BIOLOGICAL REPORT 82(10.136)

APRIL 1987 Ltbrary

~H;o:1al Wrtla .ds Rt!$ureh Cl!f'!t,r

U. S. Fisl'l and Wlldl1fl Service

7 0 C~~': l'\ c')m(" t"ol.llevard

LJfal l.~ic La. 70;:'06

HABITAT SUITABILITY INDEX MODELS:

AMERICAN ALLIGATOR

Fish and Wildlife Service

·· - ' epartment of the Interior

SK

361

. U54

no . 82 -

ID . 136
MODEL EVALUATION FO~'

Habitat models are designed for a wide variety of planning applications

where habitat infonnation is an important consideration in the decision

process. It is impossible, however, to develop a model that perfonns equally

well in all situations. Each model is published individually to facilitate

updating and reprinting as new infonnation becomes available. Assistance from

users and researchers is an important part of the model improvement process.

Please complete this fonn foll owi ng appl ication or revi ew of the model. Feel

free to include additional infonnation that may be of use to either a model

developer or model user. We also would appreciate infonnation on model

testing, modification, and application, as well as copies of modified models

or test results. Please return this form to the following address.

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Thank you for your assistance.

Species~ __ Geographic

Location. _

Habitat or Cover Type(s) --

Type of Application: Impact Analysis Management Action Analysis~ _ Baseline Other------------------------ Variables Measured or Evaluated~ _

Was the species information useful and accurate? Yes No___

If not, what corrections or improvements are needed? _
Were the variables and curves clearly defined and useful? Yes No

If not, how were or could they be improved? __

Were the techniques suggested for collection of field data:

Appropriate? Yes No

Clearly defined? Yes No

Easily applied? Yes No

If not. what other data collection techniques are needed?

Were the model equations logical? Yes No

Appropriate? Yes No

How were or could they be improved?

Other suggestions for modification or improvement (attach curves.

equations. graphs. or other appropriate information) ___

Additional references or information that should be included in the model:

Model Evaluator or Reviewer Date --------------------- ----------- Agency _

Address _

Telephone Number Comm: FTS _
Biological Report 82(10.136)

April 1987

HABITAT SUITABILITY INDEX MODELS: AMERICAN ALLIGATOR

by

John D. Newsom

Professional Wildlife Service

6973 N. Rothmer Drive

Baton Rouge, LA 70808

Ted Joanen

Rockefeller Wildlife Refuge

Louisiana Department of Wildlife and Fisheries

Grand Chenier, LA 70643

and

Rebecca J. Howard

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Performed for

National Wetlands Research Center

Research and Development

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report should be cited as:

Newsom, J.D., 1. Joanen, and R.J. Howard. 1987. Habitat suitability index

models: American alligator. U.S. Fish Wildl. Servo 8iol. Rep. 82(10.136).

14 pp.
PREFACE

The habitat suitability index (HSI) model for the American alligator is

intended for use in the habitat evaluation procedures (HEP) developed by the

U.S. Fish and Wildlife Service (1980) for impact assessment and habitat

management. The model was developed from a review and synthesis of existing

information and is scaled to produce an index of nesting alligator habitat

suitability between 0 (unsuitable) and 1 (optimally suitable). Assumptions

i nvo 1ved in deve1opi ng the HSI model and guide 1i nes for model app1i cations,

including methods for measuring variables, are described.

This model is a hypothesis of species-habitat relationships, not a statement of proven cause and effect. The model has not been fi e1d-tested. For

this reason, the U.S. Fish and Wildlife Service encourages model users to

convey comments and suggestions that may help increase the utility and effectiveness of this habitat-based approach to fish and wildlife management.

Please send any comments and suggestions you may have on the HSI model to the

following address.

Information Transfer Specialist

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

Page

PREFACE............................................................ iii

ACKNOWLEDGMENTS vi

INTRODUCTION '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Distribution and Commercial Inportance 1

Life History Overview............................................ 1

HABITAT REQUIREMENTS 2

Food 2

Cover 3

HABITAT SUITABILITY INDEX (HSI) MODEL 5

Model Applicability 5

Mode1 Descri ption 5

Suitability Index (SI) Graphs for Model Variables 7

Component Index (CI) Equations and HSI Determination 10

Field Use of Model 10

Interpreting Model Outputs .. 11

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

v
ACKNOWLEDGMENTS

Deve1opment of the habitat sui tab] 1i ty index model and narrative for the

American alligator was reviewed by L.L. McNease. Louisiana Department of

Wildlife and Fisheries. Grand Chenier. and Robert H. Chabreck. Louisiana State

University. Baton Rouge. Evaluation of the model structure and functional

relationships was provided by personnel of the U.S. Fish and Wildlife Service's

(FWS) National Wetlands Research Center. Model and narrative reviews were also

provided by representatives of the FWS Division of Ecological Services in Corpus Christi. Texas; Houston. Texas; Lafayette. Louisiana; and Panama City, Florida. Funding for model development and publication was provided by the

