U.S. Department of the Interior 

 U.S. Geological Survey 

 Fact Sheet 2004-3105 

 October 2004 

 American Alligator Ecology and Monitoring for the 

 Comprehensive Everglades Restoration Plan 

Printed on recycled paper

 Introduction 

 The American alligator once occupied all wetland habitats 

in south Florida, from sinkholes and ponds in pinelands to 

freshwater sloughs to mangrove estuaries. Nearly all aquatic life 

in the Everglades is affected by alligators (Beard, 1938). As a 

top predator in their ecosystem, they undergo an extraordinary 

change in body size and consume different prey items as they 

grow. As ecosystem engineers, alligator trails and holes provide 

refuge for wading birds and fish during the dry season, and 

their nests provide elevated areas for nests of other reptiles and 

germination of plants less tolerant of flooding. 

 Development and water-management practices have reduced 

the quantity and quality of these habitats. Less freshwater 

moves into the mangrove zone, resulting in higher salinities and the filling 

in of creeks (Craighead, 1971). In marl 

prairies, the historic 6- to 9-month 

hydroperiod has been reduced to 3 

months or less. Additionally, water levels may drop 2 feet below land surface 

during the dry season. Because of these 

habitat changes, alligators are now less 

numerous in some habitats (Mazzotti 

and Brandt, 1994). As Craighead (1968) 

declares, “probably no phenomenon 

better typifies man’s desecration of the 

southern Florida wilds than the disappearance of the alligator through his 

unknowledgeable attempts to manipulate nature’s plan of watering the Everglades.” 

 Currently, restoration is beginning 

under the Comprehensive Everglades 

Restoration Plan (CERP, www.evergladesplan.org), which includes more 

than 68 projects ranging from removing 

canals to creating alternative water-storage areas. The goals of the plan include increasing spatial extent 

of natural areas, enhancing quality of existing natural areas, and 

improving abundance and diversity of native plants and animals. 

 An important component of the plan is an adaptive assessment process that will be used to assess the performance of 

CERP and recommend changes as necessary. Part of the adaptive assessment process has included the identification of key 

fauna and habitats that can serve as indicators of success. The 

American alligator was chosen due to its ties to hydrology, 

salinity, and system productivity, all of which should be affected 

during restoration. 

 Many important questions concerning the effects of Everglades restoration on alligator populations remain unanswered 

such as impacts of canal removal, the role of alligator holes as 

aquatic refugia, and effects of hydrology on population growth 

and condition. Furthermore, methods for monitoring and evaluating restoration success have not been developed across the 

entire Everglades system. Additionally, we need to continue to 

update and validate restoration tools such as population models 

for use in alternative selection, performance measure development, and prediction of effects of restoration. This project, a 

cooperative effort between the U.S. Geological Survey (USGS) 

and the University of Florida, will directly address the questions 

outlined above, develop monitoring methods, and validate restoration tools for use in CERP. 

 Alligator Abundance 

 Changes in land use and land cover 

resulting from water management 

have affected alligator populations. 

Some locations that were alligator 

habitat have been converted to agricultural and residential development. 

Other locations are so overdrained 

that alligators only occur in permanent water bodies such as canals or 

ponds, or during periods of extremely 

high water. As Everglades restoration 

targets restoring hydrology of these 

areas, re-occurrence of alligators will 

be an excellent indicator of success. 

 At present, the only long-term 

monitoring of alligators in Everglades 

National Park are aerial surveys for 

nests. The Florida Fish and Wildlife 

Conservation Commission conducts 

limited surveys for alligators in and 

adjacent to Water Conservation Areas 

2 and 3 as part of their public hunt program. Yet, as restoration occurs in ecosystems, such as the rocky glades and the 

mangrove transition zone, it will take more than 10 years for 

dispersing juveniles to become nesting animals. Evaluating the 

relative distribution, abundance, and demography of alligators 

allows for a more rapid and geographically balanced assessment 

of the impacts of CERP projects on target systems. 

 To date, survey routes have been established in estuarine rivers, freshwater canals and marshes extending from the 

mangrove fringe of Everglades National Park (ENP) north 

through Loxahatchee NWR (fig. 1). During these surveys, we 
count, estimate size, and record locations of alligators on more 

than 300 kilometers of airboat trails, rivers, and canals in the 

Everglades. We conduct these surveys 4 times annually to estimate trends in populations. Currently, we are initiating research 

to improve our estimates of the total numbers of alligators 

present on the survey routes using radio-telemetry and statistical estimation techniques. To evaluate restoration success, this 

survey network will ultimately be able to detect change in alligator populations on the order of 5 percent annually over a 2- to 

3-year period. The alligator survey network described above is 

the first system-wide, systematic effort to look at Everglades 

alligators. 

