Library

National Wetlands Research Center

U. S. "Fish and Wildlife Service

700 Cajundome Bsulevard

Lafayette. La. 70506

BIOLOGICAL REPORT 82(10.138)

JUNE 1987

HABITAT SUITABILITY INDEX MODELS:

BULLFROG

_. ~h and Wildlife Service

S. Department of the Interior sK

361

.U54

no . 8 2-

10 .138
MODEL EVALUATION FORM

Habitat models are designed for a wide variety of planning applications where habitat information is an important consideration in the

decision process. However, it is impossible to develop a model that

performs equally well in all situations. Assistance from users and

researchers is an important part of the model improvement process. Each

model is published individually to facilitate updating and reprinting as

new i nformat i on becomes ava i 1ab1e. User feedback on model performance

will assist in improving habitat models for future applications. Please

complete this form following application or review of the model. Feel

free to include additional information that may be of use to either a

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

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

models or test results. Please return this form to:

Habitat Evaluation Procedures Group

U.S. Fish and Wildlife Service

2627 Redwing Road, Creekside One

Fort Collins, CO 80526-2899

Thank you for your assistance.

Species

Habitat or Cover Type(s)

Geographic

Location

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(10.138)

June 1987

HABITAT SUITABILITY INDEX MODELS: BULLFROG

by

Brent M. Graves

Department of Zoology and Physiology

University of Wyoming

Laramie, WY 82071

and

.Stanley H. Anderson

Wyoming Cooperative Research Unit

University of Wyoming

Laramie, WY 82071

National Ecology Center

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report should be cited as:

Graves, B.M., and S.H. Anderson. 1987. Habitat suitability index models:

bullfrog. U.S. Fish Wildl. Servo Biol. Rep. 82(10.138). 22 pp .
PREFACE

This document is part of the Habitat Suitability Index (HSI) model series

[Biological Report 82(10)J, which provides habitat information useful for

impact assessment and habitat management. Several types of habitat information

are provided. The Habitat Use Information section is largely constrained to

those data that can be used to derive quantitative relationships between key

environmental variables and habitat suitability. This information provides

the foundation for the HSI model and may be useful in the development of other

models more appropriate to specific assessment or evaluation needs.

The HSI Model section documents the habitat model and includes information

pertinent to its application. The model synthesizes the habitat use information into a framework appropriate for field application and is scaled to

produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum

habitat). The HSI Model section includes information about the geographic

range and seasonal appl ication of the model, its current verification status,

and a list of the model variables with recommended measurement techniques for

each variable.

The model is a formalized synthesis of biological and habitat information

published in the scientific literature and may include unpublished information

reflecting the opinions of identified experts. Habitat information about

wildlife species frequently is represented by scattered data sets collected

during different seasons and years and from different sites throughout the

range of a species. The model presents this broad data base in a formal,

logical, and simpl ified manner. The assumptions necessary for organizing and

synthesizing the species-habitat information into the model are discussed.

The model should be regarded as a hypothesis of species-habitat relationships

and not as a statement of proven cause and effect relationships. The model

may have merit in planning wildlife habitat research studies about a species,

as well as in providing an estimate of the relative suitability of habitat for

that species. User feedback concerning model improvements and other suggestions that may increase the utility and effectiveness of this habitat-based

approach to fish and wildlife planning are encouraged. Please send suggestions

to:

Resource Evaluation and Modeling Section

National Ecology Center

U.S. Fish and Wildlife Service

2627 Redwing Road

Ft. Collins, CO 80526-2899

iii
;v
CONTENTS

Page

PREFACE iii

FIGURES vi

ACKNOWLEDGMENTS vi i

HABITAT USE INFORMATION 1

Genera 1 1

Food 1

Water 2

Cover 3

Reproduct ion 3

I nterspersion 4

Special Considerations................................................ 4

HABITAT SUITABILITY INDEX (HSI) MODEL 4

Model Appl icabi 1ity 4

Model Description..................................................... 6

Appl ication of the Model 15

SOURCES OF OTHER MODELS 19

REFERENCES

v

19
Number

1

2

3

4

5

6

7

8

9

10

FIGURES

Geographic range of Rana catesbeiana within the continental

United States (after Conant 1975 and Stebbins 1985) .

The assumed relationship between the mean distance to deep

water and suitability index values for bullfrog food

requi rements .

The assumed relationship between percent canopy cover of

aquatic vegetation tn the littoral zone and the food suitability of a wetland for bullfrogs .

The assumed relationship between percent shoreline cover and

food suitability of a wetland for bullfrogs .

The assumed relationship between mean water transparency and

food suitability for larval bullfrogs .

The assumed relationship between silt in the substrate and the

suitability of a wetland for winter cover for bullfrogs .

Relationships between water characteristics affecting reproduction of bullfrogs and habitat suitability indices .

Relationship between distance to permanent water and habitat

suitability for the bullfrog .

Relationships of habitat variables to components of the HSI

model for the bullfrog (Rana catesbeiana) .

