roads from Texas Natural Resource Information System 

McAllen

Br

Raymondville

Rio

Grande

City

Roma

Harlingen

!

!

!

!

!

!

Rio Grande

Figure 1. South Texas Refuge Complex, including properties in the Lower Rio Grande Valley National Wildlife Refuge (NWR), Santa Ana NWR, and L

Starr, Hidalgo, Willacy, and Cameron Counties in southern Texas. 
4 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Survey of LRGV-centric Literature since 1988

While surveying the scientific literature since the 

publication of Jahrsdoerfer and Leslie (1988), priority 

was placed on relevant published research mostly in peerreviewed scientific outlets that was specifically conducted in 

the four counties of southern Texas (this research hereinafter 

referred to as “LRGV centric”) and synthesizing it relative to 

contemporary conservation challenges and needs. Research 

in the LRGV from the late 1980s through 2014 suggests that 

accomplishments have provided substantial insight in certain 

areas of conservation need but not in others. During that 

period, 388 scientific publications were identified with specific 

research findings from the LRGV (table 1). Collectively, this 

is an impressive amount of science coming from only four 

counties, and research topics are highly varied.

It is clear that certain areas, species, and disciplines 

have received more research than others; for example, birds 

in general and the northern ocelot in particular received 

considerably more research attention than other vertebrate 

categories (table 1). Seven focal areas were apparent across 

22 research topics synthesized from the 388 LRGV-centric 

publications. If all influences that drive research impinged 

equally on the probability of such research being conducted 

and published, one would expect about 17 publications per 

research topic from 1988 through 2014 (table 1). Nevertheless, 

those seven focal areas accounted for 71 percent of the 

published papers, highlighting the lack of published work, 

and presumably research, on many other topics important to 

conservation of the LRGV.

Aside from the LRGV-centric peer-reviewed literature, 

numerous stand-alone Federal and State agency reports, at 

least 66 graduate theses and dissertations, and at least 18 

books (12 of them field guides) with relevance to the flora, 

fauna, and ecology of the LRGV have been published since 

Jahrsdoerfer and Leslie (1988). A few titles and contributors 

deserve note because they are comprehensive primers 

on some of the more important and unique aspects of the 

LRGV. “Nesting Birds of a Tropical Frontier: the Lower Rio 

Grande Valley of Texas” by T. Brush (2005) is a thorough 

summary of the rich diversity of birds and their changing 

abundance, occurrence, and absence in the LRGV, based 

on the author’s and his students’ extensive field research 

and thorough review of the literature. “The Laguna Madre 

of Texas and Tamaulipas,” edited by J.W. Tunnell, Jr., and 

F.W. Judd (2002), is a definitive synthesis of 19 contributed 

chapters on biotic and abiotic characteristics of Laguna 

Madre in the United States and Mexico. “U.S.–Mexican 

Borderlands—Facing Tomorrow’s Challenges through USGS 

Science” (Updike and others, 2013) provides a starting 

point to understand the varied characteristics of the entire 

3,152-kilometer (km) border, subdivided into eight subareas of 

distinct biology, geology, and culture, including the LRGV at 

its easternmost reach. Great scientific insights from the LRGV 

also are evident in the collected works of (1) D.J. Shaver 

and colleagues on sea turtles, particularly the endangered 

Lepidochelys kempii (Kemp’s ridley); (2) M. Tewes and his 

students on the northern ocelot, mostly in and around Laguna 

Atascosa NWR; and (3) M.A. Mora and colleagues on 

contaminants in birds, including the northern aplomado falcon, 

and the northern ocelot.

Report Objectives

This compilation has three goals: (1) synthesize scientific 

literature published since 1988 related to the biology and 

management of the LRGV and its unique biota, focusing 

on communities and plant and animal species of greatest 

conservation concern and habitat restoration; (2) update 

challenges facing organizations and agencies that focus 

on conservation of key natural resources in the LRGV; 

and (3) redefine a land-acquisition strategy that is feasible 

and appropriate today, given the many new constraints 

and conservation challenges facing the LRGV. The report 

also assesses attainment of conservation goals set forth 

in Jahrsdoerfer and Leslie (1988) and chronicles changes 

in conservation challenges and advancement of scientific 

understanding, specific to the LRGV. 

Vegetation removal for agriculture, rangeland 

management, and urban development and competition for 

growingly scarce water were cited among the most important 

conservation challenges facing the LRGV in the late 1980s 

(Jahrsdoerfer and Leslie, 1988). Although these challenges 

remain, they are largely overshadowed by the exponential 

growth of the human population in the LRGV over the 

past 30 years, which shows no signs of slowing. Additional 

challenges new to the area include international border 

issues (that is, increased border-patrol agents and associated 

infrastructure such as the border fence and related lighting, 

construction of international bridges, road construction and 

expansion, and increased disturbance by illegal activities 

on USFWS property); oil, gas, and alternative energy 

development projects (e.g., proposed liquefied natural gas 

facilities and rapidly expanding windfarm development); and 

even the proposed construction of a rocket launching facility 

near the mouth of the Rio Grande within one of the largest 

contiguous tracts in the Lower Rio Grande Valley NWR. 

These activities impinge on successful conservation of the 

unique and increasingly isolated and rare natural resources of 

the LRGV. 
Introduction 5

Table 1. Numbers of publications focused on the Lower Rio Grande Valley (referred to as “LRGV-centric publications”) from 1988 

through 2014 (sources: Google Scholar and Web of Science); summary includes publications in peer-reviewed journals, agency series, 

books and book chapters, and proceedings from scientific meetings but not stand-alone agency reports or graduate theses. 

Topic 1988–99 2000–14 Total

Invertebrates (nonpest)

Total 8 2 10

Vertebrates

Amphibians and reptiles (including sea turtles) 12 7 19

Fish 5 6 11

Birds 30a 44 72

Mammals 

Leopardus pardalis albescens (northern ocelot) 

specific

17b 25 42

Other mammals 6 3 9

Total 70 85 155

Conservation and eco-processes

General 3 7 10

Habitat restoration and assessment 17 14 31

Plants (including endangered) 3 10 13

Riparian (flooding, sedimentation, etc.) 1 4 5

Seagrass (including brown tide effects) 14 9 23

Soil 0 2 2

Total 38 46 84

Human activities and impact

Agriculture (pests, irrigation, etc.) 16a 22 38

Air quality 1 4 5

Aquaculture (shrimp farming) 2 1 3

Border issues (including the North American

Free Trade Agreement)

1 4 5

Climate 0 2 2

Contaminants and pollution 13 11 24

Economic and ecotourism 3 2 5

Exotic and invasive species 6 4 10

Health issues and attitudes 19 15 34

Land use (brush management, grazing) 2 3 5

Waste management 0 3 3

Water use and management 3 2 5

Total 66 71 139

Overall totals 182 204 388

a

One relevant paper published in 1987 but not cited in Jahrsdoerfer and Leslie (1988) included here. 

b

Two papers published in 1986–87 but not cited in Jahrsdoerfer and Leslie (1988) included here. 
6 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Unique Aspects of the Lower 

Rio Grande Valley

General Description

The LRGV of southern Texas is located in the United 

States-Mexico borderland and represents a unique transition 

between the temperate conditions that prevail in most of the 

United States and the tropics of Mexico and Central America. 

These conditions give rise to a very diverse mixture of 

temperate and subtropical species of plants (e.g., Lonard and 

others, 1991; Donohue, 1992; Lonard, 1993; Lonard and Judd, 

1993, 2002; Richardson, 1995; Fulbright and Bryant, 2003; 

King and Richardson, 2011) and animals (e.g., Gehlbach, 

1987; Holt and Lane, 1992; Schmidly, 2004; Brush, 2005) 

unique in the United States. The LRGV generally lies at the 

approximate center of the east-west continental gradient of 

aridity, wetter to the east and drier to the west. Within the 

LRGV, a gradient occurs from dry conditions in its western 

reaches to wetter conditions eastward toward the Gulf of 

Mexico. This gradient exists along a very small linear reach of 

about 240 km (150 miles [mi]). 

Climate of the LRGV is both humid subtropical and 

semiarid (Thornthwaite, 1948), with very hot summers 

and mild winters, although cold fronts regularly hit the 

LRGV each winter, mainly in December–January when the 

probability of below-freezing temperatures, even severe 

freezes, is the greatest (e.g., Lonard and Judd, 1991; Cruce 

Alvarez and Plocheck, 2012). Rainfall is highly erratic, with 

single rainfall events often making up significant parts of 

the yearly rainfall (about 53–71 centimeters per year [cm/yr] 

[21–28 inches per year (in/yr)]) (Cruce Alvarez and Plocheck, 

2012), although there are typically wet periods in early 

summer and late summer/early autumn, with generally drier 

periods during the rest of the year.

The LRGV is not really a valley but is primarily a 

delta that slopes gently away from the Rio Grande, with 

rolling uplands and sandy soils away from the Rio Grande 

in Starr and northern Hidalgo Counties. The LRGV has been 

placed into different biotic and vegetative communities by 

different authors. Clover (1937) was the first to record and 

classify the unique vegetation of the LRGV consisting of 

the Chihuahuan Desert and areas with temperate, coastal, 

and tropical affinities. Blair (1950) placed the LRGV in the 

Tamaulipan Biotic Province (Judd, 2002) and further defined 

it as a separate biotic district called the Matamoran District―a 

designation that is still widely used today. This district 

was characterized by favorable hydrology and a generally 

subtropical climate that resulted in luxuriant vegetation, 

particularly bottomland hardwood species and thicker brush. 

Today, some maps place the LRGV in the South Texas Brush 

Country, extending north from the LRGV to the edge of the 

Edwards Plateau, and the Gulf Coast Prairies and Marshes, 

extending eastward along the Gulf Coast to Louisiana (Poole 

and others, 2010); parts of the LRGV have floral characteristics 

of both.

Floral Characteristics

Classification of plant communities and ecosystems 

in which they occur varies depending on available data and 

degrees of refinement, scale, and objectives to meet scientific 

inquiry and management goals. The 11 biotic communities 

presented in figure 4 in Jahrsdoerfer and Leslie (1988) were 

very generalized―descriptively and spatially―and highlighted 

priority areas for land acquisition by the USFWS in the early 

stages of the development of the Lower Rio Grande Valley 

NWR after it was established in 1979. Unfortunately, the 11 

biotic communities have been incorrectly cited in the literature 

as distinct plant associations or communities, which they are 

not. They were descriptive and intended to highlight the unique 

biodiversity and habitat variety in the LRGV that could be 

conserved through land purchases. 

Since the late 1980s, the ecoregion concept of Omernik 

(1987) and Bailey and others (1994) was developed to provide 

a broad spatial framework to the varied ecosystems (e.g., type, 

environmental characteristics, etc.) throughout North America, 

and it expanded quickly and cooperatively with partners from 

many Federal and State agencies. The ecoregion framework is 

hierarchical from broad landscape-type classifications in Level 

I to more refined regional and relatively local classifications in 

Level IV. In this hierarchical ecoregion framework, the LRGV 

is described (Omernik and Griffth, 2013) as follows: 

Level I Great Plains

Level II Texas-Louisiana Coastal Plains

Tamaulipas-Texas Semiarid Plain

Level III Western Gulf Coastal Plain

Level IV Lower Lower Rio Grande Valley

• Lower Rio Grande Alluvial Floodplain

• Coastal Sand Plain

• Laguna Madre Barrier Islands and

Coastal Marshes

Level IV Extreme Upper Lower Rio Grande Valley

• Rio Grande Floodplain and Terraces

• Texas-Tamaulipan Thornscrub

These classifications are descriptive and generally 

reasonable, but at Level IV, they fail to characterize the great 

heterogeneity in plant associations/communities that currently 

exists in the LRGV. This is particularly true in a landscape 

where 90–95 percent of the once uniform and extensive 

brushlands or coastal grasslands and about 91–98 percent of the 
Unique Aspects of the Lower Rio Grande Valley 7

mature riparian woodlands have been negatively affected by or 

lost to human activities such as water diversions, agriculture, 

industrialization, and urbanization (e.g., Jahrsdoerfer 

and Leslie, 1988; Raney and others, 2004; Tremblay and 

others, 2005).

Approaches to classification of plant communities have 

changed in the past several decades, particularly with the 

advent of remote-sensing capabilities, satellite technologies, 

geographic information system (GIS) software, and resulting 

open-access online databases. Hathcock and others (2012, 

2014) used a standardized approach to delineate six basic 

physiographic zones in the LRGV (fig. 2) from digital data 

in the Geologic Atlas of Texas (Texas Natural Resources 

Information System, 2016), a collaborative effort of the U.S. 

Geological Survey (USGS) and the Texas Water Development 

Board in 2002–7. Each physiographic zone was delineated on 

the basis of geological features resulting, for example, from 

its underlying geology, soils, and fluvial processes. Hathcock 

and others (2014) further delineated prominent geological 

features (initially 1–6 subzones, Hathcock and others, 2012) 

in 5 of the 6 zones (table 2), refining the great variability in 

geology and resulting vegetation. The Rio Grande Delta (fig. 

2) is the most diverse physiographic zone, containing six 

prominent geological features and reflecting its riverine and 

coastal characteristics. This zone also contains the greatest 

numbers of vertebrate species of concern (n = 18) and invasive 

plant species of concern (n = 8; table 2); both are discussed in 

greater detail below.

By using comprehensive and contemporary classifications 

of global vegetation, Hathcock and others (2012, appendix 

1) further characterized the vegetation in the LRGV by using 

the International Vegetation Classification (IVC; NatureServe, 

2013). They identified 38 terrestrial IVC Ecological 

Communities (that is, plant associations/communities) 

that occurred within at least 16 higher-level terrestrial IVC 

Ecological Systems, many of them occurring in more than 

one of the six physiographic zones. The resulting matrix of 

vegetation classification and occurrence provides an excellent 

depiction of the very high degree of overall complexity of 

the flora in the LRGV (table 3) and the resulting challenges 

facing ongoing conservation, including restoration. Not all 

probable Ecological Communities have been identified yet in 

the IVC system (C. Hathcock, written commun.), but the fact 

that 20 IVC Ecological Communities are unique in particular 

physiographic zones (table 4) could provide a useful way to 

prioritize management activities for restoration and acquisition 

of rare and unique plant associations and their associated 

fauna in the LRGV. In particular, 11 of the 38 IVC Ecological 

Communities identified by Hathcock and others (2012) occur 

only in the Rio Grande Delta physiographic zone, followed by 

five unique IVC Ecological Communities in the Bordas Cuesta 

physiographic zone (table 4).

This recent interpretation and consolidation of the 

classification of plant communities within physiographic zones 

in the LRGV could allow the STRC to focus management 

attention in the zones, subzones, and plant associations with 

the greatest need of conservation attention (e.g., rarity, habitat 

restoration, and land acquisition). On the basis of Hathcock 

and others’ synthesis (2012, 2014) of the diversity and 

complexity of the vegetation in the LRGV and the associated 

biota (tables 2–4), it would appear that the Rio Grande Delta 

and, in particular, the Bordas Cuesta are physiographic zones 

with the greatest future conservation challenges. In particular, 

the Rio Grande Delta faces extra challenges because human 

densities and growth are (and have been) greatest within 

its boundaries. Nevertheless, relative to management and 

conservation activities, the focus could be on the IVC 

Ecological Systems and plant associations that currently occur 

within them. Based on the hierarchy provided in table 3 and 

including the aquatic Tampaulipan Saline Lake, the following 

sections highlight unique aspects of those IVC Ecological 

Systems with the greatest numbers of described plant 

associations in the six physiographic zones of the LRGV. 

©Larry Ditto Nature Photography
laf16-0737_fig02

TEXAS

0 10 20 40 MILES

N 0 10 20 40 KILOMETERS

!

!

!

!

!

!

Rio Grande

HIDALGO

COUNTY

STARR

COUNTY

WILLACY

COUNTY

CAMERON

COUNTY

SAND SHEET

AGUILARES

 PLAIN

UPPER VALLEY

FLOODPLAIN

McAllen

Brow

Raymondville

Rio Grande City

Roma

Harlingen

BORDAS

CUESTA

RIO GRANDE

DELTA

Arroyo Colorado Base modified from the Geologic Atlas of Texas as delineated by Hathcock and others (2012, 2014), 1:250,000.

Figure 2. The six major physiographic zones in the Lower Rio Grande Valley (LRGV), uniquely colored as delineated by Hathcock and others (2012, 20

Atlas of Texas (Texas Natural Resources Information System, 2016) and historical and geological literature from the LRGV.
Table 2. Physiographic zones, prominent geological features, vertebrate species of concern, and invasive plant species of concern in the Lower Rio (Aguilares Plain) to east (Barrier Island), as outlined in Hathcock and others (2012).

Physiographic zones Prominent geological features Primary vertebrate species of concerna Primary plant species of concerna Primary in

Aguilares Plain Yegua Formation

Laredo Formation

Polioptila melanura (black-tailed 

gnatcatcher) 

Frankenia johnstonii (Johnston’s frankenia)Physaria thamnophila (Zapata bladderpod)Thymophylla tephroleuca (ashy dogweed)

Nicotiana glaCenchrus ciliTamarix ramo

Upper Valley Floodplain Patagioenas flavirostris (red-billed 

pigeon)

Psilorhinus morio (brown jay)

Arundo donax Nicotiana glaTamarix aphyTamarix ramo

Bordas Cuesta Bordas Dip Plain

Bordas BreaksBordas Escarpment

Coleonyx brevis (Texas banded 

gecko)

Phrynosoma cornutum (Texas 

horned lizard)b

Corvus cryptoleucus (Chihuahuan 

raven)b

Asclepias prostrata (prostrate milkweed)Astrophytum asterias (star cactus)Cardiospermum dissectum (Chihuahua 

balloon-vine)

Manfreda longiflora (Runyon’s huaco)Manihot walkerae (Walker’s manioc)

Dichanthium 

bluestem)

Cenchrus cili

Sand Sheet Shallow Water Flats

Wind Tidal Flats–Delta

Gopherus berlandieri (Texas 

tortoise)b

Glaucidium brasilianum

(ferruginous pygmy-owl)

Megathyrsus Cenchrus cili

©Larry Ditto Nature Photography
Table 2. Physiographic zones, prominent geological features, vertebrate species of concern, and invasive plant species of concern in the Lower Rio (Aguilares Plain) to east (Barrier Island), as outlined in Hathcock and others (2012).—Continued Physiographic zones Prominent geological features Primary vertebrate species of concerna Primary plant species of concerna Primary in

Rio Grande Delta Recent Delta Mission Terrace Beaumont Delta Clay Dunes Shallow Water Flats Wind Tidal Flats–Delta Drymobius margaritiferus (speckled racer) Drymarchon melanurus erebennus (Texas indigo snake)b Leptodeira septentrionalis septentrionalis (northern cat-eyed snake) Notophthalmus meridionalis (black-spotted newt) Siren sp. (South Texas siren, large Rio Grande form) Anas fulvigula (mottled duck) Buteo nitidus plagiatus (gray hawk) Calidris canutus rufa (rufa red knot) Camptostoma imberbe (northern beardless- tyrannulet) Chondrohierax uncinatus (hook-billed kite) Falco femoralis septentrionalis (northern aplomado falcon)b Icterus cucullatus (hooded oriole) Icterus gularis (Altamira oriole) Leptotila verreauxi angelica (white-tipped dove) Pachyramphus aglaiae (rose-throated becard) Lasiurus ega (southern yellow bat) Puma yagouaroundi cacomitli (Gulf Coast jaguarundi)b Leopardus pardalis albescens (northern ocelot)b Ambrosia cheiranthifolia (South Texas ambrosia) Ayenia limitaris (Tamaulipan kidneypetal) Echeandia texensis (Texas craglily) Esenbeckia runyonii (limoncillo) Arundo donax 

Dichanthium a

bluestem) Melia azedarac

Megathyrsus mCenchrus ciliaTamarix aphyllTamarix ramosSchinus terebin

Barrier Island Sand Dune and Beach Shallow Water Flats Wind Tidal Flats–Island Falco peregrinus (peregrine falcon)b Lepidochelys kempii (Kemp’s ridley)b Calidris canutus (red knot)

Charadrius melodus (piping plover)Charadrius nivosus (snowy plover)Charadrius wilsonia (Wilson’s plover)

Sporobolus tharpii (South Padre 

Island dropseed)

aSpecies of concern do not necessarily have official State or Federal threatened and (or) endangered designations (see tables 5 and 6), but they have management interest in a p

these are speciesthat require more field research to clarify their conservation status or are considered sensitive, rare, or declining by State and Federal agencies or professional and (or) academic 

bSome of the species occur in more than one physiographic zone.
Table 3. Numbers of terrestrial International Vegetation Classification (IVC) Ecological Communities at the Association Level (known as plant commHathcock and others (2012) in each of their six physiographic zones, tallied for each of the 17 Ecological Systems, as listed on the IVC (NatureServe, 2Grande Valley. 

 IVC Ecological Communities by physiographic zone

IVC Ecological Systems Aguilares Plain Upper Valley Floodplain Bordas Cuesta Sand Sheet Rio Grande Delta Barrier I

Tamaulipan Floodplain (22) 3 5 3 3 8 

Tamaulipan Mixed Deciduous 

Thornscrub (13)

2 6 2 3 

Texas Coast Salt and Brackish Tidal 

Marsh (11)

4 7 

Tamaulipan Closed Depression 

Wetland (9)

1 2 2 2 2 

Tamaulipan Calcareous Thornscrub 

(5)

2 3 

Tamaulipan Ramadero (5) 1 1 1 1 1 

Tamaulipan Savanna Grassland (4) 2 1 1 

Texas Coast Dune and Coastal 

Grassland (3)

1 1 1 

Rio Grande Delta Thorn Woodland 

(3)

3 

North American Desert Riparian 

Mesquite Bosque (3) 1 1 1 

Central and South Texas Coastal 

Fringe Forest and Woodland (2) 2 

Tamaulipan Loma Shrubland and 

Grassland (2)

2 

Tamaulipan Saline Thornscrub (2) 1 1 

Texas Saline Coastal Prairie (2) 1 1 

Tamaulipan Palm Grove Riparian 

Forest (1)

1 

Gulf Coast Chenier Plain Salt and 

Brackish Tidal Marsh (1) 1 

Total IVC Ecological Communities 

represented

11 8 20 13 31 9

Total IVC Ecological Systems 

represented

7 3 10 8 11 3
Table 4. Twenty of the 38 International Vegetation Classification (IVC) Ecological Communities unique to 5 of 6 physiographic zones (see fig. 2), as deothers (2012, 2014) in the Lower Rio Grande Valley. Physiographic zonesa IVC Ecological Community identifier IVC Ecological Community description Aguilares Plain CEGL007760 Vachellia rigidula – Leucophyllum frutescens – Hechtia glomerata Shrubland Blackbrush – Cenizo – Guapilla Shrubland Sand Sheet CEGL007786 Prosopis glandulosa var. glandulosa – Senegalia greggii – Celtis pallida/Paspalum setaceum – Urochloa 

Honey Mesquite – Catclaw Acacia – Spiny Hackberry/Slender Crowngrass – Fringed Signalgrass Woodla

CEGL007788 Prosopis glandulosa var. glandulosa/Colubrina texensis – Monarda fruticulosa – Waltheria indica Woodl

Honey Mesquite/Texan Hogplum – Shrubby Beebalm – Basora-Prieta Woodland Bordas Cuesta CEGL002181 Senegalia berlandieri South Texas Plains Shrubland Guajillo Shrubland CEGL004923 Helietta parvifolia – Vachellia rigidula – Ebenopsis ebano – Leucophyllum frutescens Shrubland Barreta Shrubland – Chaparro-Prieto (Blackbrush) – Texas Ebony – Cenizo Shrubland CEGL007759 Vachellia rigidula – Leucophyllum frutescens – Senegalia berlandieri Shrubland Blackbrush – Cenizo – Guajillo Shrubland CEGL007762 Prosopis glandulosa var. glandulosa – Parkinsonia texana var. macra – (Cordia boissieri, Koeberlinia sp

Honey Mesquite – Tamaulipan Palo Verde – (Anacahuita, Allthorn) Shrubland CEGL007789 Leucophyllum frutescens – Salvia ballotiflora – Lippia graveolens Shrubland Cenizo – Shrubby Blue Sage – Mexican Oregano Shrubland Rio Grande Delta CEGL002054 Ebenopsis ebano – Ehretia anacua/Condalia hookeri Forest Texas Ebony – Anacua/Bluewood Forest CEGL002056 Sabal mexicana – Ebenopsis ebano Forest Rio Grande Palmetto – Texas Ebony Forest CEGL002132 Prosopis glandulosa var. glandulosa/(Celtis pallida, Phaulothamnus spinescens, Ziziphus obtusifolia var. Honey Mesquite/(Spiny Hackberry, Snake Eyes, Lotebush) Woodland CEGL002169 Ebenopsis ebano – Phaulothamnus spinescens Shrubland Texas Ebony – Snake Eyes Shrubland CEGL002170 Citharexylum berlandieri – Yucca treculeana – Ebenopsis ebano – Phaulothamnus spinescens Shrubland Berlandier’s Fiddlewood – Spanish Dagger – Texas Ebony – Snake Eyes Shrublandb
Table 4. Twenty of the 38 International Vegetation Classification (IVC) Ecological Communities unique to 5 of 6 physiographic zones (see fig. 2), as deothers (2012, 2014) in the Lower Rio Grande Valley.—Continued

Physiographic 

zonesa

IVC Ecological 

Community identifier IVC Ecological Community description

Rio Grande 

Delta—Continued

CEGL004755 Spartina patens – Schoenoplectus (americanus, pungens) – (Distichlis spicata) Herbaceous Vegetation (M

Saltmeadow Cordgrass – (Chairmaker’s Bulrush, Common Threesquare – (Inland Seagrass) Herbaceous V

CEGL007752 Celtis laevigata – Ulmus crassifolia – (Fraxinus berlandieriana)/Rivina humilis – Chromolaena odorata 

Sugarberry – Cedar Elm – (Mexican Ash)/Pigeonberry – Crucita Forest

CEGL007764 Maytenus phyllanthoides – Prosopis reptans/Spartina patens Herbaceous Vegetation

Gutta-percha Mayten – Creeping Mesquite/Saltmeadow Cordgrass Herbaceous Vegetationb

CEGL007787 Prosopis glandulosa var. glandulosa – Celtis pallida/Opuntia spp. – Neonesomia palmeri Woodland

Honey Mesquite – Spiny Hackberry/Prickly Pear species – Texas Desert Goldenrod Woodlandb

CEGL007832 Prosopis glandulosa var. glandulosa/Acanthocereus tetragonus Woodland

Honey Mesquite/Triangle Cactus Woodlandb

CEGL008456 Typha domingensis Tidal Marsh (Herbaceous Vegetation)

Southern Cattail Tidal Marshb

Barrier Island CEGL004971 Spartina patens – Panicum amarum – Hydrocotyle bonariensis Dune Grassland (Herbaceous Vegetation) 

Saltmeadow Cordgrass – Bitter Panicgrass – Beach Marsh-pennywort Dune Grassland (Herbaceous Vege

aThe Upper Valley Floodplain physiographic zone does not contain a unique IVC Ecological Community.bCoastal mainland ecological communities unique to the Rio Grande Delta physiographic zone.
14 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

IVC Ecological Systems in the LRGV

Tamaulipan Floodplain

The Tamaulipan Floodplain represents the Rio Grande 

riparian forest ecosystem in the LRGV (fig. 3A), which 

typifies the flood plain (NatureServe, 2013). This Ecological 

System contains 22 IVC Ecological Communities in 5 of the 

6 physiographic zones (table 3), of which 11 only occur in the 

Rio Grande Delta zone (table 4; Hathcock and others, 2012). 

Once a lush, continuous, old-growth forest along the lower 

Rio Grande, the Tamaulipan Floodplain is now fragmented 

by water manipulations, agricultural practices, and other 

harmful activities of the past century or more (Jahrsdoerfer 

and Leslie, 1988). It is typically found along the Rio Grande 

in Cameron and Hidalgo Counties, and a prime example 

occurs at Santa Ana NWR (e.g., Lonard and Judd, 2002). 

Diagnostic characters include occurrence in the lowland flood 

zone of the Rio Grande, now highly modified; subtropical and 

tropical vegetation; and forest/woodland landscapes somewhat 

terraced from true riparian vegetation near the river and 

bottomland forests to more upland terraces of thorny shrubs. 

The Tamaulipan Floodplain is a mosaic of resacas, which are 

old meander channels of the Rio Grande (e.g., oxbows) and 

old natural levees containing Parkinsonia aculeata (retama), 

Sesbania drummondii (rattlebox), and Mimosa pigra (black 

mimosa); riparian areas are represented by Salix nigra (black 

willow); and upland, typically drier terraces of often thorny 

plants are represented by Leucophyllum frutescens (cenizo), 

Guaiacum angustifolium (guayacan), Ebenopsis ebano

(Texas ebony), and Sideroxylon celastrinum (coma). The true 

bottomland forest can be very conspicuous because of its 

high stature—up to 15 meters (m) (49 feet [ft])—and contains 

large Celtis laevigata (sugarberry), Ulmus crassifolia (cedar 

elm), Fraxinus berlandieriana (Mexican ash), and Leucaena 

pulverulenta (tepeguaje), often covered with epiphytes (e.g., 

Tillandsia spp.) if mature enough. Understory vegetation in 

this community can be lush and contain various species, but 

it is completely dependent on the density of the overstory 

canopy. Fire likely had a limited role in this community 

because of the usually damp conditions from periodic 

flooding, which are greatly diminished today (2016). 

A B

C

Figure 3. Vegetative diversity of inland ecological systems in the Lower Rio Grande Valley along the Rio Grande and upslope from 

it. A, Tamaulipan Floodplain. B, Tamaulipan Mixed Deciduous Thornscrub upslope from the Rio Grande. C, Tamaulipan Calcareous 

Thornscrub. ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
Unique Aspects of the Lower Rio Grande Valley 15

Tamaulipan Mixed Deciduous Thornscrub

The Tamaulipan Mixed Deciduous Thornscrub is found 

primarily in Cameron and Hidalgo Counties, and it contains 

13 Ecological Communities in 4 of the 6 physiographic 

zones (table 3), of which 5 only occur in the Bordas Cuesta 

zone (table 4; Hathcock and others, 2012). This Ecological 

System contains highly variable associations of shrublands 

and woodlands (fig. 3B), occurring on a variety of substrates 

and landforms. A mere 5 percent or less of the original 

community remains in a natural state in the United States 

(perhaps more in Mexico―Jiménez Pérez and others, 2013), 

having been lost primarily to agriculture and expanding 

industrialization and urbanization. According to the 

NatureServe classification, this system shares characteristics 

with and often abuts Tamaulipan Savanna Grasslands 

and Tamaulipan Saline Thornscrub—differences between 

them may be very subtle. Diagnostic characters include 

a tropical xeric character, shrub and (or) low woodland 

dominance, nearly closed canopy with limited herbaceous 

layer, occurrence on lowland to upland sites associated with 

past alluvial processes of the Rio Grande, and clay or clayloamy soils, sometimes associated with high salinity. The 

system is typically dominated by Prosopis glandulosa var. 

glandulosa (honey mesquite), cenizo, and species of Acacia 

(Eddy and Judd, 2003), with a wide variety of codominant or 

subdominant shrubs (e.g., Amyris texana [Texas torchwood]; 

Celtis pallida [spiny hackberry]; Karwinskia humboldtiana

[coyotillo]; and Zanthoxylum fagara [colima]). Opuntia 

engelmannii var. lindheimeri (Texas prickly pear) is often 

abundant, and the limited herbaceous layer may include 

Trichloris pluriflora (four-flower trichloris) and Setaria 

texana (bristlegrass). Emergent honey mesquite and other tall 

shrubs may reach 4 m (13 ft) in height, but the entire canopy 

is most dense at about 2 m (6.6 ft). Lack of fire currently 

maintains this community (C. Hathcock, written commun.). 

Texas Coastal Salt and Brackish Tidal Marsh

The Texas Coastal Salt and Brackish Tidal Marsh is a 

tidally influenced, marshy Ecological System that occurs on 

coastal plains of Cameron and Willacy Counties and typically 

on the lagoon side of barrier islands (fig. 4E). It contains 

11 Ecological Communities in 2 of the 6 physiographic zones, 

of which seven communities occur in the Barrier Island 

zone (table 3). It is a key habitat in Laguna Atascosa NWR 

(Judd and Lonard, 2002) and the Boca Chica tract of the 

Lower Rio Grande Valley NWR. Diagnostic characteristics 

include dominance by tidal grasses; windy tidal and estuarine 

influences; freshwater input into Laguna Madre; soils ranging 

from sand, silt, and clay depending on juxtaposition with 

geomorphic characteristics such as sheltered lagoons and 

exposed shorelines; and generally 0–2 m (0–6.6 ft) above 

mean sea level. Plant composition is largely influenced by 

soil, elevation, and salinity. When salinities are 25–35 parts 

per thousand (ppt), Spartina spartinae (Gulf cordgrass—

Cooper and Wagner, 2013) can form monotypic stands, but 

as salinity increases, open mudflats are characteristic, with 

sparse coverage of Salicornia bigelovii (dwarf glasswort), 

which can become invasive on disturbed tidal flats (Withers, 

2002b; Onuf, 2006). More moderate salinities (10–25 ppt) 

and higher elevations give rise to dominance by Spartina 

patens (saltmeadow cordgrass). As freshwater input increases 

around deltas, Paspalum vaginatum (seashore paspalum) 

and Schoenoplectus americanus (chairmaker’s bulrush) 

become more common. At the highest elevations of about 

2 m (6.6 ft), plant associations become more diverse (Judd 

and Lonard, 2002) and include Distichlis spicata (inland 

saltgrass), Distichlis littoralis (shoregrass), Sporobolus 

virginicus (seashore dropseed), Borrichia frutescens (seaox-eye daisy), Rayjacksonia phyllocephala (camphor daisy), 

and Iva angustifolia (sumpweed). Avicennia germinans

(black mangrove) is sparsely distributed in some midelevation, high-saline marsh zones in the LRGV (Everitt 

and Judd, 1989; Everitt, Judd, Escobar, Davis, and others, 

1996), but its distribution and that of the Rhizophora mangle

(red mangrove) are expanding along the Texas coast as 

temperatures increase with climate change (Montagna and 

others, 2007, 2011). Fire is generally a minor influence in 

this community because of the high moisture content of 

the vegetation. This tidal system was greatly altered by 

construction of the Gulf Intracoastal Waterway beginning in 

the 1940s, effectively increasing water depths by dredging 

and upland habitats by deposition of dredged material. 

Upstream dam construction and water-flow manipulation, 

which reduce freshwater inputs, increase downstream 

salinities and reduce nutrient and sediment input to tidal 

estuaries in the LRGV (e.g., Cooper and Wagner, 2013).

Tamaulipan Closed Depression Wetland

The Tamaulipan Closed Depression Wetland is a 

freshwater to brackish-water wetland Ecological System of 

small and internally drained depressions (e.g., ponds and 

potholes) that occurs along western shorelines of Laguna 

Madre in Cameron and Willacy Counties, and it is an 

important system in Laguna Atascosa NWR. It contains 

nine Ecological Communities occurring rather equally 

in 5 of the 6 physiographic zones (table 3; Hathcock and 

others, 2012). Diagnostic characters include occurrence 

in lowland depressions; intermittent flooding, wet and dry 

cycles, and high rates of evaporation; sandy, wind-blown 

substrates but typically lined with clay or clay-loamy soils; 

and variable water salinity depending on freshwater input, 

evaporation (drawdown), and flooding. Seasonal flooding 

affects composition of shoreline and emergent vegetation and 

the degree of exposed mudflats, which can then be invaded 

by terrestrial vegetation. On depression wetland perimeters, 

Gulf cordgrass (Cooper and Wagner, 2013), sea-ox-eye 

daisy, saltgrass, and shoregrass occur in varied abundance 

depending on soil type, salinity, and elevation. As conditions 

become more brackish, Schoenoplectus pungens (common 

threesquare) becomes more abundant; when particularly 
A B C D E F GFigure 4. Coastal ecological systems illustrating the rich landscape and floral diversity of the coastal areas in the Lower Rio Grande Valley. A, Tamauli

with Tamaulipan Floodplain. B, Tamaulipan Loma Shrubland and Grassland. C, Tamaulipan Palm Grove Riparian Forest. D, Texas Coast Dune and Coasta

and Brackish Tidal Marsh. F, Texas Coast Dune and Coastal Grassland. G, Tamaulipan Loma Shrubland and Grassland. ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography
Unique Aspects of the Lower Rio Grande Valley 17

wet, Eleocharis montevidensis (sand spikerush) can become 

prevalent. Subemergent vegetation in LRGV wetlands is 

typically dominated by the macro-alga muskgrass (Chara

spp.) and Ruppia maritima (widgeongrass) when surface 

water is present for extended periods of time. As salinity of 

wetlands decreases inland, plants with floating leaves, such 

as Nelumbo lutea (American lotus) and Nymphaea odorata

(water lily), can become established. Where not protected, 

some of these wetlands have been drained for agriculture, 

resulting in sedimentation and contamination from pesticides, 

fertilizer runoff, and water manipulation; they are also 

affected by heavy cattle grazing and oil and gas production. 

Wintering waterfowl species heavily use these wetlands; 

for example, 75 percent or more of the North American 

population of redheads feed on Halodule wrightii (shoal 

grass) in the most saline of wetlands. 

Tamaulipan Calcareous Thornscrub

Tamaulipan Calcareous Thornscrub occurs only in 

Starr County on upland, calcareous, and caliche substrates 

with shallow soils. This Ecological System contains five 

Ecological Communities in 2 of the 6 physiographic zones, 

the Aguilares Plain and Bordas Cuesta (table 3; Hathcock 

and others, 2012). This system has a unique native citrus, 

Helietta parvifolia (baretta―Best, 2011a, 2011b), that grows 

as thickets on gravel-caliche ridges, forming an ecotone with 

the flood plain (fig. 3C). Diagnostic characters include xeric 

shrub dominance, occurrence on ridges or upper slopes, 

and very shallow soils. Dominant plant species typically 

are cenizo and Senegalia berlandieri (guajillo), and the five 

unique plant associations of the Tamaulipan Calcareous 

Thornscrub occur with various abundances of the dominants 

and Vachellia rigidula (blackbrush acacia), Texas ebony, 

Salvia ballotiflora (shrubby blue sage), Lippia graveolens

(Mexican oregano), baretta, and even the bromeliad Hechtia 

glomerata (guapilla). This shrubland community has a shorter 

and more open canopy—generally less than (<) 2 m (6.6 ft)—

compared with other thornscrub communities in the LRGV, 

growing on favorable sites with more developed soils. Other 

plant species include Ziziphus obtusifolia var. obtusifolia

(lotebush), Parkinsonia texana var. macra (Tamaulipan palo 

verde), Koeberlinia spinosa (allthorn), and Castela texana

(chaparro amargosa). Fire does not have much of a role in 

this community, except on its edges when spreading from 

other communities during drought and high winds.