FWS. Patrick Lynch prepared the cover illustration for this report.

vi
AMERICAN ALLIGATOR (Alligator mississippiensis)

INTRODUCTION

Distribution and Commercial Importance

The American alligator (Alligator mississippiensis) is a North American

reptile of the order Crocodilia, family Crocodylidae. It and A. sinensis, a

species found in China, are the only known members of the genus existing in the

world (Ditmars 1964). Other closely related members of the order Crocodilia,

the crocodiles, number 21 species worldwide and are represented by a single

species in the United States, Crocodylus acutus. This species has a limited

range in southern Florida.

The American alligator is characteristically a resident of river swamps, lakes, bayous, and marshes of the Gulf and Lower Atlantic Coastal Plains from

Texas to North Carolina (Conant 1958).

Alligator skins have been a highly prized article of commercial trade in

the 1eather industry for many years, and a source of income to trappers and

landowners. Because of this demand and a relatively unrestricted harvest in

the first half of this century, the alligator population declined to its lowest

level about 1960. Growing concern for the future of the species prompted the

Louisiana Department of Wildlife and Fisheries and, ultimately, the Louisiana

Legislature to impose certain harvest restrictions in 1960. All commercial

harvest in Louisiana was prohibited in 1964. In 1967, the species was placed

on the Federal endangered species list throughout its range (National Fish and

Wildlife Laboratory 1980). Response to these actions was spectacular. By 1972

the population had increased to the point where Louisiana petitioned the U.S.

Fish and Wildlife Service to change the status of the alligator to allow

controlled harvest (Palmisano et al. 1973). The status was changed to

"Threatened by Similarity of Appearance" [T(S/A)] in a limited number of

parishes (counties) in Louisiana in 1975, and restricted harvesting began. The

current Federal status of the American alligator is as follows: T(S/A) in

Texas and Louisiana, threatened in Flori da and areas of Georgi a and South

Carolina, and endangered in the remainder of its range (Federal Register, 50

CFR 17.11 and 17.12, January 1, 1986).

In recent years alligator meat has become a highly regarded gourmet food

item. The market is developing to the extent that with adequate slaughter and

processing facilities, it may equal the commercial value of the skins.

Life History Overview

Alligators are amphibious reptiles requiring both land and water to meet

their needs; they prefer fresh to brackish water and generally avoid saline

1
habitats. Adults often dig dens at the edges of rivers or lakes and in

marshes; dens have underwater entrances that 1ead to an underground ho11 ow

(National Fish and Wildlife Laboratory 1980).

Adult male and female alligators in south Louisiana gather together in the

deeper open water 1akes, canals, and bayous as the spri ng breedi ng season

approaches. Courtship and breeding occur from April through mid-June (Joanen

and McNease 1980). Following the mating season, males remain in the deep water

areas. Males occupied summer home ranges with an average minimum size of 887

ha (Joanen and McNease 1972). The males show no fi de1i ty toward den sites

until late autumn, when they return to established dens and remain in the close

vicinity through the winter. Females disperse to nesting areas after

successful mating and remain until the following spring. The amount of habitat

used by females vari es accordi ng to season; the average home range size for

four adult females during the spring, summer and fall was 8.5 ha (range 2.6 to

16.6 ha) (Joanen and McNease 1970). The female constructs a nest of mud and

any available vegetation. Joanen (1969) described the nesting ecology of

all igators in southwest Louisi ana. The nests averaged 1. 8 min di ameter and

0.6 m in height. Egg laying began about mid June and ended usually in the

fi rst week of July. Eggs averaged 39 per cl utch and hatched after 65 days of

incubation. Females usually released the young, which averaged 23 cm in

1ength, from the egg chamber at hatchi ng by removi ng the top 1ayer of nest

material. About 50% of the young liberated themselves from two nests to which

nesting females failed to return.

Young alligators grow fairly rapidly during the first 10 years, at which

time males average 2.55 m and females 2.10 m. After 10 years, the growth rate

declines rapidly in females; males continue to grow until about age 20

(Chabreck and Joanen 1979). Joanen and McNease (1980) reported that most

alligators, both male and female, were sexually mature at 1.83 m total length.

Captive female alligators raised in outdoor enclosures first nested at 9 yr 10

mo, and 2.10 m total length.