Definition of variables and suggested measurement techniques ..

vi

5

7

8

9

10

11

13

14

16

17
ACKNOWLEDGMENTS

We would like to thank Michael Armbruster, Mel Schamberger, Adrian Farmer,

and Carl Armour (U.S. Fish and Wildlife Service, National Ecology Center) for

editorial and professional criticism, and Thomas Baskett and Ernie and Hope

Wiggers (University of Missouri) for logistical aid in field evaluation of the

model. We would also like to thank Bruce Bury and Thomas Baskett who reviewed

an earlier draft of this model. The cover of this document was illustrated by

Jenn ifer Shoemaker. Word processi ng wa s provi ded by Carolyn Gul zow, Dora

Ibarra, Elizabeth Barstow, and Patricia Gillis.

Funding for the development of this model was provided by the U.S. Fish

and Wildlife Service and the Army Corps of Engineers.

vii
BULLFROG (Rana catesbeiana)

HABITAT USE INFORMATION

General

The bullfrog (Rana catesbeiana) is a large, aquatic frog that commonly

inhabits permanent bodies of standing or slow-moving water. Bullfrogs may be

found in water bodies with swifter currents where slow backwaters are available

and may be drawn to such areas in search of prey (Rabeni 1985). Conant (1975)

states that the natural range of the bullfrog extends from Nova Scotia to

central Florida, west to Wisconsin and across the Great Plains to the Rocky

Mountains. Introduction of the bullfrog has been successful in Puerto Rico

(Perez 1951), Japan (Telford 1960), Italy (Lanza 1962), Jamaica (Mahon and

Aiken 1977), the western United States (Stebbins 1985), and elsewhere.

Most life history events are temperature, rather than photoperiod,

dependent. Male choruses (group vocalizations that attract females for mating)

generally coincide with air temperatures around 27°C (Wright and Wright 1949;

Howard 1978). Individuals begin hibernating at water temperatures below 16 °C

(Willis et al. 1956), and emergence from hibernation in the spring occurs at

air temperatures from 19 to 24°C and water temperatures of about 13 °C (Wright

1914; Willis et al. 1956). Young (smaller) frogs enter and emerge from

hibernation sooner than older (larger) frogs. A few large frogs may be seen

at air temperatures as low as 5 °C (14°C water) in the fall.

Bullfrogs are usually found on or near shorelines, but move a number of

meters into the water when water temperature is higher than air temperature in

the fall (Willis et al. 1956). Males move away from the shore in spring and

summer for mating choruses (Howard 1978). Many aspects of bullfrog ecology

are reviewed in Bury and Whelan (1984).

Food

Adult bullfrogs are omnivorous carnivores that, generally, will eat

anything that can be captured and swallowed (Dickerson 1906). Numerous studies

have been published concerning bullfrog diet, and each reflects the opportunistic feeding strategy of the species. Major components of the diet are

snails, insects, crayfish, fish, frogs, tadpoles, reptiles, and occasionally

mammals and birds, proportions of each depending upon their relative abundance

in the particular study area (Korschgen and Moyle 1955; Korschgen and Baskett

1963; Stewart and Sandison 1972; Tyler and Hoestenbach 1979; Corse and Metter

1980) .

1
Larval bullfrogs (tadpoles) are nonselective filter feeders and ingest

primarily algae, diatoms, and blue-green algae (Steinwascher 1975; Seale and

Beckvar 1980). An exception to this general statement is that bullfrog larvae

do not eat Chara, even when it is a dominant growth form (Brown 1972). Some

vascular plants may be eaten under starvation conditions (Brown 1972).

Water

Standing water is required for all stages (i .e., egg, larvae, and adult)

of bull frog 1i fe hi story. For eggs, water must be between 15 and 32 °C for

development to occur- (Moore 1942). The "adapti ve temperature" for embryos is

given as 24.3 °C by Bachman (1969). Similarly, Ryan (1978) measured the

temperature within 62 egg masses in a New Jersey pond and found the mean to be

23.8 °C (SE = 4.3). Hatching is severely disrupted at pH readings below 4.3.

This is probably due to changes in osmotic potential across the egg membrane,

and the same phenomenon probably accounts for reduced viability of eggs in

water with high salinity (Gosner and Black 1957; Mahon and Aiken 1977; Dunson

and Connell 1982).

Bullfrogs are larvae from 79 days (Corse and Metter 1980) to 3 years

(Brattstrom 1962), depending on food availability (Corse and Metter 1980) and

temperature (Moore 1942). Larval temperature preferences generally increase

with acclimation (Hutchison and Hill 1978). Lucas and Reynolds (1967) found

that the preferred temperature of Wisconsin bullfrog larvae was 24°C in May,

28 °C in June, and 30 °C in July, and that such di fferences were due to

developmental stage, not season.

Constant water level is beneficial to larvae (McAuliffe 1978). As water

level falls, available habitat decreases, hence, intraspecific competition and

predation increase. Similarly, cannibalism of larvae by adult bullfrogs

increases as population density increases. Frequent water level fluctuations,

such as in flood control reservoirs, can be detrimental to bullfrog breeding,

because eggs laid in a suitable site at one water level may be left on land

when water level is dropped. Similarly, frogs hibernating in mud may be

exposed to air and desiccated or frozen if water level drops in winter.