Tamaulipan Ramadero

The Tamaulipan Ramadero, a riparian shrublandwoodland Ecological System, occurs along drainages in 

upland areas that ultimately drain into the Rio Grande, 

primarily in Starr and Hidalgo Counties. This Ecological 

System contains five Ecological Communities, one each 

in 5 of the 6 physiographic zones (table 3; Hathcock and 

others, 2012). Intermittent flooding affects this system, 

which contains isolated riparian strips of dense brush, often 

modified by check dams in arroyos that prevent water and 

nutrients from reaching them, thus reducing vegetation height 

and density. Under ideal conditions, stature of overstory 

vegetation can be 5–10 m (16.4–32.8 ft) high and closedcanopied, limiting understory density and diversity. Some 

sites have a shrubby, often impenetrable character of 1–5 

m high (3.3–16.4 ft). Woody plant species include honey 

mesquite, granjeno, retama, Condalia hookeri (bluewood),|and 

Vachellia farnesiana var. farnesiana (huisache). Ramaderos 

provide important nesting and feeding habitats for wildlife 

and corridors to the Tamaulipan Floodplain along the Rio 

Grande (fig. 4A). 

Tamaulipan Savanna Grassland

Remnants of the Tamaulipan Savanna Grassland, a 

formerly grass-dominated system, occur only on the coastal 

plain of northern Willacy County in the LRGV, and it contains 

four Ecological Communities in 3 of the 6 physiographic 

zones (table 3; Hathcock and others, 2012). This community 

was once expansive in areal coverage, typically grew on sandy 

or sandy-loam soils, and had a patchy and clumped overstory 

dominated by honey mesquite but sometimes granjeno, brasil, 

colima, lotebush, Diospyros texana (Texas persimmon), 

and various species of Acacia (Eddy and Judd, 2003). This 

Ecological System has been degraded by intensive livestock 

grazing and variation in precipitation during the past century 

(Archer and others, 1988; Thompson, 1997) and perhaps lack 

of fire, which was intentionally used by Native Americans to 

maintain open grassland but is now uncommon. Diagnostic 

characters include grass domination, patchy shrub and tree 

overstory, and occurrence on coastal lowlands. Dominant 

herbaceous species include Bothriochloa barbinodis (cane 

bluestem), Schizachyrium scoparium (little bluestem), B. 

laguroides var. torreyana (silver beard grass), four-flower 

trichloris, Verbena halei (Texas verbena), and Rivina humilis

(pigeonberry). Today, this community is a closed shrubland, 

and the likelihood of reverting it to savanna grassland is slim 

because of the extensive restoration that would be required. 

Texas Coast Dune and Coastal Grassland

The Texas Coast Dune and Coastal Grassland is a coastal 

Ecological System that occurs on barrier islands of the Laguna 

Madre and near-coastal (0–16 km) inland areas along the Gulf 

Coast in Cameron and Willacy Counties and is represented in 

parts of Laguna Atascosa NWR and Boca Chica tract in the 

Lower Rio Grande Valley NWR (fig. 4D). It contains three 

Ecological Communities in 3 of the 6 physiographic zones 

(table 3; Hathcock and others, 2012). Diagnostic characters 

include sand dunes, interdune swales, and barrier-flat and 

tidal-flat physiography; sparse vegetation; and frequent 

high winds and exposure to sea spray, infrequent high tides 

(about 20-year intervals), and storm events, which can alter 

vegetation and the physical nature of the system and cause 

movement of primary and secondary dunes (Judd and others, 

2008). Leeward sides of dunes are typically the most vegetated 
18 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

because of protection from winds and tidal forces. Flowering 

flora is diverse, with at least 74 flowering angiosperms in 

70 genera and 28 families on South Padre Island (Lonard and 

Judd, 1989). Depending on the physical landscape, this system 

is a matrix of grasses, typically dominated by little bluestem, 

Panicum spp., and Gulf cordgrass, but many other species 

of grass occur in this system (e.g., Andropogon glomeratus 

[bushy bluestem]; A. ternarius [splitbeard bluestem]; 

Sporobolus texanus [Texas dropseed]; and tropical grasses in 

the genera Heteropogon, Paspalum, and Trachypogon). Shrubs 

include honey mesquite, huisache, and blackbrush. Because of 

a lack of fire, which might have had anthropogenic origins in 

the past, this community can become shrub-dominated. Most 

of this community on the mainland has been lost to grazing 

and cropland. 

Tamaulipan Loma Shrubland and Grassland

The Tamaulipan Loma Shrubland and Grassland occurs 

among plant associations of the Texas Coast Salt and Brackish 

Tidal Marsh system in Cameron and Willacy Counties and 

is an important feature of the Rio Grande Delta (e.g., Loma 

Preserve and Boca Chica tracts of the Lower Rio Grande 

Valley NWR and habitat in Laguna Atascosa NWR; fig. 

4B and 4G). It contains only two Ecological Communities, 

both of them in the Rio Grande Delta physiographic zone 

(table 3; Hathcock and others, 2012). Lomas are small, xeric, 

subtropical, shrubby islands scattered among tidal marshes 

and flats, resulting from wind-blown deposits of silt, clay, and 

sand—typically well drained. In places, emergent trees such 

as Texas ebony and honey mesquite occur, but mostly the 

dense, thorny shrub layer is characterized by Citharexylum 

berlandieri (Berlandier’s fiddlewood), Yucca treculeana

(Spanish dagger), Texas torchwood, Lycium berlandieri

(wolfberry), Lantana horrida (Padre Island lantana), 

blackbrush, and Cylindropuntia leptocaulis (tasajillo). The 

herbaceous understory is typically sparse because of the 

dense shrub canopy and includes pigeonberry, Heliotropium 

curassavicum (wild heliotrope), and Phyla nodiflora (Texas 

frogfruit). Hurricanes and tropical storms cause wind damage 

and saltwater inundation; outer edges of lomas can suffer from 

erosion during such storm events. 

Texas Saline Coastal Prairies

Texas Saline Coastal Prairies occur as remnant coastal 

prairies on terraces of generally level topography flanking 

the Gulf Coast and mostly on private land in northern 

Willacy County. These prairies contain only two Ecological 

Communities in the Rio Grande Delta and Barrier Island 

physiographic zones (table 3; Hathcock and others, 2012). 

Diagnostic characters include soil saturation by local rainfall 

and periodic major storm events causing flooding with saline 

water, occurrence of both upland saline prairie and wetland 

community types in depressions, and microtopographic 

features with ridge-and-swale plant associations. Upland 

dominant grass species are little bluestem, Paspalum 

plicatulum (brownseed paspalum), Sorghastrum nutans

(yellow Indiangrass), and Andropogon gerardii (big 

bluestem). Wetland dominant grasses depend on the level 

of disturbance: undisturbed sites have Panicum virgatum

(switchgrass) and Tripsacum dactyloides (eastern gramagrass) 

and disturbed sites have bushy bluestem; other species of 

Andropogon, Sporobolus, and Chloris; Carex and Juncus

(sedges); and various tropical genera such as Heteropogon, 

Paspalum, Trachypogon, and Panicum. Herbaceous forbs 

include Ratibida columnifera (prairie coneflower), Liatris 

pycnostachya (blazing-star), and Sagittaria latifolia (aquatic 

arrowhead). Honey mesquite and Quercus (oaks) invade this 

community in the absence of fire, and estimates suggest that 

99 percent of this coastal prairie has been lost because of 

agricultural practices and disruption of natural processes. 

Tamaulipan Palm Grove Riparian Forest

The Tamaulipan Palm Grove Riparian Forest is now a 

very unique riparian community in the LRGV (fig. 4C), and it 

is preserved only in the Rio Grande Delta physiographic zone 

in the Southmost Ranch and Boscaje de la Palma tracts of the 

Lower Rio Grande Valley NWR in extreme southern Cameron 

County along the Rio Grande (Everitt, Judd, Escobar, Alaniz, 

and others, 1996). The system is named after its diagnostic 

plant, Sabal mexicana (Rio Grande palmetto or sabal palm; 

fig. 4C). This palm was once much more common along the 

Rio Grande (Crosswhite, 1980). Other prominent tree species 

in this forested system include Texas ebony, Ehretia anacua

(anacua), and tepeguaje; other associated riparian plants 

can include cedar elm and Mexican ash, trees most typical 

of forests in the Tamaulipan Floodplain, as well as Texas 

persimmon and huisache. 

Tamaulipan Saline Lake

The Tamaulipan Saline Lake, a hypersaline aquatic 

system, is well represented in the East Lake/La Sal Vieja and 

La Sal del Ray/Schalaben tracts of the Lower Rio Grande 

Valley NWR in northern Willacy and Hidalgo Counties, 

respectively. Plant associations of the Tamaulipan Mixed 

Deciduous Thornscrub Ecological System typically surround 

these saline lakes. Sparse to moderate vegetation cover occurs 

on the edges of these lakes, dominated by halophytic grasses 

and small shrubs. The lakes provide migrating and wintering 

habitat for Aythya affinis (lesser scaup), Oxyura jamaicensis

(ruddy ducks), Dendrocygna autumnalis (black-bellied 

whistling ducks), Pelecanus erythrorhynchos (white pelicans), 

and sandhill crane. Ten percent of the North American 

population of the long-billed curlews winters at East Lake 

(Covington, 2014). 
Unique Aspects of the Lower Rio Grande Valley 19

Seagrass Meadows of Laguna Madre

Laguna Madre is the largest of seven estuarine systems 

on the Texas coast and, along with the Laguna Madre de 

Tamaulipas of Mexico, is 1 of only 5 hypersaline ecosystems 

in the world (Tunnell, 2002a; Webster and others, 2002). In 

the United States, Laguna Madre is protected on the west 

by Padre Island, the longest barrier island in the world. The 

lagoon has an average depth of only about 1 m (3.3 ft), and 

seagrass meadows cover about 65 percent on its bottom (Onuf, 

1996b, 2007; Tunnell and Judd, 2002; Mendoza and others, 

2011; fig. 5). Just east of the LRGV, the Lower Laguna Madre 

contains abundant seagrass meadows of shoal grass, Thalassia 

testudinum (turtle grass), Syringodium filiforme (manatee 

grass), Ruppia maritima (widgeongrass), and Halophila 

engelmannii (star grass) that provide vital resources for 

wintering waterfowl, notably shoal grass for redheads (Smith, 

2002a), and nurseries for fish, shrimp, and other marine 

life (e.g., Sheridan and Minello, 2003). Herbivory by such 

organisms is now seen as a fundamental process in seagrass 

meadows (Tolan and others, 1997; Valentine and Heck, 1999; 

Hemminga and Duarte, 2000; Withers, 2002a; Sheridan and 

Minello, 2003; Valentine and Duffy, 2006). Laguna Madre and 

associated terrestrial areas are part of UNESCO’s Man and the 

Biosphere Programme, designated as a Ramsar Convention 

Wetland of International Importance, and recognized as a 

site of international importance by the Western Hemisphere 

Shorebird Reserve Network (Tunnell and Judd, 2002; 

Mendoza and others, 2011). 

Seagrass meadows are ecologically dynamic (Duarte, 

2002; Withers, 2002a). Wind tides, tropical and northern 

storms, and phytoplankton blooms, for example, greatly 

modify seagrass meadows and change biotic communities 

that depend on them (Tunnell, 2002a, 2002b; Withers, 2002d). 

Phytoplankton-caused brown tides have likely shaped seagrass 

meadows in Laguna Madre for thousands of years, although 

Figure 5. Shallow-water seagrass meadows, a dominant feature of the Lower Laguna Madre offshore of the Lower Rio Grande Valley. 

Seagrass meadows provide critical habit for many marine and wetland bird species and can be damaged by inadvertent boating 

activity, as shown in the bottom-right image. Top and bottom-right images from ©Gywn Carmean and bottom-left image from ©Larry 

Ditto Nature Photography, all used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
20 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

historical records of their occurrences are quite uncommon 

(Withers, 2002a). A recent 6–7-year brown tide, caused by 

the alga Aureoumbra lagunensisi (DeYoe and others, 1997) 

in the Upper Laguna Madre, began in the early 1990s and 

ended in 1997 (e.g., Rhudy and others, 1999; Onuf, 2000; 

Withers, 2002d); photosynthesis, elemental metabolism, and 

productivity of seagrass meadows and changing community 

composition relative to the persistent algal bloom were studied 

in the entire Laguna Madre (appendix A). Onuf (2007) noted 

that because precipitation is generally low and surrounding 

landscapes of the Laguna Madre are flat, there is little input 

of nutrients and sediments, resulting in typically clear water 

(fig. 5). The algal bloom, however, altered the upper lagoon’s 

water clarity and light penetration and thereby reduced 

productivity and overall coverage of seagrass meadows. The 

algal bloom was likely caused by hard freezes in 1989 and 

1990 (Lonard and Judd, 1991) during drought that caused 

extensive mortalities of fish and benthic organisms―the pulse 

of nutrients that fueled the algal bloom (DeYoe and Suttle, 

1994; Rhudy and others, 1999; Withers, 2002d). Toxic red 

tides caused by dinoflagellates occur in Laguna Madre but are 

rare compared with brown tides (Withers, 2002d).

Boating and dredging activities, mainly to establish 

waterways and keep them open to boats, can directly destroy 

seagrass meadows (fig. 5) and increase turbidity that decreases 

seagrass meadow productivity and long-term survival and 

that of dependent biota (Quammen and Onuf, 1993; Onuf, 

1994; Martin and others, 2008; Larkin and others, 2010). In 

the Upper Laguna Madre, 14.5–97.6 percent of extensive 

seagrass areas, with depths generally <1 m (3.3 ft), were 

lightly scarred (<5 percent of the study area covered by 

propeller scars), moderately scarred (5–20 percent), and 

severely scarred (greater than [>] 20 percent); on Estes Flats 

alone, 97.6 percent of the 1,258.2 ha (3,109 acres) sampled 

was damaged―948.7 ha (2,344 acre) of them severely―by 

propellers on recreational boats (Dunton and Schonberg, 

2002). Recolonization of damaged areas is species-specific; 

long-lived, slow-growing seagrasses such as turtle grass 

recolonize slowly after damage, whereas fast-growing 

species such as shoal grass invade more rapidly if conditions 

are favorable (Dunton and Schonberg, 2002). Comparable 

studies have not been conducted in the Lower Laguna Madre 

(Tunnell, Withers, and Smith, 2002). 

Restoration of seagrass meadows has met with limited 

success; at disposal sites of dredging materials just north of 

Brazos Santiago Pass in the Lower Laguna Madre, winddriven resuspension of dredging materials decreased light 

penetration and increased sediment ammonium, both of 

which decreased revegetation success of shoal grass (Kaldy 

and others, 2004). Onuf (1994) noted the greatest losses of 

seagrasses due to decreased light penetration nearest disposal 

sites of dredged materials near Port Mansfield, with negative 

effects detectable up to 1.2 km (0.7 mi) away. 

Onuf (2007) provided a comprehensive summary of 

seagrass changes from the mid-1960s to 1998 in the entire 

Laguna Madre, which contains 75–80 percent of the seagrass 

meadows on the entire coast of Texas (Gutierrez and others, 

2010). A critical phase in the dynamics of seagrass meadows 

in the Lower Laguna Madre of the LRGV occurred from 1965 

to 1974 when 118 square kilometers (km2

) (45.5 square miles 

[mi2]) of meadows became bare (Onuf, 1994), as a result of 

ongoing dredging and maintenance of the Gulf Intracoastal 

Waterway, initially dug in the 1940s; such activities increase 

turbidity and decrease productivity of seagrass. Maintenance 

of the Gulf Intracoastal Waterway and developments such Port 

Mansfield Channel and Brazos Santiago Pass also allowed 

more regular exchange of water between the Upper and Lower 

Laguna Madre and the Gulf of Mexico, respectively, reducing 

the historically hypersaline characteristics of the lagoon. 

Coverage of shoal grass—the most salt tolerant seagrass—in 

the Lower Laguna Madre declined from 89 percent in the 

mid-1960s to only 46 percent in 1998, likely because reduced 

salinity allowed expansion of less salt-tolerant but more 

competitive manatee grass and turtle grass (e.g., turtle grass 

increased from a mere 436 ha [1,077 acres] in the mid-1960s 

to 11,132 ha [27,507 acres] in 1998); similar changes occurred 

in the Upper Laguna Madre (Onuf, 2007; Gutierrez and others, 

2010). Changing dynamics of seagrass meadows, particularly 

abundance of shoal grass, need to be carefully monitored 

because they are inextricably linked to viability of the 

continental population of redheads, other waterfowl, resident 

fishes, and other fauna living in them (e.g., Mitchell, 1991; 

Mitchell and others, 1994; Custer and others, 1997; Bergan, 

2002; Sheridan and Minello, 2003). 

Restoration of Threatened Plant Communities

Most, if not all, of the native forests and subtropical 

brushlands in the expansive 1.1 million ha (2.7 million acres) 

of the LRGV are threatened by ongoing human activities, 

particularly population growth, agriculture, urbanization 

(e.g., Ewing and Best, 2004; Twedt and Best, 2004; Tremblay 

and others, 2005), and illegal foot traffic and subsequent 

border patrol activities, including road construction. Native 

riparian forests of Mexican ash, cedar elm, and sugarberry 

have been greatly diminished by clearing, periodic droughts 

leading to habitat xerification (Gehlbach, 1987; Brush, 2005), 

and especially the many hydrological alterations to the Rio 

Grande, minimizing seasonal flooding on which flood plain 

vegetation depends or, on the contrary, permitting long and 

harmful inundation. The largest remaining relatively intact 

forest of this kind occurs in Santa Ana NWR in the midLRGV (Vora, 1990a, 1990b), but it can be challenged by 

persistent inundation, such as that which occurred for 5–8 

months in 2010, killing many mature trees. Communities 

of honey mesquite, granjeno, Texas ebony, and brasil once 

covered much of the alluvial plain of the LRGV, but their 

coverage has been reduced to <5 percent of what it once was 

(Parvin, 1988a, 1988b; Raney and others, 2004; Tremblay and 

others, 2005). Upland thornscrub communities have been less 

affected than plant communities of the Rio Grande flood plain, 

but heavy grazing, invasive grasses, and the expanding human 
Unique Aspects of the Lower Rio Grande Valley 21

population and its associated activities continue to threaten 

them (e.g., Vora and Messerly, 1990).

Restoration of threatened plant communities has been 

and is currently an important focus for managers of State 

and Federal properties in the LRGV (e.g., Vora, 1992; Judd 

and others, 2002; Sternberg, 2003; Adhikari and White, 

2014). Twedt and Best (2004) provided a historical overview 

of restoration activities from which most of the following 

synthesis was derived, supplemented with records of the 

Lower Rio Grande Valley NWR (B. Barry, written commun.). 

The first documented habitat restoration in the LRGV was 

conducted at the Las Palomas (meaning “doves” in Spanish) 

Wildlife Management Area (WMA) by TPWD in the late 

1950s (Judd and others, 2002), with a focus on improving 

habitat and hunting opportunities for Zenaida asiatica (whitewinged doves). 

Initial restoration work of acquired croplands in the 

Lower Rio Grande Valley NWR began in the early 1980s and 

continued through the mid-1990s (Vora, 1992; Twedt and Best, 

2004). It largely focused on direct seeding of fast-growing, 

woody legumes (Texas ebony, huisache, and tepeguaje) in 

strips in abandoned cropland; associated experiments were 

conducted on techniques to enhance seed germination (Vora, 

1989a; Vora and Labus, 1988; Vora and others, 1988). The 

goal was to provide quick cover at “a low cost, suppress 

weeds, and attract seed-dispersing fauna” and thereafter let 

secondary succession naturally restore the former cropland to 

native habitats (Twedt and Best, 2004, p. 199). Seed plantings 

were mostly abandoned in 1995 because of unreliable 

germination, availability of seeds, and low success and lack of 

diversity from planting seeds directly. 

With foresight gained from restoration of native 

shrublands and forests (Vora, 1992; Twedt and Best, 2004), the 

Lower Rio Grande Valley NWR established physical facilities 

and species-specific protocols for greenhouse production of 

native plant species to be used in restoration operations. A 

single restoration site may be planted with 30–40 species of 

trees, shrubs, and cacti (Twedt and Best, 2004), resulting in 

more diverse communities in a shorter amount of time than 

unaided communities (Judd and others, 2002; Sternberg, 

2003). It typically takes 3–5 years to judge the success of a 

planting. From 1995 to 2011, more than 3 million seedlings 

were planted in the Lower Rio Grande Valley NWR (fig. 6A); 

plantings occur in September–November when soil moisture 

Cumulative number of plants, in millions

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Years

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Acres planted

0

2,000

4,000

6,000

8,000

10,000

12,000

A

B

Acres planted

200 400 600 800 1,000

Plants per acre

0

200

400

600

800

1,000

C

2007–10

1995–2006

EXPLANATION

Individual plants planted

Acres planted

Cumulative acres planted

Figure 6. Restoration activities conducted by the U.S. Fish and Wildlife Service in the Lower Rio Grande Valley National Wildlife 

Refuge from 1995 through 2012. A, cumulative number of plants. B, acres planted. C, plants per acre and acres planted.
22 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

is typically replenished and cool weather enhances seedling 

survival. An average of 186,000 seedlings was planted 

each year, with about 127 seedlings planted per hectare 

(314 seedlings planted per acre). The Lower Rio Grande 

Valley NWR itself produces about 85,000 seedlings per year 

from more than 60 native plant species in its own greenhouse 

facility near Santa Ana NWR, and private nurseries provide 

the rest. Production and planting of refuge stock peaked in 

2008 with 121,100 seedlings produced, or 57.1 percent of 

the 212,047 total seedlings planted. An average of 567 acres 

(229.5 ha) has been planted annually in the Lower Rio Grande 

Valley NWR, with just about 10,000 acres (4,047 ha) being 

restored since 1995 (fig. 6B). Through time, the number of 

acres planted annually decreased, but the number of seedlings 

planted per acre and their survival increased (fig. 6C). Since 

1997, the Lower Rio Grande Valley NWR has held the annual 

Rio Restoration in October, using up to 1,000 volunteers to 

plant tree and shrub seedlings (now protected in tubes―

Dick, 2015). To date, volunteers have planted about 200,000 

seedlings on about 250 ha (620 acres). Availability of native 

seedlings is currently the primary limitation to restoration 

efforts. Native plants of southern Texas are not commercially 

or locally available. The only native plant growers are 

contracted by the USFWS to grow native species for 

restoration activities in the LRGV. 

Many restorations in the LRGV have involved old 

croplands (fig. 7) with soils depleted in organic matter (Vora 

and Jacobs, 1990), and site preparation can involve a no-till 

approach or a more traditional approach of shredding crop 

residue (if present), chisel plowing to a depth of greater than 

or equal to (≥) 38 centimeters (cm) (15 inches [in.]), and 

disking to break up the soil and create planting rows or beds. 

Herbicide applications before and after planting might be 

needed to control invasive species, particularly Asian and 

African grasses (e.g., Megathyrus maximus [guineagrass], 

Sorghum halepense [Johnson grass]; Cynodon dactylon

[Bermudagrass], Bothriochloa ischaemum var. ischaemum 

[King Ranch bluestem]; and Dichanthium annulatum [Kleberg 

bluestem]; Twedt and Best, 2004; Falk and others, 2012). 

Trends in community development on restored sites are 

strongly influenced by soil moisture at the time of planting, 

planting pattern, characteristics of edge communities 

associated with the restoration site, and competitive 

interactions from early, naturally invading successional 

species; survival of transplants is associated with site 

conditions at the time of planting—a balancing act of planting 

at times of optimal soil moisture but when the soil is not so 

wet that equipment has difficulty entering a site (Ewing and 

Best, 2004). Restoration methods and results vary (fig. 7): 

compared to a desired canopy cover baseline of 87 percent, 

restorations have changed from a mere 0.2 percent canopy 

cover after initial planting to 2.1–8.8 percent after 3 years 

post-planting to 17.6–33.4 percent after 6 years post-planting 

(Ewing and Best, 2004). 

Recent assessments suggested that 15–25-year-old 

revegetated tracts in the Lower Rio Grande Valley NWR 

had similar woodland bird communities as mature woodland 

habitat, except mature tall riparian forest that is hard to 

maintain and restore unless close to the Rio Grande (Brush 

and Feria, 2015). Small mammal communities in replanted 

former agricultural fields were comparable to native thornscrub 

woodlands (Sternberg and Judd, 2006; see “Small Mammals” 

subsection of this report). Relative to restoring habitat for 

northern ocelots, Young and Tewes (1994) evaluated tree 

shelters, fertilizer, tree branch trimming, and elimination of 

herbaceous growth to increase and enhance growth of woody 

seedlings. Tree shelter tubes increased survival and growth of 

seedlings but not weeding, clipping, or fertilizing (Young and 

Tewes, 1994; USFWS, 2010a). Recent research suggests that 

shelter tubes, grass-specific herbicide, and herbivore exclusion 

facilitate faster, efficient, and effective thornscrub restoration 

(Dick, 2015; Alexander and others, 2016), and increasing 

seedling density may provide positive synergistic effects 

among adjacent or grouped seedlings (M. Sternberg, written 

commun.).

Twelve cooperators still farm 2,505 ha (6,189 acres) in the 

Lower Rio Grande Valley NWR until native plants are available 

to restore the natural vegetation. Eight of these 12 farmers 

grow “dryland” crops because most farmland purchased by 

the Lower Rio Grande Valley NWR has nonfunctional or 

dilapidated irrigation systems; five farmers have access to 

irrigation and pay higher rent per unit area to USFWS than the 

others. In exchange for paying rent to use tracts in the Lower 

Rio Grande Valley NWR, farmers can purchase and plant 

seedlings of native species used in the restoration program and 

also conduct in-kind services that provide equipment and labor 

to remove invasive grasses (see “Exotic and Invasive Species” 

section) and conduct follow-up care of tracts that have been 

restored. The pace of habitat restoration is often impeded by 

limited availability of native seedlings.

Threatened and Endangered Plants 

Overview

In the LRGV, at least 24 plant species, representing 

18 families, are considered officially endangered by Federal 

and State agencies or are rare enough to be of global 

conservation concern (Poole and others, 2007, 2010; table 

5). Six of these plant species, representing five families, are 

on both Federal and State lists of endangered and threatened 

species (Poole and others, 2010): Thymophylla tephroleuca

(ashy dogweed), Ambrosia cheiranthifolia (South Texas 

ambrosia), Astrophytum asterias (star cactus), Ayenia limitaris

(Tamaulipan kidneypetal or Texas ayenia), Manihot walkerae

(Walker’s manioc), and Physaria thamnophila (Zapata 

bladderpod—Wu and Smeins, 2000). Frankenia johnstonii

(Johnston’s frankenia) was listed as endangered in 1985 and 

was proposed for delisting in 2003 and 2011 because of the 

discovery of additional populations in Texas (Starr, Webb, and 

Zapata Counties) and Mexico, including on tracts of the Lower 
Unique Aspects of the Lower Rio Grande Valley 23

A B

C D

E

Figure 7. Restoration of old cropland to native forest and brushland, a high conservation priority in the Lower Rio Grande Valley 

National Wildlife Refuge. Restoration sequence of the Sam Fordyce tract: A, preplanting in October 2007; B, seedling planting by hand in 

October 2007; C, direct seeding of grasses and herbaceous plants with a tractor; D, restoration progress by July 2008; and E, restoration 

success by March 2015. Photographs courtesy of Kim Wahl, U.S. Fish and Wildlife Service.
24 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Table 5. Endangered and rare plant species found in the four-county area of the Lower Rio Grande Valley in southern Texas, from 

the Annotated County Lists of Rare Species by Texas Parks and Wildlife Department (2014), NatureServe (2013), U.S. Fish and Wildlife 

Service Federal listings, and Poole and others (2010). 

[Federally or State listed as E, endangered; T, threatened; C, candidate species; R, rare in Texas but with no regulatory status; DL, delisted, January 2016. 

NatureServe’s G1 or S1, critically imperiled globally or subnationally, in this case Texas; G2 or S2, imperiled and very vulnerable; G3 or S3, vulnerable; G4 

or S4, apparently secure; G5 or S5, secure; T, infraspecific classifications that follow the same status as G and S; GH, SH, or H, historical occurrence/possibly 

extinct or extirpated; SX, presumed extirpated/extinct; B, breeding] 

 Listing Rank

FAMILY/Species Federal State Global State County occurrence

ACANTHACEAE

Justicia runyonii (Runyon’s water-willow) R G2 S2 Cameron, Hidalgo, Willacy

AMARANTHACEAE

Atriplex klebergorum (Kleberg saltbush) R G2G3 S2S3 Starr

ASCLEPIADACEAE

Asclepias prostrata (prostrate milkweed) R G1G2 S1S2 Starr

Matelea radiata (Falfurrias milkvine) R GH SH Hidalgo (H)

ASPARAGACEAE

Echeandia chandleri (lila de los llanos) R G2G3 S2S3 Cameron

Echeandia texensis (Texas craglily) R G1 S1 Cameron

Manfreda longiflora (Runyon’s huaco) R G2 S2 Hidalgo, Starr

ASTERACEAE

Ambrosia cheiranthifolia (South Texas 

ambrosia)

 E E G2 S2 Cameron (H)

Grindelia oolepis (plains gumweed) R G2 S2 Cameron

Thymophylla tephroleuca (ashy dogweed) E E G2 S2 Starr (H)

BRASSICACEAE

Physaria thamnophila (Zapata bladderpod) E E G1 S1 Starr

Thelypodiopsis shinnersii (Shinner’s rocket) R G2 S2 Starr (H)

BROMELIACEAE

Tillandsia baileyi (Bailey’s ballmoss) R G2G3 S2 Cameron, Hidalgo, Starr

CACTACEAE

Astrophytum asterias (star cactus) E E G2 S1S2 Cameron, Hidalgo (H), Starr

Coryphantha macromeris ssp. runyonii 

(Runyon’s cory cactus)

T2T3 S2S3 Cameron, Hidalgo, Starr

COMMELINACEAE

Tradescantia buckleyi (Buckley’s spiderwort) R G2 S2 Cameron (H)

EUPHORBIACEAE

Manihot walkerae (Walker’s manioc) E E G2 S1 Hidalgo, Starr

FABACEAE

Dalea austrotexana (dune dalea) R G2 S2 Cameron

FRANKENIACEAE

Frankenia johnstonii (Johnston’s frankenia) DL E G3 S3 Starr

MALVACEAE

Ayenia limitaris (Tamaulipan kidneypetal) E E G2 S1 Cameron, Hidalgo, Willacy

Wissadula parviflora (small-leaved yellow 

velvet-leaf)

R G1 S1 Hidalgo
Unique Aspects of the Lower Rio Grande Valley 25

Table 5. Endangered and rare plant species found in the four-county area of the Lower Rio Grande Valley in southern Texas, from 

the Annotated County Lists of Rare Species by Texas Parks and Wildlife Department (2014), NatureServe (2013), U.S. Fish and Wildlife 

Service Federal listings, and Poole and others (2010).—Continued

[Federally or State listed as E, endangered; T, threatened; C, candidate species; R, rare in Texas but with no regulatory status; DL, delisted, January 2016. 

NatureServe’s G1 or S1, critically imperiled globally or subnationally, in this case Texas; G2 or S2, imperiled and very vulnerable; G3 or S3, vulnerable; G4 

or S4, apparently secure; G5 or S5, secure; T, infraspecific classifications that follow the same status as G and S; GH, SH, or H, historical occurrence/possibly 

extinct or extirpated; SX, presumed extirpated/extinct; B, breeding]

 Listing Rank

FAMILY/Species Federal State Global State County occurrence

NYCTAGINCEAE

Abronia ameliae (Amelia’s abronia) R G2 S2 Hidalgo (H)

POLYGONACEAE

Eriogonum greggii (Gregg’s wild-buckwheat) R G2 S1 Hidalgo (H), Starr

PONTEDERIACEAE

Heteranthera mexicana (Mexican mudplantain)

R G2G3 S1 Cameron (H), Hidalgo (H)

SAPINDACEAE

Cardiospermum dissectum (Chihuahua 

balloon-vine)

R G2G3 S2S3 Hidalgo, Starr

Rio Grande Valley NWR, bringing the number of known 

populations from six in 1984 to about 84 in 2015. Johnston’s 

frankenia was just delisted in 2016 (U.S. Department of 

Interior, Fish and Wildlife Service, 2016). Two areas in the 

LRGV are hotspots of occurrence for these endangered plants: 

the Rio Grande Delta in Hidalgo and Cameron Counties and 

upland areas in Starr County, where 4 of 6 endangered taxa are 

known to occur (table 5). 

Habitat loss and modifications are at least partly to 

blame for the decline in these plant species (Wu and Smeins, 

2000). Additional threats include overcollection, particularly 

of the star cactus, and lack of genetic diversity caused by 

very small population sizes (USFWS, 2003). Currently, the 

USFWS and other conservation groups protect and restore 

endangered plants in the LRGV; conservation efforts include 

purchasing lands on which these endangered plant species 

grow, cataloging Global Positioning System (GPS) locations 

of where individual plants are growing to enhance protection 

(USFWS, 1997), and cultivating some of these plants for 

future restoration efforts. GPS technology is being used to 

identify and map potential areas of occurrence, based on 

ecological requirements of these species and areas for possible 

restoration efforts in the LRGV (Wu and Smeins, 2000). 

Ashy Dogweed

Ashy dogweed is in the sunflower family (Asteraceae) 

and was listed as federally endangered in 1984 (Poole, 1987). 

This herbaceous perennial has bright yellow flowers, and its 

stems are covered with ashy-white wooly hairs. It grows in 

dense clusters after rains and prefers disturbed sandy to sandyloam soils on gentle topography. It was first discovered in 

Starr County in 1932 but no longer occurs there (Poole, 1987). 

Currently, the only known location of the ashy dogweed is just 

west, outside the LRGV proper in Zapata County. It now grows 

in a mixed cenizo-blackbrush community―an area that may 

have been originally native grassland (Poole, 1987). Grazing, 

habitat clearing, and competition with exotic species such as 

Cenchrus ciliaris (buffelgrass) likely led to its endangered 

status. The present population in Zapata County also may be 

threatened by use of roadside herbicides because it mainly 

occurs in right-of-ways (Poole, 1987). It has been propagated 

successfully by the San Antonio Botanical Center, which may 

prove useful to its recovery. 

South Texas Ambrosia

South Texas ambrosia was federally listed as endangered 

in 1993 (USFWS, 2010c). It is a clonal, perennial member of 

the sunflower family, and each individual may have hundreds of 

stems (fig. 8B). It was historically native to Cameron County in 

the LRGV, but it is no longer found there. Its most recent 5-year 

review showed that 8 of 14 known populations had been lost; 

the 6 remaining populations are isolated in Nueces and Kleberg 

Counties, Tex., just north of the LRGV (USFWS, 2010c). 

It is also found in northern Tamaulipas, Mexico (Poole and 

others, 2010). It grows in grasslands and mesquite-dominated 

shrublands and has declined because of intense cultivation 

of the coastal prairie (Poole and others, 2010). Most remnant 

populations occur in areas that have never been plowed (e.g., 

railroad and highway rights-of-ways and cemeteries). Even in 

these areas, South Texas ambrosia is threatened by encroachment 

of invasive grasses (Poole and others, 2010), more than any 

other known threat (USFWS, 2010c). South Texas ambrosia is 

easy to propagate, enhancing recovery options.
26 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

A B

C

D E

Figure 8. Endangered plants of the Lower Rio Grande Valley. A, Ayenia limitaris (Tamaulipan kidneypetal). B, Ambrosia cheiranthifolia

(South Texas ambrosia). C, Astrophytum asterias (star cactus). D, Physaria thamnophila (Zapata bladderpod). E, Manihot walkerae

(Walker’s manioc). Photographs courtesy of Chris Best, U.S. Fish and Wildlife Service.
Unique Aspects of the Lower Rio Grande Valley 27

Star Cactus

The federally endangered star cactus (Cactaceae) was 

historically native to Starr, Hidalgo, and possibly Cameron 

Counties in the LRGV and adjacent Zapata County and in the 

Mexican States of Nuevo León, Tamaulipas, and Coahuila 

(Poole and others, 2010; fig. 8C). It is now found only in 

Starr County and a few areas in Nuevo León and Tamaulipas, 

and it does not occur on any tracts of the Lower Rio Grande 

Valley NWR (USFWS, 2003). Star cacti grow on gentle slopes 

and flats in sparsely vegetated areas in mesquite grasslands 

and shrublands (Poole and others, 2010). It is completely 

dependent on pollination by insects to reproduce (Strong, 

2005; Strong and Williamson, 2007; Blair and Williamson, 

2008), and 4 of 5 populations examined had high genetic 

diversity (Terry, 2005; Terry and others, 2006, 2012). Star 

cacti in Mexico are particularly susceptible to mortality from a 

plant pathogen (Phytophtora infestans), a cerambycid beetle, 

and Ictidomys mexicanus (Mexican ground squirrel), which 

can reduce a population by 50 percent (Martínez-Ávalos 

and others, 2007). In the United States, principal causes for 

decline are habitat clearing and overgrazing; in Mexico, very 

little suitable habitat remains, lost to orange groves and corn 

fields (USFWS, 2003). Because of its unique appearance, 

the star cactus is very popular among cacti enthusiasts, and it 

continues to be illegally harvested. 

Tamaulipan Kidneypetal

The spineless subshrub Tamaulipan kidneypetal (fig. 

8A), newly assigned to the family Malvaceae, was listed 

as federally endangered in 1994 (Poole and others, 2010; 

USFWS, 2010d) and has a recently drafted recovery plan 

(USFWS, 2014a). There are five populations in Hidalgo, 

Cameron, and Willacy Counties of the LRGV: four of 

them with 100–200 individuals (one population in the 

Lower Rio Grande Valley NWR) and the fifth with about 

1,000 individuals (USFWS, 2010d). Tamaulipan kidneypetal 

occurs only in the Mexican state of Tamaulipas at 14 sites with 

about 4,000 individuals (USFWS, 2014a). Partially shaded 

edges of well-drained subtropical thorn woodland or savanna, 

rather than dense woodland, in the Rio Grande Delta are its 

optimal habitats (Poole and others, 2010; USFWS, 2014a). 