HABITAT REQUIREMENTS

Food

McIlhenny (1935) stated that an alligator wilil eat any living thing small

enough to kill, whether it flies, walks, swims, or crawls. Delaney and

Abercrombie (1986) examined the contents of 350 alligator stomachs collected

during late summer and early fall by hunters on three lakes in north-central

Florida. They found that alligators ingested a wide variety of food items;

variation in diet was attributed to type of habitat occupied, prey species

encountered, prey vulnerability and size, and alligator size. Fish species

were the most important food item in terms of percentage volume (57%), but

ranked second to snailsin frequency of occurrence. Invertebrates (i ncl uding

snails) had a low (0.8%) perceAtage volume. Reptiles ranked second in

percentage volume (23%) and third in frequency of occurrence. Turtles

(primarily Pseudemys nelsoni, P. floridana, and Sternotherus odoratus) were the

most common reptlle and the most important food for alligators exceedin~ 3 m in

length. Subadult alligators consumed more invertebrates and terrestr-ial prey

2
(primarily round-tailed muskrat [Neofiber alleni] and marsh rabbit [Sylvilagus palustris]) than did larger alligators. Adult females also consumed more

mammals than did adult males. The variation in diet by size and sex was

probably due to the tendency of large alligators, primarily males, to spend

more time in open water. Delaney and Abercrombie noted that alligators in

Florida consumed more fish and turtles and fewer mammals than did alligators in

other parts of this species' range. Diverse wetland habitat that can provide a variety of foods was suggested as an essential component of alligator habitat.

McNease and Joanen (1977) studied adult alligator diets in various marsh

types in southwestern Louisiana and found vertebrates (mammals, birds,

reptiles, and fish) to be the most important class of foods taken. Based on

percentage weight of food items in the stomachs of 314 freshly killed

alligators in September of 1972 and 1973, the most important foods in fresh and

intermediate marshes were mammals, arthropods (crabs, shrimp and crawfish),

fish, birds, and reptiles, respectively. In brackish marshes, fish,

arthropods, mammals, birds and reptiles, respectively, were most important.

Valentine et al. (1972) examined stomach contents of 413 alligators taken

during 1961-62 and 1964 _by hunters on Sabine National Wildlife Refuge (southwest Louisiana), where water salinities range from near-fresh to

brackish. They found crustaceans and fish to be important foods for alligators 1.5 m long and greater. Reptiles, birds, and mammals were also well

represented. A comparison of this study with that of Giles and Childs (1949),

who exami ned 318 stomachs of all igators kill ed by hunters in 1946, revealed

that mammals were eaten in proportion to their availability. In 1946, remains

of muskrats (Ondatra zibethicus rivalicius) occurred i n 17% of the stomachs

collected, while.those of nutria (Myocastor cl~i6s) were absent. Muskrats were

near the peak of their population cycle in (0'Nea1 1949), but nutria had

not yet become well established in the coastal marshes. In 1961, however,

nutria remains were found in 56% of the alligator stomachs collected and

muskrat remains were absent. Muskrat populations had declined and nutria were

abundant in 1961 (Greg Linscombe, Louisiana Department of Wildlife and

Fisheries, New Iberia; pers. comm.).

Chabreck (1971) examined stomach contents of 20 young Louisiana alligators less than 1.8 m long. Ten were from saline environments and 10 from freshwater

environments. Basing his data on fresh total weight, he found crustaceans,

mostly crawfish and crabs, were the principal foods taken in both areas. This

study suggested that young alligators in saline environments eat less and grow slower than those in freshwater environments.

Investigators studying the food habits of young alligators (less than 1.2

m long) in Florida documented heavy predation on invertebrates. Fogarty and

Albury (1967) found that invertebrates made up about 98% of the diet of 36 alligators: snails (Pomacea paludosa), 66%, and crawfish (Procambarus sp.), 32%.

Cover

Reproduction. Duri ng the breedi ng season in southwest Louisiana,

alligators congregated in deep, open water areas of the marsh (e.g., ponds, lakes, canals), where courtship and copulation occurred (Joanen and McNease

3
1970, 1972). Following successful mating, females moved to marsh characterized

by small, shallow pot holes and ponds interspersed wi th wi regrass (~art ina

pateth). Few nests were placed in marshes where salinities exceeded 10 parts

per ousand (ppt) (McNease and Joanen 1978). Females at Rockefeller Refuge in

Louisiana occupi ed areas ranging from an average of about 2 ha duri ng nest

construction to about 0.1 ha during the first half of the incubation period (Joanen and McNease 1970). In areas considered to be excellent habitat on

Rockefeller Refuge, alligator nests averaged one per 5.2 ha (Ted Joanen, unpub1. data).