Larvae tend to congregate in shallow, unshaded water with minimal current

where solar radiation raises the temperature.

Adult bullfrogs require standing water but can migrate to other ponds if

water level drops considerably (Raney 1940; Schroeder and Baskett 1968; Stewart

and Sandison 1972). Stable water levels are also important because drops in

water level result in exposed banks without cover and drying of backwater

areas that norma lly serve as refugi a (McAul i ffe 1978). Bullfrogs generally

inhabit larger and deeper bodies of water than other frogs (Dickerson 1906;

Moore 1942; Moyle 1973; Conant 1975), and eutrophic waters are generally

preferred, although exceptions to these generalities are frequent. Eutrophic

waters may be favored because of the abundance of algae as food for 1arvae,

abundance of aquatic vegetation as cover for larvae and adults, abundance of

prey animals living in and feeding on the aquatic vegetation, and mud bottoms

for hibernating and escape. Additionally, since bullfrog eggs are deposited

as a film on the surface of the water, low dissolved oxygen of eutrophic

waters will not affect bullfrog eggs to the extent that it affects frogs that

2
lay their eggs in clumps, e.g., Rana plplens, B. sylvatica, (Moore 1940).

Water bodies should have both deep and shallow areas (Dickerson 1906; Moore

1942); however, ponds in Kentucky with good bullfrog populations had a maximum

depth of 80 cm (Viparina and Just 1975; Cecil and Just 1979). Hibernation in

water that is too shallow can result in freezing of animals in the bottom mud

(Manion and Cory 1952).

Cover

Availability of overhead cover for protection and escape from predators

is an important factor influencing the quality of an area as bullfrog habitat.

Indeed, Cecil and Just (1979) concluded that population size for R. catesbeiana

in permanent ponds is not controlled by food availability but-by predation.

Larvae rely on aquatic vegetation for cover (Brown 1972). Aquatic vegetation,

logs, stumps, and brush are also important as cover for adults (Raney 1940;

Currie and Belles 1969; Brown 1972; Moyle 1973; McAuliffe 1978). Adults often

retreat to deep water when disturbed (Raney 1940; Smith 1961).

Many descriptions of bullfrog habitat have emphasized the importance of

heavily vegetated banks (Dickerson 1906; Raney 1940; Wright and Wright 1949;

Wiewandt 1969; McAuliffe 1978). Bullfrogs spend much of their time sitting on

the shore 1i ne and uti 1i ze such vegetation for concealment from predators.

Willis et al. (1956) studied movements of bullfrogs and found that no specific

cover types were preferred, a finding congruous with observations of bullfrogs

using vegetation, overhanging banks, muskrat (Ondatra zibethica) burrows,

stumps, logs, branches, and other debris as shoreline cover. Thrall (1971)

found young bullfrogs using pits in the mud of bare pond banks as cover.

Apparent ly, the frogs dug these themselves, because pi ts were not found at

nearby ponds with vegetated banks. In contrast, Moyle (1973) found bullfrogs

most abundant in waters devoid of surrounding vegetation. His study, however,

dealt with mountain streams and probably reflects a need for solar radiation

to warm the water, overshadowing a need for shoreline cover.

Reproduction

R. catesbeiana is a "warm-adapted" species (Bachman 1969). Eggs wi 11 not

hatch-and larvae will not develop below 15°C (Moore 1942; Viparina and Just

1975). The critical factors necessary for breeding are permanent, calm water

and air temperatures above 27°C (Wright and Wright 1949; Howard 1978). (Of

course, water temperature is the primary factor; however, only air temperature

data are available in the literature). Therefore, earliest spring breeding

dates vary from February along the Gulf of Mexico coast to May in northern

parts of the bullfrog's range (Willis et al. 1956).

Males establish territories a few meters from shore and call at night to

females that stay near the shore and approach selected males to mate (Emlen

1968; Wiewandt 1969). Small males often do not call but stay near calling

males and attempt to mate with females that calling males attract (Howard

1978). Sites defended by males are used as oviposition sites and the largest

males defend the best oviposition sites (Howard 1978). Each female may produce

10,000 to 20,000 eggs (Wright 1920); thus, populations can be maintained or

3
ponds repopulated if only a few frogs breed (Wiewandt 1969; McAuliffe 1978).

Eml en (1977) has presented evi dence that fema 1e bull frogs can produce two

clutches in a season; however, the usual number is one.

Interspersion

All habitat requirements for bullfrogs are usually found within and

around a single pond. Emigration to other ponds, however, is common (Raney

1940; Wiewandt 1969). Corse and Metter (1980) found that virtually all bullfrogs in a pond in Missouri dispersed from their natal pond within 1 to 2 days

after metamorphosis. And, bullfrogs have been observed on land by others in

situations that suggest emigration (Bohnsack 1952; Schroeder and Baskett

1968). Such movements between ponds may be important for reestabl i shi ng

populations in ponds that periodically dry up (Cohen and Howard 1958; Wiewandt

1969; Tyler and Hoestenbach 1979). Territoriality is apparent only during the

breeding season (Blair 1963). Because only males defend territories, and

males that cannot establish and defend a territory usually become satellites,

it is not thought that such behavior limits population density.