The Lower Rio Grande Valley NWR has actively participated 

in its recovery with successful germination, propagation, 

and reintroduction activities. Tamaulipan kidneypetals were 

successfully reintroduced at 3 of 4 locations in the LRGV, 

with one population growing from 84 seedlings in 1999 to 

295 individuals in 2008 (USFWS, 2014a). Main threats to 

Tamaulipan kidneypetals are brush clearing for agriculture 

and urbanization and competition with exotic grasses, 

mainly guineagrass.

Walker’s Manioc

Walker’s manioc is in the family Euphorbiaceae (Poole 

and others, 2010) and was listed as federally endangered in 

1991 (USFWS, 2009). In the United States, it is limited to 

Hidalgo and Starr Counties in the LRGV and Duval County 

just to the north; it also occurs in Tamaulipas, Mexico (Poole 

and others, 2010). Walker’s manioc is a perennial vine-like 

subshrub (fig. 8E) that grows in semiarid, shaded shrublands on 

xeric slopes and uplands, often overexposed caliche outcrops 

(Clayton, 1993; Poole and others, 2010). It occurs at only 

9 sites in the United States, with 3 of the largest sites in the 

Lower Rio Grande Valley NWR, and 24 sites in Tamaulipas, 

Mexico. Population sizes range from only one individual 

to about 90 individuals (USFWS, 2009). Invasive grasses, 

chemical runoff, and herbicides from nearby cultivated fields 

threaten remnant populations (Clayton, 1993). Major causes 

for decline of Walker’s manioc are clearing for agriculture, 

oil and gas development, surface mining for caliche, and 

urbanization (Clayton, 1993; USFWS, 2009). The San Antonio 

Botanical Garden and the Lower Rio Grande Valley NWR 

have developed techniques to propagate Walker’s manioc from 

seeds and tubers, but reintroduction criteria relative to recovery 

objectives have not been formalized (USFWS, 2009). A 5-year 

status review was initiated in April 2015.

Zapata Bladderpod

The Zapata bladderpod is part of the mustard family 

Brassicaceae (Poole and others, 2010; fig. 8D) and was listed as 

endangered in 1999. In the United States, it occurs only in Starr 

County of the LRGV and adjacent Zapata County; in Mexico, it 

occurs in Tamaulipas (USFWS, 2004; Poole and others, 2010). 

Only eight populations were known in 2005 (Sternberg, 2005). 

This short-lived perennial grows in clumped distributions on 

upland terraces of gravelly to sandy-loam soil above the Rio 

Grande flood plain. It is typically occurs with canopy associates 

(22 canopy species at one site), preferring a moderate degree 

of overstory cover, particularly as seedlings (Sternberg, 

2005; Fowler and others, 2011). Population sizes vary 

considerably; one population in Starr County varied from about 

826 individuals to 8,351 individuals over an 8-year period 

(Sternberg, 2005). In contrast, 2 of the 4 populations studied 

by Fowler and others (2011) had 2,000–78,000 individuals, but 

none of them were reproductive in 2006. Road construction, 

oil and gas exploration, agriculture, and livestock grazing 

are principal causes of decline (USFWS, 2004). Recovery 

of the Zapata bladderpod may be best achieved by providing 

sufficient litter cover to enhance germination and seedling 

growth, reducing dense shrubby overstory to moderate levels 

with a minimum of soil disturbance, and eliminating invasive 

grasses (Fowler and others, 2011). 
28 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Faunal Characteristics

Conservation-related resources of the USFWS list 

429 species of birds, 44 species of mammals, 115 species of 

reptiles and amphibians, and 31 species of fishes occurring 

in the STRC at some time during the year (Perez, 2014). 

Invertebrates of the LRGV include about 300 species of 

butterflies (about 280 confirmed and about 20 hypothetical) 

and more than 100 species of dragonflies and damselflies, 

as well as many freshwater, brackish, and marine aquatic 

invertebrates such as mollusks and shrimp. Invasive exotic 

species also occur in the STRC (see “Exotic Species” 

section), with varying degrees of impact and managerial 

concern: vertebrates including Myocastor coypus (nutria), 

Sus scrofa (feral hog), and Boselaphus tragocamelus (nilgai) 

and invertebrates including Solenopsis spp. (fire ant), Apis 

mellifera (Africanized honey bee), and Litopenaeus vannamei

(Pacific shrimp).

Eighty-five species of invertebrates and vertebrates 

that occur in the STRC, or have historically, are federally 

or State listed as endangered, threatened, or rare: 3 mussels, 

13 insects, 6 amphibians, 7 fishes, 15 reptiles, 29 birds, and 

12 mammals (table 6). Several species are only historically 

known from the LRGV and are effectively now extirpated; for 

example, Quadrula mitchelli (false spike mussel), Cicindela 

nigrocoerulea subtropica (subtropical black sky tiger beetle), 

C. obsoleta neojuvenilis (neojuvenile tiger beetle), Eximacris 

superbum (superb grasshopper), Hybognathus amarus

(Rio Grande silvery minnow), Numenius borealis (Eskimo 

curlew, perhaps extinct throughout its entire distribution), and 

Panthera onca (jaguar). The endemic Opilidia chlorocephala 

smythi (Smyth’s tiger beetle) from Cameron County is likely 

extinct. Critical habitat has been designated for the federally 

threatened Charadrius melodus (piping plover) within the 

STRC in Cameron and Willacy Counties. Although the 

Gulf Coast jaguarundi is extremely rare, one was killed on a 

highway just east of Brownsville, Tex., in 1986, and several 

sightings have been made since then (B.R. Winton, oral 

commun.). In the late 1980s, the list of endangered species 

in the LRGV included the Pelecanus occidentalis (brown 

pelican), Haliaeetus leucocephalus (bald eagle), and Falco 

peregrinus (peregrine falcon); all three species recovered and 

were removed from the USFWS Endangered Species List. 

Invertebrates

Diversities of aquatic and terrestrial invertebrates in the 

LRGV and the associated Lower Laguna Madre are no doubt 

high but little studied. Economically important terrestrial 

species, such as those considered as agricultural pests, have 

been studied the most (see “Pesticides” section). In the only 

study on freshwater bivalves in the Rio Grande since the 

late 1980s, nine species of unionids (e.g., Anodonta spp. and 

Uniomerus spp.), two species of fingernail clams (Sphaerium 

spp.), and the introduced Corbicula fluminea (Asian clam) 

were most closely associated with remnant free-flowing 

parts of the Rio Grande and associated resacas (Neck and 

Metcalf, 1988). Research on other aquatic invertebrates in 

the Laguna Madre has been limited to several species of 

mollusks (Mercenaria spp. [hard clams]―Dillon and Manzi, 

1989; invasive Perna perna [brown clam]―Hicks and 

Tunnell, 1993; and Crassostrea virginica [American oyster]―

Fredensborg and others, 2013) and crabs (Callinectes sapidus

[blue crab]―Kordos and Burton, 1993—and five species 

of Uca spp. [fiddler crabs]―Thurman 1998a, 1998b), with 

new records of occurrence for natant decapods (Pontonia 

domestica and P. floridanus—Strenth and Chace, 1995) and 

Aplysia cervina (sea hare—Strenth and Littleton, 1994). 

The most economically important commercial and 

artisanal fisheries in Laguna Madre of Texas (Withers and 

Dilworth, 2002) and Mexico (Pérez-Castañeda and others, 

2012) focus on penaeid shrimp. Farfantepenaeus duorarum

(northern pink shrimp) and F. aztecus (northern brown shrimp) 

are most abundant in the Lower Laguna Madre east of the 

LRGV, and Litopenaeus setiferus (white shrimp) are found 

mostly in areas from Port Mansfield to South Bay off Willacy 

and Cameron Counties in the LRGV. Post-larval and juvenile 

penaeid shrimp are a foundational part of the food web and 

most common in lagoons, with adults found mainly offshore; 

pink shrimp in particular depend on productive, dense seagrass 

meadows in waters east of the LRGV (Withers and Dilworth, 

2002). Commercial shrimp farming of primarily exotic Pacific 

shrimp (Balboa and others, 1991; Wakida-Kusunoki and 

others, 2011) has expanded in the LRGV since the late 1980s

(e.g., Baker, 1997; Ritvo and others, 1998), bringing the risk 

of environmental contamination (e.g., hypernutrification 

of estuarine systems where they are located) (Hopkins and 

others, 1995; Samocha and others, 2004). 

Some tropical and subtropical insects reach their 

northern distribution in the LRGV, such as the Brachygastra 

mellifica (Mexican honey wasp), which lives in colonies of 

tens of thousands, often constructed in riparian areas but 

also urban areas (Sugden and McAllen, 1994). Six of the 

13 insects of highest conservation priority to Federal and State 

agencies, and now historically extirpated in the LRGV, are 

tiger beetles: 5 species in the genus Cicindela, with 1 likely 

extinct throughout its distribution, and 1 species of Tetracha

(table 6). No recently published information was found on 

the ecologies of these species in the LRGV. Tiger beetles in 

the family Cicindelidae are globally distributed, and of the 

120 species in the United States, 50 are endemic. Cicindelidae 

is a globally distributed and diverse family of insects and can 

have important value in conservation planning, even at small 

spatial scales (Cassola and Pearson, 2000). For example, many 

species of tiger beetles have strict ecological requirements, 

often with high habitat specificity during some part of their life 

cycle and therefore are good indicators of ecological threats 

(Cassola and Pearson, 2000). Reassessment of their status in 

the LRGV could provide broad landscape-level planning.
Unique Aspects of the Lower Rio Grande Valley 29

Table 6. Endangered and rare invertebrate and vertebrate species found in the four-county area of the Lower Rio Grande Valley in 

southern Texas, from the Annotated County Lists of Rare Species by the Texas Parks and Wildlife Department (2014), NatureServe 

(2013), and U.S. Fish and Wildlife Service Federal listings.

[Federally or State listed as E, endangered; T, threatened; C, candidate species; R, rare in Texas but with no regulatory status. NatureServe’s G1 or S1, critically 

imperiled globally or subnationally, in this case Texas; G2 or S2, imperiled and very vulnerable; G3 or S3, vulnerable; G4 or S4, apparently secure; G5 or S5, 

secure; T, infraspecific classifications that follow the same status as G and S; GH, SH, or H, historical occurrence/possibly extinct or extirpated; SX, presumed 

extirpated/extinct; B, breeding]

Species

Listing Rank

Federal State Global State County occurrence

Invertebrates–mussels

Fusconaia mitchelli (false spike mussel) T GH SH Cameron (H), Hidalgo (H), Starr (H)

Popenaias popeii (Texas hornshell) C T G1 S1 Hidalgo (H), Starr (H)

Potamilus metnecktayi (Salina mucket) T G1 S1 Cameron, Hidalgo, Starr

Invertebrates–insects

Agapema galbina (Tamaulipan agapema) R G1 SH Cameron, Hidalgo

Calephelis rawsoni (Rawson’s metalmark) R G4 SNR Willacy

Campsurus decoloratus (mayfly) R G3 SNR Hidalgo

Cicindela cazieri (Cazier’s tiger beetle) R G2 S2 Starr

Cicindela nigrocoerulea subtropica

(subtropical black sky tiger beetle)

R G5T2 SH Cameron (H)

Cicindela obsoleta neojuvenilis (neojuvenile tiger 

beetle)

R G5T1 SH Hidalgo (H), Starr (H)

Ellipsoptera nevadica olmosa (Los Olmos tiger 

beetle)

R G5T2 S1S2 Hidalgo, Willacy

Eximacris superbum (superb grasshopper) R G5 SH Willacy (H)

Opilidia chlorocephala smythi (Smyth’s tiger 

beetle)

R GUTH SX Cameron (H)

Rhionaeschna dugesi (arroyo darner) R G4 SNR Hidalgo

Sphingicampa blanchardi (royal moth) R G1 SNR Cameron, Hidalgo

Stallingsia maculosus (Manfreda giant-skipper) R G1 S1 Cameron, Hidalgo

Tetracha affinis angustata (upland big-headed tiger 

beetle)

R G5T4 SNR Hidalgo, Starr, Willacy

Amphibians

Hypopachus variolosus (sheep frog) T G5 S2 Cameron, Hidalgo, Starr, Willacy

Leptodactylus fragilis (Mexican white-lipped frog) T G5 S1 Cameron, Hidalgo, Starr

Notophthalmus meridionalis (black-spotted newt) T G1 S1 Cameron, Hidalgo, Starr, Willacy

Rhinophrynus dorsalis (Mexican burrowing toad) T G5 S2 Starr

Siren sp. (South Texas siren, large Rio Grande form) T GNRQ SNR Cameron, Hidalgo, Starr, Willacy

Smilisca baudinii (Mexican treefrog) T G5 S3 Cameron, Hidalgo, Starr, Willacy

Fishes

Anguilla rostrata (American eel) R G4 S5 Cameron, Hidalgo, Willacy

Awaous banana (river goby) T G5 S1 Cameron

Ctenogobius claytonii (Mexican goby) T GNR S1 Cameron, Hidalgo

Hybognathus amarus (Rio Grande silvery minnow) E E G1 SX Cameron (H), Hidalgo (H), Starr (H)

Microphis brachyurus (opossum pipefish) T G4G5 S1 Cameron, Willacy

Notropis jemezanus (Rio Grande shiner) R G3 S3 Cameron, Hidalgo, Starr

Pristis pectinata (smalltooth sawfish) E E G1G3 SNR Cameron, Willacy
30 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Table 6. Endangered and rare invertebrate and vertebrate species found in the four-county area of the Lower Rio Grande Valley in 

southern Texas, from the Annotated County Lists of Rare Species by the Texas Parks and Wildlife Department (2014), NatureServe 

(2013), and U.S. Fish and Wildlife Service Federal listings.—Continued

[Federally or State listed as E, endangered; T, threatened; C, candidate species; R, rare in Texas but with no regulatory status. NatureServe’s G1 or S1, critically 

imperiled globally or subnationally, in this case Texas; G2 or S2, imperiled and very vulnerable; G3 or S3, vulnerable; G4 or S4, apparently secure; G5 or S5, 

secure; T, infraspecific classifications that follow the same status as G and S; GH, SH, or H, historical occurrence/possibly extinct or extirpated; SX, presumed 

extirpated/extinct; B, breeding]

Species

Listing Rank

Federal State Global State County occurrence

Reptiles

Caretta caretta (loggerhead) T T G3 S2 Cameron, Willacy

Cemophora coccinea lineri (Texas scarlet snake) T G5T2 S2 Cameron, Willacy

Chelonia mydas (green turtle) T T G3 S1 Cameron, Willacy

Coniophanes imperialis (black-striped snake) T G4G5 S2 Cameron, Hidalgo, Willacy

Crotaphytus reticulatus (reticulate collared lizard) T G3 S2 Hidalgo, Starr

Dermochelys coriacea (leatherback) E E G2 S1 Cameron, Willacy

Drymarchon melanurus erebennus (Texas indigo 

snake)

T G5T4 S3 Hidalgo, Starr, Willacy

Drymobius margaritiferus (speckled racer) T G5 S1 Cameron, Hidalgo, Willacy

Eretmochelys imbricata (Atlantic Hawksbill) E E G3T3 S1 Cameron, Willacy 

Gopherus berlandieri (Texas tortoise) T G4 S3 Cameron, Hidalgo, Starr, Willacy

Holbrookia lacerata (spot-tailed earless lizard) R G3 S3? Hidalgo, Starr, Willacy

Holbrookia propinqua (keeled earless lizard) R G4 S3? Cameron, Willacy

Lepidochelys kempii (Kemp’s ridley) E E G1 S1 Cameron, Willacy

Leptodeira septentrionalis septentrionalis

(northern cat-eyed snake)

T G5 S2 Cameron, Hidalgo, Starr, Willacy 

Phrynosoma cornutum (Texas horned lizard) T G4G5 S4 Cameron, Hidalgo, Starr, Willacy

Birds

Anthus spragueii (Sprague’s pipit) C R G4 S2 Cameron, Hidalgo, Starr, Willacy

Athene cunicularia hypugaea (western burrowing 

owl)

R G4T4 S2B Cameron, Willacy

Buteo albicaudatus (white-tailed hawk) T G4G5 S4 Cameron, Hidalgo, Starr, Willacy

Buteo albonotatus (zone-tailed hawk) T G4 S3 Cameron, Hidalgo, Starr, Willacy

Buteo nitidus plagiatus (gray hawk) T GNR S2 Cameron, Hidalgo, Starr

Buteogallus anthracinus (common black-hawk) T G4G5 S2 Cameron, Hidalgo, Starr, Willacy

Calidris canutus rufa (rufa red knot) T G4T2 Cameron, Hidalgo

Camptostoma imberbe (northern beardlesstyrannulet)

T G5 S3 Cameron, Hidalgo, Starr, Willacy

Charadrius nivosus (snowy plover) R G3 S3B Cameron, Hidalgo, Willacy

Charadrius melodus (piping plover) T T G3 S2 Cameron, Willacy

Charadrius montanus (mountain plover) R G3 S2 Hidalgo

Chondrohierax uncinatus (hook-billed kite) R G4 S2 Hidalgo, Starr

Egretta rufescens (reddish egret) T G4 S3 Cameron, Hidalgo, Willacy

Falco femoralis septentrionalis (northern aplomado 

falcon)

E E G4T2 S1 Cameron, Hidalgo, Willacy

Falco peregrinus (peregrine falcon) T G4 S3 Cameron, Hidalgo, Starr, Willacy
Unique Aspects of the Lower Rio Grande Valley 31

Table 6. Endangered and rare invertebrate and vertebrate species found in the four-county area of the Lower Rio Grande Valley in 

southern Texas, from the Annotated County Lists of Rare Species by the Texas Parks and Wildlife Department (2014), NatureServe 

(2013), and U.S. Fish and Wildlife Service Federal listings.—Continued

[Federally or State listed as E, endangered; T, threatened; C, candidate species; R, rare in Texas but with no regulatory status. NatureServe’s G1 or S1, critically 

imperiled globally or subnationally, in this case Texas; G2 or S2, imperiled and very vulnerable; G3 or S3, vulnerable; G4 or S4, apparently secure; G5 or S5, 

secure; T, infraspecific classifications that follow the same status as G and S; GH, SH, or H, historical occurrence/possibly extinct or extirpated; SX, presumed 

extirpated/extinct; B, breeding]

Species

Listing Rank

Federal State Global State County occurrence

Birds—Continued

Geothlypis trichas insperata

(Brownsville common yellowthroat)

R G5T2 S1B Cameron, Hidalgo, Starr

Glaucidium brasilianum cactorum

(cactus ferruginous pygmy-owl)

T G5T3 S3 Cameron, Hidalgo, Starr, Willacy

Icterus cucullatus cucullatus (Mexican hooded 

oriole)

R G5 S4B Starr

Icterus cucullatus sennetti (Sennet’s hooded oriole) R G5 S3B Cameron, Willacy

Icterus graduacauda audubonii (Audubon’s oriole) R G5T4 S3B Cameron, Hidalgo, Starr, Willacy

Mycteria americana (wood stork) T G4 SH Cameron (H), Hidalgo, Starr, Willacy

Numenius borealis (Eskimo curlew) E E GH SH Cameron (H), Willacy (H) 

Onychoprion fuscatus (sooty tern) T G5 S2 Cameron, Willacy

Pachyramphus aglaiae (rose-throated becard) T G4 Cameron, Hidalgo, Starr, Willacy

Parula pitiayumi (tropical parula) T G5 S3 Cameron, Hidalgo, Starr, Willacy

Peucaea botterii texana (Texas Botteri’s sparrow) T G4T4 S3 Cameron, Hidalgo, Willacy

Plegadis chihi (white-faced ibis) T G5 S4 Cameron, Hidalgo, Willacy

Psilorhinus morio (brown jay) R G5 S2B Starr

Sternula antillarum athalassos (interior least tern) E E G4T2 S2S3 Hidalgo, Starr

Mammals

Choeronycteris mexicana (Mexican long-tongued 

bat)

R G4 S1 Cameron, Hidalgo, Starr, Willacy

Lasiurus ega (southern yellow bat) T G5 S1 Hidalgo, Willacy

Leopardus pardalis albescens (northern ocelot) E E G4 S1 Cameron, Hidalgo, Starr, Willacy

Mormoops megalophylla (Peter’s ghost-faced bat) R G4 S2 Cameron, Hidalgo, Starr, Willacy 

Myotis velifer (cave myotis) R G5 S4 Hidalgo, Starr

Myotis yumanensis (Yuma myotis) R G5 S4 Starr

Nasua narica (white-nosed coati) T G5 S2? Cameron, Hidalgo, Starr, Willacy

Oryzomys couesi (Coues’ rice rat) T G5T2/4 S2 Cameron, Hidalgo, Starr, Willacy

Panthera onca (jaguar) E E G3 SH Cameron (H), Willacy (H)

Puma yagouaroundi cacomitli (Gulf Coast 

jaguarundi)

E E G4T3 S1 Cameron, Hidalgo, Starr, Willacy

Spilogale putorius interrupta (Plains spotted skunk) R G4T4 S3 Cameron, Hidalgo, Starr, Willacy

Trichechus manatus (West Indian manatee) E E G2 Cameron, Willacy
32 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

 The LRGV is considered one of the most species-rich 

butterfly areas in the United States (Wauer, 2004; table 7). 

Diversity of butterflies is typically greatest in neotropical areas 

(Robbins and Opler, 1997), and many neotropical butterflies 

find their way northward into the LRGV (fig. 9). The larval 

caterpillar stages of many lepidopterans are agricultural pests 

and targets of pesticides, which could be harmful to reptiles 

and birds that rely on them for food―a potentially pernicious 

impact little studied in the LRGV. The South Texas Chapter 

of the North American Butterfly Association (2015) maintains 

a checklist of butterfly occurrences in the LRGV. More than 

280 species of butterflies from at least 6 families and 20 

subfamilies have been confirmed in the LRGV, and many 

other species are considered hypothetical and likely to be 

observed in the future (table 7). Notably, >50 percent of the 

recorded butterfly species are considered LRGV specialists, or 

species that rarely occur anywhere else in the United States. 

Table 7. Numbers of species of butterflies in 6 families and 20 subfamilies found in Starr, Hidalgo, and Cameron Counties of south 

Texas from the North American Butterfly Association Checklist of Rio Grande Valley Butterflies (South Texas Chapter of the North 

American Butterfly Association, 2015).

Family (common names) Subfamily (common names)

Number 

of species 

confirmed

Number of species

not seen elsewhere in 

the United States

Number of species

not found north of the

Lower Rio Grande Valley

Papilionidae (Swallowtails) Papilioninae (Swallowtails) 14 10 5

Pieridae (Whites and 

Sulphurs) Pierinae (Whites) 9 6 3

Coliadinae (Sulfurs) 20 7 1

Dismorphiinae (Mimic-Whites) 1 1 1

Lycaenidae (Gossamar-wing) Theclinae (Hairstreaks) 32 24 15

Polyommatinae (Blues) 7 0 0

Riodinidae (Metalmarks) Riodinae (Metalmarks) 10 7 6

Nymphalidae (Brushfooted) Libytheinae (Snouts) 1 0 0

Heliconiinae (Heliconians and 

Fritillaries) 9 0 0

Nymphalinae (True Brushfoots) 30 11 2

Limenitidinae (Admirals) 19 18 7

Charaxinae (Leafwings) 6 3 2

Apaturinae (Emperors) 5 2 1

Satyrinae (Satyrs) 2 0 0

Danainae (Clearwings) 4 4 4

Danainae (Monarchs) 4 0 0

Hesperiidae (Skippers) Heteropterinae (Skipperlings) 1 1 1

Pyrginae (Spread-wing Skippers) 72 50 31

Hesperiinae (Grass-Skippers) 36 19 12

Hesperiinae (Giant Skippers) 2 0 0

Totals 283 163 91
Unique Aspects of the Lower Rio Grande Valley 33

A B

C D

E F

Figure 9. Diversity of butterflies in the Lower Rio Grande Valley, which is unparalleled with nearly 300 confirmed species, of which 

163 are generally not found elsewhere in the United States. Striking representatives are A, Heliconius charithonia (zebra heliconian); 

B, Agraulis vanillae (Gulf fritillary); C, Chlosyne janais (crimson patch); D, Papilio polyxenes (black swallowtail butterfly); E, Lasaia sula

(blue metalmark); and F, Diaethria anna (Anna’s eighty-eight). ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
34 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Amphibians and Reptiles

Although six amphibians in the LRGV are listed as 

State threatened (table 6), only the relatively common Rana 

berlandieri (Rio Grande leopard frog―Parker and Goldstein, 

2004) and the large Siren sp. (South Texas siren―LaFortune, 

2015) have received limited research attention since the late 

1980s. Summer and autumn diets of Rio Grande leopard frogs 

from Santa Ana NWR, various tracts in the Lower Rio Grande 

Valley NWR, and a private ranch in Willacy County were 

dominated by grasshoppers, beetles, moths, and butterflies. 

Notably, frogs shifted their diets with the autumn emergence 

of Spodoptera frugiperda (fall armyworms), which composed 

78 percent of their diets at the ranch and 90.4 percent at Santa 

Ana NWR. Fall armyworms are agricultural pests in the 

LRGV (Raulston and others, 1990, 1992; Wolf and others, 

1995; see “Pesticides” section of this report), and pesticides 

used to control them could negatively affect the Rio Grande 

leopard frog. Recent work on sirens in southern Texas suggests 

that the large form in the LRGV may be genetically distinct 

enough from Siren lacertina (greater siren) and S. intermedia

(lesser siren) to warrant recognition as a separate species 

(LaFortune, 2015). Overall, lack of information on amphibians 

in the LRGV limits understanding of their conservation needs.

Ten species of reptiles (5 snakes, 2 lizards, 2 sea turtles, 

and 1 tortoise) in the LRGV are considered State threatened, 

and 2 lizard species are considered rare, with a majority 

occurring in Cameron and Willacy Counties (table 6). The 

State-threatened Gopherus berlandieri (Texas tortoise; fig. 4E) 

prefers semidesert shrubland, and its population ecology and 

reproductive biology have been studied in Cameron County 

(Bury and Smith, 1986; Rose and Judd, 1989, 1991, 2014; 

Judd and Rose, 2000), where it occurs on nearly all tracts in 

the Lower Rio Grande Valley NWR and Laguna Atascosa 

NWR (Bury and Smith, 1986). Current threats include habitat 

loss to agriculture and urbanization, resulting in a highly 

fragmented distribution; potential conflicts with grazing 

interests; capture as pets or for the pet trade; need for better 

enforcement of its protected status; and lack of a conservation 

plan outlining recovery objectives by local, State, and Federal 

agencies (e.g., Rose and Judd, 2014). Studies north of the 

LRGV demonstrated that Texas tortoises can use a variety 

of shrub-dominated habitats but avoid extremes in canopy 

cover, riparian areas, and old fields; furthermore, common 

range-management practices (e.g., mechanical brush clearing 

and root plowing) create unusable habitats for Texas tortoises 

(Kazmaier, Hellgren, and Ruthven, 2001; Kazmaier, Hellgren, 

and Synatzske, 2001; Kazimaier, Hellgren, Synatzske, and 

Ritledge, 2001).

There are five federally listed species of sea turtles in 5 

of the 6 living genera that occur in gulf waters, bays, lagoons, 

and channels east of the LRGV. Federally endangered sea 

turtles of the LRGV are Kemp’s ridley, the most endangered 

of all sea turtles (National Marine Fisheries Service [NMFS], 

USFWS, and SEMARNAT, 2011); Eretmochelys imbricata

(hawksbill―NMFS and USFWS, 1993); and Dermochelys 

coriacea (leatherback―NMFS and USFWS, 1992). 

Threatened species are Caretta caretta (loggerhead―Plotkin 

and others, 1993; NMFS and USFWS, 2008) and Chelonia 

mydas (green sea turtle―NMFS and USFWS, 1991). All 

five species have been listed since the 1970s. Recovery plans 

for all sea turtles highlight nest-site destruction, overharvest, 

ingestion of oceanic human debris, and mortality in fishing 

nets (e.g., shrimp trawls) as principal causes of endangerment 

(Smith and Childs, 2002). Mandatory turtle excluder devices 

in shrimp trawls have greatly reduced this cause of mortality 

(Lewiston and others, 2003).

Sea turtles nest on beaches of Texas, principally on Padre 

Island National Seashore, South Padre Island, and Boca Chica 

tract in the Lower Rio Grande Valley NWR (fig. 10). Recent 

nesting records from the Kemp’s Ridley Sea Turtle Recovery 

Project on South Padre Island (Cameron and Willacy Counties 

of the LRGV) show that Kemp’s ridleys are the most common 

nesters, with 57 nests documented in 2012; 105 additional 

nests were found just north in Padre Island National Seashore. 

Nesting records of the other species of sea turtles in southern 

Texas are considerably lower: 13 or fewer loggerhead nests 

per year, 15 green sea turtle nests per year, 1 hawksbill 

nest, and 1 leatherback nest. The most abundant sea turtles 

in Gulf of Mexico waters off the southern Texas coast are 

green turtles, Kemp’s ridleys, and loggerheads, and the 

least abundant are hawksbills (perhaps mostly pelagic stage 

juveniles from Yucatan nesting areas—Amos, 1989; Bowen 

and others, 2007) and leatherbacks (only two strandings off 

Cameron County in 1969 and 1990―Judd and others, 1991); 

green turtles vastly outnumber other sea turtle species in 

southern Texas inshore waters (Caillouet and others, 1991; 

Shaver, 2000; D.J. Shaver, written commun.). 

Pioneering work on Kemp’s ridley by D.J. Shaver 

and her colleagues under cooperative agreements with the 

governments of Texas, Mexico, and the United States began 

in 1978. Prior to that, the only major nesting site of Kemp’s 

ridleys was a 26-km stretch of beach at Rancho Nuevo, 

Mexico; sporadic nesting near Corpus Christi, Tex., was 

known (Shaver, 1992; Smith and Childs, 2002). A head-start 

program began in 1978; eggs were collected at Rancho Nuevo, 

hatched at a facility at Padre Island National Seashore, reared 

to 6–11 months of age at the National Marine Fisheries Lab 

in Galveston, Tex., and released (e.g., Caillouet and others, 

1995; Shaver, 1991, 1992, 1996; Shaver and Caillouet, 1998). 

Numbers of nests of Kemp’s ridleys on the Texas coast have 

steadily increased from nine in 1996 to a peak of 209 in 2012, 

decreasing to 119 in 2014 (National Park Service, 2013).

The Lower Laguna Madre provides critical habitat for 

juvenile green turtles; only very occasionally are hawksbill, 

Kemp’s ridley, or loggerhead turtles found there (D.J. Shaver, 

written commun.). Although created by humans to provide 

access from the Gulf of Mexico into the Lower Laguna 

Madre, Port Mansfield Channel and Brazos Santiago Pass 

provide young turtles with foraging and resting opportunities 

and access to the Lower Laguna Madre (Renaud and others, 

1995; Shaver, 1994; Shaver and others, 2013). Recent sighting 
Unique Aspects of the Lower Rio Grande Valley 35

Figure 10. An endangered Lepidochelys kempii (Kemp’s ridley) nesting on South Padre Island. Photograph courtesy of the National 

Park Service.

frequencies of green turtles near jetties at Brazos Santiago 

Pass in 2009 suggest that recruitment of juveniles has 

increased nine times since comparable sightings in 1992–93 

and that the lower Texas coast is an important “developmental 

foraging area” for juvenile green turtles (Metz and Landry, 

2013, p. 293). 

Fishes 

Seven species of fishes (4 freshwater, 2 marine, and 1 

anadromous), part of or historically part of the LRGV fauna, 

are considered endangered, threatened, or rare by Federal and 

State agencies (table 6). The Federal- and State-endangered 

freshwater Rio Grande silvery minnow (Bestgen and Platania, 

1991; Hubbs and others, 2008; USFWS, 2010b) and the 

marine Pristis pectinata (smalltooth sawfish―NMFS, 2009) 

are effectively extirpated in areas associated with the LRGV. 

The Rio Grande silvery minnow was described taxonomically 

from a collection near Brownsville, Tex., but it has not been 

collected anywhere below Falcon Dam since 1961. The likely 

extinct Notropis orca (phantom shiner) once occurred at Boca 

Chica Beach in the mouth of the Rio Grande near Boca Chica 

Beach into New Mexico, but it was last collected from the 

Mexican side of the lower Rio Grande in 1975 (Miller and 

others, 1989). 

Historically, the freshwater fish assemblage in the lower 

Rio Grande has been remarkable, with 142 species in 49 

families (e.g., Contreras-Balderas and others, 2002; Hubbs 

and others, 2008). Striking changes in the fish community 

of the lower Rio Grande involve a shift of freshwater 

forms retreating upstream (as many as 32 species) and their 

replacement with marine and estuarine forms (as many 

as 54 species), resulting from increased salinity, turbidity, 

temperatures, and pollution and decreased freshwater flows 

into the lower reaches of the Rio Grande (Treviño Robinson, 

1959; Edwards and Contreras-Balderas, 1991; ContrerasBalderas and Lozano-Vilano, 1994; Anderson and others, 

1995; Contreras-Balderas and others, 2002; Mathis, 2005). 

The community now includes four exotics: Oreochromis 

aureus (Nile tilapia―Edwards and Wood, 1991; Martin and 

others, 2010), Morone chrysops (white bass), Carassius 
36 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

auratus (freshwater goldfish), and Cyprinus carpio (common 

carp—Contreras-Balderas and others, 2002). 

Typically, fish communities in enclosed hypersaline 

lagoons are characterized by low diversity and big swings 

in abundance, dependent on heavy rainfall and “hurricaneopened passes” that together lower a lagoon’s salinity and 

allow surges in productivity (Tunnell, 2002a, 2002b; Withers 

and Dilworth, 2002, p. 223). Densities and biomasses of 

fishes and decapods are high in healthy seagrass meadows 

and fundamental to high coastal productivity (Sheridan 

and Minello, 2003). Now (2016), the hypersaline character 

of Laguna Madre has been reduced—perhaps more so in 

the Upper Laguna Madre—by construction of permanent 

passages such as the Gulf Intracoastal Waterway and Port 

Mansfield Channel (e.g., Tunnell, 2002d), generally altering 

fish diversity. Based on limited research comparing fish 

communities in the Upper and Lower Laguna Madre, overall 

numbers of species are similar, but two families are apparently 

unique to the Lower Laguna Madre: Chaetodontidae 

(butterflyfishes) and Pomacentridae (damselfishes—Withers 

and Dilworth, 2002).

The estuarine-dependent marine fishes Sciaenops 

ocellatus (red drum), Cynoscion nebulosus (spotted sea trout), 

and Pogonias cromis (black drum) are the most sought after by 

anglers east of the LRGV (McKee, 2008). Past depleted stocks 

of commercially harvested red drum (Withers and Dilworth, 

2002; Carson and others, 2013) and recent genetic research 

(Rooker and others, 2010) highlight the need to identify and 

conserve unique subpopulations of many organisms. Based 

on analyses of isotopic carbon and oxygen in otoliths (fish 

ear bones), stocks of red drum along the Texas coast can be 

differentiated into regional nurseries of origin, with Laguna 

Madre being 1 of 4 uniquely identifiable natal areas; natal 

homing for subsequent reproduction also highlights the 

importance of “spatially explicit management” of red drum 

to perpetuate its ecological and genetic diversity (Rooker and 

others, 2010, p. 195). 

Birds

Thirty-four percent of all invertebrate and vertebrate 

species of conservation concern to Federal and State agencies 

in the LRGV are birds (table 6). The 29 species include 5 

Federal- and State-threatened or endangered species, 14 Statethreatened species, and 10 rare species listed as of concern 

by the State of Texas. The riparian zone of the Rio Grande 

once harbored a lush forest, apart from the more upland 

and extensive areas of thornscrub and grasslands. These 

characteristics made the LRGV and associated Laguna Madre 

important stopovers for avian migrants traveling between 

wintering areas in the Neotropics and breeding areas farther 

north in the United States and Canada (Brush, 2005; Mehlman 

and others, 2005; Ruth and others, 2008). The Central Flyway 

converges in the LRGV, and some bird species are funneled 

through it from the Mississippi Flyway along the Texas coast, 

resulting in the tremendous diversity of birds that breed and 

winter in and migrate through the LRGV (Brush, 2005).

Only 2–5 percent of the native habitat remained in most 

of the LRGV by the late 1980s. Nevertheless, the great and 

ever-changing variety of abiotic and biotic conditions and 

resulting habitats in the LRGV―even those caused by human 

activities―result in the richest bird community in the United 

States, a mix of more than 400 species (Brush, 2008a) and 

one of the top 10 destinations for bird watchers (Gehlbach, 

1987; Brush, 2005). Birds symbolic of the LRGV and near 

neighboring lands, with common names as unique or colorful 

as they are, include the northern aplomado falcon, Cyanocorax 

yncas (green jay―Gayou, 1986), Ortalis vetula (plain 

chachalaca―Peterson, 2000), and Pitangus sulphuratus (great 

kiskadee―Brush, 1993, 2005; Brush and Fitzpatrick, 2002). 

Many bird species in the LRGV are specialists in 

remnant habitat types that have experienced the greatest loss. 

Especially affected are birds that specialize on mature riparian 

forests such as the Leptotila verreauxi angelica (white-tipped 

dove), which has declined significantly in the LRGV (Flood 

and others, 2002; Brush, 2005; Rupert and Brush, 2006; 

Breeden and others, 2009; Brush and Feria, 2015). Similar 

changes are occurring southward in Mexico, where tropical 

species are moving north and temperate species are moving 

south (Brush, 2009a). These changes in the avian community 

of the LRGV are likely a result of complex interactions of 

human-caused, localized habitat changes and very large-scale 

effects such as climate change (Brush, 2005, 2008a; Rupert 

and Brush, 2006; Rappole and others, 2007). The following 

sections provide syntheses of important avian resources in the 

LRGV, with the substantial, supportive LRGV-centric research 

conducted on them since the late 1980s (appendix B). Little or 

no supportive research has been conducted on some taxonomic 

groups; for example, richness of raptors in the LRGV is 

high (Brush, 2005) and includes species of concern in the 

United States such the Buteo nitidus plagiatus (gray hawk),

B. albicaudatus (white-tailed hawk), and Chondrohierax 

uncinatus (hook-billed kite) (table 6). Nevertheless, aside 

from the northern aplomado falcon and the peregrine falcon, 

no studies have been undertaken on other raptor species in 

the LRGV.