Most (80%, n = 315) nestin~ females studied at Rockefeller Refuge from

1964-68 used the natural marsh (l.e., outside impounded areas) and constructed

nests of wiregrass (Joanen 1969). When other plants, including Scirpus robustus, Phragmites communis, Spartina alterniflora, and Scirpus 01neY1t were

present they were used along wlth wlregrass. Baccharis halmlfolla was usually mixed with other nest materials when nests were on or near levees. Goodwin and

Marion (1978) found that alligators in Florida nested close to permanent water

and used a wide variety of plant materials for nest construction.

Alligators in lakes of inland Louisiana nested on sloping banks, usually withi n a few meters of the waterI s edge but sometimes as far as 54 mfrom the

edge. Swamp-dwelling alligators nested on elevated areas including old road

beds and canal banks. Nesting materials consisted of whatever vegetation was

available; in most cases, alligators in swamps use more soil in nest construction than did marsh-dwelling alligators (Dave Taylor, Louisiana Department of Wildlife and Fisheries, Bernice; pers. comm.).

In the Okefenokee National Wildlife Refuge, 90% (n =99) of the alligators studied nested in the wet prairie and 10% in elevated areas of the swamp forest

(Metzen 1977). About two-thirds of nests in the wet prai ri e were on peat batteries (formed from sections of peat that break away, float to the surface,

and become vegetated with various wetland species, primarily maidencane

[Panicium hemitomon]). The remaining nests in the wet prairie were supported by tree ~ouses (elevated areas densely vegetated with various shrubs and

vines). As in other alligator nesting studies, the most readily available

vegetation was used as nest material.

Hatchlings and Immatures. Cover requirements for alligators newly hatched

to about one year of age appear to be met by habitat used by females as nesting and den sites because they usually remain in the vicinity in close association

with the female (McIlhenny 1935).

McNease and Joanen (1974) found immature alligators (from 1.1 to 1.8 m) of

both sexes utilizing deep water areas during summer, autumn, and winter and

moving into shallower areas and the emergent marsh in the spring. They believed

that temperature, interspersion of water and vegetated areas, water levels, and

the availability of food influenced habitat use by immatures. A preference for the intermediate marsh type (as defined by Chabreck 1970) was noted.

Adults. Adult all-igators apparently find suitable cover where permanent fresh to brackish water is available, including lakes, streams~ canals, swamps, and marshes. In coastal Louisiana, the intermediate marsh type contained a

4
greater density of alligators than fresh or brackish marshes (McNease and

Joanen 1978). Joanen and McNease (1970, 1972) reported that adult male

alligators preferred deep water canals and lakes throughout the year. They

bel ieved that the deep waters buffered the effects of extreme temperatures

duri ng the vari ous seasons. Adul t females, however, spent the entire year,

except during the breeding season, in and around their dens in the marsh. In a

north-central Florida lake, Goodwin and Marion (1979) noted that female

alligators were more sedentary than males throughout the year. During summer,

males preferred open lakes while females restricted themselves to surrounding

swamps.

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area and season. The habitat suitability index (HSI) model for

the American alligator is applicable in northern Gulf of Mexico coastal

marshes, principally in Louisiana and Texas. It provides an index to habitat

sui tabi 1i ty duri ng the all i gator nest; ng season, from the onset of mating to

hatching of the eggs. Variables should therefore be sampled during May through

September.

Cover types. This model was developed to evaluate fresh, i ntermedi ate,

and brackish marshes in coastal Texas and Louisiana as habitat for nesting

alligators. These wetlands are described by Cowardin et al. (1979) as

palustrine emergent (fresh) and estuarine emergent (intermediate and brackish).

The fresh marsh is dominated by Panicum hemitomon. Alternanthera, Eleocharis,

and Sagittaria are also common in the fresh marsh. The brackish and

intermediate marshes are dominated by Spartina patens; other common genera are

Sagittaria, Eleocharis, Scirpus, Juncus and Distichlis (Chabreck 1972). The

model is not designed to evaluate riverine or interior lacustrine habitats.

Minimum habitat area. Minimum habitat area is considered to be the

minimum amount of contiguous habitat required in order to be occupied by a

species. Minimum habitat area for the alligator has not been described in the

literature. However, an average of one nest per 5.2 ha of habitat considered

to be excellent has been found on Rockefeller Refuge in southwest Louisiana.

Therefore, it is recommended that the model be app1i ed to areas 1arger than

5 ha.

Verification level. The model was evaluated by biologists in Louisiana

who are active in alligator management or research along the coastal marshes of

the northern Gulf of Mexico. L. McNease, Louisiana Department of Wildlife and

Fisheries, Grand Chenier, and R. Chabreck, Louisiana State University, Baton

Rouge, reviewed the model. Their comments were considered in preparing the

model, but the final version is the responsibility of the authors. The model

has not been field-tested.