Special Considerations

Bullfrogs hibernate during cold winter months. Both adults and larvae

spend the winter buried in soft mud at the bottom of permanent wetlands,

although adults have been reported to hibernate on land (Bohnsack 1952). Soft

mud pond bottoms that remain below ice level and are oxygenated throughout the

winter may be important in this regard (Manion and Cory 1952).

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. The model presented here has been developed for use in

the midwestern United States. Within other areas of the geographic range of

R. catesbeiana in North America (Figure 1), caution should be used when applying the model. The model may require modifications when applied in the

northern extremes, throughout the southeastern United States (i. e., ice-free

climates), or in areas of the range where the bullfrog did not originally

occur, but has been introduced.

Season. Bullfrogs are year-round residents within their habitats; hence,

this model is intended to assess an area's ability to support bullfrogs during

all seasons. Variables should be measured during the summer, except for an

assessment of ice depth as described below.

Cover types. The model may be used to evaluate bullfrog habitat quality

in and around permanently or semi permanently (containing standing water yearround during most years) flooded, riverine, lacustrine, or palustrine wetlands

as defined by Cowardin et al. (1979). .

4
Figure 1. Geographic range of Rana catesbeiana within the continental

United States (after Conant 1975 and Stebbins 1985).

Minimum habitat area. No information is available to quantify this

value. However, the senior author has observed several adult and larval bullfrogs at the University of Missouri1s Ashland Wildlife Research Area near

Ashland, Missouri, living in a permanent pond with a diameter of approximately

1.5 m. This single observation has limited value, but in general, bullfrogs

seem to occur in permanent wetlands at least several meters in diameter.

Conversely, a much larger (approximately 20 m diameter) ephemeral wetland

at the Harry S. Truman State Park (Missouri) contained no frogs or tadpoles.

Hence, the size and depth of a wetland may influence desiccation and winter

ice thickness rather than indicate spatial requirements of the species.

Verification level. Sensitivity analyses were performed to indicate if

the influence of each variable on model output was appropriate to the

variable's assumed biological significance in determining bullfrog habitat

suitability. The model was evaluated at various sites throughout Missouri,

Kansas, and Nebraska during May 1984. Twenty-nine sites were surveyed and

variables contained in an earlier version of this model were quantified at

each. An estimate of bullfrog population densities at each site was also

made. The results of this work and the senior author's impressions of

bullfrog-habitat relationships were employed to revise the model to more

accurately portray our interpretations of bullfrog habitat suitability.

5
The model is a set of hypotheses descri bi ng assumed bull frog-habi tat

relationships, but no attempt has been made to address all causal relationships

affecting population densities. The standard of comparison for this model is

bullfrog use of a site, as reported in the literature, and as interpreted from

limited observations of bullfrog populations at the 29 surveyed sites. Potential model users should realize that the demographic histories of surveyed

populations in Missouri, Kansas, and Nebraska were unknown. Without such

information, it is impossible to evaluate correspondence between model output

and habitat-use patterns at locations that were not surveyed, or at survey

sites in other years.

Constructive reviews of an earlier draft of this model were provided by

Drs. T.S. Baskett and R.B. Bury. Modifications suggested by these reviewers

have been incorporated into the model where possible, and their assistance is

gratefully acknowl edged. Use of the rev i ewers I names, however, does not

necessarily imply that they concur with each section of the model, or the

entire model.

Model Description

Overview. This model assumes that food, winter cover availability, and

water characteristics associated with reproduction and migration are the

primary factors determining habitat suitability for bullfrogs. The following

sections provide documentation of the logic and assumptions used to extrapolate variables and suitability index relationships from information on

bull frog habitat use and 1i fe hi story features presented in the Habi tat Use

Information section and surmised from the 1984 application of an earlier

version of this model. Specifically, these sections address: (1) identification of habitat-related variables, (2) definition and justification of

suitability levels for each variable, and (3) des-criptions of the assumed

relationships between variables.

Food component. Variables selected to characterize food availability are

often associated with cover availability. For example, it is not known whether

bullfrog populations in areas hav inq excellent cover thrive because of the

protection cover provides from predators, or because prey items are usually

found in such cover. Although bullfrogs are dietary generalists, adults feed

on animal life and larvae feed primarily on phytoplankton. Because animal

prey abundance is difficult to assess, it is assumed that prey availability

will be commensurate with cover availability for adult bullfrogs. Therefore,

the variables included in the food component are assumed to quantify cover for

both bullfrogs and their prey, as well as food availability for tadpoles.

Both shallow and deep water are required by bullfrogs. Some shallow

water around the edge of a wetland is desirable to provide warm (due to solar

radiation) water for tadpoles and adults and an area for growth of aquatic

vegetation. Also, shallow water is required for many prey items. Deep water

(>1.5 m) should be located relatively close to shore so that adults and tadpoles can use such areas for escape and hibernation.