Northern Aplomado Falcon

One of three subspecies of the wide-ranging aplomado 

falcon, the federally endangered northern aplomado falcon 

(fig. 11), occurs in the LRGV. It is a rare, resident, mediumsized, and slender neotropical raptor, generally inhabiting 

open habitats (USFWS, 2014b). Its historical distribution in 

the United States encompassed borderlands from southern 

and west-central Texas to Arizona (Keddy-Hector 1990, 

2000; Perez and others, 1996; Truett, 2002; Brush, 2005). 

Early reports from the LRGV suggested that the northern 

aplomado falcon was relatively common in eastern Willacy 

and Cameron Counties (Brush, 2005), particularly in coastal 

grasslands between Brownsville and Port Isabel, Tex., with 
Unique Aspects of the Lower Rio Grande Valley 37

Figure 11. The endangered Falco femoralis septentrionalis (northern aplomado falcon). More than 800 northern aplomado falcons 

have been successfully reintroduced in the Lower Rio Grande Valley, with 16–18 nesting pairs now found each year. ©Larry Ditto Nature 

Photography, used with permission. ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

Because early studies of the northern aplomado falcon 

were limited, our understanding of its ecology in the LRGV 

is tied to efforts to reintroduce it and follow up on newly 

established individuals (Perez and others, 1996; Brown 

and others, 2003, 2006; Jenny and others, 2004; Brown 

and Callopy, 2008, 2012), supplemented with information 

from Mexico (Keddy-Hector, 2000). Habitat types used by 

northern aplomado falcons in the LRGV and Mexico include 

coastal prairies, particularly those with scattered yuccas 

and mesquites (Perez and others, 1996), oak woodlands, 

and riparian forests amidst desert grasslands; in other parts 

of their distribution farther south, they occur in coastal 

savannas, marshlands, and even cut-over pasturelands 

(Keddy-Hector, 2000). 

As of 2004, in partnership with The Peregrine Fund, 

more than 800 northern aplomado falcons were released in 

the LRGV, with 16–20 nesting pairs now found each year; the 

recovery plan’s goal of 60 breeding pairs in the United States 

includes 30–35 pairs in the LRGV (Keddy-Hector, 2000; 

USFWS, 2014a). Northern aplomado falcons are now found 

in and around Laguna Atascosa NWR and the easternmost 

tracts of the Lower Rio Grande Valley NWR. Some concern 

scattered woody vegetation such as mesquite and yuccas 

(Perez and others, 1996). This area, known as the Palo Alto 

Prairie, seems to have been the principal location for breeding 

northern aplomado falcons in the LRGV, but they also 

occurred northward along the Texas coast to the King Ranch 

in Kleberg County (Keddy-Hector, 2000). 

The northern aplomado falcon was listed as endangered 

in the United States in 1986 (Keddy-Hector, 1990, 2000; 

Perez and others, 1996). Years of habitat loss and degradation 

from overgrazing of coastal grasslands and savannas, 

resulting in woody brush encroachment (initially mostly 

by sheep―Thompson, 1997; Truett, 2002; Brush, 2005), 

and probably overcollection of skins and eggs from 1890 

through 1910 (Hector, 1987; USFWS, 2014b) effectively 

extirpated the northern aplomado falcon from the LRGV 

as early as the 1930s (Perez and others, 1996; Truett, 2002; 

Jenny and others, 2004; Brush, 2005). Although there were 

sporadic sightings of northern aplomado falcons, indicating 

at least ephemeral presence in the coming decades, they did 

not breed again in the United States until captive breeding 

and reintroduction efforts began in the 1980s (Perez and 

others,1996; Jenny and others, 2004). 
38 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

has been expressed about quality of available nesting habitat; 

some pairs might be using poor-quality nest sites, suggesting a 

shortage of suitable nest sites in the LRGV relative to current 

population size (Brown and Callopy, 2008, 2012). Addled eggs 

of northern aplomado falcons in the LRGV have been regularly 

assessed for contaminants, which have generally been found 

in background levels, but mercury (Hg) levels appear to be 

elevated (Mora and others, 1997, 2008, 2011).

Orioles

Five of the nine North American orioles occur in the 

LRGV (Brush, 2005), and two of them, Icterus graduacauda

(Audubon’s oriole; fig. 12B) and I. cucullatus cucullatus and I. 

c. sennetti (both known as the hooded oriole), are considered 

rare in Texas (table 6). Loss or alteration of habitats by 

humans, drought, and periodic freezes (Lonard and Judd, 1991; 

Brush and Cantu, 1998; Flood and others, 2002; Rupert and 

Brush, 2006) and parasitism by range-expanding Molothrus 

ater (brown-headed cowbirds) and M. aeneus (bronzed 

cowbirds—Monk, 2003; Kostecke and others, 2004; Monk and 

Brush, 2007; Janecka and Brush, 2014) have affected oriole 

abundances in the LRGV. The bronzed cowbird has expanded 

northward and occurs in relatively high densities (3.25 

cowbirds per hectare [1.32 cowbirds per acre]) in the LRGV, 

often most closely associated with agricultural areas (Carter, 

1986; Warren, 2002). Cowbirds are known to negatively affect 

nest success of I. gularis (Altamira orioles—Brush, 2005; fig. 

12A), Audubon’s orioles (Monk and Brush, 2007), and hooded 

orioles (Brush, 2000c) in the LRGV. They also parasitize 

nests of green jays (fig. 12E), Toxostoma longirostre (longbilled thrashers), Cardinalis cardinalis (northern cardinals), 

and Arremonops rufivirgatus (olive sparrows―Brush, 1998b; 

T. Brush, written commun.) and rarely nests of Coccyzus 

americanus (yellow-billed cuckoos—Clotfleter and Brush, 

1995), Tyrannus verticalis (western kingbirds), and Tyrannus 

couchii (Couch’s kingbirds—Brush, 1999b).

Audubon’s orioles have disappeared from large parts of 

the LRGV (Sennett, 1879; Brush, 2000d; Flood and others, 

2002; Monk and Brush, 2007; Brush and Feria, 2015). They 

now occur only in the western and northern parts of the LRGV 

in areas less fragmented by agriculture and urbanization, and 

they have disappeared from central and eastern parts of the 

LRGV where they were once found along the Rio Grande 

(Flood and others, 2002; Brush, 2005). Audubon’s orioles do 

reasonably well in second-growth conditions in Mexico, so 

they may persist in the LRGV if they acclimate to changing 

second-growth habitats (Flood and others, 2002). Given 

their high vulnerability to brood parasitism, it is thought that 

parasitism by bronzed and brown-headed cowbirds is the 

main negative factor affecting their populations. Similarly, 

hooded orioles were once the most abundant oriole species 

in the LRGV but have declined greatly, now occurring as an 

uncommon breeder in urban areas (Brush, 2005). Hooded 

orioles are no longer common in riparian areas of the LRGV 

but still occur in and near Santa Ana NWR. They are known 

to forage in agricultural areas, which could expose them to 

pesticides (Brush, 2005), and they are very susceptible to 

cowbird parasitism (Brush, 2000c). 

Among the other oriole species in the LRGV, the 

Altamira oriole—noted for its long, pendulous nest (Brush, 

1998c; fig. 12A)—was not found during the earliest bird 

surveys in the LRGV (Sennett, 1879; Brush, 2005). It spread 

through much of the LRGV in the 1950s and quickly became 

the most common oriole (Brush, 1998c, 2005; Brush and 

Pleasants, 2005). The increase in Altamira orioles in the 

LRGV coincided with declines in other sympatric oriole 

species, but since the 1980s, Altamira orioles have steadily 

declined in the LRGV (Brush, 1998c; Hathcock and Brush, 

2004). Icterus spurius (Orchard orioles) once bred in the 

LRGV but have retreated northward perhaps because of 

parasitism from cowbirds and interspecific interactions with 

other species of orioles (Brush, 2005, 2008a). I. bullockii

(Bullock’s orioles) are an arid-land species, most common 

in the western United States. In the LRGV, they once bred in 

downstream areas of Hidalgo County (last occurrence in 1996) 

but are now restricted to less fragmented areas in Starr County 

and above Falcon Dam (Brush, 2005).

Other Riparian Dependents

Several bird species are at particular risk of extirpation 

or continued rarity in their limited United States distributions, 

particularly in the LRGV (e.g., Brush, 2005; Brush and Feria, 

2015). Among those species, Camptostoma imberbe (northern 

beardless-tyrannulet) is a small flycatcher that only occurs in 

the United States in extreme southern Texas and a similarly 

small area of southern Arizona. In the LRGV, it has lost much 

of its preferred riparian forest habitat, but some evidence 

suggests that it tolerates second-growth habitats. Nevertheless, 

northern beardless-tyrannulets could benefit from management 

that favors revegetation of large riparian trees that support 

epiphytic mosses preferred for nesting (Brush, 1999a, 2005), 

particularly cedar elm where 93 percent of 28 nests were 

found in a recent study (Werner, 2004; Werner and others, 

2015). Other forest species similarly affected by loss of 

riparian forests along the Rio Grande, and are now rare as a 

result, include Pachyramphus aglaiae (rose-throated becard―

Howell and Webb, 1995; Brush, 2000a, 2005; Miller and 

others, 2015) and Parula pitiayumi (tropical parula), which 

also prefer epiphytic mosses in tall trees for nesting (Regelski 

and Moldenhauer, 1997; Brush, 1999a). In recent years, the 

few nesting attempts of rose-throated becards have only been 

seen in Anzalduas Park and Santa Ana NWR in Hidalgo 

County, but these attempts were unsuccessful (T. Brush, 

written commun.). 

In the United States, the Amazilia yucatanensis (buffbellied hummingbird) breeds only in the LRGV, primarily in 

Hidalgo and Cameron Counties, and seems to prefer scrubby 

and broken forests along and near the Rio Grande (Brush, 

2005; fig. 12C). Its distribution is relatively restricted for a 

hummingbird, from southern Texas along the Gulf Coast of 
Unique Aspects of the Lower Rio Grande Valley 39

A B

C

D

E

Figure 12. Prominent species of birds found in the Lower Rio Grande Valley (LRGV). With more than 420 species of birds 

documented in the LRGV, riparian forest and brushland species have high conservation value. A, the iconic Icterus gularis

(Altamira oriole); B, I. graduacauda (Audubon’s oriole); C, Amazilia yucatanensis (buff-bellied hummingbird); D, Ortalis vetula

(plain chachalaca); and E, Cyanocorax yncas (green jay). ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
40 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Mexico into the Yucatán in Mexico, Guatemala, and Belize; 

it is unique among North American hummingbirds because it 

regularly moves north in autumn and winters in low numbers 

in much of the southeastern United States (Chavez-Ramirez 

and Moreno-Valdez, 1999). Clearing of thornscrub and brushy 

habitats in the LRGV was probably detrimental to buff-bellied 

hummingbirds (Chavez-Ramirez and Moreno-Valdez, 1999; 

Brush, 2005). It appears to use residential and park areas in 

the LRGV and is more common in southern Texas than it once 

was (Brush, 2005).

Doves and Pigeons

Species richness of doves and pigeons in the relatively 

small LRGV is hard to match elsewhere in United States. 

Six native species (two exotic species are discussed under 

“Exotic or naturalized birds”) are common to scarce, and 

three of them readily acclimate to human-altered agricultural 

and urban landscapes (e.g., Brush, 2005, 2008b; Collins and 

others, 2010). In the United States, Patagioenas flavirostris

(red-billed pigeon―Lowther, 2002; fig. 13C) occurs only 

in the LRGV (fig. 13), and it was formerly widespread in 

relatively large numbers at mid-valley locations like Santa Ana 

NWR, nesting in riparian forests (Brush, 1998a, 2005, 2008a; 

Brush and Feria, 2015). Loss of riparian woodland and native 

brush and changes in the flood regime of the Rio Grande 

have greatly reduced preferred habitat of red-billed pigeons 

(Breeden and others, 2009). As a result, they are currently 

restricted to remnant riparian woodland near Falcon Dam 

in Starr County, where they still breed in mature stands of 

Mexican ash, willow, and Texas ebony (Brush, 2005; Breeden 

and others, 2009).

Among urban-shy and increasingly uncommon species, 

the white-tipped dove is secretive and relatively sedentary 

(fig. 13B) and occurs in low densities throughout the LRGV 

(Boydstun and DeYoung, 1988). It is more selective of 

nesting habitat than most other doves and consistently 

prefers increasingly rare and isolated forests of Texas ebony, 

sugarberry, and cedar elm in the LRGV, although they have 

been known to nest in citrus groves and urban parks in the 

past (Boydstun and DeYoung, 1985, 1987, 1988; Hayslette 

and others, 2000; Brush, 2005). Columbina passerina

(common ground doves) are found primarily in dry, rural 

mesquite savannas and grassland/thornscrub and have the 

fewest interactions with other doves and pigeons in the LRGV 

(Brush, 2005).

White-winged doves have retained their importance 

to sportsman in the LRGV (Jahrsdoerfer and Leslie, 

1988; Swanson and Rappole, 1992; Hayslette and others, 

1996, 2000; Small and others, 2004; fig. 13A). Along with 

congeneric Zenaida macroura (mourning doves), whitewinged doves are comfortable in agricultural and urban 

settings and readily nest in available offerings (Collins and 

others, 2010). Numbers of white-winged doves in the LRGV 

have varied through time, influencing harvest regulations 

(Small and Waggerman, 1999; Hayslette and others, 2000; 

Brush, 2005). They were much more abundant in 1930s and 

declined after being forced to shift nesting from diminishing 

native thorn brushlands to expanding citrus orchards, which 

were severely damaged by freezes in 1951, 1962, and 1983 

(Lonard and Judd, 1991; Swanson and Rappole, 1992) and 

more recently in 1989 and 2011. Heavy use of growing urban 

areas in and north of the LRGV have benefited populations of 

white-winged doves. Columbina inca (Inca doves) thrive in 

urban areas in the LRGV (Brush, 2008b). 

Thornscrub Species

The United States breeding distribution of the Passerina 

versicolor (varied bunting) is limited and concentrated in 

the United States-Mexico borderland (Howell and Webb, 

1995), typically occurring in arid thornscrub habitat (Ehrlich 

and others, 1988). It was once common in coastal thickets in 

Cameron County in the eastern LRGV but no longer occurs 

there (Brush, 2005). Varied buntings still occur locally in 

the western LRGV, particularly in Starr County. Reasons for 

their disappearance in the eastern LRGV and general decline 

elsewhere are not understood, but habitat fragmentation and 

destruction are likely causes (Brush, 2005). 

The Micrathene whitneyi (elf owl) is the smallest 

owl species in North America. In the United States, it is 

concentrated along the Mexican border from Arizona into 

Texas (Ehrlich and others, 1988). It was first reported in the 

LRGV in the 19th century but then disappeared for more than 

70 years (Brush, 2005). It now occurs in the western one-half 

of the LRGV, primarily from Bentsen Rio Grande Valley State 

Park in Starr County to Santa Ana NWR and in tracts of the 

Lower Rio Grande Valley NWR adjacent to Anzalduas Park 

(Gamel, 1997; Gamel and Brush, 2001; Brush, 2005). Elf owls 

need large snags for nesting and prefer open chaparral in Santa 

Ana NWR, which could theoretically support as many as 

800 elf owls with each pair residing in about 1 ha (Gamel

and Brush, 2001).

Waterfowl, Colonial Waterbirds, and Shorebirds/Seabirds

The LRGV and associated Laguna Madre are well known 

for supporting diverse assemblages of well-studied waterfowl 

(Smith, 2002a), colonial waterbirds (Smith, 2002b), and 

shorebirds/seabirds (Brush, 1995; Withers, 2002c) (appendix 

B). A great diversity of wetland habitats occurs in southern 

Texas to support these species: seagrass meadows, estuarine 

bays, coastal salt and brackish tidal marshes and flats, closed 

depression wetlands, freshwater ponds, inland saline lakes, 

resacas, small riparian streams, backwater areas of the highly 

human-modified Rio Grande flood plain, and even commercial 

rice fields. Associated terrestrial habitats are essential for 

resting, nesting, and, for some species, feeding. Numerous 

dredge and barrier islands in Laguna Madre support mixed 

nesting rookeries of colonial waterbirds and seabirds; species 

richness of these rookeries is high, ranging from a single 

species up to 20 species nesting at the same location in the 

Lower Laguna Madre—highest where human disturbance is 
Unique Aspects of the Lower Rio Grande Valley 41

A

B C D

E

Figure 13. Wetland birds and doves, which are notably diverse and abundant in the Lower Rio Grande Valley. A, Zenaida asiatica

(white-winged dove), an important game species; B, the growingly rare Leptotila verreauxi (white-tipped dove) and C, Patagioenas 

flavirostris (red-billed pigeon); D, Dendrocygna autumnalis (black-bellied whistling ducks); and E, wintering Aythya americana

(redheads). ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
42 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

the lowest (Smith, 2002b). The Lower Rio Grande Valley NWR, 

in partnership with various water districts and cooperative 

farmers, manages seven wetlands totaling 162.2 ha (401 acres), 

with five potentially manageable wetlands of 80.9 ha (200 

acres), to benefit wetland-dependent species.

Conservation of wetlands in the LRGV and associated 

Laguna Madre is fundamental to maintaining rich avian 

assemblages. Tens of thousands wintering redheads (fig. 13E) 

depend on meadows of shoal grass, typically at depths of 

12–30 cm (4.7–11.8 in.—Mitchell, 1992), in the highly saline 

Lower Laguna Madre, associated wetlands of Laguna Atascosa 

NWR, and coastal freshwater ponds (Mitchell and others, 

1994; Woodin, 1994, 1996; Custer and others, 1997; Michot 

and others, 2006; Woodin and others, 2008). Redheads prefer 

ponds with greater than average depths and percentages of 

open water and lower than average salinities—conditions that 

vary depending on annual rainfall and availability of ponds; 

redheads stay closest to lagoons in wet years but must travel 

farther in dry years (Ballard and others, 2010). Anas clypeata

(northern shovelers) have a similar dependence on two habitats, 

freshwater ponds and estuarine wetlands, in the LRGV (Tietje 

and Teer, 1996). Conservation of coastal wetlands, sometimes 

compromised by local ranching operations and naturally 

occurring dry periods, is essential to maintaining viable 

wintering habitat for redheads and other waterbirds.

Freshwater and marine wetlands in the LRGV host a 

variety of special waterbirds. Inland, 10 percent of the North 

American population of long-billed curlews winter at the saline 

East Lake. Unique and rarely seen waterfowl throughout the 

LRGV include Nomonyx dominicus (masked ducks—Anderson 

and Tacha, 1999), Dendrocygna bicolor (fulvous whistling 

ducks), Anas fulvigula (mottled ducks), and Mergus serrator

(red-breasted mergansers—Rupert and Brush, 1996). In 

riverine areas of the western LRGV, the Mexican race of A. 

platyrhynchus diazi (mallard) and naturally occurring (versus 

[vs.] domestic escapees) Cairina moschata (Muscovy ducks—

Brush and Eitniear, 2002) can be seen. The largest known 

colony of the nesting Egretta rufescens (reddish egrets) occurs 

north of Port Isabel and off the coasts of Willacy and Cameron 

Counties (Brush, 2005). 

Important to the conservation of coastal and barrier-island 

habitats, four distinct zones of critical wintering habitat have 

been formally designated for the federally threatened piping 

plover (U.S. Department of Interior, Fish and Wildlife Service, 

2009) along the entire length and width of Padre Island from 

the northern boundary of Padre Island National Seashore south 

to the mouth of the Rio Grande and considerable areas of the 

eastern shores of Willacy and Cameron Counties along the 

Lower Laguna Madre. Piping plovers show strong site fidelity 

to their wintering home ranges, which average about 12 km2

(4.6 mi2

), and they are most dependent on algal flats in autumn 

(arrival) and spring (departure) and exposed sand flats in winter 

(K.L. Drake, 1999; K.R. Drake, 1999b; Drake and others, 

2001). Sterna hirundo (common terns) were once regular 

breeders in the coastal Rio Grande Delta but are now rare or 

absent (Brush, 2008a).

Mammals

Of the 143 native mammal species and 12 introduced 

exotic mammal species in Texas, about 50 of them occur in the 

LRGV (e.g., Jones and Jones, 1992; Jones, 1993; Schmidly 

2003, 2004), including south Padre Island (e.g., Goetze and 

others,1999; Jones and Frey, 2013). Twelve (14 percent) of 

the 85 invertebrate and vertebrate species of conservation 

concern to Federal and State agencies in the LRGV are 

mammals (table 6). Four of the 12 species are federally and 

State-endangered species, and 3 threatened species and 5 rare 

species are listed by the State of Texas.

Bats

The rich bat fauna of the LRGV includes 14 resident 

and (or) migratory species (Chapman and Chapman, 1990; 

Schmidly, 2004), or nearly 30 percent of the total number 

of bat species in the United States and 30 percent of the 

overall number of mammal species in the LRGV. Five of 

the 12 (42 percent) mammals of conservation concern in the 

LRGV are bats (table 6), and two species reach their northern 

distributional limits in the LRGV: Choeronycteris mexicana

(Mexican long-tongued bat), listed as near threatened on 

the IUCN Red List (Arroyo-Cabrales and Perez, 2008), and 

Lasiurus ega (southern yellow bat). Aside from museum 

specimens collected over the past century (e.g., Chapman 

and Chapman, 1990), no information exists on the status of 

individual bat species in the LRGV. 

Small Mammals

Information on small mammal communities in the LRGV 

is limited. Sternberg and Judd (2006) found that three habitat 

types in the LRGV (native thornscrub woodland, agricultural 

field replanted in 1995, and unaided secondary succession 

following termination of farming in 1985) supported up 

to 10 rodent species, typically 5–9, with very high overall 

densities of 269–388 rodents per hectare. Sigmodon hispidus

(cotton rats), Peromyscus leucopus (white-footed mice), and 

Liomys irroratus (Mexican spiny pocket mice) were the most 

abundant rodents in native thornscrub woodland (88 percent) 

and replanted habitats (90 percent). Oryzomys couesi (Coues’ 

rice rat) is a Mexican form reaching its northern distributional 

limits in the LRGV and distinguished as a unique species by 

Schmidt and Engstrom (1994). It is semi-aquatic, preferring 

marsh/grass zones near resacas, for example, and loss of such 

habitats and the general xerification of the riparian corridor in 

the LRGV have likely contributed to the Coues’ rice rat being 

State listed as threatened in Texas (table 6). 

Most recently, Dolman (2015) studied small mammals 

in 14 tracts of the mid-LRGV and found that species richness 

was positively correlated with tract size. Species composition 

was comparable to findings of Sternberg and Judd (2006), but 

in Dolman’s study, capture rates of interior-adapted species 

were higher than edge-adapted species. This suggested that 

habitat restoration of natural woodland and thornscrub habitats 
Unique Aspects of the Lower Rio Grande Valley 43

in the areas studied likely had positive effects on some interioradapted small mammals. Genetic assessments of white-footed 

mice in the 14 tracts studied by Dolman (2015) suggested a 

lengthy period of low population size during fragmentation and 

conversion of natural habitats to agricultural and urban areas 

beginning in the early 1900s. This was eventually followed 

by population expansion likely associated with restoration 

activities of the USFWS beginning in the late 1970s (Dolman, 

2015).

Northern Ocelot in the LRGV

The ocelot was first listed as endangered by the USFWS 

throughout its foreign distribution in 1972, and the United 

States population of the northern ocelot was listed as federally 

endangered in 1982 (U.S. Department of Interior, Fish and 

Wildlife Service, 1982). The ocelot is a small, mainly nocturnal 

felid (fig. 14), native to subtropical and tropical regions 

in North, Central, and South America, currently ranging 

from the LRGV and southeastern Arizona (E. Fernandez, 

pers. commun.) south through Mexico to Peru and northern 

Argentina (Tewes and Schmidly, 1987; Murray and Gardner, 

1997; USFWS, 2010a). In the United States, the ocelot once 

occurred in other parts of Arizona (López González and others, 

2003; Holbrook and others, 2011) and much of Texas (Brown, 

1990; Stangl and Young, 2011) into Arkansas and Louisiana, 

and fossil records exist from Florida (Navarro-Lopez and 

others, 1993; Murray and Gardner, 1997). Today, northern 

ocelots (subspecies albescens) in the United States occur in 

two populations in the LRGV, numbering about 50 individuals 

(Chappell, 2010; USFWS, 2010a; H. Swart, pers. commun.), 

and since 2009, a total of five Sonoran ocelots (subspecies 

sonoriensis) have been detected in Arizona (E. Fernandez, pers. 

comm.). 

Ocelots use a wide variety of habitats in Mexico, Central 

America, and South America, including tropical dry forests, 

tropical humid forests, marshy areas, mangroves, riparian 

forests, and dry scrublands (Tewes and Schmidly, 1987; 

Murray and Gardner, 1997). Northern ocelots have been well 

studied in the LRGV (appendix C) and are now restricted to 

the densest remnants of thornscrub in Cameron and Willacy 

Counties, mainly Laguna Atascosa NWR and private ranches 

(Tewes and Everett, 1986; Harwell, 1990; Shindle, 1995; 

Shindle and Tewes, 1998; Haines, Tewes, Laack, and others, 

2005; Haines, Janečka, and others, 2006; USFWS, 2010a). 

A population occurs in and around private ranches such as 

the Yturria Ranch, in which 3,201 ha (7,910 acres) are in 

conservation easements for northern ocelots (Chappell, 2010; 

B.R. Winton, 2015, pers. commun.), and the San Perlita and El 

Jardin tracts of the Lower Rio Grande Valley NWR in northcentral Willacy County (Sternberg and Chapa, 2004; Haines, 

Grassman, and others, 2006). Presence of northern ocelots is an 

indicator of relative quality and connectivity of dense, remnant 

thornscrub habitat, making its preservation and restoration 

a critical conservation goal in the LRGV. Nevertheless, no 

Federal critical habitat has been designated for the northern 

ocelot in the United States (USFWS, 2010a).

Research since the 1980s, mainly under the direction of 

M.E. Tewes and through the Endangered Cat Research and 

Monitoring Program at Laguna Atascosa NWR, has uncovered 

many aspects of the biology and ecology of the northern ocelot 

to aid conservation and recovery (appendix C). Although 

data are still limited for some reproductive characteristics, 

parturition of northern ocelots around Laguna Atascosa NWR 

occurs from mid-April to late December, with typical litters of 

1–2 kittens, each with a survival rate of 68 percent from birth 

to 1 year of age. Females select den sites generally within 

10 m from or right amongst dense thornscrub; use 2–4 dens, 

110–280 m apart, for each litter; and occupy dens for 3–64 

days (Laack, 1991; Laack and others, 2005). Annual survival 

of northern ocelots from 1983 to 2002 differed significantly 

for resident (87 percent) and transient individuals (57 percent) 

but not by sex; transient northern ocelots were more likely 

to die from natural causes such as disease (e.g., Mercer and 

others, 1988; Pence and others, 1995, 2003) than residents, 

but both were equally affected by human impacts, mainly 

being hit by vehicles (Haines, Tewes, and Laack, 2005). 

Echoing early comments by Tewes and Miller (1987), more 

recent population viability analyses identified northern ocelot 

vehicle mortality as the single most important variable in 

long-term survival of northern ocelots in southern Texas, and 

without implementation of other recovery objectives (USFWS, 

2010a), there was a 33 percent chance the population would 

be lost in 50 years; supplementation of the Texas population 

by translocation of northern ocelots from elsewhere also 

improved theoretical models of long-term viability (Haines, 

Tewes, Laack, and others, 2005; Haines, 2006; Haines, 

Tewes, and others, 2006; Haines and others, 2007). To enact 

the much-needed translocation, the Ocelot Recovery Team 

developed an Ocelot Translocation Plan (Translocation 

Working Group, 2009), and considerable research has been 

conducted in Mexico to support this effort (Stasey, 2012; 

Conservación y Desarrollo de Espacios Naturales, 2014).

Conservation of northern ocelots in the LRGV has 

benefited substantially from research on techniques to restore 

essential habitats, such as the correlations among preferred 

habitats with >95 percent canopy cover and particular soil 

series (Young and Tewes, 1994; Harveson, 1996; Harveson 

and others, 2004; Alexander and others, 2016); chemical 

immobilization protocols, applicable to similar felids around 

the world (Beltrán and Tewes, 1995; Shindle and Tewes, 

2000); multiple approaches to remote sensing, video imaging, 

GIS, and landscape metrics to identify and assess essential 

habitats (Anderson and others, 1997; Tewes and others, 1999; 

Jackson, 2002; Harveson and others, 2004; Haines, Caso, and 

others, 2005; Jackson and others, 2005); scented hair snares 

to entice facial rubbing for collection of DNA in hair follicles 

(Weaver and others, 2005); first application of satellite-based 

telemetry on northern ocelots (Haines, Grassman, and others, 

2006); and rapid whole genome amplification of DNA for 

conservation genetics work (Janečka, Grassman, and others, 

2007; Janečka and others, 2008).
Figure 14. The endangered Leopardus pardalis albescens (northern ocelot). Perhaps more than any other species, the northern ocelot is emblematiconservation challenges facing the biota of the Lower Rio Grande Valley (LRGV). About 50 northern ocelots likely remain in the LRGV. Left and bottom Photography, used with permission; top-right photograph courtesy of D. Martinez, U.S. Fish and Wildlife Service.

©Larry Ditto Nature Photography

©Larry Ditto Nature Photography
Unique Aspects of the Lower Rio Grande Valley 45

Northern ocelots prefer the densest thornscrub habitats 

in the LRGV (Shindle and Tewes, 1998; Harveson and 

others, 2004; Jackson and others, 2005; Horne and others, 

2009). These habitats are associated with certain soil types 

(Harveson and others, 2004) and have been severely depleted 

and fragmented, now making up <1 percent of the LRGV 

(Tewes and Everett, 1986). Loss of habitat, particularly large 

contiguous patches (Jackson and others, 2005), is the principal 

cause of decline of the northern ocelot in the LRGV, and its 

effects are multifaceted (Harveson and others, 2004; Haines 

and others, 2005a; Jackson and others, 2005). Throughout 

most of southern Texas, dense thornscrub has been eliminated 

or degraded by agriculture and urban development. With the 

loss of habitat, northern ocelots have been forced into smaller 

and smaller fragments of available habitat (Harveson and 

others, 2004; Haines, Tewes, Laack, and others, 2005; Jackson 

and others, 2005). To move between these small patches of 

habitat, northern ocelots often have no choice but to cross 

the increasing number of roads that have resulted from the 

continued human development in the LRGV (Tewes and 

Hughes, 2001). Northern ocelots also are occasionally killed 

directly by poachers or domestic dogs and after eating illegal 

poisoned baits used for “predator control” (Haines, Tewes, and 

Laack, 2005, p. 258; Laack and others, 2005). 

Loss of habitat probably results in more interactions 

between transient and resident northern ocelots defending 

territories, resulting in death or injury (Haines, Tewes, and 

Laack, 2005). Interactions with Lynx canadensis (bobcats) 

also might negatively affect northern ocelots. Northern ocelots 

prefer dense brush, and bobcats generally occur in more open 

areas; however, it is unclear if this partitioning is the result 

of habitat preferences or competitive exclusion between the 

species (Horne, 1998; Horne and others, 2009). Most recently, 

camera traps found northern ocelots, living sympatrically 

with bobcats in Cameron County, mostly associated with 

corridors of brush, resaca edge, and drainage ditches and not 

brushy patches, which are typically thought of as a preferred 

core habitat of northern ocelots (Nordlof, 2015). Large, exotic 

nilgai have created trails through dense brush in areas that 

could be occupied by northern ocelots, permitting intrusion 

by bobcats and other mesocarnivores (Leslie, 2008). Recent 

dietary comparisons between northern ocelots and bobcats 

in southern Texas suggest some differentiation of prey size, 

with bobcats eating larger rodents and more lagomorphs 

than northern ocelots, but there was no clear partitioning of 

habitats, with both carnivores selecting prey about equally 

from thornscrub and grassland habitat (Booth-Binczik and 

others, 2013). Environmental contaminants have been found 

in northern ocelot tissue from the LRGV, but levels did not 

appear to affect their health (Mora and others, 2000)—a 

situation that could change with the expanding human 

population and warrants future monitoring.

Numbers of northern ocelots in the LRGV are very small 

and basically isolated in two locations about 30 km apart, 

and there is evidence of overall low nucleotide diversity 

(Janečka, 2006; Janečka, Walker, and others, 2007) and 

genetic differentiation between them (Janečka and others, 

2011). Based on microsatellite analyses, effective population 

sizes of these remnant breeding populations (2.9 and 

8.0 individuals) are well below the minimum level of about 

50 individuals needed to maintain genetic health, suggesting 

that both populations will suffer (or have already suffered) 

from inbreeding depression and loss of adaptive variation, 

making them much more likely to be extirpated (Janečka and 

others, 2008). Prospects for colonization of additional habitat 

by transient northern ocelots are limited unless more thornscrub 

habitat associated with existing populations can be connected 

and protected from agricultural uses and urbanization (Janečka 

and others, 2011). As with population viability models, genetic 

deficiencies in the two populations suggest that translocation of 

taxonomically similar, but more genetically diverse, northern 

ocelots from Tamaulipas, Mexico, could improve recovery 

outlook (Translocation Working Group, 2009; Janečka and 

others, 2011).

Conservation strategies for the northern ocelot in the 

LRGV include habitat protection and restoration, land 

acquisition, creation of corridors linking populations and 

disjunct habitat that will encourage establishment of new 

populations, reducing automobile collisions, and introducing 

wild-caught northern ocelots from Mexico into the LRGV 

to bolster genetic diversity (e.g., Tewes and others, 1995; 

Harveson and others, 2004; Haines, Tewes, and others, 2005; 

Jackson and others, 2005; Haines, Tewes, and others, 2006; 

Janečka and others, 2011; Alexander and others, 2016). The 

urgent nature of reducing mortality from vehicle collisions was 

exemplified by the loss of five adult northern ocelots (about 10 

percent of the known population) in an 8-month period in 2015 

and early 2016 (H. Swarts, written commun.). An important 

means of minimizing vehicle collisions could be culverts 

under roads in areas near known territories, corridors, or other 

areas frequented by northern ocelots (Hewitt and others, 1998; 

Tewes and Hughes, 2001; Haines, Tewes, and Laack, 2005). 

Another strategy to minimize collisions is to use wildlifecrossing signs along roads where northern ocelots are known 

to occur, encouraging drivers to be cautious and slow down; 

northern-ocelot-crossing signs are not suggested because they 

could draw unwanted activities such as inadvertent harassment 

by nature enthusiasts and even poaching (Tewes and Hughes, 

1991). Because almost 96 percent of Texas is privately owned, 

it will be essential that private landowners participate in habitat 

restoration and recovery efforts. Conservation easements and 

other incentives could engage private landowners to assist with 

habitat restoration efforts to benefit northern ocelots and other 

wildlife (Haines, Janečka, and others, 2006; Leggett, 2009).

Construction of the border fence by the U.S. Department 

of Homeland Security on the United States side of the Rio 

Grande has compromised recovery efforts for northern 

ocelots and other wildlife in the LRGV (see “Border Issues 

and Homeland Security” section). The fence could impede 

north-south movements of transient northern ocelots and other 

wildlife from Mexico and the United States along the lower Rio 

Grande and thereby restrict dispersal from extant populations 
46 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

and the possibility of establishment of a new resident population 

(e.g., Grigione and others, 2009; Abhat, 2011). Even artificial 

lighting along the United States-Mexico border associated with 

the wall could affect activity patterns and behavior of northern 

ocelots’ prey (Grigione and Mrykalo, 2004). The last northern 

ocelot at Santa Ana NWR in the early 1990s reared two kittens 

there, and genetic analyses suggested that she originated from 

Mexico (Walker, 1997). How the border fence will affect such 

future movements is uncertain. Success in conserving northern 

ocelots in the LRGV will require integration of cross-border 

strategies (Grigione and others, 2009).

Ongoing Challenges Facing the LRGV

Since the arrival of Europeans in the 17th century, 

landscapes of the LRGV have been intensively modified, 

initially from overgrazing by livestock (Thompson, 1997; 

Tiefenbacher, 2001; Brush, 2005), which promoted growth 

of woody, thorny, and unpalatable plants and resulted in 

the gradual conversion of grasslands along the Gulf Coast 

and the northern part of the LRGV into areas dominated by 

mesquite and other woody species (Crosswhite, 1980). After 

the American Civil War, water manipulation (e.g., irrigation 

and flood control) and agriculture were initiated in the LRGV 

(Thompson, 1997; Tiefenbacher, 2001; Stubbs and others, 

2003; Brush, 2005). Irrigation allowed settlers to grow crops 

that would have been impossible given the often dry conditions 

in the LRGV. Settlers cleared thornscrub, forests, and riparian 

woodlands to make room for cotton and citrus crops. 

Tremblay and others (2005) calculated that native 

woodland loss in Cameron County from the 1930s to 1983 was 

91 percent, as a result of agricultural expansion. This expansion 

greatly accelerated throughout the 20th century, and farm 

subsidies in the 1960s and 1970s encouraged clearing of more 

brush, even in Starr County which was generally thought to be 

too dry for farming. Human population growth in the LRGV 

also accelerated during the latter part of the 20th century, 

expanding housing developments, shopping centers, roads, 

and other aspects of human settlement and further impacting 

remnant native habitats (table 10.1 in Tiefenbacher, 2001).

Construction of dams, levees, and channels in the mid20th century changed the flow and character of the Rio Grande 

and surrounding landscapes (Thompson, 1997; Stubbs and 

others, 2003). In particular, construction of Falcon Dam in 

1953–54 reduced natural flooding that had regularly occurred 

in the river, causing changes in riparian forests, wetlands, and 

other areas that were dependent on freshwater and nutrient 

input from frequent floods; normal successional changes after 

such flooding disturbance were altered or ceased, resulting 

in loss of unique plant communities (e.g., Small and others, 

2009). Reductions in flooding greatly affected permanence of 

an expanding human population in early decades of the 20th 

century (Thompson, 1997).