Model Description

Overview. This model has been developed for determining the suitability

of coastal marsh as habitat for nesting alligators. Habitat requirements of

5
alligators not nesting are assumed to be provided by an environment that is

suitable for nesting. The model evaluates habitat in terms of a single life

requisite--cover--for breeding adults and for nesting females. As described

previously in this report, the alligator's diet varies greatly and its composi- tion depends on the local abundance of prey species. Food was assumed, therefore, to be adequate and readily available in all areas with appropriate cover;

food as a life requisite was not included in this model. The relationship of

five habitat variables to the cover life requisite for both breeding adult and

nesting female alligators and to the overall HSI value is illustrated in Figure

1. The model should only be applied in wetlands with salinity less than 10 ppt.

Flooding of alligator nests during incubation is a major cause of egg mortality. Flooding may result in almost complete absence of an age class for

coastal populations in areas where storm tides or heavy rainfall occur during the critical nesting period. These factors are, however, unpredictable and

unmanageable and therefore not addressed in the model.

Breeding cover. Alligators breed in relatively deep, open water. Suitability of an area as breeding habitat is influenced by the amount and type of

open water. Open water, defi ned as that wi th 1ess than 10% canopy cover of

emergent vegetation, over 20% to 40% of the wetland area (VI) is assumed to be

optimal for both breeding and nesting. Bayous, canals, and deeper water areas of lakes and ponds are the preferred areas for breeding, but shallow open water

is important for prey species and is used by young alligators. Habitat is

optima1 when 10% to 20% of the open water is bayous, canals or deeper than

1.2 m in ponds or lakes (V~). Suitability decreases as this value increases

above 20%, and habitat becomes unsuitable when bayous, canals, and deep water

represent 100% of the open water.

Habitat Variable Component

VI Percentage of wetland

that is open water

over-breeding

V2 Percentage of open water

that is bayous or canals

VI Percentage of wetland HSI

that is open water

V3 Interspersion over-nesting V4 Percentage of ponded area

with water 15 cm deep

Vs Percentage of substrate

exposed at mean low tide

(Tidal areas only)

Figure 1. Relationship of habitat variables and the cover life requisite for

breeding adults and nesting females in the alligator HSI model.

6
Nesting cover. As mentioned above, 20% to 40% of the wetland area covered

by open water (VI) is assumed to be optimal for nesting female alligators as

well as breeding adults. Two additional variables--the interspersion of

vegetation and open water and the amount of ponded area that retains water--

affect nesting habitat suitability in nontidal wetlands. Optimal habitat has a

hi gh i nterspersi on of water and vegetati on (V3 ) . The categori es provided in

the model for determining the 51 value for V3 are subjective (i.e., high,

medium, or low interspersion). Information on the number of ponds greater than

or equa1 to O. 2 ha per 6 ha of wet1and habi tat is prov i ded as one way to

quantify the categories. The percentage of ponds that retain water should also

be consi dered; it is assumed that at 1east 15 cm of water must be present

throughout the nesting period for alligators to use a pond. Suitability

increases as the percentage of ponds retaining this water depth increases (V4 ) .

Fluctuating water levels have an adverse impact on alligator nesting

success (Joanen, pers. obs.). A fifth variable--the percentage of substrate

exposed at mean low tide (Vs)--is added to the model for use in tidal wetlands.

Suitability for this variable decreases as the percentage of exposure at mean

low tide increases; the 51 becomes 0 at 100% exposure.

Suitability Index (51) Graphs for Model Variables

The graphs presented in this section illustrate the relationship of each

variable to alligator habitat suitability in coastal palustrine emergent (PEM)

and estuarine intertidal emergent (E2EM) marshes and associated open water

areas with a mean annual salinity less than 10 ppt. The 51 values

(1.0 =optimum suitability, 0.0 = unsuitable) for each variable are read

directly from the graphs. Assumptions involved in developing the graphs are

summarized in Table 1.