6
It is assumed that the relationship between the availability of both

sha 11 ow and deep water can be represented by a determi nat i on of the mean

distance from shore at which water depth >1.5 m occurs (SIV1, Figure 2).

Based on field observations of various midwestern wetlands, the optimal

di stance from shore for water depth >1.5 m is assumed to range from 10 to

20 m. Distances <10 m are assumed to have a decreased value because of a

reduced littoral zone. No habitat value exists if a mean water depth >1.5 m

is reached <1.0 m from shore. Suitability is assumed to decrease between 20

and 30 m because of increased travel distances required to reach water >1.5 m

in depth.

...-.. 1.0 ......

>.-.

V'l 0.8

x

(])

"'0 0.6 .-.

>,

of-l 0.4

..0

", 0.2 of-l

::s

V'l 0.0

0 10 20 30 40

Mean distance from shore

to water >1.5 m deep

Figure 2. The assumed relationship between the mean distance to deep

water and suitability index values for bullfrog food requirements.

Some users may wish to redefine the above relationships. The maximum

depth of ponds in Kentucky supporting bullfrog populations was 80 cm (Viparina

and Just 1975; Cecil and Just 1979).

7
Aquat i c vegetation, both emergent and submergent forms, provi des escape

cover, ambush cover, and hiding cover for bullfrogs. Such vegetation also is

employed by prey items and provides attachment sites for egg masses. Hence,

it is assumed that this type of cover improves habitat suitability for bullfrogs unless vegetation becomes so dense that it impedes locomotion. This

variable may have its greatest effect on habitat suitability in relatively

shallow water, because this is where most foraging, sunning, and escape

behavi or occur.

It is assumed that the relationship between aquatic vegetation and hiding

cover for both bullfrogs and their prey can be characterized by a measure of

the percent canopy cover of aquatic vegetation (both emergent and submergent

hydrophytes) occupying the littoral zone (water <1.5 m deep) (SIV2, Figure 3).

Optimal conditions are assumed to exist when canopy cover ranges from 55% to

80%. Suitability is assumed to decrease to 0 when no aquatic vegetation is

present, and to 0.2 at 100% canopy cover. The assumption that aquatic vegetation density affects habitat suitability is derived from literature sources;

specific values are derived from field observations in midwestern wetlands.

N 1.0

:::-

......

V> 0.8 x

Q)

s:: 0.6 ......

>,

+-> .,... 0.4

oD

ttl 0.2 +-> .,...

:::l

V> 0.0

0 25 50 75 100

Percent canopy cover of

aquatic vegetation in the

1i ttora1 zone

Figure 3. The assumed relationship between percent canopy cover of aquatic

vegetation in the littoral zone and the food suitability of a wetland for

bullfrogs.

8
Adult bullfrogs usually sit directly along the land-water interface.

Shore 1i ne vegetation may provi de both concealment for bull frog sand thei r

prey. Bullfrogs are rarely found along sections of shoreline devoid of cover

unless no shoreline with cover is available. Shoreline cover is defined as

vegetation, debris, or overhanging banks sufficient for frog concealment. A

direct linear relationship is assumed between percent of shoreline with cover

and habitat suitability (SIV3, Figure 4).

M 1.0 >-

......

Vl 0.8 x

OJ

"C

c 0.6 ......

>,

+oJ 0.4

..c

+J

1'0 0.2

::l

Vl 0.0

0 25 50 75 100

Percent shoreline cover

Figure 4. The assumed relationship between percent shoreline cover and

food suitability of a wetland for bullfrogs.

The last variable included in the food component is a measure of food

availability for the larval stage of the bullfrog life cycle. This variable

attempts to quantify phytoplankton abundance using a Secchi disk. It is

assumed that a mean Secchi disk depth ranging from 100 to 300 cm corresponds

to optimal phytoplankton abundance for larval bullfrogs (SIV4, Figure 5). It

is assumed that too little phytoplankton (high water transparency) will not

provide sufficient food for tadpoles and that too much phytoplankton (low

water transparency) will be associated with algal blooms. Blooms typically

have high proportions of cyanophyta, which can secrete toxic substances.

However, too little or too much phytoplankton are not thought to be limiting.

Water transparency should be measured in midsummer to minimize the influences

of other suspended particles, such as silt.

9
'd" 1.0

>.....

(/) 0.8

x

OJ

"0

s:: 0.6 .....

....

+-' 0.4

....

..c

to 0.2 +-'

::l

(/)

0.0

a 100 200 300 400

Mean water transparency (crn)

Figure 5. The assumed relationship between mean water transparency

and food suitability for larval bullfrogs.

A value for the food component (SIF) is obtained by combining suitability

indices SIV1 through SIV4, as shown in equation 1. Suitability indices are

combined with an arithmetic mean because frogs may sti 11 occupy sites that

exhibit a suitability for any of the identified variables.