Human Population Growth and Associated 

Challenges

Since 1940, the population in the four-county area of 

the LRGV has increased by more than a million people, 

making it one of the fastest growing regions in the United 

States. In the 1960s through the 1980s, the population in the 

LRGV grew from about 400,000 people to nearly 700,000 

(table 10.1 in Tiefenbacher, 2001; fig. 15), and since then, it 

has nearly doubled to an estimated 1,317,156 people in 2013 

(U.S. Census Bureau, 2013). The McAllen-Edinburg-Mission 

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

Cameron Hidalgo Starr Willacy

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1940 1950 1960 1970 1980 1990 2000 2008 2013

Year Counties

Human population in the Lower Rio Grande Valley

Population 1990

Population 2000

Population 2008

Population 2013

EXPLANATION

Figure 15. Nearly exponential growth of the human population in the Lower Rio Grande Valley from 1940 to 2013, projected to reach 

about 3 million people by 2050 (Stubbs and others, 2003; U.S. Census Bureau, 2013).
Ongoing Challenges Facing the LRGV 47

and Brownsville-Harlingen-San Benito areas have been, in 

recent years, among the fastest growing metropolitan areas 

in Texas and the United States (Chang and Davila, 2008). 

On the Mexican side of the Rio Grande, growth has been 

even greater; for example, Reynosa (633,730 people) and 

Matamoros (462,157 people) are considerably larger than their 

American counterparts, McAllen, Tex. (129,877 people), and 

Brownsville, Tex. (172,023 people), respectively (U.S. Census 

Bureau, 2010; Parcher and others, 2013). 

From 2000 to 2010, human populations increased 36 

percent in Hidalgo County, 21 percent in Cameron County, 

14 percent in Starr County, and 10 percent in Willacy County, 

which experienced a slight decrease of about 1 percent from 

2010 to 2013 (U.S. Census Bureau 2013). Growth trajectories 

are difficult to predict with certainty, but if growth from April 

2010 to July 2013 in the two most populous counties is any 

indication, increases of 0.83 percent per year in Cameron 

County and 1.63 percent per year in Hidalgo County can be 

expected (U.S. Census Bureeau 2013). If this rate is realized 

in Hidalgo County, for example, the countywide population 

would rise from 815,996 people in 2013 to more than 1 

million people in 2026. Stubbs and others (2003) predicted 

that populations in the LRGV will more than double by 2050 

to as many as 3 million people.

Recent growth in the human population in the LRGV can 

be attributed to increased trade with Mexico and other parts of 

Central America. The North American Free Trade Agreement 

(NAFTA, U.S. Public Law 103–182), implemented in 1994, 

facilitated growth of trade throughout the Americas and 

Canada, and the LRGV “emerged as a [significant] warehouse 

and transportation center between Central America and the US 

[sic]” (Davila and others, 2005; Chang and Davila, 2008, p. 

777). Rapid expansion of the human population and economy 

in the LRGV is related, in large part, to the strategic location 

next to a large manufacturing base in northern Mexico 

(Wynne, 1994; Chang and Davila, 2008). The LRGV in the 

United States has become a major shopping and vacation 

destination for Mexican citizens. Rapid development of the 

LRGV has not come without environmental consequences 

related to infrastructure to deal with air quality, water supply, 

wastewater treatment, solid waste disposal, and human health 

(e.g., Wynne, 1994; Ellenson and others, 1997; Mukerjee, 

2001, 2002; Mukerjee and others, 2001; Davila and others, 

2005; Chang and Davila, 2008). 

As the human population continues to increase in the 

LRGV, urbanization will outpace agriculture as the principal 

cause of loss and fragmentation of native habitats (Marzluff 

and Ewing, 2001). Some estimates predict that 70 percent of 

humans―some 5 billion or more―will live in urban areas 

within the next 30 years (World Health Organization, 2013; 

Shanahan and others, 2015). The increase in the total amount 

of urban landscapes is problematic because it often has greater 

effects on wildlife than agricultural landscapes; urban areas 

persist longer and are more dissimilar to and disjunct from 

native habitats (Marzluff and Ewing, 2001). The almost certain 

increase in human numbers and associated urbanization in the 

LRGV will make it increasingly difficult for wildlife species to 

find enough habitat to maintain sustainable populations. Thus, 

maintaining and increasing protected areas in the LRGV―

amidst an ever-increasing urban landscape―will become 

even more important in the years and decades to come. 

Importantly, wise planning of urban residential areas and parks 

can improve conditions for some plants, butterflies, birds, and 

other animals, providing more wildlife-friendly habitat than 

intensively cultivated areas.

Urbanization, Industrialization, and Economic 

Development 

Urban landscapes now dominate many parts of the LRGV 

(fig. 16). From 1993 to 2003, total area of urbanized landscapes 

increased by 46 percent and total area of irrigated landscapes 

decreased by only 7.6 percent in Hidalgo (59.7 percent and 

10.2 percent, respectively), Cameron (52.8 percent and 6.7 

percent, respectively), and Willacy (25.7 percent and 5.9 

percent, respectively) Counties (Huang and Fipps, 2006). 

This change clearly demonstrates that the landscape cost 

of urbanization in the LRGV has been at the expense of 

remaining natural habitats, not irrigated agricultural areas, 

which is also reflected in population densities: 1,277 people 

per square kilometer (493 people per square mile) in Hidalgo 

County, 1,181 people per square kilometer(456 people per 

square mile) in Cameron County, 129 people per square 

kilometer (49.8 people per square mile) in Starr County, and 

97.1 people per square kilometer (37.5 people per square mile) 

in Willacy County (U.S. Census Bureau 2013). 

The economy of the LRGV during early European 

settlement was based on cattle, sheep, and goat ranching, 

with other types of agriculture considered less important 

(Thompson, 1997). With enhanced irrigation and increased 

transportation capabilities such as railroads and major 

highways, cash crops like citrus fruits and sugarcane gradually 

replaced ranching as the most important component of the 

LRGV economy (Thompson, 1997; Mier and others, 2004). In 

recent years, however, there has been a shift from an agrarian 

economy to one based on services and trade with Mexico. 

In the early 2000s, 30 percent of jobs were in the service 

sector, 25 percent were in government, and 25 percent were 

in manufacturing, construction, and transportation (Mier and 

others, 2004). 

A principal reason for the rise in trade with Mexico 

and subsequent boom in the human population and 

economic growth in the LRGV area was the establishment 

of maquiladoras―“twin plants,” as they are known―

beginning in the mid-1960s and greatly expanding after 

NAFTA was implemented in 1994 (Wynne, 1994; Gruben, 

2001). Maquiladoras are manufacturing plants located on 

the Mexican side of the border. The Mexican government 

created a 20-km-wide strip of land along the border where 

duty-free foreign parts and goods could be imported; the intent 

was to increase employment, wages, and industrialization 
laf16-0737_fig16

STARR

COUNTY

HIDALGO

COUNTY

CAMERON

COUNTY

WILLACY

COUNTY

TEXAS

Laguna Atascosa National Wildlife RefugeLower Rio Grande Valley National Wildlife RefugeSanta Ana National Wildlife Refuge

EXPLANATION

0 10 20 40 MILES

N 0 10 20 40 KILOMETERS

Urban

281

83

281

77

2

186

10

Base map image is the intellectual property of Esriand is used herein under license. Copyright © 2014 Esriand its licensors. All rights reserved.Refuge boundaries from U.S. Fish and Wildlife Service.Urban areas, county borders, and roads fromTexas Natural Resource Information System 

McAllen

Bro

Raymondville

Rio

Grande

City

Roma

Harlingen

!

!

!

!

!

!

Rio Grande

Figure 16. Urbanized areas in the Lower Rio Grande Valley, which form an almost continuous band from Roma, Texas, to beyond Brownsville, Tex. (TInformation System, 2016).
Ongoing Challenges Facing the LRGV 49

in northern Mexico and decrease illegal immigration into 

the United States (Douglas, 2009). Maquiladoras typically 

import goods, mostly from the United States, which are then 

processed quickly and brought back to the United States for 

more processing or sale (Gruben, 2001; Douglas, 2009). The 

number of maquiladoras has risen significantly since NAFTA, 

and there has been corresponding growth in communities of 

northern Mexico and particularly in the LRGV (Chang and 

Davila, 2007; Douglas, 2009).

With increases in population and urbanized landscapes, 

the demand for new and larger roads has increased (McCray, 

1998). Commerce with Mexico has also increased the need 

for more international bridges in the LRGV. The number of 

bridges has doubled from 5 international bridges in 1980 to 

10 international point-of entry bridges by 2015; the PharrReynosa International Bridge, south of McAllen, Tex., carried 

93 percent of the 452,821 commercial trucks entering the 

United States in 2011, followed distantly by the Veterans 

International Bridge near Brownsville, Tex., at 5 percent 

(Rajbhandari and others, 2012). Collectively, this steadily 

increasing traffic puts added pressure on fragile riparian 

habitats and creates more barriers for wildlife and challenges 

to conservation activities (e.g., Dolman, 2015). Because the 

human population in the LRGV has increased, and will likely 

continue to do so, the numbers of roads and vehicles traveling 

on them will also increase, bringing more pressure on LRGV 

wildlife from collision-related injuries and mortalities (Loss 

and others, 2014b).

Energy Development

Oil and Gas

Energy demands in the LRGV will continue to increase 

with growing populations (e.g., Gray and others, 2013). 

Although the U.S. Government owns surface lands in the 

Lower Rio Grande Valley NWR, in many cases private 

parties own subsurface mineral rights and have the legal right 

to reasonable access to explore for and extract oil and gas.

Ramirez and Baker Mosley (2015, p. 1) recently summarized 

increases in oil and gas development on USFWS properties, 

noting that they increase the “burden on lands set aside for 

natural resource conservation” and the likelihood of “impacts 

from brine, oil, and other hydrocarbon spills ... [and] habitat 

alteration associated” with exploration and development of 

wells and pipelines. Across the entire NWR system of fewer 

than 560 properties and facilities in 2003, the Lower Rio 

Grande Valley NWR had the fifth highest number of oil and 

gas wells: oil, 4 active and 19 inactive; gas, 60 active and 

79 inactive; oil and gas combined, 2 active and 8 inactive; and 

172 in total. It ranked third in the number of injection wells 

with 5 active, 4 inactive, and 1 plugged. 

Inactive, plugged, and improperly plugged wells can be 

overlooked relative to their potential ongoing environmental 

impact. In 2011, three wells posing potential hazards to East 

Lake in the Lower Rio Grande Valley NWR had to be plugged 

at a cost of $1.2 million to taxpayers because the owner/

operator of the site could not be located (Covington, 2014). In 

March 2013, 25 barrels (1,050 gallons) of drilling mud were 

spilled at a drilling operation in the Lower Rio Grande Valley 

NWR (Ramirez and Mosley, 2015). Drilling and processing 

sites for oil and gas on Padre Island negatively impacted 

vegetative recovery by creating hard surfaces with oyster 

shells and caliche for the oil wells and roadways to them and 

alteration of site elevation, locally modifying the typically low 

relief of barrier islands (Carls and others, 1990). 

Because of the high level of oil and gas development, 

personnel from the Lower Rio Grande Valley NWR have 

had substantial and ongoing roles in developing oil and gas 

management protocols and national policies that have refined 

techniques to reduce impacts to Federal resources and the 

general public and still afforded access to refuge properties. 

The need for protocols―consistent with current laws, 

policies, and industry practices―to assess impacts of oil and 

gas operations and follow-up inventory and monitoring on 

USFWS properties nationally (Ramirez and Mosley, 2015) 

has led to proposed changes to the 50-year-old regulations 

governing such activities. These changes are currently under 

public review before implementation (U.S. Department of 

Interior, Fish and Wildlife Service, 2015). 

Wind Power

In the late 1980s, wind-power facilities did not exist 

in the LRGV. Since then, and particularly in the past 10 

years, development of alternative energies to oil and gas has 

increased substantially in the United States. As of May 2015, 

wind-power facilities in the LRGV had 1,373 wind turbines of 

various heights and dimensions, with 132 proposed turbines 

(table 8). As maintained by the USFWS (2016a), the Federal 

Aviation Administration data in proximity to various cities in 

the LRGV show that most wind turbines occur near Harlingen, 

Raymondville, Rio Grande City, and Brownsville, Tex., with 

new developments proposed near Raymondville, Brownsville, 

Table 8. Total number of wind turbines in proximity to various 

cities in the four-county area of the Lower Rio Grande Valley 

through May 2015 (U.S. Fish and Wildlife Service, 2016a).

City Existing

Proposed/New 

location

Brownsville 108 58

Harlingen 512 0

Los Fresnos 120 0

Raymondville 345 74

Rio Grande City 228 0

South Padre Island 3 0

Sullivan City 57 0

Total 1,373 132
50 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

and Los Fresnos (fig. 17). Fifteen companies were actively 

developing wind-power facilities in the LRGV in mid-2015, 

but three companies owned and operated 90 percent of the 

wind turbines: FPL Energy, Inc., alone or partnered with 

Horse Hollow Wind Energy, 586 turbines; Renewable Energy 

Systems (Americas), Inc., 423 turbines; and Buffalo Gap 

Wind Farm, LLC, 345 turbines.

Mortalities of birds and bats occur from collisions with 

manmade structures, including wind turbines (National 

Research Council, 2007; Sovacool, 2009; Loss and others, 

2012). Estimates of wind turbine losses for birds in the 

United States (140,000–368,000 birds per year—Loss and 

others, 2013a; Erickson and others, 2014) are far less than 

estimated mortalities from collisions with communication 

towers (4–5 million birds per year—Erickson and others, 

2005), power lines (12–64 million birds per year―Loss and 

others, 2014a), vehicles (89–340 million birds per year—

Loss and others, 2014b), buildings (365–988 million birds 

per year: 56 percent at low-rise buildings of 4–11 stories, 

44 percent at residences of 1–3 stories—Loss, Will, Loss, 

and Marra, 2014), and feral/domestic cats (1.4–3.7 billion 

birds per year and 6.9– 20.7 billion mammals per year—Loss 

and others, 2013b). Unfortunately, no research on effects of 

wind turbines in the LRGV (or other like hazards) has been 

published and presumably collected, but the fact that two 

major migratory flyways (Mississippi and Central) converge 

in the LRGV is of concern. 

Graham and Hudak (2011) calculated a potential hazard 

index (PHI) from wind-power facilities for 31 rare, threatened, 

or endangered bird species and 10 bat species in Texas; PHI 

was a weighted average of a species’ percentage of known 

distributional area in each of six wind-speed classifications at 

a height of 50 m in Texas. Mean PHI for birds was 2.1 (range 

= 1–3.5), and species of conservation interest in the LRGV 

had relatively high PHIs: tropical parula (2.29), rose-throated 

becard (2.29), northern beardless-tyrannulet (2.24), Peucaea 

botterii texana (Texas Botteri’s sparrow―2.23), and northern 

aplomado falcon (2.23). Mean PHI for bats was 2.29 (range = 

1–2.74), and species of conservation interest in the LRGV had 

relatively high PHIs: southern yellow bat (2.36) and Mexican 

long-tongued bat (2.15). This index could not incorporate 

actual mortalities of birds and bats because such data are not 

widely available (Graham and Hudak, 2011), but it did provide 

a generalized indication of potential hazards to species of 

concern in the LRGV and elsewhere in Texas. 

Recently, the USGS proposed a standardized 

methodology to assess regional and national impacts of 

wind-power development on birds and bats; it relies on oftenlimited, species-specific data to assess probability of risk 

to overall populations of birds and bats before developing 

wind power in a specific area (Diffendorfer and others, 

2015). Future applications of this methodology could benefit 

conservation relative to wind-power development near the 

STRC and elsewhere in the LRGV.

Figure 17. Wind-power development in the Lower Rio Grande Valley, which has grown from zero in the late 1980s to about 1,400 wind 

turbine towers operated by 15 companies and often juxtaposed near or in important brushlands and coastal habitats; photos show wind 

turbine towers north of Los Fresnos, Cameron County, Texas. Photographs courtesy of Chris Perez, U.S. Fish and Wildlife Service.
Ongoing Challenges Facing the LRGV 51

Pollution and Waste Management

Significant effects of border growth and manufacturing 

activities are associated with water and air pollution, waste 

management, sanitation, and human health problems (e.g., 

Warner, 1991; Akland and others, 1997; Garcia and others, 

2001; Peterson and others, 2001; Callegary and others, 2013). 

Air pollution in the Matamoros-Reynosa region of Mexico 

in the LRGV came mainly from vehicular sources (80.8 

percent) and secondarily from a power plant in Rio Bravo 

(17.7 percent) and industry (1.5 percent―Mejia-Velazquez 

and Rodriguez-Gallegos, 1997). Various agricultural 

practices can be significant sources of water pollution. Recent 

monitoring of the Arroyo Colorado in the LRGV suggested 

that, despite a 30-percent loss of irrigated water from runoff, 

pollutants reaching the Arroyo Colorado, particularly soluble 

nitrogen compounds, were reduced by the combination of 

residue management, subsurface tile drainage, and education 

of farmers to closely monitor irrigation applications relative 

to real-time runoff (Enciso and others, 2014). Agricultural 

burning is another factor affecting air quality and is routinely 

done on sugarcane fields from November through March in 

the LRGV (Dennis and others, 2002).

Solid waste management is an ever-increasing challenge 

in the LRGV as urbanization expands on either side of the 

United States-Mexico border (Davila and others, 2005; 

Chang and Davila, 2008; Chang and others, 2008). Due to 

rapid population growth and increasingly limited landfill 

space, the LRGV region, particularly the highly urbanized 

areas in Hidalgo and Cameron Counties, will face increasing 

pressure to successfully manage human waste while 

maintaining environmental standards in the coming decades. 

Annual waste production in Hidalgo, Cameron, and Willacy 

Counties in the early 2000s amounted to 410,000 tons per 

year, handled by six landfills with “only 12 more years of 

landfill life” (Chang and Davila, 2008, p. 778). Substantial 

energy recovery could be possible from recycling of plastic 

and paper in the waste stream of the LRGV (Chang and 

Davila, 2008). Somewhat paradoxically, authorized tire 

disposal costs $5 per tire. Some residents in the LRGV would 

rather keep the $20 disposal fee for a new set of tires than 

pay it, so they stockpile or dump old tires, often on remote 

tracts of the Lower Rio Grande Valley NWR. Dumped tires 

create sanctuaries for exotic Africanized honey bees and 

nesting sites for mosquitoes―both of which are a nuisance to 

humans (see “Exotic and Invasive Species” section).

Human Health and Associated Benefits

of Land Conservation

Communities in the LRGV have long been considered 

among the poorest in the United States, with weak healthcare 

opportunities and delivery (Maril, 1989) and poor living 

conditions for some in unregulated settlements, or “colonias” 

(Rivera, 2014). As of 2012, 34.9–39.9 percent of people 

in the LRGV live under the poverty level, in contrast to 

an average of 17.4 percent throughout Texas (U.S. Census 

Bureau, 2013). A dual system of human healthcare exists in 

the LRGV, with contemporary medical services mixed with 

traditional remedies from folk healers, or “curanderos,” and 

midwives, or “parteras” (Maril, 1989; Thompson, 1993; Burk 

and others, 1995; Keegan, 1996; Richardson and others, 

2012). Workers in the LRGV are often dependent on jobs that 

can be injurious to their health, pay low wages, and do not 

provide adequate health insurance. Healthcare literacy (Olney 

and others, 2007) and language barriers to understanding 

healthcare options and procedures (Martinez, 2008) have also 

been challenging for some residents of the LRGV. 

As a result of healthcare limitations in the LRGV (e.g., 

Mier and others, 2008), a significant increase in Federal and 

State healthcare dollars and programs were provided in the 

1970s and 1980s (Maril, 1989), yet healthcare costs and 

opportunities are still out of reach of many of residents of the 

LRGV, unless they are covered under assistance programs like 

Medicare. In 2009, McAllen, Tex., in Hidalgo County had one 

of the most expensive healthcare markets in the United States, 

with Medicare costs of $15,000 per enrollee in 2006—twice 

the national average and $3,000 above the average per capita 

income of only $12,000 (Gawande, 2009). Although medical 

facilities in the area of McAllen, Tex., were of equal quality, if 

not better than, those in comparable cities, excessive testing, 

surgery, hospital stays, and homecare, perhaps coupled with 

fear of lawsuits, seemed to cause overuse of medical care in 

McAllen, Tex. (Gawande, 2009). 

The growing population in the LRGV has resulted in 

associated medical issues, particularly among MexicanAmericans who composed 87.2–95.7 percent of the 

populations in Cameron, Hidalgo, Starr, and Willacy Counties 

in 2010 (U.S. Census Bureau, 2013). Mexican-American 

residents of the LRGV have experienced high rates of clinical 

anxiety (Glover and others, 1999); the highest rates of and 

deaths from diabetes in Texas (Larme and Pugh, 2001; 

Brown and others, 2002; Mier and others, 2007); low rates 

of mammography in farmworker communities (Palmer and 

others, 2005); low frequencies of prenatal diagnoses of birth 

defects (Waller and others, 2000); higher than average rates 

of adolescent obesity (Lacar and others, 2000); barriers to 

adolescent mental healthcare services (Pumariega and others, 

1998); and correlation of poverty and country of origin with 

adolescent depression, drug use, suicide (Swanson and others, 

1992), and insomnia (Roberts and others, 2004). Among 

adults, the obesity rate is 38 percent, and heavy drinking is 

60 percent higher than the national average (Gawande, 2009).

Open spaces such as city parks, outdoor classrooms, 

nature centers featuring birds and butterflies, and closely 

associated wildlife refuges and State and national parks 

enhance human health by encouraging physical activity 

and providing psychological well-being and community 

engagement (Maller and others, 2008, 2009; Shanahan and 

others, 2015; fig. 18). These areas also serve to fundamentally 

connect people with biota and ecological processes that they 
52 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Figure 18. Some of the many outdoor open-space recreational and educational opportunities available in the Lower Rio Grande Valley. 

©Larry Ditto Nature Photography, used with permission.

depend on, serving as educational platforms to reconnect 

people with their evolutionary past and ecological future 

(Maller and others, 2009). Community-based multidisciplinary 

models exist that attempt to enhance physical activity, and 

therefore overall health, by addressing social and physical 

environments relative to community layout, opportunity, and 

policy (Sallis and others, 2006). The various cities in the LRGV 

are rapidly becoming a unified metropolis, stretching nearly the 

entire length of the LRGV. Fortunately, many open spaces still 

remain in the LRGV, and their enhanced use by local people 

could improve physical and mental health (e.g., Bratman and 

others, 2015) and foster an appreciation of conservation goals 

of the various city, county, State, and Federal agencies (U.S. 

Fish and Wildlife Service, 2016c). The challenge will be to 

vigorously engage citizens of the LRGV when many people are 

detached from the natural world and more inclined to focus on 

sedentary activities that do not foster good health (Shanahan 

and others, 2015). 

Agriculture

Changes in Crop Production

Crop production is still very important to the economy of 

the LRGV, enhanced by the long growing season of more than 

300 frost-free days and still relatively rich deltaic soils (Stubbs 

and others, 2003). As it was in the late 1980s (Jahrsdoerfer and 

Leslie, 1988), Hidalgo County ranks first among Texas counties 

for 90 percent of its cash receipts, totaling $314.3 million per 

year, coming from crops, primarily sugarcane, grain, produce 

(e.g., cantaloupes, onions, broccoli, and tomatoes), and citrus; 

Cameron County ranks second in sugarcane production and 

generates about $112 million per year; and Starr and Willacy 

Counties generate considerably less agricultural revenue at 

$64.4 million per year and $51.2 million per year, respectively 

©Larry Ditto Nature Photography ©Larry Ditto Nature Photography

©Larry Ditto Nature Photography
Ongoing Challenges Facing the LRGV 53

(Cruce Alvarez and Plocheck, 2012). Most of these crops 

require irrigation to be profitable in the seasonally dry climate 

of the LRGV, although some dryland crops such as grain 

sorghum are also produced. 

Since the late 1980s, the total area of the LRGV in crop 

production decreased as urbanization increased, and the 

importance of various crops has changed (Ellard and Patrick, 

1988; Cruce Alvarez and Plocheck, 2012). Severe freezes in 

1983 and 1989 destroyed thousands of hectares of citrus trees 

and started the trend of decreasing agriculture, associated 

with expanding urbanization (Stubbs and others, 2003). The 

recent arrival of “citrus greening” carried by the Asian citrus 

psyllid Diaphorina citri, capable of causing rapid die-off of 

trees (French and others, 2001), has resulted in quarantines 

in expanding areas of the LRGV, reduced production, 

and acreages removed from citrus production (www.

texascitrusgreening.org/psyllid-control-treatments). 

From 1993 to 2003, irrigated land generally decreased 

in Cameron, Willacy, and Hidalgo Counties (Huang and 

Fipps, 2006). In Cameron and Hidalgo Counties, there 

were more than 142,854 ha (353,000 acres) of irrigated 

cropland in 1997, which decreased 16 percent or 22,258 ha 

(55,000 acres) by 2007. Concomitantly, farms and ranches 

in Cameron and Hidalgo Counties decreased by 35,622 ha 

(88,025 acres), and only 2–3 percent of Willacy and Starr 

Counties were irrigated—both counties best known for 

livestock production (Wilkins and others, 2009). Along with 

losses of native habitats to agricultural activities, considerable 

escalation of the use of complex chemicals and biological 

agents to fight agricultural pests and diseases occurred from 

the 1940s onward, resulting in growing concern about and 

documentation of the toxicity of many of these chemicals 

to the biota, including humans, of the LRGV (Tiefenbacher, 

2001). 

Pesticides, Herbicides, and

Associated Contamination

Research on effects of insecticides, herbicides, and 

fungicides to nontarget biota in the LRGV was just beginning 

in the 1980s. With the creation of the U.S. Environmental 

Protection Agency (EPA) in 1970, some of most harmful 

pesticides, particularly those that were organochlorine based, 

were banned early (e.g., dichlorodiphenyltrichloroethane 

[DDT], dieldrin, mirex, alrin, and endrin). Of the 98 

“commonly used” pesticides in the LRGV up until the late 

1980s (table 4 in Jahrsdoerfer and Leslie, 1988), many have 

been banned since or registered with restricted use by the 

EPA (e.g., application by certified/professional applicators, 

or designated use on only particular crops and areas). Six of 

the 35 organophosphate insecticides listed by Jahrsdoerfer 

and Leslie (1988) are now banned by the EPA, and 6 

have restricted use permits; 2 of the 8 n-methyl carbonate 

insecticides are banned, and 4 are in restricted use; 14 of the 

18 organochlorine insecticides―a particularly problematic 

class of chemicals relative to lethality and potential to 

bioaccumulate in nontarget organisms―are banned, and 2 

have restricted use; 1 of 25 herbicides is banned, and 2 have 

restricted use; and none of the 12 fungicides are banned or 

have restricted use, but 2 are no longer marketed. 

Impacts of many contaminants can be evident for 

decades (e.g., Mora and Wainwright, 1998), and despite 

improvement of regulations for pesticide use in the United 

States and banning those most harmful, deleterious effects 

of early use of some pesticides have been documented in the 

LRGV (Papoulias and Parcher, 2013; table 9). Origins of these 

contaminants were largely agricultural, and the highest levels 

were typically found in aquatic systems from agricultural 

runoff. Fish in the lower Rio Grande can still contain high 

levels of contaminants (e.g., DDT metabolites, chlordanerelated compounds, dieldrin, and toxaphene); maximum 

concentrations were evident in Ictalurus punctatus (channel 

catfish) from near Mission and Brownsville, Tex. (Schmitt 

and others, 2005). Micropterus salmoides (largemouth bass) 

and common carp in the lower Rio Grande had burdens and 

reproductive impairments consistent with chronic exposure to 

contaminants (Schmitt and others, 2005).

Some research on avian species since the late 1980s 

suggests that effects of past pesticide contamination in the 

LRGV have abated. Levels of 57 contaminants, including 

trace elements, organochlorines, and hydrocarbons, were 

generally in background levels in redheads from the Upper 

and Lower Laguna Madre (Michot and others, 1994); 

dichlorodiphenyldichloroethylene (DDE) declined in eggs 

of Rynchops niger (black skimmers) from 1979 to 1984 near 

Laguna Vista in south Lower Laguna Madre (Custer and 

Mitchell, 1987); and significant declines from the 1970s and 

1980s were reported in levels of organochlorines (e.g., DDE 

and polychlorinated biphenyls [PCBs]) and trace elements in 

nesting colonial waterbirds (Hydroprogne caspia [Caspian 

terns], Ardea herodias [great blue herons], Egretta thula 

[snowy egrets], and Egretta tricolor [tricolored herons]) in the 

Lower Laguna Madre (Mora, 1996a, 1996b). Similar results 

of low to background levels of contaminants have been noted 

for Plegadis chihi (white-faced ibis—Custer and Mitchell, 

1989), and white-winged doves collected at eight sites in the 

LRGV during summer 2003 had only background levels of 

potentially harmful elements (e.g., arsenic, chromium, and 

lead) and organochlorines (e.g., DDE, dieldrin, and chlordane) 

known to impair survival and reproduction in birds (Fredricks 

and others, 2009). 

DDE decreased about 66 percent in adult and subadult 

peregrine falcons migrating through the LRGV from 1978 

(1.0 microgram per gram [μg/g] wet weight) to 1994 

(0.34 μg/g); no other organochlorine pesticides were detected 

in 1994 but were routinely detected prior to that (Henny and 

others, 1996). Addled eggs of northern aplomado falcons in 

the LRGV have been assessed for contaminants for many 

years. Although organochlorine byproducts such as DDE have 

been variously reported in addled eggs of northern aplomado 

falcons, they generally have been detected in low background 
54 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Table 9. Research in the Lower Rio Grande Valley and nearby Mexico since the late 1980s indicating elevated and (or) higher-thanthreshold levels of contaminants known to cause deleterious effects to organisms.

[DDE, dichlorodiphenyldichloroethylene; PCB, polychlorinated biphenyl]

Location or organism studied Contaminant References

Abiotic

Sediments in Laguna Madre

Heavy metals: cadmium and lead Sharma and others (1999)

Petroleum hydrocarbons (benzene,

methyl and dimethyl naphthalene)

Sharma and others (1997)

Soil contamination from oil and gas

on Padre Island

Heavy metals: barium, chromium,

lead, and zinc

Carls and others (1995)

Petroleum hydrocarbons Carls and others (1995)

Biotic–avian

Butorides virescens (green heron) eggs Organochlorine: DDE and Toxaphenea Wainwright and others (2001)

Falco femoralis septentrionalis (northern aplomado 

falcon) eggs (United States and Mexico)

Organochlorine: DDE and PCB Mora and others (1997, 2008)

Heavy metals: mercury Mora and others (1997, 2008)

Northern aplomado falcon prey

Organochlorine: DDE (Quiscalus mexicanus

[great-tailed grackle])

Mora and others (1997)

Heavy metals: mercury (Sturnella magna [eastern 

meadowlark])

Mora and others (1997)

Petrochelidon fulva (cave swallow) and 

Petrochelidon pyrrhonota (cliff swallow) liver 

and muscle

Organochlorine: Toxaphenea Maruya and others (2005)

Organochlorine: DDE and Toxaphenea Mora and others (2005, 2006)

Tringa semipalmata (willet) liver Arsenic Custer and Mitchell (1991)

Overview/synthesis Heavy metals: mercury Mora and Wainwright (1998)

Trace elements: selenium Mora and Wainwright (1998)

Biotic–aquatic

Carp in JAS Farm Lake (Cameron County) Organochlorine: DDE Wainwright and others (2001)

Fish in lower Rio Grande Basin, particularly 

Micropterus salmoides (largemouth bass) and 

Cyprinus carpio (common carp)

Chlordane related Schmitt and others (2005)

Organochlorine: DDE, Dieldrin,b

 and Toxaphenea Schmitt and others (2005)

Fish in resacas (United States and Mexico)

Arsenic Mora and others (2001)

Organochlorine: DDE Mora and others (2001)

Shrimp and Lagodon rhomboids (pinfish) Heavy metals: nickel and chromium Custer and Mitchell (1993)

Overview/synthesis Heavy metals: mercury Mora and Wainwright (1998)

Trace elements: selenium Mora and Wainwright (1998)

a

Banned for most uses in the United States in 1982 and all uses in 1990 and globally by the Stockholm Convention on Persistent Organic Pollutants in 2001.

b

Banned for all uses in the United States in 1987.

levels beginning in the early 1990s (Mora and others, 1997, 

2008), and similar detections were found more recently in 

neighboring Chihuahua and Veracruz, Mexico

(Mora and others, 2011). Troubling, however, is that these 

studies suggest that Hg is elevated in addled eggs (Mora and 

others, 2008), which may be traceable to the falcon’s prey 

(Mora and others, 1997; table 9). It has been recommended that 

Hg levels be continually monitored in addled eggs of northern 

aplomado falcons in the LRGV (Mora and others, 2008).

Primary research on invertebrates in the LRGV has been 

largely focused on insect pests, many of them lepidopterans. 

Most research has been ecological rather than pesticide based 

and has focused on lepidopteran cabbage pests (Cartwright 

and others, 1987); citrus pests such as Gonodonta nutrix

(citrus leafminer) (Legaspi, French, and others, 1999a); potato 

pests such as sharpshooters and leafhoppers (both family 

Cicadellidae) and jumping plant lice (family Psyllidae—

Goolsby, Bextine, and others, 2007; Munyaneza and others, 
Ongoing Challenges Facing the LRGV 55

2007); basic research and parasitoid control of Bemisia spp. 

(silverleaf whiteflies—Norman and Sparks, 1997; Riley 

and Ciomperlik, 1997; Liu, 2000; Goolsby and Ciomperlik, 

2008); cotton pests such as Anthonomus grandis (boll 

weevils—Summy and others, 1992; Showler and Cantú, 

2005; Greenberg and others, 2007), Euschistus spp. (stink 

bugs—Hopkins and others, 2009), and Spodoptera exigua

(beet armyworms—Pair and others, 1991, 1995; Summy 

and others, 1996); corn pests such as Helicoverpa zea (corn 

earworms—Raulston and others, 1990, 1992; Lingren and 

others, 1994; Pair and others, 1995) and fall armyworms 

(Raulston and others, 1992; Wolf and others, 1995); sugarcane 

pests and their parasites such as invasive pyralid stem borers 

(Youm and others, 1990; Legaspi and others, 1997, 2000; 

Legaspi, Legaspi, and others, 1999; Meagher and others, 

1998; Sétamou and others, 2002) and the sugarcane tingid 

Leptodictya tabida (Sétamou and others, 2005); and general 

control of pests such as aphids by treating cotton with kaolin 

(Showler and Sétamou, 2004). Some research has focused on 

biological control of pests with exotic and potentially invasive 

parasitic organisms (e.g., Cartwright and others, 1987; Summy 

and others, 1992; Legaspi and others, 1997; Meagher and 

others, 1998; Goolsby and Ciomperlik, 2008).

There has been limited research on effects of current 

pesticide use on the LRGV’s biota. Some life stages of these 

insects might be important to birds, reptiles, and amphibians 

(e.g., Parker and Goldstein, 2004), and those that feed in 

agricultural areas may be harmed by chemical control of pests 

(e.g., Corson and others, 1998). Therefore, USFWS places 

significant importance on minimizing effects of pesticides 

on pollinators by avoiding microencapsulated pesticides 

similar in size to pollen, reducing drift from applications by 

encouraging ground versus aerial equipment, and minimizing 

the time pesticides remain on plant parts, which can reduce 

effects on nontarget insects (USFWS, 2016b). The USFWS 

also cooperates with 26 other agencies and organizations 

to improve conservation of Danaus plexippus (monarch 

butterflies), which involves reducing pesticide exposure 

(Monarch Joint Venture, 2015). 

Under current Federal law, chemical companies that want 

to market a new pesticide for use in the United States must 

register it (and reregister it for any new use) with the EPA and 

provide assurances that it poses no risk to human health and 

no unreasonable risks to the environment (U.S. Environmental 

Protection Agency, 2016a). The EPA also encourages 

producers to develop Integrated Pest Management (IPM) 

plans to minimize damage to crops while reducing possible 

pesticide hazards to people, property, and the environment 

(U.S. Environmental Protection Agency, 2016b). Various IPMs 

dating back to the 1960s were implemented in the LRGV, 

now often resulting in reduced need for repeated pesticide 

applications on crops such as cotton (Castro and others, 

2007; Greenberg and others, 2012), cabbage (Cartwright and 

others, 1987), and potatoes (Goolsby, Adamczyk, and others, 

2007). The USFWS’s Pesticide Use Proposal System requires 

regional or national oversight to approve use of various 

chemicals on its land, and the Lower Rio Grande Valley NWR 

authorizes and closely manages pesticide and herbicide use 

on its land to ensure minimal harm to desirable plants and 

animals. 

Despite positive progress to minimize pernicious 

effects of pest management, continued vigilance is needed 

to conserve all aspects of the LRGV’s biotic richness. Tacha 

and others (1994) found chronic and widespread exposure 

of white-winged doves to anticholinesterase compounds 

(many of them banned now) in the LRGV, but the effects of 

long-term, sublethal exposure to pesticides are still largely 

unknown. Relatively recent comprehensive profiles of 20 

organochlorines, 21 polychlorinated biphenyl congeners, 

and toxaphene in tissue of white-winged doves showed 

that exposure in the LRGV does not impair reproduction 

or survival (Fredricks and others, 2009). In sharp contrast, 

neonicotinoid insecticides, registered and approved for use 

in more than 120 countries including the United States, have 

recently been implicated in colony collapse disorder of honey 

bees (Lu and others, 2014; Rundlöf and others, 2015); in 2014, 

the USFWS became the first Federal agency to announce plans 

to phase out the use of all neonicotinoids on its properties.

Continued Agricultural Pressure on Native 

Habitats and Water Resources

European explorers and settlers in the LRGV introduced 

domestic grazing animals as early as the 1700s. Organized 

ranches soon followed, and by the Mexican-American War in 

the mid-19th century, there was intense grazing pressure from 

free-ranging cattle across much of the LRGV (Thompson, 

1997). After the American Civil War, thousands of veterans 

and others came to the LRGV, in part to round up free-ranging 

cattle and drive them north to waiting railroads where they 

were shipped to growing urban centers elsewhere in the 

United States or used to stock significant parts of the American 

West. This “cowboy model” remains one of the most enduring 

symbols of the American “Wild West” and owes much of 

its genesis and early history to what occurred in the LRGV 

(Thompson, 1997). The importance of livestock is not what it 

once was, perhaps related to quarantines imposed to control 

tick fever (see “Exotic Mammals” section). Nevertheless, 

between 1997 and 2007 in Cameron and Hidalgo Counties, 

the area of nonnative pasture for livestock—some of which 

represents plantings of exotic grass (K. Wahl, pers. commun.) 

at the expense of native grassland—increased 31.4 percent 

from 31,270 ha (77,270 acre) to 41,089 ha (101,534 acres) 

(Wilkins and others, 2009).