Habitat Variable

PEM,E2EM VI Percentage of wetland

that is open water

(ponds, bayous, canals). )(

Q)

'0

.5

7

Suitability Graph

0.8

0.4

0.2

0.0 .....-,....,,.....,.~--r....,...~......... o 20 40 60 80 100

"
Habitat Variable Suitability Graph

PEM,E2M V2 Percentage of open water 1.0

area in bayous, canals,

or greater than 1.2 m

)( 0.8 deep in lakes and ponds. CD

C

- 0.6 >- ::

:ti 0.4 as

:!:

(/) 0.2

0.0

0 20 40 60 80 100

"

PEM,E2EM V3 Interspersion class 1.0

l. High interspersion;

10-15 ponds (~0.2 )( 0.8 ha) per 6 ha. CD

2. Medium interspersion; c

3-9 ponds per 6 ha or - 0.6. >- 15-20 ponds per 6 ha. :: 3. Low interspersion; 2 :a 0.4 - or fewer ponds per 6 as

:!: ha, or highly eroded

and fragmented marsh. (/) 0.2-

0.0 1 2 3

Class

PEM,E2EM V4 Percentage of ponded area 1.0

with water 15 cm deep

from May to September. )( 0.8

CD

C

- 0.6 >- ::

:ti 0.4 as

..

-;

(/) 0.2

0.0

0 20 40 60 80 100

"

8
Habitat Variable Suitability Graph

0.2

0.4

1.0.................."...........-.....L...........

0.0 __~,....,.~.....,..__.................

o 20 40 80 80 100

"

M 0.8

."

•

.s

>- 0.8

::

=.a

::

•

tI)

Percentage of substrate

exposed at mean low tide

from May to September

(used in tidal areas

only).

PEM,E2EM

Table 1. Data sources and assumptions for variables used to calculate the HSI

for nesting American alligators.

Variable and source Assumptions

VI Joanen (pers. obs.) Optimal nesting alligator habitat

is composed of 20% to 40% open water

and 60% to 80% vegetated wetland.

V2 Joanen and McNease 1970, 1972

McNease and Joanen 1974

Deepwater areas in bayous, canals,

ponds, and lakes are essential

habitat components for adult

alligators during the breeding

season and for immatures year round.

However, shallow water must also be

present to support prey species.

V3 McNease and Joanen 1974

V4 Joanen and McNease 1970

Nesting alligator habitat quality is

directly related to the degree of

interspersion of water bodies within

the vegetated wetlands.

Ponds that dry out during the spring

and summer tend to restrict the

movements of alligators and increase

the vulnerability of the young to

predation.

(continued)

9
Table 1. (Concluded).

Va~iable and source

Vs Joanen (pers. obs.)

Assumptions

In marsh areas affected by tidal

influences, nesting alligator

habitat is negatively impacted by

any degree of substrate exposure at

mean low tide.

Component Index (CI) Equations and HSI Determination

Before applying the alligator HSI model, the user should review the model

assumptions to decide the validity of the model for the area in question.

Following acceptance of the model, suitability index values for each variable

can be read off the appropriate suitability index graph by using collected

field data. The following equations are suggested for aggregating the individual SI values into component values and the overall HSI value.

Component Equation

Cover-Breeding (Cb) (SI Vl x SIV2)~

Cover-Nesting, Nontidal wetlands (Cnn) (SIVl x SIV3 X SIV4)l/3

Cover-Nesting, Tidal wetlands (Cnt) [SI Vl x SIV3 X (SI V4 X SIVs)~]l/3

HSI = (Cb x Cnn or nt)~

Sample data sets representing tidal and nontidal areas are presented in

Table 2. The outputs derived by using the HSI model equations are believed to

be reasonable representations of the value of hypotheti ca1 study areas as

habitat for American alligators.

Field Use of Model

Although not included as a variable in the model, flooding may have a

serious impact on the quality of alligator nesting habitat (Joanen et al.

1977). Local information may be available concerning the tendency of a marsh

to flood to a depth above the average height of the nest cavity. Despite the

HSI value obtained from application of the model, the HSI becomes 0 if the

marsh is flooded above the egg chamber for more than 2 h at any time from June

through August.

While field sampling of all variables is desirable for greatest

reliability in the determination of HSI values, it should be kept in mind that

10
the level of field sampling required to address all the variables would likely

be cost prohibitive. The use of infrared photography and water-Teve l

information will greatly facilitate the use of the model. It is therefore

suggested that field reconnaissance with intensive use of available maps and

water level data will provide the most reliable output from this model.

Suggested methods for measuri ng model vari ab1es are presented in Table 3.

Interpreting Model Outputs

This model will only provide information on the potential of a coastal

marsh area to support alligators. Other factors (e.g., poaching, storm tides)

may, within a particular time frame, exert a greater influence on the alligator

population than any of the habitat variables discussed. This model cannot,

therefore, be used to calculate alligator populations in any given area. It

can be used to compare the potential of different habitats to support

alligators at a single point in time or to compare the potential of a single

area to support alligators at two points in time.

Table 2. Calculations of suitability indices (S1) and habitat suitability

indices (HSI) for three hypothetical data sets on the basis of habitat

variables (V) and model equations.