SIF = SIV1 + SIV2 + SIV3 + SIV4 --'-------'-'=----:------'-----

4 (1)

Winter cover component. No characterization of year-round habitat would

be complete without an attempt to address the suitability of a wetland in

terms of winter survival for bullfrogs. Bullfrogs overwinter by hibernating

in the bottom substrates of permanent wetlands. The bullfrog 1iterature

provides only limited general descriptions of winter hibernation requirements.

10
We have elected to address this aspect of the bullfrog's biology as winter

cover needs, and describe two variables that can be used to assess sites at a

level of resolution representing presence versus absence.

Water depth sufficient to prevent a wetland from freezing completely to

its bottom is assumed to be necessary for winter survival of hibernating

bullfrogs. This depth will vary with local conditions and must, therefore, be

determined for each wetland in an evaluation area. We suggest that the relationship can be characterized with a binary variable (SIV5) providing values

of either 1 or 0, depending upon the following conditions:

If winter water depth is greater than maximum ice depth then SIVS = 1

If winter water depth is less than maximum ice depth then SIVS = 0

Composition of a wetland's bottom substrate is assumed to influence the

bullfrog's abilities to burrow into the bottom for hibernation. We have

assumed that the composition of the bottom substrates, as they relate to

winter cover suitability, can be represented by particle size. Of all

potential sized particles available, fine silt was selected to represent our

interpretation of an ideal substrate for burrowing (a 63-micron sieve allows

passage of fine silt but not larger particles, such as sand, gravel, etc.).

Although bullfrogs may not require 100% silt for optimum conditions, a linear

relationship (0 to 100) is assumed between the percent of the substrate

composed of silt (or finer particles) and the suitability of the substrate for

burrowing by bullfrogs (SIV6, Figure 6).

1.0 I.D

>-

.....

Vl 0.8

x

aJ

"t:l 0.6 c::

.....

>,

..., 0.4 .....

..Cl 0.2 rtl

...,

.....

:J

Vl 0.0

a 25 50 75 100

Percent silt in substrate

Figure 6. The assumed relationship between silt in the ~ubstrate

and the suitability of a wetland for winter cover for bullfrogs.

11
It is assumed that the suitability of a wetland as winter cover for

bullfrogs can be expressed as the product of the suitability indices for

winter water depth (SIV5) and the percent silt in the bottom substrates (SIV6).

The suitability index for winter cover (SIWC) can be determined by using

equation 2.

SIWC = SIV5 x SIV6 (2)

Reproduction component. Variables describing habitat suitability for

bullfrog reproduction address water characteristics required for successful

breeding and hatching of eggs, and include current velocity, pH, temperature,

and water level constancy.

Bullfrogs are not found in streams with a strong current, although the

relationship between habitat suitability and current velocity has not been

systematically quantified. Values used in the suitability index graph for

this variable (SIV7, Figure 7a) have been drawn from observations of bullfrog

abundance in various streams in Missouri and Kansas and from literature

sources. The relationship is based on the hypothesis that bullfrogs preferentially inhabit wetlands with still or slow-moving water and that habitat

suitability decreases as current velocity increases.

Hydrogen ion concentration (expressed as pH) also affects bullfrog habitat

suitability. Gasner and Black (1957) and Dunson and Connell (1982) found

severe inhibition of hatching at pH readings of approximately 3.9-4.0 [high

salinity (-0.250 gm/l00 cc H20) would produce the same effect] and about 50%

of eggs did not hatch at pH of 4.1-4.3 (Gasner and Black 1957). Hatching

occurs normally at pH 5.9. The relationship in Figure 7b (SIV8) uses the

above i nformat i on to assume ali near increase in hatchi ng from pH 4 to 5.5.

Maximum hatching is assumed to occur from pH 5.5 to 8.5 and a decrease in

hatching is assumed to be caused by high pHI s.

Because bullfrogs are aquatic ectotherms, water temperature directly

affects activity and metabolic rates. Water temperature also influences

hatching of eggs. The relationship between mean water temperature (measured

at mid-depth during summer) and habitat suitability (SIV9, Figure 7c) is

derived from data presented in Moore (1942) and Bachman (1969). Development

was abnormal at 14°C, and the minimum temperature at which normal development

occurred was 15 °C, although developmental rate was approximately eight times

slower at 15°C than at 30 °C. At 34°C eggs were killed in the blastula

stage and 24.3 °C was determined to be the "adaptive temperature."

Frequent large fluctuations in water level can be detrimental to bullfrog

populations by leaving eggs or hibernating frogs above water level, thereby

inducing desiccation. Also, large water level fluctuations in riverine

habitats may indicate flooding and associated excessive current velocity,

which could wash eggs, tadpoles, and adults out of the area. Infrequent

fluctuation in water level of ~2 m per year is regarded as normal. More

frequent fluctuations of such a magnitude could result in detrimental effects

as described above, although these are not seen as strictly limiting (SIVI0,

Figure 7d).