Most remnant patches of native habitat in the LRGV 

that existed in the late 1980s were protected by NWRs, State 

parks, and private conservation organizations (e.g., the Nature 

Conservancy), and they largely remain protected as of early 

2016. Efforts to acquire more intact native habitats and restore 

former agricultural lands in the Lower Rio Grande Valley 

NWR have met with some success. Nevertheless, despite 
56 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

tax incentives for private landowners in Texas to manage 

their land to benefit wildlife, no land had been officially 

designated for tax appraisal purposes in the LRGV up until 

2007, perhaps a reflection of greater value when land is in 

agricultural production or sold outright (Wilkins and others, 

2009). Average value of “farms, ranches, and forestland” in 

Texas in 2007 ($1,196 per acre) was about 2.4 times lower 

than its average value in the LRGV ($2,900 per acre) (Wilkins 

and others, 2009, p. 2). Positive progress notwithstanding, 

native habitats in the LRGV, particularly in the riparian 

corridor, remain small and fragmented—even more so with 

construction and completion of the border fence (see “Border 

Issues and Homeland Security” section). Thus, continued 

pressure from agricultural use in the LRGV, along with 

expanding urban areas, makes it important to acquire, protect, 

and restore native habitat before it becomes increasingly 

unsuitable or, worse, unavailable.

Flood Control and Water Development

The Rio Grande is the fifth largest river in North America 

and the longest border river between two countries in the 

world (Stubbs and others, 2003; Mathis, 2005; Updike and 

others, 2013). The LRGV has the smallest major watershed 

(26,522 km2) along the United States-Mexico border, with 

about 60 percent (15,942 km2) of the watershed draining from 

Mexico and about 40 percent (10,580 km2) from the United 

States (Parcher and others, 2013). Regular flood-pulse cycles 

of large river systems such as the Rio Grande are an essential 

form of natural disturbance, providing regular selection 

pressure on organisms living with them (e.g., Bayley, 1995; 

Molles and others, 1998; Small and others, 2009). When 

a flood-pulse cycle is disrupted, it can decrease plant and 

animal richness (Mathis and others, 2004; Small and others, 

2009). Maintenance and restoration of the flood-pulse cycles 

in these waterways are critically important to conservation 

of ecological function, quality, and biodiversity (e.g., Richter 

and Richter, 2000; Small and others, 2009). Much of the 

flood-pulse cycling has been lost in the LRGV, rendering 

some habitat restoration projects difficult or impossible to 

accomplish without some type of prescribed flooding (Richter 

and Richter, 2000; Small and others, 2009).

Flood-control Activities

Prior to large-scale water-control efforts in the late 

1930s, the Rio Grande overflowed its banks and flooded the 

LRGV on at least 23 separate occasions, often associated with 

hurricanes and significant loss of life and property (Thompson, 

1997; Tiefenbacher, 2001; Stubbs and others, 2003). As a 

result, the International Boundary and Water Commission 

(IBWC), established in 1889 in cooperation with Mexico, 

initiated numerous flood control projects on the lower Rio 

Grande from Peñitas in Hidalgo County to the Gulf of Mexico; 

by 1950, the IBWC had completed 75 percent of planned 

projects that included 233 km (145 mi) of floodways (e.g., 

Main and North Floodways) and 459 km (285 mi) of levees 

along the river (Tiefenbacher, 2001; Stubbs and others, 2003). 

Falcon Dam was constructed in 1953–54 and is the 

largest dam on the lower Rio Grande; however, it was not 

the last dam constructed to control the Rio Grande but did 

provide dependable water for irrigation (fig. 19). In 1960, 

the Anzalduas Dam was completed downriver from Falcon 

Dam; it was designed to divert floodwaters into floodways on 

the United States side of the river and into irrigation canals 

on the Mexican side. The third major dam, Retamal Dam, 

was constructed in the 1970s downriver from Anzalduas 

Dam; it further diverted floodwaters into floodways on the 

Mexican side of the river. Numerous pumping stations have 

been built along the river between Anzalduas Dam and 

Brownsville, Tex., to take water out of the Rio Grande for 

irrigation and municipal and (or) industrial uses. Numerous 

weirs have been constructed below Retamal Dam to raise 

water levels for pumping into canals for irrigation purposes 

along the lower reaches of the Rio Grande. Currently, the 

IBWC manages Anzalduas and Retamal Dams, 500 irrigation 

and drainage structures, and 435 km (270 mi) of levees, 

among other projects (International Boundary and Water 

Commission, 2016).

As immigrants continued to settle in the LRGV 

throughout the 1900s, new and varied forms of agriculture and 

crops were introduced (table 10.1 in Tiefenbacher, 2001). As 

agrotechnology advanced, waterways were manipulated more 

and more for flood control, agriculture, industry, and drinking 

water. Apart from IBWC, various irrigation districts were 

set up and managed separate irrigation systems consisting 

of massive pump stations along the Rio Grande itself and 

ditches, canals, and weirs to transport water to rapidly growing 

needs of farms and towns of the LRGV (Stubbs and others, 

2003). The Arroyo Colorado, the primary distributary of 

the lower Rio Grande in the United States, was dredged and 

altered to satisfy demands of irrigation, navigation, and flood 

control (fig. 19); it no longer connects to the Rio Grande 

and is completely diverted into Laguna Madre (Small and 

others, 2009).

Irrigation and Water Rights

A landmark 1904 amendment to the Texas Constitution 

allowed public development of the State’s water resources. 

After that, farmers bought out private irrigation companies and 

established the first public irrigation districts (Tiefenbacher, 

2001; Stubbs and others, 2003). The districts were organized 

to provide “irrigation, drainage, flood control, and wholesale 

water and untreated water supply” (Stubbs and others, 2003, p. 

15). Initially, irrigation districts encompassed all or part of one 

county, including cities and towns, but eventually cities and 

towns were not included in the irrigation-district system unless 

requested. There are now 29 irrigation districts in Cameron 

and Hidalgo Counties that supply all irrigation and municipal 

water needs in the LRGV (Stubbs and others, 2003).
laf16-0737_fig19

TEXAS

FreshwaterIntermediate salineMarine

EXPLANATION

0 10 20 40 MILES

N 0 10 20 40 KILOMETERS

STARR

COUNTY

HIDALGO

COUNTY

CAMERON

COUNTY

WILLACY

COUNTY

Falcon Reservoir

OTHA HOLLAND CANAL/

WILDLIFE CORRIDOR

NORTH FLOODWAY

MAIN FLOODWAY

FALCON DAM

ANZALDUAS DAM

RETAMAL DAM

ARROYO COLORADO

Rio Grande

Base map image is the intellectual property of Esriand is used herein under license. Copyright © 2014 Esriand its licensors. All rights reserved.Water features and county borders from Texas Natural Resource Information System 

Figure 19. The three major dams, water-control canals, and the Arroyo Colorado in the Lower Rio Grande Valley (LRGV) that negatively affect remnarestoration efforts along the LRGV by altering, and even eliminating, natural flow and flooding regimes.
58 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

For much of the 19th century, Texas had a riparian 

water rights system that simply gave rights to those whose 

land abutted surface waters (Stubbs and others, 2003). After 

1895, the State used a dual system by which owners of land 

purchased before 1895 maintained their riparian water rights. 

With the passage of 1895 Irrigation Act, land purchased 

after 1895 fell under State appropriation rights in which 

individuals had to file water claims and obtain permits from 

the State to divert water. Essentially, this created a first-come, 

first-served system because overappropriation of water hurt 

those who filed late for particular water rights (Stubbs and 

others, 2003). In the 1950s, the courts decided that the old 

“riparian water rights,” granted by the Spanish Crown or 

Mexico, were invalid. Additionally, water use from a shared 

borderland resource such as the Rio Grande involves complex 

international cooperation (e.g., Patiño-Gomez and others, 

2007).

Today, water in Texas falls under a State licensing 

system that resulted from the 1969 lawsuit State of Texas v. 

Hidalgo County Water Control and Improvement District 

No. 18, often called the Lower Rio Grande Valley Water 

Suit, which established the water-rights system now used 

in the LRGV (Stubbs and others, 2003). Under this system, 

“Domestic, Municipal, and Industrial rights have the highest 

priority in the allocation procedures, with irrigation rights 

holding a residual claim on inflows to the reservoirs” 

(Characklis and others, 1999; Stubbs and others, 2003, p. 17; 

Levine, 2007), and water can be marketed by willing sellers 

(e.g., Schoolmaster, 1991; Wurbs, 1995). There are two types 

of irrigation rights: Class A and Class B. Class A irrigation 

rights are given to those that had proven water rights under 

the provisions of an old system, and Class B rights are given 

to those who can prove a history of diversion from the Rio 

Grande (Stubbs and others, 2003). During dry years, Class A 

water rights receive a greater allocation of water than those 

provided under Class B rights (Stubbs and others, 2003). The 

Lower Rio Grande Valley NWR has Class A and B water 

rights, but if water is scarce, allocations may be curtailed, 

making water shortages an issue for conservation of habitats 

such as rare flood-plain riparian forests. Although the legal 

settlement in 1969 attempted to fix problems with water 

allocation, water in the LRGV is still often overallocated, 

which can create conflicts among users (Stubbs and others, 

2003) and even an uncertain international future as human 

populations and needs grow (Schmandt, 2002).

Detrimental Effects of Water Development

on LRGV Biota

Modifications along the lower Rio Grande have vastly 

changed its flow and thereby altered native plant communities. 

The naturally occurring high- and low-flood periods that once 

characterized the lower Rio Grande have been eliminated or 

greatly reduced in frequency and duration, which has resulted 

in severe modification of its riparian corridor (Small and 

others, 2009). From the time Falcon Dam was completed 

in 1954 through 2004, mean annual flow of the Rio Grande 

past Brownsville, Tex., decreased 75 percent from 105.3 

cubic meters per second (m3/s) before completion of the dam 

to 25.77 m3/s; similar declines were noted for daily event 

frequency: high-flow pulse, 28.3 percent before vs. 10.4 

percent after; small flood, 14.5 percent before vs. zero after; 

and large flood, 4.9 percent before vs. zero after (Small and 

others, 2009). Major floods still occur and overtop the bank 

of the lower Rio Grande―sometimes with considerable 

ecological and long-term damage. For example, major 

flooding resulted when ongoing storage of water at Falcon 

Dam (at or near capacity) coincided with Hurricane Alex, 

causing the IBWC to release massive amounts of water from 

July through November 2010. Water breeched many levees, 

leaving Santa Ana NWR and 6,819 ha (16,850 acres) of river 

tracts in the Lower Rio Grande Valley NWR impounded 

with water for 5–8 months. Riparian vegetation that has 

evolved with flood-pulse cycles is not generally adapted to 

long periods of standing water; many trees died, permitting 

significant invasion of exotic saltcedar (see “Exotic and 

Invasive Species” section below).

Mature bottomland riparian forests of the lower Rio 

Grande Delta are adapted to varying degrees of high- and 

low-water flow (Small and others, 2009). With the loss of 

this naturally occurring flood-pulse cycle, mature bottomland 

forests are no longer an easily sustainable or restorable 

habitat type in the LRGV. Even in Santa Ana NWR, which 

has been protected for decades, mature bottomland forests 

composed of Texas ebony, Mexican ash, cedar elm, and 

sugarberry are changing from riparian bottomlands that 

evolved with regular sheet flooding to habitats able to 

tolerate extended dry periods (Small and others, 2009). 

Comparisons of studies conducted at Santa Ana NWR during 

the 1970s (Gehlbach, 1987) and 1990s (Brush and Cantu, 

1998) show how remarkably disparate the forest types in the 

riparian zone became after just 30 years (Small and others, 

2009). In the 1970s, a mature closed-canopy forest existed, 

with large-trunked trees and low densities (Gehlbach, 1987). 

Die-offs stemming from loss of the natural flood-pulse cycle 

left a vastly different forest by the 1990s, with a broken, 

not closed, canopy; canopy height decreased by 10 m 

(33 ft) in the riparian zone, stem densities of trees increased 

by 250 percent since the 1970s, and trunk diameters of 

individual trees decreased substantially (Brush and Cantu, 

1998; Small and others, 2009). 

“Riparian-dependent birds have been negatively 

impacted by habitat loss and severe deterioration of this 

habitat, over the past half century” (Rupert and Brush, 2006, 

p. 48), directly associated with the construction of Falcon 

Dam and other water-control activities that have severely 

reduced flooding along the Rio Grande (Small and others, 

2009). The tall and mature riparian forests that once lined 

the Rio Grande and adjacent waterways no longer occur or 

have suffered severe die-offs because of the lack of flooding 

(Brush, 2005; Rupert and Brush, 2006). Riparian-dependent 
Ongoing Challenges Facing the LRGV 59

bird species that were reported as common in the 1870s 

either no longer occur in the LRGV or have become very 

rare (Sennett, 1879; Brush, 2005; Rupert and Brush, 2006). 

Some species like the Geothlypis poliocephala (graycrowned yellowthroat) and Icteria virens (yellow-breasted 

chat), for example, have almost completely disappeared as 

breeding species in the LRGV (Brush, 2005; Rupert and 

Brush, 2006). Other once-abundant riparian species like 

the Audubon’s oriole, red-billed pigeon, and tropical parula 

have become very rare and only occur in scattered locales 

where some mature riparian forests remain; these species can 

still be seen upriver near Falcon Dam where some elements 

of the historical flood-pulse cycle still occur (Brush, 2005; 

Rupert and Brush, 2006). 

Although missing some of the mature riparian-forestobligate avian species mentioned above, even depauperate 

riparian forest in tracts of the Lower Rio Grande Valley 

NWR can support higher avian richness and nesting densities 

than comparable upland tracts (Rupert and Brush, 2006). 

Unfortunately, the prevailing trend on many tracts in the 

Lower Rio Grande Valley NWR is that without restoration of 

some flooding, riparian forests will continue to become thorn 

forests that lack tall canopy trees (Brush and Cantu, 1998; 

Rupert and Brush, 2006). If this trend continues, there will 

be continued loss of riparian-dependent bird species in the 

LRGV. Some evidence suggests that large and wide riparian 

corridors can support more bird species at higher densities, 

raising the possibility that some loss from the drying of 

riparian areas can be offset if more land can be used to widen 

and expand riparian corridors (Rupert and Brush, 2006; 

Brush and Feria, 2015).

Native freshwater fish communities in the lowest 

reaches of the Rio Grande suffer from detrimental effects 

of flow alterations, increasing estuarine conditions, 

contaminants from industry and agriculture, and introduction 

of exotic, often invasive, fish and plant species (ContrerasBalderas and Lozano-Vilano, 1994; Contreras-Balderas and 

others, 2002; Tunnell, 2002d; Small and others, 2009; Martin 

and others, 2010). The lower Rio Grande and its delta have 

changed substantially in the past 50–60 years (e.g., Cooper 

and Wagner, 2013) to the point that unless flows are very 

high after heavy rain, little (if any) freshwater reaches the 

delta, being held by Amistad and Falcon Reservoirs and 

used for agriculture, municipalities, and industry in the 

LRGV (Small and others, 2009). The broad agricultural and 

urbanized deltaic plain is mostly drained into the Lower 

Laguna Madre by an artificial network that delivers water to 

the channelized, dredged, and variously impounded (small 

dams/diversions) Arroyo Colorado and North Floodway in 

Texas and South Floodway in Tamaulipas, Mexico (Tunnell, 

2002a, 2002c; fig. 19). Under current low-flow conditions, 

and aside from limited rainwater runoff, little freshwater 

now reaches the lower Rio Grande, resulting in a negative 

freshwater inflow into the Lower Laguna Madre of -93 

× 106 cubic meters per year (Tolan, 2013) and upstream 

advancement of estuarine conditions. The delta was formerly 

shallow water of the southern Lower Laguna Madre in Texas 

and the northern Laguna Madre de Tamaulipas in Mexico, 

but it is now a 1,550-km2 bulge of “meander streams, oxbow 

lakes (resacas), playa lakes, wetlands, mudflats, lomas, and 

clay dunes” that effectively separates the two international 

lagoons (Tunnell, 2002b, p. 31).

If the long-gone, flood-pulse cycles of the Rio Grande 

cannot be mimicked by returning some aspects of the 

seasonal hydrologic regime (e.g., timing, duration, and 

extent), management activities such as land acquisition 

efforts of the Lower Rio Grande Valley NWR could have 

little effect on maintaining riparian-dependent wildlife 

diversity because deterioration of mature riparian forests will 

continue (Small and others, 2009). Although returning even 

some level of the flood-pulse cycle is a difficult prospect, 

especially considering the wide and varied stakeholders 

involved in the LRGV (including two different countries 

with different priorities), other major river systems, including 

the Colorado River system in Arizona, have achieved some 

level of success in this endeavor (e.g., U.S. Department of 

Interior, Bureau of Reclamation, 2001; Small and others, 

2009).

Exotic and Invasive Species

Natural barriers (e.g., oceans and mountain ranges) and 

species-specific capabilities typically impede the spread of 

species beyond areas where they evolved, but humans have 

altered that. Exotic (or nonnative), invasive species—those 

that seriously affect populations of native species—threaten 

biodiversity, economic development, and human health 

(Lowe and others, 2004; Pimentel and others, 2005). Native 

species are often outcompeted or killed by invasive species; 

in fact, competition with invasive species is often cited as a 

principal reason for declines of many species now considered 

threatened and endangered, second only to habitat loss 

(Lowe and others, 2004; Pimentel and others, 2005). About 

42 percent of threatened and endangered species in the 

United States are imperiled in some way by invasive species, 

and closer to 80 percent are imperiled elsewhere in the world 

(Pimentel and others, 2005). 

All native aquatic and terrestrial habitats in the LRGV 

are affected to some degree by exotic species (e.g., Williams 

and Baruch, 2000; Smith, 2010; Mendoza and others, 2011). 

Exotic plant species, in particular, pose serious threats to 

the ecological integrity of biotic communities in the LRGV. 

Twelve grass species, 11 of which are exotic and 3 of which 

are priority target species for control, and 6 exotic woody 

plant species, 4 of which are priority target species, are 

variously invasive in the LRGV (table 10). Nine percent of 

the “100 of the World’s Worst Invasive Alien Species” as 

listed by the Invasive Species Specialist Group of the World 

Conservation Union occur in the LRGV: 1 aquatic plant, 4 

land plants, 2 fish, and 2 mammals (Lowe and others, 2004).
60 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Table 10. Exotic and native invasive terrestrial grasses and woody 

plant species in the South Texas Refuge Complex of the Lower Rio 

Grande Valley.

Grass species Origin

Arundo donax (giant reed)a Mediterranean 

Bothriochloa ischaemum var.

ischaemum (King Ranch 

bluestem)

Europe/Asia

Cenchrus ciliaris (buffelgrass)b dry eastern–southeastern 

Africa

Cynodon dactylon (Bermuda grass) Asia (probable)

Dichanthium annulatum

(Kleberg bluestem)b

Africa/Asia

Dichanthium aristatum (Angelton 

bluestem)

India

Echinochloa crus-galli

(barnyard grass)

tropical Asia

Megathyrsus maximus

(guineagrass)b

tropical/subtropical Africa

Melinis repens

(natal grass)

tropical/subtropical Africa

Panicum antidotale

(blue panicum)

Himalayas

Sorghum halepense

(Johnson grass)

Mediterranean northern 

Africa

Urochloa platyphylla

(broadleaf signalgrass)

southeastern–southern 

United States

Woody plant species Origin

Leucaena leucocephala

(lead tree)

southern Mexico/Central 

America

Melia azedarach (chinaberry)b southeastern Asia/Australia

Prosopis glandulosa var. 

glandulosa

(honey mesquite)a

southwestern United States/

Mexico

Schinus terebinthifolius

(Brazilian pepper)a,b

tropical/subtropical South 

America

Tamarix aphylla

(athel tamarisk)

eastern–central Africa to 

western–southern Asia

Tamarix ramosissima (saltcedar)a,b Europe/Asia

Triadica sebifera

(Chinese tallow)b

eastern Asia

Aquatic plant species Origin

Eichhornia crassipes

(water hyacinth)a

South America

Hydrilla verticillata (hydrilla) southeastern Asia

a

Listed among the 100 worst invasive species globally (Lowe and others, 2004).

b

Priority target species for control/eradication in the South Texas Refuge Complex.

Plants

Water Hyacinth

Eichhornia crassipes (water hyacinth) is an aquatic 

plant native to South America (Lowe and others, 2004). Its 

large purple flowers make it popular in ornamental plantings, 

and as such, it is now found in more than 50 countries on 

five continents (Lowe and others, 2004). Water hyacinth can 

become invasive very quickly, doubling its biomass in as little 

as 5 days. It is detrimental to native plants and wildlife and 

forms vast mats covering once open waterways, rendering 

them unfit for boating, swimming, and fishing. Mats of water 

hyacinth prevent light and oxygen from diffusing into the 

water column, which can cause die-offs in native plants and 

animals (Lowe and others, 2004). Water hyacinth occurs in the 

LRGV, with some attempts at biological control (Grodowitz 

and others, 2000), but currently not in the STRC.

Hydrilla

Hydrilla verticillata (hydrilla), native to Southeast 

Asia, is an aquatic plant often sold for use in aquaria and 

first discovered in the wild in the United States in Florida in 

the 1960s. It is a major threat to native aquatic ecosystems 

(Owens and others, 2005) because it grows rapidly, blocks 

sunlight in the water column, depletes oxygen, and causes 

massive die-offs of native plant and animal species. 

Waterways choked with hydrilla are not suitable for fishing, 

boating, and swimming and can cause mechanical and systemmaintenance problems during water treatment, hydroelectric 

power generation, and water delivery (Grodowitz and others, 

2000; Owens and others, 2005; Douglas, 2009). It was first 

reported in the LRGV in 1990 near Brownsville, Tex., and 

now occurs beyond Falcon Reservoir (Owens and others, 

2005), with some attempts at biological control (Grodowitz 

and others, 2000), but not in the STRC. 

Giant Reed

Arundo donax (giant reed) is a large invasive grass that 

occurs in dense, extensive stands up to 6 m (12 ft) high along 

the lower Rio Grande (fig. 20A), including the LRGV (Owens 

and others, 2005; Seawright, Rister, Lacewell, McCorkle, 

and others, 2009; Seawright, Rister, Lacewell, Sturdivant, 

and others, 2009). Giant reed displaces native plant species, 

forms dense monocultures in riparian areas and other wetland 

habitats, and transpires large amounts of water (Owens and 

others, 2005; Seawright, Rister, Lacewell, McCorkle, and 

others, 2009). Current research in the LRGV is exploring 

possible biological control agents (e.g., Tetramesa romana 
Ongoing Challenges Facing the LRGV 61

A

B

C

D

E

Figure 20. Invasive and (or) nonnative plant and animal species that challenge conservation efforts in the Lower Rio Grande Valley: 

A, Arundo donax (giant reed); B, Cenchrus ciliaris (buffelgrass); C, Tamarix aphylla (athel tamarisk); D, Aratinga holochlora (green 

parakeet); and E, Amazona viridigenalis (red-crowned parrot). Photographs on the left courtesy of Kim Wahl, U.S. Fish and Wildlife 

Service; photographs on the right, ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
62 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

[galling wasp], Rhizaspidiotus donacis [arundo scale], and 

Lasioptera donacis [arundo gall midge]) that might be used to 

reduce giant reed (e.g., Goolsby and Moran, 2009; Seawright, 

Rister, Lacewell, McCorkle, and others, 2009; Seawright, 

Rister, Lacewell, Sturdivant, and others, 2009; Moran and 

Goolsby, 2010; Poinar and Thomas, 2014). In June 2015, 

the Governor of Texas signed a bill directing the Texas State 

Soil and Water Conservation Board to develop a program 

to eradicate giant reed along the Rio Grande (K. Wahl, 

pers. commun.).

Buffelgrass

Buffelgrass is a perennial warm-season bunchgrass 

that was widely planted to revitalize overgrazed areas, 

particularly in semiarid Australia and the southwestern 

United States (Jackson, 2005; Flanders and others, 2006; fig. 

20B). Buffelgrass often displaces native herbaceous species, 

simplifying a plant community (Jackson, 2005; Flanders and 

others, 2006; Abella and others, 2012). Loss of plant diversity 

can cause a trophic cascade, decreasing invertebrate diversity 

with eventual loss of vertebrates dependent on them; areas 

dominated by buffelgrass have lower diversity of plants and 

lower abundance of breeding birds, particularly those that feed 

on or near the ground (Flanders and others, 2006). Buffelgrass 

grows in dense monocultures on drier tracts of the Lower Rio 

Grande Valley NWR (fig. 20B), resulting in direct competition 

for space and light and allelopathic effects (Hussain and 

others, 2011) and poor conditions for restoration of native 

species (Abella and others, 2012). A dense canopy of native 

plants can shade out buffelgrass, but much of that has been 

lost to brush clearing and livestock grazing.

Guineagrass

Guineagrass is particularly invasive, and like other 

introduced African grasses, it creates dense monocultures, 

especially in disturbed areas, that displace native plant 

species and lead to impoverished native systems (Smith, 

2010). Guineagrass can persist during dry periods. As with 

giant reed and buffelgrass, monocultures of guineagrass can 

build up large amounts of biomass, causing fire hazards. 

When guineagrass stands burn, high levels of biomass create 

intense fires detrimental to native plants not adapted to fire, 

further enhancing encroachment of guineagrass. In the LRGV, 

guineagrass often colonizes wet lowland areas, in contrast to 

buffelgrass that is typically found in dry upland areas. Both 

grasses cause problems for restoration of native plants because 

they can quickly take over a site (Jackson, 2005; Smith, 2010), 

making control and restoration difficult.

Saltcedar

Three exotic species of Tamarix―a Eurasian genus of 

considerable variation―are found in the LRGV: Tamarix 

aphylla (athel tamarisk, fig. 20C), a large tree capable of 

growing to about 18 m (about 60 ft) high, and two shrubby 

forms, the most invasive T. ramosissima (saltcedar; table 10) 

and T. canariensis (Canary Island tamarisk). Particularly stress 

tolerant, saltcedar can be highly invasive in riparian corridors 

of dry-country streams and other waterways, and it often forms 

dense monocultures that alter and decrease flows in areas 

that have already been altered by dams, water diversions, etc. 

(Glenn and Nagler, 2005; Douglas, 2009). In the LRGV, it 

is established along the Rio Grande, canals, and resacas and 

has spread into upland areas around stock ponds and culverts. 

Considerable effort has been focused on trying to mechanically 

and biologically control or eradicate species of Tamarix in the 

western United States because of early conclusions that they 

were “water-wasting foreign monsters” (Chew, 2009, p. 231); 

however, recent publications suggest that perceptions might 

be changing and that species of Tamarix might play roles in 

riparian stabilization and provide useful avian habitat in riparian 

areas (Sogge and others, 2008; Stromberg and others, 2009; 

Paxton and others, 2011). Nevertheless, the highly invasive and 

dominant tendencies of species of Tamarix were expressed after 

Hurricane Alex in 2010, when athel tamarisk and saltcedar, 

in particular, proliferated extensively on disturbed sites in 

the LRGV.

Animals

Red Imported Fire Ant

Solenopsis invicta (red imported fire ants) were accidently 

introduced into the United States in Alabama in the late 1930s. 

They rapidly spread across most of the southern United States 

and were found in potted plants at a Brownsville, Tex., nursery 

in 1991 (Allen and others, 1993) at the western edge of their 

aridity tolerance (LeBrun and others, 2012). Red imported 

fire ants can damage agricultural crops such as citrus, kill 

newborn livestock and ground-dwelling wildlife, and decrease 

biodiversity of invertebrates (Allen and others, 1998; Wojcik 

and others, 2001). In central Texas, exclusion of red imported 

fire ants increased nest survival of two Vireo spp. (vireo) by 

86–210 percent (Campomizzi and others, 2009). Red imported 

fire ants variously affect native ants, depending in part on soil 

moisture. Red imported fire ants reduce overall ant diversity 

in undisturbed wet areas but have a weaker effect in disturbed 

dry areas, and controlled burning reduces abundance of red 

imported fire ants (LeBrun and others, 2012). Concern has been 

expressed about the loss of certain ant species caused by red 

imported fire ants relative to dietary requirements of the Statethreatened Phrynosoma cornutum (Texas horned lizard), but 

differential aridity tolerances might minimize this interaction.

Africanized Honey Bee

All Apis mellifera (honey bees) in the Americas are 

exotic, introduced by humans from the Old World―Europe in 

most of the United States and Canada. European honey bees 

did not do well in tropical Central and South America, and 
Ongoing Challenges Facing the LRGV 63

honey production was poor; therefore, an African subspecies 

(scutellata), better adapted to the tropical conditions, was 

introduced near Sao Paulo, Brazil, in 1956 (Winston, 1992). 

Since then, Africanized honey bees have spread rapidly, up to 

300–500 kilometers per year, and reached very high densities 

in some areas (Winston, 1992). Absent in the LRGV in the 

late 1980s (Rubink and others, 1990), the first Africanized 

honey bees seen in the United States were caught in Hidalgo 

County in the LRGV in 1990 (Kaplan, 1990; Winston, 

1992). Mitochondrial haplotypes of honey bees in southern 

Texas now show that the maternal origin of most colonies 

is from Africanized honey bees (Pinto and others, 2004). 

Unfortunately, Africanized honey bees show exceptionally 

high levels of defensive behavior, with large attacks and 

massive stinging after minimal disturbance (Winston, 1992). 

Exotic or Naturalized Birds

Introduced Columba livia (rock pigeons) and more 

recently Streptopelia decaocto (Eurasian collared-doves) 

thrive in urban areas of the LRGV (Brush, 2008b). Eurasian 

collared doves were first seen in the LRGV in 1998 (Brush, 

2005, 2008a) and are of concern due to their explosive 

population patterns as they spread from release sites in Florida 

in the late 1970s, and their potential to outcompete native 

doves (Poling and Hayslette, 2006). Two parrots, Aratinga 

holochlora (green parakeet; fig. 20D) and the Amazona 

viridigenalis (red-crowned parrot; fig. 20E), are now common 

in urban areas of the LRGV (Butler, 2005). Nevertheless, there 

are no historical records to suggest that they were ever native 

in the LRGV. The TPWD considers the red-crowned parrot 

an indigenous species, subject to protection under Texas law 

(Shackelford and Hanks, 2016). Because both parrots have 

parts of their breeding distributions in northern Tamaulipas, 

Mexico, not far from the LRGV (Enkerlin-Hoeflich and 

Hogan, 1997; Howell and Webb, 1995; Brush, 2009a), some 

have speculated about natural range extension; alternatively, 

both species have been sold in the pet trade, so those in the 

LRGV could be escapees or have been deliberately released 

(Brush, 2005; Rappole and others, 2007). Urban settings 

benefit both species by providing dependable sources of 

food (e.g., acorns, figs, and palm fruits) and safe nesting 

sites (Brush, 2005). The red-crowned parrot is listed as 

endangered by the IUCN and as a foreign candidate species 

by the USFWS. Serious declines in numbers of red-crowned 

parrots in Tamaulipas, Mexico, because of the pet trade 

and destruction of native habitats (Enkerlin-Hoeflich and 

Hogan, 1997) could mean that relatively new populations 

in the LRGV could have a role in their future conservation 

(Brush, 2005). 

Exotic Mammals

Free-ranging nilgai—native to India and marginally 

Pakistan and Nepal, and introduced in Texas in the 1930s 

(Leslie, 2008)—occur in Cameron, Hidalgo, and Willacy 

Counties after having escaped from ranches north of the 

LRGV. Although Schmidly (2004, p. 285) stated that the nilgai 

“does not appear to harm native species,” its spread into the 

LRGV, particularly in areas that support the northern ocelot 

and perhaps the Gulf Coast jaguarundi, is of concern because 

heavily used nilgai trails might alter dense shrubby habitats 

that are critical to these endangered felids (Leslie, 2008). 

Because of the possibility of transmission of tickborne blood 

parasites from Mexico to United States cattle via nilgai, the 

U.S. Department of Agriculture routinely samples nilgai in the 

Boca Chica tract in the Lower Rio Grande Valley NWR and 

the Bahia Grande tract in Laguna Atascosa NWR. Although 

native, Odocoileus virginianus (white-tailed deer) also plays 

a role in dispersal and maintenance of infected ticks in the 

LRGV (Pound and others, 2010). Such parasites transmit tick 

fever, fatal to cattle, and detection has resulted in a permanent 

and expanding quarantine zone of 2,233 km2 (862 mi2

) along 

the lower Rio Grande in the LRGV. 

Domestic pigs are a variant of Sus scrofa (Eurasian 

wild pig) and were first introduced in the United States in 

Florida in 1539 (Wood and Barrett, 1979). Introductions 

continued in the southeastern United States, particularly from 

the Carolinas west to Texas. Over time, pigs escaped, often 

tended in free-ranging groups, and became feral. Feral pigs 

reproduce quickly, spread rapidly, damage native habitats, are 

highly omnivorous, compete with or eat native wildlife, and 

cause up to $52 million per year in damage to agricultural 

crops (Wood and Barrett, 1979; Taylor and others, 1998; 

Timmons and others, 2012). Terrestrial vertebrates ranging 

from salamanders to birds and small mammals (and their 

nests) have been found in stomachs of feral pigs (Wood and 

Barrett, 1979). In southern Texas, it is possible that feral pigs 

compete with white-tailed deer and Pecari tajacu (collared 

peccaries―Taylor and others, 1998). More than a million feral 

pigs occurred in Texas by the early 1990s (Taylor, 1991). As 

with nilgai, populations of feral pigs are established in the 

LRGV and are actively controlled in the STRC with permits to 

professional trappers and sportsmen during designated hunting 

periods, with unlimited harvest. Regular drought may be a key 

reason why domestic pig populations have not “exploded” in 

the LRGV (B.R. Winton, pers. commun.)

Climate Change

In 2001, the Union of Concerned Scientists and the 

Ecological Society of America provided a comprehensive 

synthesis of climate-related challenges facing the gulf coastal 

region of the United States (Twilley and others, 2001). 

Key among them is sea-level rise, which is projected to be 

more dramatic in Gulf Coast States than elsewhere because 

of local shoreline erosion and land subsidence, caused by 

groundwater withdrawal and oil and gas production, which 

decrease pore pressures in underlying sediments, causing 

land to sink (Montagna and others, 2011). Globally, sea levels 

have risen 10.2–20.3 cm (4–8 inches) over the past 100 years 

compared to 20.3–101.6 cm (8–40 inches) throughout the 
64 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

gulf coastal region (Twilley and others, 2001). With the same 

100-year horizon, Schmandt (2011, p. 257) concluded that 

“temperatures [in Texas] will rise, heat waves will occur more 

frequently, it will be drier west of the Interstate 35 corridor, 

severe weather will become more frequent, in-stream flows 

will fall, biodiversity will decline, and the sea level will 

rise.” All of these events will be accompanied by increases in 

flooding, heat-related illnesses, ground-level-ozone-caused 

respiratory diseases, and harmful algal blooms (Schmandt, 

2011; Schmandt and others, 2011). 

In southern Texas, increases in sea level and sea 

temperatures are already part of the historical record; for 

example, “sea-level rise along the South Texas coast is caused 

by natural and human-induced land surface subsidence 

and a global rise in ocean level, ... sea level has risen 

2.05 millimeters (0.08 inches) per year at Port Mansfield 

since 1963 and 3.44 millimeters (0.14 inches) per year at 

South Padre Island since 1958, [and] after estimates for 

local land subsidence are added, the amount of projected 

relative sea level rise by the year 2100 is 0.2–0.61 meters 

(0.66–2 feet) at Port Mansfield and 0.34–0.75 meters 

(1.1–2.5 feet) at South Padre Island” (Montagna and others, 

2011, p. 102–103). Sea-level rise could substantially change 

the character of the coastal LRGV, but changes could be slow 

and perhaps easily overlooked. As sea levels rise, tidal flats 

and seagrass meadows could initially decrease as they respond 

to increasing water depth and then, if conditions permit, 

slowly advance toward newly inundated land (Montagna 

and others, 2007). Some of these changes have already taken 

place; from the 1950s to 2004, tidal flats decreased from 

13,647 ha (33,722 acres) to 6,121 ha (15,125 acres), but 

seagrass meadows increased from 4,167 ha (10,297 acres) to 

8,398 ha (20,752 acres) in the Texas Coastal Bend (table 3 in 

Montagna and others, 2007). 

Generally, climate change will cause a northward 

advancement of the tropical temperature zone, with associated 

biotic, but at present difficult-to-predict, changes (Brennan, 

2007; Norwine and John, 2007; North, 2011; Fitzpatrick and 

others, 2013). Rappole and others (2007) assessed changes 

in distributions of tropical, subtropical, and warm-desert bird 

species and concluded that at least 70 species have already 

expanded their breeding ranges northward and eastward 

into, within, and beyond southern Texas―many moving 

into temperate grassland and forested habitats. Although 

specific correlative data were lacking, increasing mean annual 

temperature was hypothesized to be responsible; many species 

moved into areas where availability of preferred habitats 

was declining (Rappole and others, 2007). From a wildlife 

standpoint, species dependent on high-latitude habitats could 

suffer the most under a planetary warming scenario, but 

vigilance over the long term could be necessary everywhere 

to identify and respond appropriately to likely species-specific 

and localized conservation challenges as they arise (Root and 

Schneider, 2002; Schneider and Root, 2002; Inkley and others, 

2004; Brennan, 2007).

The remarkable diversity of butterflies in the LRGV 

(table 7) could be used to establish methods to monitor 

long-term effects of climate change because populations of 

terrestrial invertebrates such as butterflies, with very short 

generation times, quickly respond to subtle changes in climate, 

particularly those that affect their interactions with other 

species such as plants on which they depend (Crozier, 2002; 

Hellman, 2002). Beginning in the late 1960s and correlated 

with warming temperatures, the Atalopedes campestris

(Sachem grass skipper), which also occurs in the LRGV, 

expanded its distribution northward by about 700 km (435 mi) 

from its historical distribution in California as far north as 

south-central Washington by 1998 (Crozier, 2002). The 

Sachem grass skipper is in the most speciose butterfly family 

(Hesperiidae), with 111 skipper species in LRGV (table 7), 

and populations of the most common among them could be 

monitored to provide a long-term record of association with 

climate change, perhaps efficiently and inexpensively. 

Border Issues and Homeland Security

Humans have crossed the Rio Grande for millennia. 