 Alligator Condition 

 Body condition (a relationship between body length and 

weight) of alligators can change over short time periods when 

the amount of prey available changes. In the Everglades, this 

is closely tied to hydrology. As water depth increases, prey 

becomes dispersed across the landscape, alligators find less 

prey, and body condition decreases. As water depth decreases, 

prey becomes concentrated in alligator holes, alligators capture 

more prey, and body condition increases. Of course, if water 

depth decreases too low or remains low for too long, prey 

become less numerous and alligator body condition suffers or 

cannibalism occurs, or both. 

 Condition has been of interest to researchers because of its 

ability to assess how crocodilians are coping with their environment. Measures of body condition can provide a measure of restoration success through an examination of alligators spatially 

and temporally in the Everglades. Condition can be viewed as 

a measure of the quality and accessibility of prey species and 

provide a linkage to lower trophic levels and their success during restoration. Monitoring of condition in both newly restored 

habitats such as the rocky glades and in existing population 

centers is critical to an understanding of effects of restoration. 

 We have established monitoring sites throughout the Everglades to assess body condition and to develop relationships 

between hydrology and body condition. To date, we have captured more than 1,000 alligators (fig. 2) to evaluate body condition across the Everglades. Over the first 4 years of monitoring, 

Figure 1. Alligator survey routes in the Everglades. Figure 2. Location of alligators captured for assessment of body condition in the Everglades.

ARM Loxahatchee NWR

WCA2A WCA3

Tower Camp

WCA3

Holiday Park

WCA3 South

NE Shark Slough

Shark Slough

Shark River

WCA3B

Canal and River

Marsh

0 10

KILOMETERS

EXPLANATION

ARM Loxahatchee NWR

WCA3A WCA2

Tower Camp

WCA3

Holiday Park

WCA3 South

NE Shark Slough

Shark Slough

Shark River

WCA3B

Capture Locations

EXPLANATION

0 8

KILOMETERS
Everglades to assess change in number, size, and types of alligator 

holes and the presence of alligators. Increases in number of alligator holes and alligators that maintain them would be an indication 

of restoration success in overdrained habitats. 

Figure 3. Alligator body condition of alligators in the Everglades from 

1999 to 2004.

Figure 4. Alligator hole density in Water Conservation Area 3.

we have seen a decrease in all areas (fig. 3). While this may be 

a short-term effect due to rainfall patterns in southern Florida, 

it does point out the sensitivity of the monitoring program to 

changes in hydrology and the usefulness of these data during 

restoration. 

 Alligator Holes 

 Alligator holes are excavated from the muck and peat that 

make up the Everglades soil, often down to the limestone bed. 

These depressions provide an aquatic refuge for alligators 

and other aquatic organisms, nest sites for other reptiles, and 

colonization sites for plants in the Everglades. Alligator holes 

provide foraging sites for wading birds, turtles, and snakes and 

also may provide refuge sites for fish during the dry season. 

Fish that make it through the dry season in an alligator hole are 

one source of future fish populations. Furthermore, alligator 

trails free of vegetation around gator hole sites may result in a 

firebreak that provides protection to woody vegetation and various animal species (Craighead, 1968). 

 Although alligator holes and other dry-season refugia have 

long been recognized as a critical component of the Everglades 

ecosystem, only one alligator hole until recently had been studied in detail (Kushlan, 1972). Current studies have concentrated 

on the ecological role and landscape pattern of alligator holes 

in the Everglades. While this understanding can reiterate the 

importance of alligator holes in the Everglades and provide linkages between species necessary for refining ecological models, 

monitoring density and occupancy of alligator holes is essential 

to the evaluation of restoration success. 

 Currently, we are mapping alligator holes throughout the 

Everglades as a baseline for ecological models of alligators, 

fish, wading birds and other species. This effort has identified 

several key relationships critical to restoration, including a 

decrease in density of alligator holes near canals (fig. 4). Fewer 

alligator holes lead to fewer refuges, and feeding sites for wading birds, fish, and other reptiles as canals, for example, are too 

deep for wading bird foraging. Furthermore, we are developing 

a monitoring program to track trends in alligator hole density 

and occupancy. We are planning aerial transects throughout the 

0 10 20 KILOMETERS

Alligator Alley

Miami Canal

Tamiami Trail

EXPLANATION

High density value

Low density value

Density of alligator holes

Canals

WCA3 South

ARM Loxahatchee NWR

ENP Shark Slough

ENP Estuaries

WCA3 Holiday Park

WCA2

Fall

1999

Spring

2000

Fall

2000

Spring

2001

Fall

2001

Spring

2002

Fall

2002

Spring

2003

Fall

2003

Spring

2004

Everglades National Park (ENP)