Model Data set 1 Data set 2 Data set 3

component Data SI Data SI Data SI

Vl % 15 0.75 30 1. 00 80 0.33

V2 % 30 0.88 60 0.-50 70 0.38

V3 Class 1 1. 00 2 0.50 3 0.20

V4 % 80 0.80 40 0.40 60 0.60

Vs % 55 0.45

Cb 0.81 0.71 0.35

Cnn

0.84 0.34

Cnt 0.60

HSI 0.82 0.65 0.34

11
Table 3. Suggested methods for measuring habitat variables included in the

American alligator HSI model.

Variable Method

The percentage of open water can be determined from

aerial photography by planimetry.

Canals and bayous can be identified on aerial

photographs, and their area determined by using a

planimeter. The area of ponds and lakes with water

exceeding 1.2 m in depth may be available from

existing maps, or it may need to be determined in

the field. The total area can then be expressed as

a percentage of the area determined for VI.

A grid of 6-ha cells can be superimposed on an

aerial photograph. The number of cells in each of

the three interspersion categories is counted.

These numbers are multiplied by the corresponding

SI for the category. The three products are added

and then di vi ded by the total number of cells to

determine the SI for V3 .

The percentage of the ponded area with water > 15

crn in depth must be determined by field measurement

at the probable lowest water 1eve1 duri ng the May

to September period. In tidal areas, measurements

should be made at low tide. Biologists familiar

with the study area may be able to provide this

information.

In those areas affected by t ida1 i nfl uences , the

amount of exposed substrate is determined by field

observation at predicted mean low tide (National

Ocean Survey tide tables) during May to September.

The area exposed is expressed as a proportion of

total study site.

12
Chabreck,

coastal

Rouge.

REFERENCES

R.H, 1970. Marsh zones and vegetative types in the Louisiana

marshes. Ph. D. Dissertation. Louisiana State University, Baton

113 pp.

Chabreck, R.H. 1971. The foods and feeding habits of alligators from fresh

and saline environments in Louisiana. Proc. Southeast Assoc. Game Fish Comrn.

25:117-124.

Chabreck, R.H. 1972. Vegetation, water and soil characteristics of the

Louisiana coastal region. La. State Univ. Agric. Exp. Stn. Bull. 664. 72

pp.

Chabreck, R.H., and T. Joanen. 1979. Growth rates of American alligators in

Louisiana. Herpetologica 35 (1):51-57.

Conant, R. 1958. A field guide to reptiles and amphibians of the United

States and Canada east of the 100th meridian. Houghton Mifflin Co., Boston,

Mass. 366 pp.

Cowardin, L.M., V. Carter, F.C. Golet, and E. L LaRoe. 1979. Classification

of wetlands and deepwater habitats of the Uni ted States. U.S. Fish Wi 1dl.

Servo Biol. Servo Program FWS/OBS-79/31. 103 pp.

Delany, M. F., and C. L. Abercrombie. 1986. American alligator food habits in

northcentral Florida. J. Wildl. Manage. 50(2):348-353.

Ditmars, R.L. 1964. Reptiles of the world. Macmillan, New York. 321 pp.

Fogarty, M.J., and J.D. Albury. 1967. late summer foods of young alligators

in Florida. Proc. Southeast Assoc. Game Fish Comm. 21:220-222.

Giles, L.W., and V.L. Childs. 1949. Alligator management on the Sabine

National Wildlife Refuge. J. Wildl. Manage. 13(1):16-28.

Goodwin, LM., and W.R. Marion. 1978. Aspects of the nesting ecology of

American alligators in north central Florida USA. Herpetologica 34(1):43-47.

Goodwin, LM., and W.R. Marion. 1979. Seasonal activity ranges and habitat

preferences of adult alligators in a north-central Florida lake. J.

Herpetol. 13:157-164.

Joanen, T. 1969. Nesting ecology of alligators in Louisiana. Proc. Southeast

Assoc. Game Fish Comm.23:141-151.

13
Joanen, T., and L. McNease. 1970. A telemetric study of nesting female

alligators on Rockefeller Refuge, Louisiana Proc. Southeast Assoc. Game Fish

Comm. 24:175-193.

Joanen, T., and L. McNease. 1972. A telemetric study of adult male alligators

on Rockefeller Refuge, Louisiana. Proc. Southeast Assoc. Game Fish Comm.

26:252-275.

Joanen, T., and L. McNease. 1980.

alligator in southwest Louisiana.

Herpetol. 1:153-159.

Reproducti ve bi 01ogy of the Ameri can

Soc. Study Amphib. Reptiles. Contrib.

Joanen, T., L. McNease, and G. Perry. 1977. Effects of simulated flooding on

alligator eggs. Proc. Southeast Assoc. Game Fish Comm. 31:33-35.