Prior to European arrival and settlement, Coahuiltecan Native 

Americans camped up and down the lower Rio Grande in 

what is the present day LRGV (Thompson, 1997). After 

Mexican and Spanish colonization in the 16th century, people 

established settlements on both sides of the lower Rio Grande. 

Crossing the river became a growing obstacle when the Rio 

Grande became the official border between the United States 

and Mexico after the Mexican-American War in the 1850s. 

Many people have crossed the Rio Grande illegally since the 

1850s, moving contraband, notably during the American Civil 

War, the Mexican Revolution, and Prohibition in the United 

States (Thompson, 1997). 

Security of the current United States-Mexico border 

was relatively relaxed from the mid-1800s until 1993 when 

the United States instituted a major policy shift to enhance 

enforcement and minimize illegal border crossings and 

associated illegal activities (Cornelius, 2001; Parcher and 

Page, 2013). Informal and underground illegal activities are 

now common along the southern Texas border, perpetuated 

in general by a large gap in wages between Mexico and the 

United States (Dávila and others, 2002). Ongoing illegal 

activities, recently ranked by their potential to generate 

income, were drug dealing, human trafficking, dog and cock 

fighting, prostitution, gambling, and stealing and selling 

pirated or counterfeited goods and services (Richardson and 

Pisani, 2012). The U.S. Customs and Border Protection (CBP) 

routinely intercepts drugs, primarily marijuana and cocaine but 

also heroine and methamphetamines, at international bridges 

and on United States land adjacent to the Rio Grande.

Border-security activities are usually most intense in 

and around centers of human activity, so the rapidly growing 

cities of McAllen, Brownsville, and Harlingen, Tex., have 
Ongoing Challenges Facing the LRGV 65

experienced substantial change with regard to security issues. 

Presence of border patrol in the LRGV increased 633 percent 

from 418 agents in 1992 to 3,064 agents in 2014 (U.S. 

Customs and Border Protection, 2015). More border patrol 

agents mean more motor vehicles, all-terrain vehicles, horse 

patrols, helicopters, boats, camera towers, lights, and other 

forms of surveillance tools. There is ever-growing pressure to 

accommodate these agents and their equipment on tracts of the 

Lower Rio Grande Valley NWR, where isolated remnants of 

native vegetation provide cover for illegal activities. 

Through the 1980s and 1990s, border-patrol activities 

deterred illegal immigration and other illegal activities in 

the short term but had “non-existent” effects in the longterm (Dávila and others, 2002, p. 459). As a result, in part, 

the United States passed the Secure Fence Act (Public Law 

109–367) in 2006. The idea of a fence evolved into border 

obstructions of concrete walls, 6-m-high (20-ft high) steel 

fences, and (or) earthen levees (Abhat, 2011; fig. 21). The 

border fence in the LRGV is only one part in about 1,127 

km (700 mi) built along the 3,141-km (1,952-mi) United 

States-Mexico border. At least 36 Federal environmental and 

cultural laws were waived to allow rapid construction of the 

border fence (Urreiztieta and Harris, 2011). In the LRGV, 18 

of a proposed 21 segments have been built, with the longest 

segment of 21 km (13.1 mi) in Brownsville, Tex., in Cameron 

County. There are 56 km (35 mi) of border fence in Cameron 

County, 35 km (22 mi) in Hidalgo County, and 21 km (13 mi) 

planned in Starr County but currently not completed.

Segments of the border fence in Cameron County were 

built on the north side of the IBWC flood-control levee and 

outside of the flood plain. Inside the IBWC flood-control 

levee, known as the restricted use zone, no development is 

allowed because it would affect flood control. Segments of 

the border fence in Cameron County were built with 21.6- by 

28-cm (8.5- by 11-inches) openings every 0.4 km (0.25 mi) 

to facilitate wildlife being able to move north of the fenced 

areas. Wildlife (e.g., Texas tortoises and coyotes) have used 

these openings. The border fence in Hidalgo County was 

built on the south side of the IBWC levee and inside the flood 

plain because it could satisfy flood-control requirements of 

U.S. Federal Emergency Management Agency. Segments of 

the border fence in Hidalgo County have about 5 m (16 ft) 

of concrete from the ground up and steel fence on top (fig. 

21). These segments do not have openings and serve as true 

barriers to movements of nonvolant animals and will likely be 

most pernicious to sensitive and rare species such as northern 

ocelots and Texas tortoises. 

Numbers of roads in the LRGV have increased 

substantially to provide access to the border fence and 

surveillance along the border. Many of these roads traverse 

important tracts of the Lower Rio Grande Valley NWR, 

some directly along the Rio Grande, and resulted in the 

loss of native habitat when vegetation was cleared to build 

the roads and enhance surveillance. These activities have 

created fragmentation of habitats critical to native wildlife, 

directly conflicting with the conservation mission of the 

Lower Rio Grande Valley NWR. The border fence isolated 

already fragmented tracts in the Lower Rio Grande Valley 

NWR, making it hard for refuge personnel to access them for 

monitoring and other management activities such as habitat 

restoration. 

Numbers of apprehensions of unauthorized immigrants 

on the United States-Mexico border in 2014 were as low 

as they have been since the mid-1970s; proportions of 

Mexican nationals apprehended fell to historic lows and, for 

the first time on record, were less than proportions of nonMexico apprehensions (Krogstad and Passel, 2014). Such 

improvements have not come without conservation costs. 

The border fence now covers much of the length of the 

LRGV, albeit in intermittent segments, from Peñitas, Tex., 

to Brownsville, Tex., (some small sections in Starr County 

are yet to be completed). In many areas, sections of the fence 

are ≥4 m (13 ft) high and have vehicle barriers and barbed 

wire. Fence segments often have cleared vegetation and roads 

≥25 m (82 ft) wide (e.g., Flesch and others, 2009), creating a 

substantial gap between native habitats over wide stretches of 

the United States-Mexico border (fig. 21). 

Construction of roads in forested habitats increases 

habitat fragmentation, and multiple studies show that even 

small amounts of habitat fragmentation increase edge habitat, 

which can adversely affect certain bird species (e.g., Askins 

and others, 1990; Askins, 1994; Paton, 1994; Ortega and 

Capen, 1999). Even small increases in edge habitat can lead to 

increased nest predation and parasitism by cowbirds (Ortega 

and Capen, 1999). For example, fragmentation and perhaps 

subsequent increases in cowbird parasitism have been linked 

with the decline of the Audubon’s oriole, one of the highprofile bird species in the LRGV (e.g., Brush, 2005). Increased 

traffic along the border-patrol roads could have negative 

effects on other species of wildlife, such as the endangered 

northern ocelot, either through direct mortality or through 

disruption of their normal activities.

Flesch and others (2009) hypothesized that the new 

obstructions along the United States-Mexico border could 

become a substantial barrier to wildlife movements. They 

demonstrated that dispersal and movements of Glaucidium 

brasilianum (ferruginous pygmy-owls) in Arizona were 

impeded by large gaps in native vegetation on either side 

of the border fence, and with enhanced disturbance, these 

habitat gaps resulted in a lower rate of successful colonization. 

Ferruginous pygmy-owls typically fly lower than the height of 

the border fence, and effects of habitat gaps and disturbance 

were particularly disruptive to juvenile owls (Flesch and 

others, 2009). This species occurs in the LRGV and is listed 

as threatened in Texas. In the LRGV, the border fence could 

similarly affect endangered northern ocelots and Gulf Coast 

jaguarundi, as well as even more common bobcats, coyotes, 

and white-tailed deer (e.g., Bies, 2007; Abhat, 2011). 

Lasky and others (2011) conducted the most 

biogeographically comprehensive assessment of risks to 
Figure 21. The border fence along the Lower Rio Grande Valley, which has been constructed in various ways and now extends in 18 of 21 proposed 

91 kilometers (57 miles) in Cameron and Hidalgo Counties. Note that considerable brushland habitat, visible beyond the fence, was permanently lost dto permit regular vehicular assess along the fences. Photographs courtesy of Chris Perez, U.S. Fish and Wildlife Service (top left and right), and Vince Wildlife Service.
Conservation Opportunities for the LRGV in the 21st Century 67

313 amphibian, reptile, and nonvolant mammal species 

affected by the fence along the United States-Mexico border. 

They found that 50 species and three subspecies, globally or 

federally threatened in the United States or Mexico, occur 

within 50 km of the border. The Gulf Coast fenced area, which 

was only represented in their analysis along the border by the 

LRGV, had 6 amphibians, 1 reptile, and 2 nonvolant mammals 

and was among 3 of 7 ecoregions of particular conservation 

concern along the entire border. 

Conservation Opportunities for the 

LRGV in the 21st Century

Although the LRGV has experienced substantial 

alterations to its native habitats and continues to experience 

demands from an increasing human population, opportunities 

to enhance conservation in the LRGV still exist. Many of the 

international, Federal, State, local, and private partnerships 

that have helped conserve aspects of the unique biota of the 

LRGV are still in place and could be refocused on areas of 

immediate concern. For example, new corridors that connect 

native habitats critical to the endangered northern ocelot could 

enhance the entire conservation network of properties already 

conserved in the LRGV. 

Productive Partnerships

Ecotourism and Conservation Activities

Wildlife-related recreation (i.e., hunting, fishing, and 

wildlife watching) is very popular in the United States, with 

more than 87 million people participating and spending about 

$122 billion in 2006 (U.S. Department of Interior, Fish and 

Wildlife Service, and U.S. Department of Commerce, 2007). 

Hunting and fishing have been historical mainstays of wildliferelated recreation in the United States, but in recent decades, 

wildlife watching, particularly bird watching, has become 

increasingly popular and now surpasses numbers of people 

involved in and economic contribution of consumptive hunting 

and fishing―both very popular in the LRGV (fig. 22). In 

2006, 48 million bird watchers spent $36 billion on trips and 

equipment in the United States, which generated $82 billion 

in total direct and indirect industry output (Carver, 2009). 

There were 4.2 million bird watchers that spent $2.9 billion 

in Texas in 2006, which exceeded numbers participating in 

hunting (more than 1 million hunters spending $2.2 billion) 

and fishing (2.5 million anglers spending $3.2 billion―Mathis 

and Matisoff, 2004; U.S. Department of Interior, Fish and 

Wildlife Service, and U.S. Department of Commerce, 2007; 

Carver, 2009).

Nature tourism in the LRGV is a very important and 

rapidly growing sector of the economy, contributing $100–170 

million annually in the early 2000s (Mathis and Matisoff, 2004; 

Mathis and others, 2004) to more than $300 million by 2011 

(Woosnam and others, 2011). “Winter Texans” have long been 

part of the senior travel market to the LRGV, ranking the mild 

climate and friendliness of the people as the most important 

attractions in choosing their “winter home”; relative to the 

USFWS mission, 55 percent of visitors ranked wildlife refuges 

as important attractions in the LRGV, only behind visiting 

Mexico, beaches, and area zoos (tables 2 and 3 in Vincent and 

Santos, 1990; Crompton and others, 1992). Winter visitors 

ranked availability of “beautiful wildlife refuges” second only 

to “plentiful array of festivals” relative to the attractiveness of 

the LRGV (Fakeye and Crompton, 1991, p. 13). 

Texas is often the number one state for bird watching, 

and the LRGV is often in the top 10 birding destinations in 

the United States (e.g., Mathis and Matisoff, 2004). With so 

many active bird watchers living in and visiting the LRGV 

and the popularity of the Rio Grande Valley Birding Festival 

(table 11)―held in November for the last 21 years―TPWD, 

in partnership with USFWS, nine valley municipalities, and 

other government agencies, initiated a plan to develop a World 

Birding Center in 1998 to “serve as the model for the future 

development and application of conservation, education, and 

economic growth” (Vincent and Thompson, 2002, p. 154). By 

2005, the World Birding Center consisted of a central visitor 

facility at Bentsen Rio Grande Valley State Park, 2 gateway 

visitor facilities, 2 major interpretive areas, and 5 destination 

locations, located along 193 km (120 mi) of the LRGV. Initial 

assessments of community support and sustainability were 

positive (Vincent and Thompson, 2002), but it has become 

apparent more recently that many bird watchers―some 

from foreign countries―want to be outside looking for rare 

birds rather than inside visitor centers. As a result, some of 

the centers are no longer staffed by USFWS personnel (e.g., 

Roma Bluffs, formerly operated by the USFWS). Bird- and 

other wildlife-related festivals of 1 to several days long were 

estimated to contribute >$1 million to the local economy 

(Mathis and Matisoff, 2004).

The rich variety of terrestrial and aquatic plant and 

animal communities, outlined in the first part of this report, 

provides substantial attractions to residents and visitors to 

the LRGV. Various festivals to celebrate this diversity have 

been established through time, and numerous related activities 

and conservation groups are dedicated to enhance, preserve, 

and appreciate it (table 11). With more than 100 species of 

dragonflies and damselflies in the LRGV, it is not unexpected 

that a unique festival celebrates their late-spring peak 

abundance. During Dragonfly Days in Weslaco, Tex., species 

new to the LRGV, and even the entire United States, have 

been discovered; for example, the Planiplax sanguiniventris

(Mexican scarlet-tail dragonfly), Tauraphilia argo (bow-tailed 

or arch-tipped glider), and Anax concolor (blue-spotted comet). 
68 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Abundance and exceptional diversity of butterflies in 

the LRGV (Wauer, 2004; table 7) led the North American 

Butterfly Association to establish the National Butterfly 

Center and open the International Butterfly Park. Both are 

located near Bentsen Rio Grande Valley State Park, along 

with the headquarters of the World Birding Center, and are 

dedicated to education, conservation, and scientific research 

on butterflies. The Texas Butterfly Festival, held annually for 

the last 20 years, increases tourism in mid-autumn (table 11), 

particularly to the National Butterfly Center and other areas 

offering butterfly-viewing opportunities (South Texas Chapter 

of the North American Butterfly Association, 2015). The Texas 

Butterfly Festival also increases interest in establishing butterfly 

gardens at schools and in public places in the LRGV, further 

fostering conservation. Ecotourists interested in butterflies 

are also drawn to Santa Ana NWR, Laguna Atascosa NWR, 

and private reserves that have developed gardens to attract the 

varied butterflies of the LRGV. If butterfly-related activities are 

continually highlighted by local, State, and Federal agencies 

and organizations and become part of regular public discourse 

in the LRGV, they could become as locally iconic as the 

monarch butterfly has become internationally (Gustafsson and 

others, 2015). 

The Ocelot Conservation Day Festival has been held 

annually in March since 1997 and focuses on conservation 

of the endangered northern ocelot in the LRGV by providing 

educational presentations by experts, activities for children, and 

opportunities to see a live ocelot. Since 2007, the Cincinnati 

Zoo’s Cat Ambassador Program has brought trained ocelots to 

the LRGV. The 1-day festival was initially held at the Marine 

Military Academy in Harlingen, Tex., but in recent years, it has 

been held and hosted by the Gladys Porter Zoo in Brownsville, 

Tex., and cohosted by Friends of Laguna Atascosa NWR and 

the USFWS. It has attracted more than 1,500 participants in 

recent years, and proceeds support important research and 

conservation of the northern ocelot in the LRGV. 

Agency and Nongovernmental

Organization Cooperation

Many State, Federal, and international agencies and 

nongovernmental organizations (NGOs) work cooperatively 

on common conservation objectives in the LRGV (table 11). 

The TPWD still owns and manages a number of areas in the 

LRGV to conserve native brush habitat and support hunting, 

particularly for white-winged doves, and the Lower Rio Grande 

Valley NWR now manages eight TPWD wildlife-management 

areas. The Las Palomas Wildlife Management Area consists of 

1,340 ha (3,311 acres) in 18 units of 0.8–244 ha (2–604 acres), 

scattered throughout the LRGV, primarily in Cameron and 

Hidalgo Counties (Mathis and Matisoff, 2004). The TPWD 

continues to manage Bentsen Rio Grande Valley, Resaca de 

la Palma, and Estero Llano Grande State Parks in the LRGV. 

Figure 22. Surf fishing (left) and birdwatching (right), just two examples of the diverse recreational opportunities in the Lower Rio 

Grande Valley. ©Larry Ditto Nature Photography, used with permission. ©Larry Ditto Nature Photography ©Larry Ditto Nature Photography
Table 11. Prominent annual festivals, activities, and locations related to wildlife and conservation in the four-county area of the Lower Rio Grande Va[LRGV, Lower Rio Grande Valley; USFWS, U.S. Fish and Wildlife Service; NWR, national wildlife refuge]

Name Purpose Sponsor and (or) location Related link (acc

Festivals

Ocelot Conservation Day Festival Conservation of the endangered Leopardus 

pardalis albescens (northernocelot) in the LRGV

Gladys Porter Zoo, Brownsville, 

and USFWS 

www.fws.gov/refuge/laguna_

activities/ocelot_conservat

Rio Grande Valley Birding Festival Celebrating the renowned bird diversity

in the LRGV

Harlingen www.rgvbirdfest.com

Rio Reforestation Annual public event focused on restoration 

of native habitats in the LRGV

USFWS www.fws.gov/refuge/Lower_

activities/special_events.ht

Texas Butterfly Festival Celebrating the great butterfly diversity

in the LRGV

Mission www.texasbutterflyfestival.co

Wild in Willacy Natural biodiversity of Willacy County Raymondville and Port Mansfield www.wildinwillacy.com

Related to conservation

Anzalduas County Park Outdoor recreation with opportunities

for bird and butterfly watching

Mission www.missiontexas.net/attract

and www.thedauphins.net/

Chihuahua Woods Preserve,

The Nature Conservancy

Preservation of Tamaulipan thornscrub 

habitat and a unique community of cacti

Mission www.nature.org/ourinitiatives

unitedstates/texas/index.htm

Frontera Audubon Society Dedicated to conserving native birds and 

habitats of the LRGV

Weslaco www.fronteraaudubon.org

Friends of Laguna Atascosa National 

Wildlife Refuge

Enhance conservation and education at 

Laguna Atascosa NWR and operate visitor center store

Los Fresnos http://friendsoflagunaatascosa

Friends of the Wildlife Corridor Enhance conservation and education at Santa 

Ana and Lower Rio Grande Valley NWRs and operate visitor center store at Santa Ana

Alamo https://friendsofthewildlifeco

Gladys Porter Zoo Animal display, conservation, and education Brownsville www.gpz.orgHugh Ramsey Nature Park Once a landfill now dedicated to habitat 

conservation and associated with Harlingen Arroyo Colorado World Birding Center

Harlingen www.trails.com/tcatalog_trail

Las Estrellas Preserve Protecting the population of the endangered

Astrophytum asterias (star cactus)

The Nature Conservancy, Starr 

County

www.nature.org/ourinitiatives

unitedstates/texas/index.htm

Lennox Foundation Southmost Preserve Protecting remnant Sabal mexicana (sabal 

palm) forest along the lower Rio Grande

The Nature Conservancy, 

Brownsville

www.nature.org/ourinitiatives

unitedstates/texas/index.htm

Los Palomas Wildlife Management 

Area: 18 tracts, including Prieta Unit, MacWhorter Unit, and Chapote Unit

Wildlife management, particularly habitat 

conservation for Zenaida asiatica (whitewinged dove)

Weslaco www.tpwd.state.tx.us
Table 11. Prominent annual festivals, activities, and locations related to wildlife and conservation in the four-county area of the Lower Rio Grande V

Texas.—Continued [LRGV, Lower Rio Grande Valley; USFWS, U.S. Fish and Wildlife Service; NWR, national wildlife refuge] Name Purpose Sponsor and (or) location Related link (acce

Related to conservation—Continued McAllen Nature Center Educational activities and family nature walks McAllen www.mcallen.net/parks-recrea

National Butterfly Center Dedicated to education and conservation for butterflies in the LRGV Mission www.nationalbutterflycenter.o

Palo Alto Battlefield Preservation of a Mexican-American War battlefield, providing access to undeveloped grassland bound by thickets of mesquite and cactus Brownsville www.nps.gov/paal/learn/histo

Rio Grande Valley Chapter of Texas Master Naturalist Dedicated to volunteer education and outreach to enhance conservation of natural resources in the LRGV San Benito www.rgvctmn.org Rio Grande Delta Audubon Society Dedicated to conservation of bird diversity in the LRGV Brownsville www.riograndedeltaaudubons

Sabal Palm Sanctuary Protection of a rare community of sabal palms and associated wildlife along the Rio Grande, in cooperation with the Gorgas Science Foundation Brownsville www.sabalpalmsanctuary.org 

www.gsfinc.org Texas State Parks Bensten-Rio Grande Boca Chica Recreation and conservation of unique habitats in the LRGV Mission and Brownsville (vicinity), respectively www.tpwd.texas.gov Valley Proud Environmental Council Sponsor of Rio Restoration for past 24 years, enhancing native habitats of the LRGV Harlingen www.valleyproud.org Valley Land Fund Expanding and enhancing native wildlife habitat in the LRGV McAllen www.valleylandfund.com World Birding Centers Bentsen-Rio Grande Valley Edinburg Scenic Wetlands Estero Llano Grande Harlingen Arroyo Colorado Old Hidalgo Pumphouse

Quinta MazatlanResaca de la PalmaRoma Bluffs South Padre Island and Nature 

Center

Highlighting the rich bird communities of the 

LRGV, focusing on education

Mission at Bentsen-Rio Grande 

State Park; nine centers providing access to a great variety of habitats and wildlife from Roma to South Padre Island in the LRGV

www.theworldbirdingcenter.co
Conservation Opportunities for the LRGV in the 21st Century 71

Adjacent to the latter two State parks, the TPWD manages

the Mercedes and Noriega tracts of the Lower Rio Grande 

Valley NWR.

The Nature Conservancy owns three conservation 

preserves in the LRGV, but management is limited because of 

border security issues. Close to the Sabal Palm Sanctuary, the 

Nature Conservancy’s Lennox Foundation Southmost Preserve 

protects 414 ha (1,023 acre) of sabal palm forest, in which rare 

plants and animals are protected. The Nature Conservancy also 

has been active in Starr County (and a few places in Mexico) 

by purchasing land and acquiring conservation easements 

to protect the endangered star cactus. The 168-ha (415-acre) 

Las Estrellas Preserve protects the star cactus and associated 

species. In Hidalgo County, the Nature Conservancy’s 142-ha 

(350-acre) Chihuahua Woods Preserve protects a community 

of cacti, unlike anywhere else in the LRGV, and it might 

transfer ownership of the preserve to the Lower Rio Grande 

Valley NWR. The Nature Conservancy actively partners with 

other public and private concerns in the LRGV, using habitat 

restoration, landowner outreach, and biotic inventories to 

further conservation goals. 

The National Audubon Society has several active 

chapters in the LRGV that promote conservation of birds and 

their habitats. The National Audubon Society protected the 

225-ha (557-acre) Sabal Palm Sanctuary, one of the largest 

remaining groves of sabal palms in Texas, but after completion 

of the border fence that affected the sanctuary, the National 

Audubon Society turned over management and oversight of 

the site to Gorgas Science Foundation which operates a gift 

shop and streams a live video camera of birds at a feeder in the 

sanctuary, with support from the Rio Grande Valley Chapter of 

Texas Master Naturalist.

The Friends of the Wildlife Corridor is a nonprofit 

organization that was founded in the LRGV in 1997. Its 

primary functions are to protect and restore native habitat in 

the last 322 km (200 mi) of the lower Rio Grande. The Friends 

of the Wildlife Corridor purchase and temporarily hold land 

until the USFWS can purchase it from them (these purchases 

are typically small acreages but can serve very important 

roles in connecting existing NWR land); operate the gift 

store at Santa Ana NWR (profits from sales of educational, 

interpretive, and promotional materials support NWR 

projects); support the volunteer program, construction of trails 

and boardwalks, and purchases of outreach and educational 

materials; secure grants to help build, for example, the 

overlook tower at Santa Ana NWR; conduct outreach through 

special events, a Web site, brochures, and other promotional 

materials; and finally, lobby at local and national levels for 

land acquisition funding and other issues that affect wildlife 

conservation in the LRGV. The Friends of Laguna Atascosa 

NWR provide similar services (table 11), often with a focus on 

the largest population of northern ocelots that occurs there.

The Valley Land Fund owns the Salineño tract, adjacent 

to the Kepler tract of the Lower Rio Grande Valley NWR; 

a management agreement allows the USFWS to manage 

the Valley Land Fund’s property. This site is a top birding 

destination in the LRGV where three oriole species are 

regularly visible and where the rare Psilorhinus morio

(brown jay) and red-billed pigeon can be sighted on a fairly 

reliable basis. The Valley Proud Environmental Council 

has cosponsored the annual Rio Reforestation event for 

the past 24 years. Rio Reforestation is the USFWS annual 

opportunity to engage the public, primarily middle and high 

school students, with planting former farmland back to 

native vegetation, often in the flood plain areas of the LRGV. 

Supporting Rio Reforestation and other restoration activities 

in the LRGV, American Forests of Washington, DC, has 

provided nearly $1 million over the past 10 years to leverage 

the reforestation programs of the Lower Rio Grande Valley 

NWR. Funds received from donations are transferred to the 

Lower Rio Grande Valley NWR to purchase seedlings. The 

Conservation Fund of Arlington, Virginia, has played a large 

role in land acquisition, particularly in Laguna Atascosa NWR.

Cooperation with Mexico

Personnel from USFWS Ecological Services-Corpus 

Christi and STRC have worked with agencies in Mexico to 

enhance conservation in the LRGV borderlands since at least 

1994. In 2001, the Lower Rio Grande/Rio Bravo Binational 

Ecosystem Group was established with representatives from 

USFWS, TPWD, the Nature Conservancy, the Mexican 

State of Tamaulipas, Comisión Nacional de Áreas Naturales 

Protegidas (CONANP, Mexican equivalent to the USFWS), 

and Pronatura Noroeste México (major Mexican conservation 

NGO). This group produced a binational ecosystem 

management plan that addressed pernicious habitat impacts 

on both sides of the international border and common specific 

concerns about natural resources and wildlife conservation.

In 2005, the Lower Rio Grande/Rio Bravo Binational 

Ecosystem Group added eight partners from Mexico, and a 

Memorandum of Understanding (MOU) was signed by the 

USFWS, TPWD, and Mexican agencies including wildlifeand conservation-related agencies from Tamaulipas and Nuevo 

Leon, Comisión Nacional para el Conocimiento y Uso de la 

Biodiversidad, CONANP, and Pronatura Noroeste México. 

A major goal of the MOU was to establish international 

habitat corridors linking the protected area of Sierra Picachos 

in Nuevo León, Mexico, with similarly protected areas in 

Tamaulipas, Mexico, and along the lower Rio Grande in 

Mexico, eventually connecting with tracts in the Lower Rio 

Grande Valley NWR in the United States. The group has 

sponsored international symposia over the years to share 

information and highlight acquisition and restoration efforts 

to establish international habitat corridors. These efforts have 

been challenged since the border fence was completed. 

A connected international wildlife corridor along the 

Rio Grande could benefit a variety of species of migratory 

and resident birds, bats, mammals, and butterflies. To 

be successful, such a corridor will require restoration, 

conservation easements and land acquisition, and research 

on flora and fauna in the United States and Mexico. Northern 
72 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

ocelots, in particular, could benefit from a corridor that 

connects its small United States population to the larger 

populations in Tamaulipas, Mexico. One current binational 

effort by at least 12 United States and Mexican agencies, 

organizations, and universities is led by the Translocation 

Working Group, a subcommittee of the Ocelot Recovery 

Team that includes, among others, the Tamatán Zoo (Ciudad 

Victoria), Gladys Porter Zoo, Dallas Zoo, and Environmental 

Defense Fund, and is focused on introduction of northern 

ocelots from Tamaulipas, Mexico, into the United States 

population to bolster its low genetic diversity (Translocation 

Working Group, 2009). Until a functional wildlife corridor 

can be established between the United States and Mexican 

populations, this is the only option to augment low genetic 

diversity of northern ocelots in the United States and decrease 

their high probability of extirpation (Haines, Tewes, Laack, 

and others, 2006).

Conservation and Land Acquisition

The USFWS acquires land in the LRGV from willing 

sellers. Fee purchase, conservation easements, lease 

agreements, management agreements, donations, gifts, and 

administrative transfer or exchanges are used to acquire, 

manage, and protect land from habitat loss and ultimately 

conserve the land in perpetuity. For example, the Monte Cristo 

tract in the Lower Rio Grande Valley NWR was exchanged 

for the 3,006-ha (7,428-acre) Yturria easement in 2015 to 

enhance conservation of northern ocelots. Nevertheless, 

funding for land acquisition is complicated and depends on 

many factors, including Federal administrative and political 

interest. As mitigation for habitat loss from construction of 

the border fence, the USFWS anticipated $20 million from the 

U.S. Department of Homeland Security for land acquisition in 

the Rio Grande Valley Sector. Nearly $3 million was received 

by the end of 2014, with little assurance then that more 

would be forthcoming. The U.S. Department of Homeland 

Security must rely on the U.S. Army Corps of Engineers to 

purchase land to mitigate for impacts, and in early 2015, it was 

announced that the U.S. Department of Homeland Security 

could not commit to the full $20 million. 

Originally, State and Federal agencies and NGOs agreed 

to establish a wildlife corridor adjacent to the Rio Grande in 

the United States from the Boca Chica tract on the Gulf Coast 

to Falcon Dam in Starr County to benefit endangered species 

and migratory birds. Since the border fence was completed 

in 2008, some conservation organizations and agencies in the 

LRGV have transferred their land and management activities 

to the Lower Rio Grande Valley NWR or private foundations. 

As a result, previous efforts to establish a wildlife corridor 

along the lower Rio Grande have been thwarted. Seven 

wildlife management areas and Boca Chica State Park owned 

by TPWD were transferred under long-term agreements in 

July 2007 to the Lower Rio Grande Valley NWR; most of the 

management areas are relatively small and close to the river, 

with no public access, fishing, or hunting opportunities that 

could generate income for Texas. The Nature Conservancy’s 

Sabal Palm Sanctuary protects a vestige of a unique forest 

type in the LRGV, and its management was transferred to the 

Gorgas Science Foundation in 2009.

The STRC owns or manages lands and water rights 

within jurisdictional boundaries of 13 of 29 water-irrigation 

districts in the LRGV. Since its establishment in 1979, the 

Lower Rio Grande Valley NWR has maintained a beneficial 

management agreement with the Delta Lake Irrigation District. 

In June 1990, a cooperative agreement was established to 

protect and manage a strip of wildlife habitat associated with 

the Willacy Canal system along 44.2 km (27.5 mi) from 

the Santa Maria tract on the Rio Grande northward to Delta 

Lake and the area midway between the Rio Grande and 

the East Lake/La Sal Vieja tract―a stretch called the Otha 

Holland Canal/Wildlife Corridor (fig. 19). It represents the 

only corridor where wildlife can safely travel from the Rio 

Grande north of Interstate 2—the major east-west, 4–6-lane 

highway bisecting the entire LRGV; culverts enable safe 

passage of species such as northern ocelots beneath Interstate 

2. In exchange for management and protection by the Lower 

Rio Grande Valley NWR, the Delta Lake Irrigation District 

agreed to allow native habitat on at least one bank of the canal 

to afford wildlife a vegetated corridor close to freshwater 

in which to travel. This agreement was renewed, with 

modification, in June 2015. The new agreement requires the 

Lower Rio Grande Valley NWR to contribute part of its annual 

water-rights allocation to the Delta Lake Irrigation District for 

preserving habitat along its 122-m-wide (400-foot-wide) canal 

easement, of which about 61 m (200 ft) is vegetation. 

The new agreement between the Delta Lake Irrigation 

District and Lower Rio Grande Valley NWR is precedent 

setting and will permit the USFWS to leverage its water 

rights to irrigation districts, landowners, or other entities by 

providing them water in exchange for conservation benefits, 

primarily planting or allowing the vegetation to recover along 

their linear canal systems. These agreements could increase 

the extent of vegetated wildlife corridors to accommodate 

movements of terrestrial mammals between the broader 

LRGV and Laguna Atascosa NWR and among larger tracts in 

the Lower Rio Grande Valley NWR. The Lower Rio Grande 

Valley NWR has 2.71 x 107 cubic meters (22,000 acre-feet) of 

water rights. Expanding agreements similar to the agreement 

renegotiated with the Delta Lake Irrigation District could 

enhance conservation in the LRGV. 

Currently, most water-irrigation districts, particularly 

those in Cameron County, regularly clear vegetation on both 

banks of their canals, which provides no habitat benefits and, 

in fact, is a detriment to most wildlife. Acquiring equipment 

that could help water-irrigation districts do necessary 

maintenance without having to clear all bankside vegetation 

might be an incentive to protect habitat for wildlife while 

continuing to effectively move water as needed. District canal 

systems typically hold water for all or part of the year. When 

these canals cross a major road or highway, they typically have 
Future Management Directions and Needs 73

culverts or provide opportunities for wildlife to pass safely 

beneath the road. Permitting terrestrial species to travel along 

canal networks is a viable way to protect essential wildlife 

habitat in the LRGV. Such habitat protection and restoration 

opportunities could provide a network of corridors throughout 

the LRGV, offering wildlife opportunities to move about in 

an otherwise largely cleared and fragmented agricultural and 

urbanized landscape.

Future Management Directions 

and Needs

Nearly 30 years ago, Jahrsdoerfer and Leslie (1988, 

p. 40) summarized the future “resource protection and 

management strategy” of the Lower Rio Grande Valley NWR 

with a five-step integrated approach to land conservation: (1) 

give “high priority in acquisition and preservation planning” 

to communities under particular threats; (2) acquire, repair, 

and maintain a riparian corridor along the Rio Grande; (3) 

maintain and acquire large anchor tracts (e.g., East Lake/

La Sal Vieja, Falcon Woodland, and Boca Chica tracts) to 

conserve “biological material to safeguard gene pools and 

replenish wildlife populations throughout the corridor”; 

(4) strategically locate management units “to provide food, 

water, and cover for selected target populations”; and (5) 

conserve unique, intact islands of important wildlife value left 

untouched when the Rio Grande Delta was cleared. Under this 

integrated approach to land management and conservation in 

the LRGV, Jahrsdoerfer and Leslie (1988) provided 18 specific 

recommendations as templates for future directions. Perhaps 

the best measurement of progress since then is the fact that 

the Lower Rio Grande Valley NWR increased 320 percent in 

size from 50 tracts of about 9,817 ha (27,300 acres) in 1988 to 

147 tracts of 39,035 ha (96,458.4 acres) in 2015. Although the 

Lower Rio Grande Valley NWR is less fragmented than it was 

in the late 1980s, it remains, in general, a series of small and 

often isolated tracts with limited connectivity, which leaves 

some tracts vulnerable to local extirpations and ecosystem 

degradation. 

To help fulfill the overall mission of the USFWS refuge 

system and parallel its vision from the late 1980s (Jahrsdoerfer 

and Leslie, 1988), the main goal for habitat management in 

the Lower Rio Grande Valley NWR is to enhance, restore, 

and protect native communities unique to the LRGV. To meet 

needs of trust-resource species, the USFWS and its partners 

developed land protection plans in 1983 (USFWS, 1983), 

1985 (USFWS, 1985), and 1999 when Laguna Atascosa NWR 

amended its plans for land acquisition (USFWS, 2010e). 

Since these plans were established, many State, Federal, and 

private partners have been constrained by multiple factors 

(e.g., increasing land prices, limited funding, and border fence 

issues) in their abilities to acquire properties in the LRGV. 

Nevertheless, the USFWS continues to focus, in part, on 

broad landscape-level partnerships to complete its strategic 

acquisitions, habitat protection, and habitat-restoration 

objectives. Five large landscape corridors in the LRGV and 

three international corridors between the United States and 

Mexico are now major foci of conservation efforts for STRC 

and its partners (fig. 23). 

Strategic land-acquisition planning conducted by STRC 

takes into account many current and projected impacts (table 

12) such as sea-level rise from climate change, transportation 

and wildlife-crossing needs, urban growth, border issues, 

physical barriers such as border security infrastructure, 

the border fence, and the floodway levee, balanced by the 

USFWS’s mandatory requirements to protect trust-resource 

species of the United States. The endangered northern 

ocelot provides an example of the urgent need for additional 

conservation and actions to reach its recovery goals and those 

of other endangered plants and animals in the LRGV (e.g., 

tables 5 and 6). Without human intervention, the northern 

ocelot could become extirpated in the United States within 

50 years (Haines, 2006: Haines, Tewes, Laack, and others, 

2006). The two most important factors limiting recovery 

of the northern ocelot are lack of connection of remaining 

suitable habitat and vehicle mortality. Northern ocelots 

in Texas are found in two subpopulations with no known 

physical connections between them and larger remnant 

populations in Mexico. Achieving recovery goals for the 

northern ocelot in the United States requires strategic habitat 

planning that crosses international boundaries (Translocation 

Working Group, 2009; USFWS, 2010a).

As described in the 2001 USFWS Policy on Biological 

Integrity, Diversity, and Environmental Health (USFWS, 

2001), the goal of habitat management on NWRs is to ensure 

long-term maintenance and, where appropriate, restoration 

of ecosystems while considering management’s contribution 

at various landscape scales. Because of the extreme diversity 

of species and habitats within the LRGV, the approach to 

acquisition and management of habitats by STRC considers 

current and historic conditions of an area and constraints on 

present-day management of these resources.

Resources of concern in the Lower Rio Grande 

Valley NWR, and therefore the entire STRC, and activities 

associated with their conservation encompass international, 

national, regional, and local conservation goals; State fish 

and wildlife conservation plans; recovery plans for threatened 

and endangered species; and habitat needs of migratory 

birds. These concerns are outlined in approved refuge 

resource-management plans (e.g., USFWS, 1983, 1985, 

1997, 2010e), most recently following the 2000 USFWS 

Policy on the Comprehensive Conservation Planning 

Process (USFWS, 2000). Management and conservation of 

resources by the STRC support purposes and missions of 

the USFWS refuge system and conserve biological integrity, 

diversity, and environmental health of the LRGV, giving 

special consideration to rare, declining, or unique natural 

communities and species.
laf16-0737_fig23

TEXAS

EXPLANATION

South Texas Refuge ComplexRio Grande CorridorCoastal CorridorRanchito Corridor

North CorridorRanchland CorridorLaguna Atascosa National Wildlife Refuge acquisition boundaryUrban 0 10 20 40 MILES

N 0 10 20 40 KILOMETERS

Rio Grande

281

83

281

77

2

186

100

STARR

COUNTY

HIDALGO

COUNTY

CAMERON

COUNTY

WILLACY

COUNTY

Base map image is the intellectual property of Esriand is used herein under license. Copyright © 2014 Esriand its licensors. All rights reserved.Refuge boundaries and wildlife corridors from U.S. Fish and Wildlife Service.Urban areas, county borders, and roads from Texas Natural Resource Information System 

Figure 23. Planned wildlife corridors to connect critical tracts and habitats in the United States and Mexico, including the future acquisition bounda

Atascosa National Wildlife Refuge.
Table 12. Present-day conservation and management challenges facing the Lower Rio Grande Valley and management concerns and options, geneleast challenging or severe.[LRGV, Lower Rio Grande Valley; LNG, liquefied natural gas; FERC, Federal Energy Regulatory Authority; NWR, national wildlife refuge; USFWS, U.S. Fish and Wildlife Sekm, kilometer; STRC, South Texas Refuge Complex; cm, centimeter; ROW, right of way; %, percent; m, meter; <, less than]

Challenges Key impacts and metrics Management concerns and 

Land acquisition • Conflicts with rezoning in response to expanding urbanization and

conservation objectives throughout the LRGV (likely most problematicin Cameron County).