12
a b

1.0 ......... 1.0 r-, 00 I , :> :> .... .... V'l 0.8 V'l 0.8 -

x x

ClJ ClJ "0 0.6 "0 c c 0.6 - .... .... >, >,

+-> 0.4 +-> 0.4 -

.0 .0

0.2 0.2 - +-> +->

:::J :::J

V1 j 0.0· V'l 0.0

0 25 50 75 100 0 2 4 6 8 10

Mean current velocity at pH mid-depth during summer (cm/s)

Frequency of water level

fluctuations >2 m

"'"

· "'"

· "'"

·

c

1.0 a 1.0 CJ) I ...-1

:> :> .... .... V'l 0.8- V'l 0.8

x x

ClJ ClJ "0 0.6 - "0 c c 0.6 .... .... >, >,

+-> 0.4 - ..... +-> 0.4

.0 0.2- .0

0.2 +-> +->

:::J J \ :::J

V'l 0.0 V'l 0.0

0 20 40 60

Mean water temperature at

mid-depth during summer (DC)

d

<1 2 3+

Figure 7. Relationships between water characteristics affecting

reproduction of bullfrogs and habitat suitability indices.

13
A value for the reproduction component (SIR) is obtained by combtrn nq

SIV7 through SIV10, as shown in equation 3. A geometric mean is used because

a value of 0 for any variable would render a wetland unsuitable for bullfrogs.

SIR = (SIV7 x SIV8 x SIV9 x SIVIO)1/4 (3)

Interspersion component. When wetlands are permanently flooded and

exhibit some level of suitability as bullfrog habitat, all requirements for

each life history stage are usually found within a single contiguous wetland.

However, in semi permanently flooded wetlands, adult bullfrogs may need to

migrate to other aquatic sites to avoid desiccation. In such situations,

alternative permanently flooded wetlands should be nearby, since bullfrogs

will desiccate enroute if the migration is too long. Also, young bullfrogs

may disperse after metamorphosis and would face the same restrictions.

Although Willis et al. (1956) reported movements of 0.16 to 2.8 km for a small

proportion of tagged bullfrogs, it is assumed that the bullfrog's saltatorial

locomotory pattern and high rate of water loss places limits on migration.

Hence, a linear decrease in habitat suitability is assumed as distance to

permanent water i ncrea ses (Fi gure 8). I f the wetland under eva1uat ion is

permanently flooded, SIVll is designated as 1.0. However, since it is not

known how far bullfrogs can migrate, this variable is not treated as exclusionary. The suitability index derived from Figure 8 will equal the

interspersion component value (SII) if the permanent water exhibits values >0

for SIF, SIWC, and SIR.

--- ......

...... 1.0 >-

......

Vl 0.8 x

OJ

-0

l:: 0.6 ......

>,

0.4

..0

ro 0.2

:::l

Vl 0.0

0 400 800 1200 1600

Distance to permanent

water (m)

Figure 8. Relationship between distance to permanent water and habitat

suitability for the bullfrog.

14
HSI determination. Because of the limited mobility exhibited by most

bullfrog populations, we have assumed that year-round habitat suitability is a

reflection of the characteristics of individual wetlands, or those in close

proximity to permanent water. We have assumed that within a wetland (or

within a short distance), food (SIF), winter cover (SIWC), and reproductive

(SIR) requirements must be met. Limitations in one component are assumed to

be compensated for by other components, but a 0 value for food, winter cover,

or reproduction indicates an unsuitable wetland in terms of year-round habitat

for bullfrogs. Permanent water is assumed critical not only in terms of

habitat suitability, but for basic survival of bullfrog populations. Because

of this assumed importance, the interspersion index (SII) is used to penalize

a semipermanently flooded wetland (Cowardin et al. 1979) that is not located

within 100 m of permanent water that exhibits some habitat value for bullfrogs

(i.e., SIF, SIWC, and SIR of the permanent wetland must each be >0). These

assumed relationships are described in equation 4.

HSI = (SIF x SIWC x SIR)1/3 x SII

Application of the Model

(4 )

Summary of model variables. Eleven habitat variables are used in this

model to determine food, winter cover, reproduction, and interspersion values

for the bullfrog. Relationships between habitat variables, model components

(life requisites), and overall HSI values are summarized in the flow diagram

presented in Figure 9.

Figure 10 provides definitions and suggested measurement techniques for

each of the 11 variables in this model. Care and judgment must be exercised

in determining when to measure variables. Variables involving water depth and

velocity should be measured during average midsummer conditions (i .e., not

after heavy rains or during severe drought). Similarly, temperature should be

measured at mid-day during seasonable weather.

Because bull frog habitat may be extremely heterogeneous with regard to

suitabil i ty , it may be necessary to determine several HSI values for various

parts of the evaluation area. This would be necessary if variables (other

than those that have mean values) have different values in different areas of

a single location (e.g., a lake, pond, swamp, or section of stream). Similarly, if the permanently or semi permanently flooded wetland under consideration

is large, and the area in which maximum water depth is greater than maximum

ice depth is small, this should be considered. For example, Manion and Cory

(1952) found that all frogs killed by freezing were in the shallow half of a

pond while those in the deep half survived. The pond was only 23 m by 6 m in

size.

15
Va r iab Ie

Mean distance from shore to water :>1.5 m deep

Percent canopy cover of aquatic vegetation in

the I ittoral zone

Component

I Food ,

I Reproduction

......