Mel 1henny, E. A. 1935.

House, Boston, Mass.

The alligator's life history.

117 pp.

The Christopher Publ.

McNease, L.,

Rockefeller

28:482-500.

and T.

Refuge,

Joanen. 1974. A study of immature alligators on

Louisiana. Proc. Southeast Assoc. Game Fish Comm.

McNease, L., and 1. Joanen. 1977. Alligator diets in relation to marsh

salinity. Proc. Southeast Assoc. Game Fish Comm. 31:36-40.

McNease, L., and 1. Joanen. 1978. Distribution and relative abundance of the

alligator in Louisiana coastal marshes. Proc. Southeast. Assoc. Fish Wildl.

Agencies Conf. 32:182-186.

Metzen, W.O. 1977. The nesting ecology of alligators on the Okefenokee

National Wildlife Refuge. Proc. Southeast Assoc. Game Fish Comm. 31:29-32.

National Fish and Wildlife Laboratory. 1980. Selected vertebrate endangered

species of the seacoast of the United States: the American alligator. U.S.

Fish Wildl. Servo Biol Servo Program FWS/OBS-80/01.39. 9 pp.

O'Neal, T. 1949. The muskrat in the Louisiana coastal marshes. Louisiana

Department of Wildlife and Fisheries, New Orleans. 152 pp.

I

I

Palmisano, A.W., T. Joanen, and L. McNease. 1973. An analysis of Louisiana1s

1972 experimental alligator harvest program. Proc. Southeast Assoc. Game

Fish Comm. 27:184-206.

U.S. Fish and Wildlife Service. 1980. Habitat evaluation procedures (HEP).

ESM 102. Washington, D.C. n.p.

Valentine, J.M., J.R. Walther, K.M. McCartney, and L.M. Ivey.

diets on the Sabine National Wildlife Refuge, Louisiana.

36(3):809-815.

14

1972. Alligator

J. Wildl. Manage.
S0272-IOI

REPORT DOCUIiENTAnON II. UI'O,", NO. Report 82(10.136) II. I. _1....ftC'. Accft_ "0.

PAGE Biological .. TItle _ lulltltle L ....... DaCe

Habitat Suitability Index Models: American Alligator Apri 1 1987 .. 1. AuCIlerC-' a b Howardc ...........,1,. Orp"IDt"" IIept. .... J.D. Newsom, T. Joanen, and R.J. ,. .....,.",.... Orp"l._ ........nd _ .... IG. ,.".....1T....1Wortl Unit ....

U. or Gront(Q) ....

1C!

ll. ~,.O..."lIot"" ".m••nd AcId.... (G)

National Wetlands Research Center 11 Ty... of _port & ....... e-rod

Research and Development Fish and Wildlife Service

U.S. Department of the Interior 14.

Washington, DC 20240

1.. lupplomento" .._ Authors' affiliations:

aprofessional Wildlife Service bRockefeller Wildlife Refuge CNational Wetlands Research

6973 N. Rothmer Drive Louisiana Dept. of Wil dl ife Center

Baton Rouge, LA 70808 and Fisheries NASA-Slidell Computer

Grand Chenier, LA 70643 Complex

1010 Gause Boulevard

Slide11 , LA 70458

·'1" Abetroc:t IUmlt: 200 _nls)

A review and synthesis of existing i nformation were used to develop a model for

evaluating American alligator habitat quality. The model is applicable in

marshes along the northern Gulf of Mexico. It is scaled to produce an index

between 0 (unsuitable habitat) and 1. 0 (optimal habitat. Habitat suitabil ity index models are designed for use with the Habitat Evaluation Procedures previously developed by the U.S. Fish and Wildlife Service. Guidelines for model

application and techniques for measuring model variables are described.

11. Document An.lye'. •. Doocrlpto...

Mathematical models

Reptiles

Wildlife

b. Idontlli..../Os>o"·Endod T.rm.

Habitat Suitability Index American alligator

Impact assessment Alligator mississippiensis

Habitat

c. COSATI R.ld/Group

1.. Anl/.blllty St.t.m.nt 19. Security CI••• (TIli. R.port) 21. "0.01 P....

Unclassified 14 -- Unlimited 20. Security CI••• (Till. Pop) 22. Pric.

Unclassified (Soo ANSI-Z39.11l

U. S. GOVERNMENT PRINTING OFFICE: 1987-761-070/40010

~ALFORM 272 (4-7n

(Fo.-!y "TI$-3!!)

Dep.rtment of Comm.rc.
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Preserve OurNatural Resources

DEPARTMENT OF THE INTERIOR

u.s. FISH AND WILDLIFE SERVICE

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