• Land values and speculation in the LRGV among the highest in Texas

(for example, $2,900 per acre for farm and [or] ranch land vs. an average of $1,196 per acre in Texas in 2007). 

• Adequate funding (for example, from the Land 

Fund) and help from nonprofit partners are the conservation tools.

• Acquisition of key lands necessary to establish c

of sufficient in size to protect LRGV diversity needed to complete the North Corridor.

• Maintain an active role in local urban planning 

critical wildlife habitats.

Energy development • At least 15 wind farms companies, with 1,373 turbines and 132 planned or 

proposed as of May 2015 (see table 8).

• LNG facilities proposed along Brownsville Ship Channel near Port Isabel;

three facilities had “prefiling” applications with the FERC as of April 2015.

• Risk of shale development in the Eagle Ford Shale Region.

• Port development and coastal wind farms could 

Coastal Corridor between Boca Chica and BahLower Rio Grande Valley NWR), and Laguna 

• USFWS needs to be aware of the FERC process 

maintain its role in proposal reviews and mitig

• Oceanic contamination could increase if LNG d

Brownsville Shipping Channel.

• Lower Rio Grande Valley Development Counci

Government Alliance, to explore shale oil devedevelop a working relationship with the Counc

Industrial development • Space X, a private launch facility for rockets (payloads about 4,500 kg

[9,920 pounds]) adjacent to the 8,903-ha (22,000-acre) Boca Chica tract. 

• Proposed STARGATE facility associated with Space X near Boca Chica 

tract, a public-private association between the University of Texas Brownsville’s Center for Advanced Radio Astronomy and Space X.

• Continued growth of the industrial border zone.

• Space X launches will require periodic closure o

(3-mile) radius of launch site.

• If Space X buys private inholdings in the 5-km 

connectivity of Boca Chica with non-USFWS conservation activities. 

• Monitoring effects of industrial growth on STR

along the Rio Grande.

• Maquiladoras (Mexican factories) constantly ex

bridges.

Border issues • 18 of 21 segments of the LRGV border fence (91 km [57 miles])

completed by 2008. 

• Number of border-patrol agents increased from 418 in 1988 to 3,064 in

late 2014.

• In 2014, the number of apprehensions of Mexicans illegally entering the

United States fell to a low of 229,178 after peaking at 1,600,000 in 2000.

• USFWS to work cooperatively with border patr

growing disturbance (for example, increased vhorse patrols).

• More patrol disturbance means less illegal hum

Lower Rio Grande Valley NWR, but ongoing dminimized.
Table 12. Present-day conservation and management challenges facing the Lower Rio Grande Valley and management concerns and options, geneleast challenging or severe.—Continued [LRGV, Lower Rio Grande Valley; LNG, liquefied natural gas; FERC, Federal Energy Regulatory Authority; NWR, national wildlife refuge; USFWS, U.S. Fish and Wildlife Sekm, kilometer; STRC, South Texas Refuge Complex; cm, centimeter; ROW, right of way; %, percent; m, meter; <, less than] Challenges Key impacts and metrics Management concerns and 

Human population growth • Increase in United States human population in the LRGV from about 400,000 in 1960s to 1,317,156 in 2013, perhaps reaching 3 million by 2050. • Similar increase in Mexico; for example, 633,730 people in Reynosa and 462,157 in Matamoros in 2013. • Increased tourism to the LRGV and South Padre Island, with proposal to build a second causeway near the south end of Laguna Atascosa NWR. • Little can be done to curb population growth, bu

zoning and restrictions to protect open spaces coutcomes and improve human health. • Added pressure to convert agricultural land, witurbanized areas. • Increased danger from Cartel activity in Mexicoto legally move to the United States LRGV. • Need for expanded transportation infrastructure fragments habitat and increases wildlife-vehicldeath for Leopardus pardalis albescens (northe

• Plans to complete STRC’s Coastal Corridor com

second causeway to South Padre Island. Invasive species • At least 20 invasive plant species in the LRGV (see table 10). • Two major exotic large mammal species (Boselaphus tragocamelus [nilgai] and Sus scrofa [feral pig]). • Native species outcompeted by invasive species

• Major flooding in 2010 facilitated spread of dist

ramosissima (saltcedar) in the lower Rio Grandriparian tree species, greatly modifying the Rio 

• Invasive grasses often develop monocultures in 

increasing risk of fast-moving and fine-fuel fire

• Exotic nilgai are controlled by the U.S. Departm

minimize transmission of tick-borne diseases t

Climate change • Gulf Coast of the United States has experienced greater sea-level rises over the past 100 years (20.3–101.6 cm [8–40 inches]) than global averages (10.2–20.3 cm [4–8 inches]). • Projected relative sea-level rises by the year 2100 are 0.2–0.61 m (0.66–2 feet) at Port Mansfield and 0.34–0.75 m (1.1–2.5 feet) at South Padre Island. • Temperatures will continue to rise, with more severe droughts and more erratic fluctuations. • Elimination or modification of coastal habitat fo

marine turtle nesting from sea-level rise. • Laguna Madre could become too deep for tidal 

• Erratic climate cycles, related to temperature an

normal, hampering restoration efforts in the LoNWR. 
Table 12. Present-day conservation and management challenges facing the Lower Rio Grande Valley and management concerns and options, geneleast challenging or severe.—Continued[LRGV, Lower Rio Grande Valley; LNG, liquefied natural gas; FERC, Federal Energy Regulatory Authority; NWR, national wildlife refuge; USFWS, U.S. Fish and Wildlife Sekm, kilometer; STRC, South Texas Refuge Complex; cm, centimeter; ROW, right of way; %, percent; m, meter; <, less than]

Challenges Key impacts and metrics Management concerns and 

Habitat restoration • Habitat fragmentation a threat from human development pressure (more and 

wider roads, more and larger utility infrastructure, etc.).

• Restoration is a key to habitat management in the LRGV; for example, more 

than 3 million seedlings were planted in the Lower Rio Grande Valley NWRfrom 1995 to 2011.

• Land purchases by USFWS more complicated now because of increasing

land value and speculation with expanding urbanization (see above).

• Need to develop a strategy for ecological restor

Rio Grande Valley NWR.

• Retired farmlands do not revert to lush diverse f

in the LRGV was so widespread and years of alittle to no seed bank is left and soils tend to be 

• Ability to reforest former agricultural land near 

by limited availability of native seedlings of th

• Lack of or limited availability of commercially 

ROW plantings for use by other agencies and (

• Nonnative grasses or annual grasses still used fo

etc.) plantings, further impacting diversity of n

Water availability • Water will eventually be limited in the LRGV, but sufficient water rights for 

STRC’s needs are currently available.

• Farms and ranches in Cameron and Hidalgo Counties decreased by 35,622 

ha (88,025 acres) from 1997 to 2007―most loss to urbanization.

• Irrigated crops still important demand on water but irrigated land in 

Cameron and Hidalgo Counties decreased 16% from 1997 to 2007―most loss to urbanization.

• Lower Rio Grande Valley NWR has 2,713.7 hec

feet) of Class B water rights for annual use.

• 13 cooperative farmers use Lower Rio Grande V

have access to water for irrigation. 

• Cooperative farmers not allowed to plant crops 

(for example, cotton or sugarcane).

Pesticides, 

contamination, and pollution

• 98 pesticides commonly used in the LRGV in 1998; today, at least 23 of 

those banned and 8 others under restricted use by the U.S. Environmental Protection Agency.

• In 2008, six landfills with an average of 12 more years of use handled 

410,000 tons per year from the LRGV.

• Air quality in the LRGV is affected by vehicle exhaust, industrial/power 

plant emissions, and seasonal burning of mostly sugarcane.

• Fast-growing human population in the LRGV re

landfills (with a shortage of places to put them)other depositories for human wastes. 

• Increased risk of environmental contamination a

pipelines to move gas and (or) oil from wells wLRGV. 

• Intensive use of pesticides and other chemicals 

because of their effects on biological resources
78 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Wildlife Corridors

Wildlife corridors of native habitats connecting 

nearby and even isolated tracts in the STRC have been 

consistently identified as important assets in need of 

expansion, connection, and restoration (fig. 23); for example, 

in the comprehensive conservation plans for Lower Rio 

Grande Valley NWR and Santa Ana NWR (USFWS, 1997) 

and for Laguna Atascosa NWR (USFWS, 2010e). These 

corridors are critically important for reestablishment of 

habitats used by trust resources in the LRGV and beyond. 

Acquisition, protection, and restoration of optimal and even 

currently suboptimal habitats are critical in management of 

migratory birds and recovery of numerous rare, threatened, 

or endangered species in southern Texas. Given significant 

changes in the landscape of southern Texas since earlier 

conservation plans were written, including but not limited to 

exponential population growth and urban expansion, landacquisition authorities of the STRC’s three NWRs may not 

be enough to provide habitat resources needed to support 

trust resource responsibilities indefinitely; thus, an alternate 

expansion-planning document (land protection plan) is being 

prepared to support STRC’s strategic habitat planning. 

The USFWS uses a variety of information to assess 

where strategic acquisition of additional land would be the 

most useful and logistically possible to enhance wildlife 

corridors (e.g., interpretation of aerial imagery, existing and 

predicted barriers to wildlife dispersal, and models of sealevel rise predicted from climate change). Five corridors for 

strategic acquisition have been identified as critical areas 

to support wildlife conservation in the LRGV: Rio Grande 

Corridor, Ranchito Corridor, Coastal Corridor, North Corridor, 

and Ranchland Corridor (fig. 23). From its creation in 1979, 

the Lower Rio Grande Valley NWR has been familiarly called 

the “Wildlife Corridor,” reflecting the still-apt priority to 

conserve an east-west linear corridor along the Rio Grande 

from the Gulf inland to benefit the unique riparian plant 

communities and animal species, such as the endangered 

northern ocelot.

The USFWS has also worked with agencies in Mexico 

to establish three international wildlife corridors that are 

anchored to State- and federally protected natural areas in 

northern Mexico and Federal properties in southern Texas, 

(i.e., Sierra Picachos-Rio Grande International Wildlife 

Corridor, Rio Grande Corridor [in this case, on both sides or 

either side of the Rio Grande from the Boca Chica tract on the 

Gulf to Falcon Reservoir in the western LRGV], and United 

States-Mexico Coastal Corridor). The Canada/Mexico/U.S. 

Trilateral Committee for Wildlife and Ecosystem Conservation 

and Management has endorsed these international corridors.

The Rio Grande Corridor serves as a major artery for 

the long-term sustainability of the diverse ecosystems in 

the LRGV (fig. 23) and, in fact, motivated the creation of 

the Lower Rio Grande Valley NWR in 1979 (Jahrsdeorfer 

and Leslie, 1988). Protecting riparian habitats along the Rio 

Grande and in coastal areas could provide buffers against 

climate change. Riparian habitats along the Rio Grande 

provide habitat for some of the most rare migratory bird 

species or species with the most restricted distributions in 

the United States (table 6). The Rio Grande Corridor could 

permit dispersal of northern ocelots between conservation 

areas in Mexico and the United States, as demonstrated by 

the female that moved from Mexico to Santa Ana NWR and 

raised two offspring in 1992–96 (B.R. Winton, oral commun.). 

Genetic analyses showed that the female was genetically more 

similar to northern ocelots in Mexico than those in Cameron 

or Willacy Counties (Walker, 1997). She presumably had a 

mate nearby, but he was never documented. Another male was 

found near the Rio Grande east of Brownsville, Tex., in 1998 

that travelled >30 km (>18 mi) to Laguna Atascosa NWR. The 

full development of the Rio Grande Corridor could also protect 

essential habitat for several endangered or rare plants (table 5). 

The Coastal Corridor will complete the United States 

contribution of the binational corridor to connect the northern 

ocelot population in Laguna Atascosa NWR to the population 

in Tamaulipas, Mexico, through the Natural Protected Area 

in the Lower Laguna Madre area in the United States and the 

Delta del Rio Bravo in Mexico, separated by the Rio Grande 

Delta (fig. 23). Acquisition of land to complete the Coastal 

Corridor could protect some of the highest priority areas that 

have had recent use by northern ocelots. It could provide an 

opportunity for dispersal of northern ocelots between Laguna 

Atascosa NWR and Mexico through natural dispersal events. 

The Ranchito Corridor could provide an alternative route 

for northern ocelots from the Coastal Corridor to disperse to 

other protected areas along the Rio Grande Corridor and along 

existing, but perhaps tenuously protected, corridors in northern 

Mexico (fig. 23). The area has abundant wetlands, including 

many resacas. Major routes within the Ranchito Corridor could 

protect habitat from some of the modeled predictions of sealevel rise (Intergovernmental Panel on Climate Change, 2007, 

2014). If sea-level rise is worse than predicted in southern 

Texas (e.g., Schmandt and others, 2011), inland movement 

of terrestrial species such as the northern ocelot and the Gulf 

Coast jaguarundi could be possible through the Ranchito 

Corridor, where disturbed habitats are very restorable. 

The North Corridor is also essential to the long-term 

conservation of northern ocelots because it could reconnect the 

two known populations of northern ocelots in the United States, 

one of which was recently documented on private lands and 

tracts of the Lower Rio Grande Valley NWR (fig. 23). These 

areas contain patches of dense but isolated brush and many 

patches of savannah, open brushlands, and coastal grasslands 

that provide usable habitat for the northern aplomado falcon 

and perhaps transitory and foraging habitat for the northern 

ocelot. The Nature Conservancy, USFWS, and TPWD plan to 

reestablish brushlands on these lands. Some of the successional 

brushlands of this area were cleared periodically to maintain 

grazing land and habitat for game species. Favorable soils 

allow quick reestablishment of brushlands through natural 

processes, and northern ocelots use these areas to forage at 

night. Restoring parts of these areas to dense brushlands could 
Conclusion 79

provide more core-area habitat of northern ocelots and the 

Gulf Coast jaguarundi but still contain adjacent grasslands as 

productive foraging areas for northern aplomado falcons. 

The Ranchland Corridor is in the recently discovered 

range of northern ocelots around the saline lake area of the 

LRGV, and conservation in this area could connect northern 

ocelots and other terrestrial species with the North Corridor 

(fig. 23). The North Corridor connects Laguna Atascosa 

NWR northward to the Lower Rio Grande Valley NWR 

conservation easements, where northern ocelots reside, and 

onto the La Sal Vieja area where a future crossing on U.S. 

Highway 77 will be constructed to facilitate safe wildlife 

movements. The North Corridor will expand westward to 

the protected area adjacent to the northern ocelot population 

in Willacy County and is critical for increasing long-term 

survival and recovery of the northern ocelot in the United 

States. The Ranchland Corridor has much in common with 

the North Corridor and is critical to the recovery of the 

northern ocelot and northern aplomado falcon because it 

provides similar brushy and grassy habitats. Continuing to 

connect and expand these corridors with new land acquisition 

could greatly enhance recovery of endangered species and a 

multitude of other unique flora and fauna in the LRGV. 

Staff of the STRC frequently evaluates and modifies, if 

needed, its preferred strategy to spend future funds on land 

acquisition. These evaluations take into account those lands 

already under STRC management, connectivity opportunities 

to complete corridors including the Rio Grande Corridor, 

ongoing development pressures such as Space X, proposed 

liquefied natural gas facilities near Port Isabel, new and 

expanding wind farms, a second access causeway to South 

Padre Island, associated land-value speculation, and impacts 

the border fence will have on any future efforts (table 12). 

Infrastructure associated with some of these activities 

probably affects north-south movements of terrestrial 

wildlife near the Rio Grande. Where the border fence and 

associated border-patrol activities are sufficiently north of 

the Rio Grande, vegetated areas remain along the river itself. 

Therefore, in such areas, it seems most useful and efficient to 

acquire properties that remain available adjacent to the Rio 

Grande and existing tracts of the Lower Rio Grande NWR. 

This effort could further the ability of the STRC to provide 

a contiguous stretch of habitat that will allow restoration 

and preservation of riparian forests. Concurrently, the STRC 

could divest itself of tracts with low biodiversity value 

that are isolated from adjacent tracts or are surrounded by 

urbanization with no reasonable expectation that the USFWS 

will acquire additional lands within the area. Examples 

include the Goodfields tract and certain tracts in the 

Brownsville, Tex., metropolitan area (e.g., Jeronimo Banco, 

Champion Bend, and Villa Nueva) that are significantly 

degraded or where they no longer serve a corridor function. 

These tracts could be exchanged for tracts that better meet 

conservation goals of the STRC, unless they could be used 

for public education and involvement.

Conclusion

As this synthesis of mostly LRGV-centric peer-reviewed 

literature since the late 1980s attests, the LRGV is one of the 

most biologically unique areas in the entire United States, 

with much of the flora and fauna found nowhere else north of 

Mexico. Scientists have long been attracted to the LRGV to 

study its varied abiotic and biotic characteristics on land and 

in fresh, estuarine, and marine waters. Very few, if any, fourcounty areas in the United States have been the site of such 

varied scientific investigation and resulting publications (table 

1). The LRGV also draws ecotourists from across the country 

and abroad, who come to experience subtropical to semiarid 

wildlands and wildlife-watching opportunities that can be 

found only in the four southernmost counties of Texas. Visitors 

support many local businesses, contributing significant capital 

into an otherwise impoverished region.

It is relatively straightforward to list, and even rank, the 

conservation challenges in the LRGV (table 12), but for many 

challenges, it is very difficult to predict outcomes precisely 

and likely more a matter of dealing with them as they present 

themselves―some slowly and insidiously, others quickly and 

more obviously. Climate change can only be measured over a 

long term, but loss of wildlands to urbanization and resulting 

fragmentation can be assessed quickly. Climate change 

challenges our approach to conservation of biodiversity, 

and it is becoming clear that integration of disciplines (e.g., 

social science, urban planning, and ecology) is required 

and that planning must extend “beyond reserves and into 

human-occupied landscapes” (Heller and Zavaleta, 2009, 

p. 14). Precise prediction of how climate change will affect 

a particular area and its ecosystems is still very difficult, 

but “identifying primary local climate drivers,” climate 

sensitivities, and risk tolerances of ecosystems/communities 

with conservation concern is easier to accomplish (Snover 

and others, 2013, p. 1147). In the LRGV, for example, the 

conservation value of existing tracts of protected land in the 

STRC might be compromised if species shift to areas not 

in the STRC (Carroll and others, 2010; Monzón and others, 

2011), so expanding property and corridors could be useful 

conservation objectives.

In many conservation plans, fragmentation and lack of 

connectivity of important wild landscapes are understood to be 

negative impacts, but only 30 percent of States in the United 

States have specific criteria and (or) plans to identify and 

link important wildlife areas (Lacher and Wilkerson, 2013). 

In sharp contrast, the USFWS at the STRC has a clear plan 

to establish five wildlife corridors with viable connections 

of key protected habitats in the United States and Mexico. 

Well-defined goals of corridors need to be expressed in the 

context of six ecological functions that can be very species 

specific; a corridor can represent a conduit, a habitat, a 

filter, a barrier, a source, and (or) a sink, or a combination 

depending on its dimensions, location, and habitat quality 

(Hess and Fischer, 2001). A recent meta-analysis of 35 

studies demonstrated that movement of organisms increased 
80 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

about 50 percent between habitats connected by corridors; 

movements were more evident for plants, invertebrates, and 

nonbird vertebrates than for bird species (Gilbert-Norton and 

others, 2010). Nevertheless, monitoring of newly established 

corridors could be needed because even well-defined and 

well-intentioned corridors could cause negative effects, such 

the spread of invasive species (e.g., fire ants―Resasco and 

others, 2014) or decrease nest success of particular birds 

susceptible to nest predation (Weldon, 2006). Generally, 

however, wide corridors with edges that meld into the 

surrounding vegetation matrix tend to minimize these 

problems (Haddad and others, 2014).

Overall, LRGV-centric research published since 

Jahrsdoerfer and Leslie (1988) was driven by a few species 

(e.g., ocelots) or topics (e.g., seagrasses and agricultural 

pests), and it generally lacked interdisciplinary outlooks (e.g., 

how human activities and impacts impinge on conservation 

needs and success, table 1). Some collected references in this 

report (e.g., appendixes A–C) could be used by managers 

of the STRC and other State and Federal agencies with 

responsibilities for, or activities than impinge on, natural 

resources in the LRGV to direct future research in areas of 

priority and deficiency. Currently, there is little interagency 

prioritization of integrated research needs and collective 

direction reflected in the published research reviewed herein, 

and as a result, each agency typically proceeds by itself, with 

research foci only on its mission. Correcting this deficiency 

could bring research focus to areas of mutual concern and 

need, particularly with regard to conservation in the LRGV.

A rapidly increasing human population in the LRGV 

will be the major conservation challenge for many years to 

come. As the population in the LRGV rises in the coming 

decades, threats to conservation that we see now (table 12) 

will only become more severe. As more people make the 

LRGV their home, resources such as water and land will 

become increasingly difficult to obtain. Land prices have 

already risen sharply in the LRGV since the 1990s, and as 

such, government land-acquisition guidelines now limit 

the ability of the STRC to acquire additional land. As the 

human population and land prices in the LRGV increase, 

it will become more difficult for the STRC to acquire land. 

Because of increased conservation threats and the likelihood 

of continually increasing land prices in the LRGV, this could 

be the last and best time to purchase land to complete the 

wildlife corridors associated with the Lower Rio Grande 

Valley NWR, Laguna Atascosa NWR, and international 

interests. 

The LRGV obviously has great biological richness, but 

it now faces great conservation challenges from a century 

or more of use and modification by increasing numbers 

of humans, leaving behind aquatic and terrestrial “novel 

ecosystems” (Hobbs and others, 2006, p. 1; Marris, 2009). 

These ecosystems “arise either from the degradation and 

invasion of ‘wild’ or natural/seminatural systems or from 

the abandonment of intensively managed systems” (Hobbs 

and others, 2006, p. 2). Marris (2009, p. 450) referred to the 

resulting matrix of natural, managed, and urbanized areas as 

our “ragamuffin Earth.” Because such matrices dominate so 

many areas around the world, particularly those juxtaposed 

with rapidly growing or already large human populations, as 

is the case in the LRGV, a growing number of ecologists and 

restoration specialists are calling for theoretical and practical 

ways to consider “novel ecosystems” valuable, to various 

degrees, for conservation efforts; this is in part because of 

great cost and difficulties in completely restoring such areas 

to some notion of what they were in the past (Hobbs and 

others, 2006)―a challenge that restoration efforts in the 

LRGV have faced. Along with conservation agencies such 

as the USFWS, urban ecologists and planners have growing 

responsibilities to help maintain and restore biodiversity and 

the ecosystem services it provides (e.g., Marzluff and Ewing, 

2001; McKinney, 2002; Hobbs and others, 2006, 2011; Pickett 

and others, 2013; Palomo and others, 2014).

Conservation challenges facing the LRGV are among 

the most difficult in the United States. Continued careful 

syntheses of existing and future information collected in the 

LRGV are needed on many biological and sociological topics 

to avoid spending precious time and resources rediscovering 

something that it is already widely known and accepted. 

Quick response would be beneficial to address contemporary 

ongoing challenges such as climate change, water availability, 

energy and industrial development, spread of invasive species, 

and habitat loss and fragmentation caused by urbanization. 

Rapidly growing human populations compete with extensive 

agricultural lands for increasingly rare resources in an already 

compromised natural landscape in the LRGV. Complexities of 

a guarded international borderland add pressure to the small 

patches of native habitat that remain along the lower Rio 

Grande. Large connected corridors of restored native habitat 

could perhaps be the best chance of maintaining, and even 

enhancing, the exceptional biodiversity in the LRGV in the 

face of exceptional human demand. 

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Appendixes 113

Appendixes
114 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley
Appendix A 115

Appendix A. Research conducted on seagrass of the Lower and Upper Laguna Madre since the early 1990s.

Laguna Madre

Main topical emphasis Lower Upper

Algal “brown tide” Onuf (2000) Dunton (1990, 1996);

Montagna and others (1993);

Whitledge (1993); Onuf (1996a); 

Buskey and others (1997, 2001);

Street and others (1997);

Rhudy and others (1999);

Ward and others (2000) 

Elemental metabolism Opsahl and Benner (1993);

Ziegler (1998);

Ziegler and Benner (1998);

Lee (1998);

Lee and Dunton (1999a, 1999b);

Major and Dunton (2000)

Koepfler and others (1993) 

Productivity and biomass/recovery Dunton and Tomasko (1994);

Herzka (1996);

Herzka and Dunton (1997, 1998);

Kaldy (1997);

Hicks and others (1998);

Kaldy and others (1999, 2002); 

Kowalski (1999);

Kaldy and Dunton (2000);

Lee and Dunton (2000);

Ziegler and Benner (2000);

Kowalski and others (2009)

Dunton (1994);

Czerny and Dunton (1995);

Tomasko and Dunton (1995);

Burd and Dunton (2001);

Gutierrez and others (2010)

Changing interspecific seagrass

community dynamics

Onuf (1996b, 2007);

Kaldy and Dunton (1999)

Onuf (1996c, 2007)

Damage (dredging, propeller, etc.)

and restoration

Quammen and Onuf (1993);

Onuf (1994);

Kaldy and others (2004)

Dunton and Schonberg (2002);

Martin and others (2008);

Larkin and others (2009)

Number of papers (by seagrass species) 27 21

 Halodule wrightii (shoal grass) 6 7

 Thalassia testudinum (turtle grass) 11 0

 Syringodium filiforme (manatee grass) 1 0

 Ruppia maritima (widgeongrass) 0 1

 In general or mixed-species research 9 13

Appendix A
116 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Appendix B. Peer-reviewed publications focused on the Lower Rio Grande Valley (referred to as “LRGV centric”) (not including books, 

book chapters, and theses but including some relative monographs in the “Birds of North America” series) on 58 bird species since the 

late 1980s.

Species Number of

peer-reviewed papers Referencesa

Amazilia yucatanensis (buffbellied hummingbird)

1 Chavez-Ramirez and Moreno-Valdez (1999)

Amazona viridigenalis (redcrowned parrot)

1 Enkerlin-Hoeflich and Hogan (1997)

Ammodramus maritimus (seaside 

sparrow)

1 Phillips and Einem (2003)b

Ardea herodias (great blue heron) 1 Mora (1995)

Arremonops rufivirgatus (olive 

sparrow)

2 Wright (1996); Brush (1998b)

Calidris mauri (western 

sandpiper)

1 White and Mitchell (1990)

Camptostoma imberbe (northern 

beardless-tyrannulet)

3 Brush (1999a)b

; Werner (2004); Werner and others (2015)

Cardinalis sinuatus (pyrrhuloxia) 1 Patrikeev (2006)

Charadrius melodus (piping 

plover)

5 Drake (1996a, 1996b); Garza (1997); Drake and others (2001); Mehl and 

others (2003)

Charadrius nivosus (snowy 

plover)

2 Rupert (1997b); Mehl and others (2003)

Coccyzus americanus (yellowbilled cuckoo)

1 Clotfelter and Brush (1995)

Cyanocitta cristata (blue jay) 1 Brush (2000b)b

Cyanocorax yncas (green jay) 3 Clotfelter and Brush (1995); Gayou (1986, 1995)

Egretta rufescens (reddish egret) 1 Huysman (1995)

Egretta thula (snowy egret) 1 Mora (1995)

Egretta tricolor (tricolored heron) 1 Mora (1995)

Falco femoralis septentrionalis

(northern aplomado falcon)

11 Perez (1995); Perez and others (1996); Mora and others (1997, 2008, 2011); 

Keddy-Hector (2000); Brown and others (2003, 2006); Jenny and others 

(2004); Brown and Callopy (2008, 2012)

Falco peregrinus (peregrine 

falcon)

3 Chavez-Ramirez and others (1994); Enderson and others (1995); Henny and 

others (1996)

Geothlypis poliocephala (graycrowned yellowthroat)

1 Lorenz and others (2006)b

Glaucidium brasilianum

(ferruginous pygmy-owl)

3 Wauer and others (1993); Proudfoot and others (1999); Proudfoot and 

Johnson (2000)

Hydroprogne caspia (Caspian 

tern)

1 Mora (1995)

Icterus cucullatus (hooded oriole) 1 Brush (2000c)

Icterus graduacauda audubonii 

(Audubon’s oriole)

4 Brush (2000d); Flood and others (2002); Monk (2003); Monk and Brush 

(2007)

Icterus gularis (Altamira oriole) 5 Brush (1998c); Hathcock and Brush (2004); Werner (2004); Brush and 

Pleasants (2005); Werner and others (2007) 

Leptotila verreauxi (white-tipped 

dove)

6 Boydstun and DeYoung (1985, 1987, 1988); Hayslette (1996); Hogan 

(1999); Hayslette and others (2000)

Limnodromus scolopaceus (longbilled dowitcher)

1 White and Mitchell (1990)

Megaceryle torquata (ringed 

kingfisher)

1 Brush (2009b)

Appendix B
Appendix B 117

Appendix B. Peer-reviewed publications focused on the Lower Rio Grande Valley (referred to as “LRGV centric”) (not including books, 

book chapters, and theses but including some relative monographs in the “Birds of North America” series) on 58 bird species since the 

late 1980s.—Continued

Species Number of 

peer-reviewed papers Referencesa

Micrathene whitneyi (elf owl) 2 Gamel (1997); Gamel and Brush (2001)

Molothrus aeneus (bronzed 7 Clotfelter and Brush (1995); Brush (2000c); Hathcock (2000); Warren 

cowbird) (2002); Kostecke and others (2004); Monk and Brush (2007); Gorton 

(2010); Janecka and Brush (2014)

Ortalis vetula (plain chachalaca) 1 Peterson (2000)

Pachyramphus aglaiae (rose- 2 Brush (2000a)b

; Miller and others (2015)

throated becard)

Parula pitiayumi (tropical parula) 2 Regelski and Moldenhauer (1997); Brush (1999a)b

Patagioenas flavirostris (red- 3 Brush (1998a); Lowther (2002); Breeden and others (2009)

billed pigeon)

Pelecanus erythrorhynchos 1 Chapman (1988) 

(American white pelican)

Petrochelidon fulva (cave 3 Musquiz (2003); Mora and others (2005, 2006)

swallow)

Petrochelidon pyrrhonota (cliff 3 Musquiz (2003); Mora and others (2005, 2006)

swallow)

Pitangus sulphuratus (great 3 Brush (1993); Gorena (1995); Brush and Fitzpatrick (2002)

kiskadee)

Plegadis chihi (white-faced ibis) 1 Custer and Mitchell (1989)

Quiscalus mexicanus (great-tailed 3 Glahn and others (1997); Johnson and others (1989); Wehtje (2003)

grackle)

Recurvirostra americana 1 White and Mitchell (1990)

(American avocet)

Rynchops niger (black skimmer) 2 Custer and Mitchell (1987); King and others (1991)

Sayornis nigricans (black phoebe) 1 Brush (2001)b

Spiza americana (dickcissel) 1 Larkin and others (2002)

Sterna forsteri (Forster’s tern) 1 King and others (1991)

Tiaris olivaceus (yellow-faced 1 Brush (2003)b

grassquit)

Tringa semipalmata (willet) 1 Custer and Mitchell (1991)

Turdus grayi (clay-colored robin) 1 Brush (2000a)b

Tyrannus couchii (Couch’s 2 Clotfelter and Brush (1995); Brush (1999b)

kingbird)

Tyrannus verticalis (western 1 Clotfelter and Brush (1995)

kingbird)

Waterfowl 

Anas acuta (northern pintail) 1 Ballard and others (2004)

Anas clypeata (northern 2 Tietje and Teer (1996); Tietje and Vreeland (1997)

shoveler) 

Aythya americana (redhead) 11 Mitchell (1991, 1992); Moore (1991); Mitchell and others (1992, 1994); 

Woodin (1994, 1996); Custer and others (1997); Skoruppa and Woodin 

(1997); Woodin and others (2008); Ballard and others (2010)

Cairina moschata (Muscovy 1 Brush and Eitniear (2002)

duck)

Dendrocygna autumnalis (black- 2 Fedynich and others (1996); Edmonds and Stolley (2008)

bellied whistling duck)
118 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

Appendix B. Peer-reviewed publications focused on the Lower Rio Grande Valley (referred to as “LRGV centric”) (not including books, 

book chapters, and theses but including some relative monographs in the “Birds of North America” series) on 58 bird species since the 

late 1980s.—Continued

Species Number of

peer-reviewed papers Referencesa

Diving ducks (general) 1 Adair (1990)

Mergus serrator (red-breasted 

merganser)

1 Rupert and Brush (1996)b

Nomonyx dominicus (masked 

duck) 

2 Anderson and Tacha (1999); Eitniear (1999)

Zenaida asiatica (white-winged 

dove)

11 Swanson and Rappole (1992); Tacha and others (1994); Schacht and others 

(1995); Hayslette (1996); Hayslette and others (1996, 2000); Small and 

Waggerman (1999); Sepúlveda and others (2006); Fredericks and others 

(2009); Collins and others (2010); Ruiz (2012)

Zenaida macroura (mourning 

dove)

4 Hayslette (1996); Hayslette and others (2000); Collins and others (2010); 

Ruiz (2012)

Nonspecies specific

Community 16 Gehlbach (1987); Bauer (1993); Muehl (1994); Brush (1995, 2008a, 

2008b); Adair and others (1996); Balin (1996); Cantu (1996); Castillo 

(1997); Rupert (1997a); Brush and Cantu (1998); Fernandez (1999); 

Gallegos (2001); Rupert and Brush (2006); Rappole and others (2007) 

Contaminants (waterbirds) 6 Mora (1996a, 1996b); Corson and others (1998); Wainwright (1998); 

Wainwright and others (2001); Maruya and others (2005) 

Range expansion 1 Brush (2009a)

Total 166

a

White and Mitchell (1990) involved American avocet, long-billed dowitcher, and western sandpiper; King and others (1991) involved black skimmer and 

Forster’s tern; Mora (1995) involved Caspian tern, great blue heron, snowy egret, and tricolored herons; Brush (1999a) involved northern beardless-tyrannulet 

and tropical parula; Brush (2000a) involved clay-colored robin and red-throated becard; Brush (2000c) involved bronzed cowbird and hooded oriole; Hayslette 

(1996) and Hayslette and others (2000) involved mourning dove, white-tipped dove, and white-winged dove; Mehl and others (2003) involved piping plover and 

snowy plover; Musquiz (2003) and Mora and others (2005, 2006) involved cave swallow and cliff swallow; Monk and Brush (2007) involved Audubon’s oriole 

and bronzed cowbird; and Collins and others (2010) involved white-winged dove and mourning dove. These 12 publications were double-counted.

b

New nesting records for the LRGV.
Appendix C 119

Appendix C. Peer-reviewed publications focused on the Lower Rio Grande Valley (referred to as “LRGV centric”) (not including 

popular publications or graduate theses) on northern ocelots by subject categories since the late 1980s.

Subject Number by subjecta Referencesb

Conservation

General and habitat improvement 10 Tewes (1986); Tewes and Everett (1986); Laack (1991); Young (1992); 

Young and Tewes (1994); Haines, Caso, and others (2005); Haines, Tewes, 

Laack, and others (2005); Haines (2006); Haines, Janečka, and others 

(2006); Grigione and others (2009)

Corridors 3 Tewes and others (1995); Tewes and Hughes (2001); Nordlof (2015)

Highway/bridges/artificial lights 4 Fischer (1998); Hewitt and others (1998); Tewes and Blanton (1998); 

Grigione and Mrykalo (2004)

Contaminants 1 Mora and others (2000)

Demography 

Population viability 3 Haines, Tewes, Laack, and others (2006); Haines and others (2007); 

Sternberg and Mays (2011)

Reproduction 1 Laack and others (2005)

Survival 1 Haines, Tewes, and Laack (2005)

Diseases and parasites 3 Mercer and others (1988); Pence and others (1995, 2003)

Drought 1 Tewes and Hornocker (2008)

Field techniques 

Hair snares 1 Weaver and others (2005)

Immobilization 2 Beltrán and Tewes (1995); Shindle and Tewes (2000)

Telemetry (Global Positioning 

System) 

1 Haines, Grassman, and others (2006)

General 7 Navarro-Lopez (1985); Tewes and Everett (1986); Tewes and Miller (1987); 

Tewes and Schmidly (1987); Brown (1990); Navarro-Lopez and others 

(1993); Murray and Gardner (1997)

Genetics and phylogenetics 7 Walker (1997); Janečka (2006); Janečka, Grassman, and others (2006, 2007); 

Janečka, Walker, and others (2007); Janečka and others (2008, 2011)

Habitat use and assessment 12 Shindle (1995); Harveson (1996); Anderson and others (1997); Fischer 

(1998); Shindle and Tewes (1998); Tewes and others (1999); Jackson 

(2002); Shinn (2002); Harveson and others (2004); Haines, Caso, and 

others (2005); Jackson and others (2005); Connolly (2009)

Interspecific (bobcats) 4 Horne (1998); Horne and others (2009); Booth-Binczik and others (2013); 

Nordlof (2015)

Total 61

a

References can appear in more than one category depending on content.

b

A few references published in or just prior to 1988 were not included in Jahrsdoerfer and Leslie (1988) but are included here.

Appendix C
120 An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley

For more information about this publication contact:

David Leslie

U.S. Geological Survey

Oklahoma Cooperative Fish and Wildlife Research Unit

007 Ag Hall

Oklahoma State University

Stillwater, OK 74078

405-744-6342

Publishing support provided by

Lafayette Publishing Service Center

ISBN 978-1-4113-4060-2

9 7 81411 340602

Leslie—An International Borderland of Concern: Conservation of Biodiversity in the Lower Rio Grande Valley—Scientific Investigations Report 2016–5078

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20165078