FULTON’S K BODY CONDITION INDEX Y

C

O

N

DITIO

N IN

DEX ON’S K BOD

12.0

11.5

11.0

10.5

10.0

9.5

9.0

12.5

12.0

11.5

11.0

10.5

10.0

9.5

Water Conservation Areas (WCA’s)
INCREASE

DECREASE

INCREASE OR DECREASE

IN ALLIGATOR POPULATION

DENSITY UNDER THE

RESTORATION SCENARIO

(CERP or D13R) OVER THE

BASE DENSITY (1995 Base)

EXPLANATION

INCREASE

DECREASE

INCREASE OR DECREASE

IN ALLIGATOR POPULATION

DENSITY UNDER THE

RESTORATION SCENARIO

(CERP or D13R) OVER THE

BASE DENSITY (F2050)

EXPLANATION

 Alligator Models 

 Predicting how restoration alternatives will influence wildlife 

populations and community patterns is a task that the USGS and 

its cooperators have undertaken in the “Across Trophic Levels 

System Simulation” (ATLSS) project. This large-scale, spatially 

explicit set of landscape models of hydrologic conditions and 

populations of plants and animals includes the American alligator population model (APM). By applying the APM to proposed 

restoration projects, current hydrologic management schedules, 

and historic conditions, we can examine and compare model 

predictions of effects on alligator population density, survival, 

reproduction, and dispersal. Restoration managers can use model 

output to make decisions about which alternatives to im-plement 

and monitor. 

 Comparisons of base conditions (1995 and 2050) to the 

selected CERP alternative, D13R, suggest that alligator populations would increase after restoration in many of the overdrained 

wetlands of the Everglades (figs. 5 and 6). However, the model 

predicted that some habitats, such as Shark River Slough in ENP, 

which is projected to have increased water depths under CERP, 

will either maintain populations at present levels or experience 

slight declines. Our results concur with predictions made by 

crocodilian biologists working in the Everglades. 

 References Cited 

 Beard, D.B., 1938, Everglades National Park Project: Wildlife Reconnaissance: U.S. 

Department of the Interior, National Park Service. Washington, D.C. 

 Craighead, F.C., Sr., 1968, The role of the alligator in shaping plant communities and 

maintaining wildlife in the Southern Everglades: Florida Naturalist, 41:2-7, 

p. 69-74. 

 Craighead, F.C., Sr., 1971, The trees of south Florida: The natural environments and 

their succession: Miami, Florida, University of Miami Press. 

 Kushlan, J.A., 1972, An ecological study of an alligator pond in the Big Cypress 

Swamp of southern Florida: MS Thesis, University of Miami, Coral Gables, 

Florida. 

 Mazzotti, F.J., and Brandt, L.A., 1994, Ecology of the American alligator in a seasonally fluctuating environment; in S. Davis and J. Ogden, (eds.), Everglades: The 

Ecosystem and its Restoration: Delray Beach, Florida, St. Lucie Press, p. 485-505. 

 For further information on USGS alligator 

research in the Everglades, please contact :

 Ken Rice 

 U.S. Geological Survey 

 3205 College Ave 

 Davie, FL 33314 

E-mail: ken_g_rice@usgs.gov

For further information on University of 

Florida alligator research in the Everglades, 

please contact: 

 Frank Mazzotti 

 University of Florida 

 3205 college Ave 

 Davie, FL 33314 

E-mail: fjma@ufl.edu

Figure 5. Alligator Population Model 

simulations of CERP versus 1995 Base.

Figure 6. Alligator Population Model 

simulations of CERP versus F2050.

 We will continue to update and validate the model as further 

research results become available. For example, we have developed “virtual” night-light survey routes that correspond with our 

monitoring network to track how well the model predicts actual 

conditions. As new alternatives are proposed, the model can be 

used to assist managers in making informed decisions. 

 Restoration Success 

 During Everglades restoration, managers will require predictions about how proposed changes might affect populations of 

indicator species and monitoring data to ensure that success 

has occurred. This information will be required over short- and 

long-term periods and across the spatial extent of the Everglades. Our program provides a set of tools including predictive 

models (APM) and monitoring over short (body condition), 

medium (alligator distribution), and long (alligator hole) time 

periods that occurs across the breadth of the Everglades system. 

The USGS Priority Ecosystem Science (PES), Program that 

supports this research has been instrumental in acquiring further 

funding from the U.S. Army Corps of Engineers and the CERP 

Monitoring and Assessment Plan to continue this research 

including new monitoring components such as alligator 

nesting.