CT\

Percent shorel ine cover I

Mean water transparency (em)

Maximum water depth greater than maximum ice l

depth.

1---------- Wi n t e r

Percent si It in substrate ------------

Mean current velocity at mid-depth during I

summer (cm/s)

pH --------------------- I

Mean water temperature at mid-depth I

during summer (DC)

Frequency of water level fluctuations -------l

:>2 m

cove r --I-I------. --------- HSI

Distance to permanent water (m) Interspersion

Figure 9. Relationships of habitat variables to components of the HSI model for the

bullfrog (Rana catesbeiana).
Variable (definition)

Mean distance from shore to

water >1.5 m (4.9 ft) deep.

Percent canopy cover of aquatic

vegetation in the littoral zone

(the percent of the aquatic

substrate in the littoral zone

that is shaded by a vertical

projection of submergent and

emergent vegetation).

Percent shoreline cover [the

percent canopy cover of overhanging shrubs, tree crowns,

woody downfall, herbaceous

vegetation, and debris within

1 m (3.3 ft) of a wetland's

shore].

Mean water transparency (cm)

[the average depth at which a

weighted white disk 20 cm

(8 inches) in diameter disappears from view].

Maximum water depth greater

than maximum ice depth.

Suggested measurement technique

Weighted line with gradations marked

along it. Drop to bottom at various

points along shore to find points at

which water becomes >1.5 m (4.9 ft)

deep.

Emergent vegetation can be observed

from the shore but submergent vegetation distribution will be more

difficult to assess. If water is

clear, submergent vegetation may

be mapped from a boat. Otherwise,

wading and a tactile survey may be

required. A convex polygon may be

drawn around vegetation patches to

segregate vegetation vs. no vegetation areas.

This variable can be measured by

pacing off the total shoreline

distance and dividing this value

into total shoreline distance with

cover adequate for concealment of

bullfrogs (assessed visually).

Standard methods for Secchi disk use

are covered in Lind (1979).

Establish where deepest water areas

are within wetland during summer.

Return when ice is thickest (or

sample throughout winter) and

determine ice thickness by

drilling a hole with an auger.

Figure 10. Definition of variables and suggested measurement techniques.

17
Variable (definition)

Percent silt in substrate

(silt is defined as material

0.004-0.06 mm in diameter).

Mean current velocity at middepth during summer (cm/s).

pH

Mean water temperature at

mid-depth during summer (OC).

Frequency of water level

fluctuations >2 m (6.6 ft).

Distance to permanent water (m).

Figure 10.

Suggested measurement technique

A spring loaded dredge (see Lind

1979) may be lowered on a line for

sampling from deepest water areas

within wetland. Samples should be

thoroughly dried, then sifted

through a 63-micron sieve. Weight

of material passing through the

sieve should be divided by the

total sample weight to obtain a

percent value.

Speed of a neutrally buoyant

object.

Negative log of the hydrogen ion

concentration in the water as

measured with pH meter or pH paper.

Temperature can be accurately

measured with a thermometer.

Determine annual frequency from

water records, interviews with

residents, frequent measurement

over a number of years.

Records, interviews, or observation

over a number of years to determine

permanency of vicinal wetlands.

Pace or measure distances between

wetlands. If the wetland under

consideration is permanent,

distance = O.

(Concluded)

18
Model assumptions. Output from this model should be interpreted as an

indication of the ability of an area to support bullfrogs. The model does not

predict population density in an area because more factors influence population

density than habitat suitabil ity. For instance, McAul iffe (1978) found that

the factor havi ng greatest effect on bull frog popul ations in Nebraska ponds

was the extent of predation by man. Hence, many areas that may be deemed

excellent habitat for bullfrogs by this model, may actually have relatively

sparse bullfrog populations as a result of nonhabitat influences.

Most relationships proposed in the model are not empirically derived.

They are based on fnferences drawn from the literature, supported by what

empirical data is available, and the impressions and data obtained from

application of an earlier version of this model in Missouri, Kansas, and

Nebraska during May 1984. Therefore, personal judgment should be used in

determining the applicability of this model. Model users should be aware that

bullfrogs are not necessarily desirable species in all regions currently

occupied, especially in the western United States. Bury and Whelan (1984)

reviewed several studies documenting the reduction or elimination of native

species by bullfrogs.

Potential users may wish to make some modifications before using the

model. For example, 1.5 m was selected as a reference point in SIV1 because

it fit our objectives in the sites surveyed in 1984. Bullfrog populations

occur in wetlands with maximum depths <1.5 m, and some users may wish to

modify SIV1 to more accurately characterize local conditions. SIV4 assumes a

relationship between water transparency and algae abundance, but the values

presented in Figure 5 may be inappropriate for some localities within the

model's assumed area of applicability. Silt was selected to represent the

suitability of bottom substrates for burrowing, but other sized particles may

have suitability, and some percent composition <100% silt may represent optimum

conditions. The relationship between current velocity and suitability should

be carefully scrutinized before being incorporated into an assessment of

habitat. Inherent in SIV7 is the assumption that still- or slack-water areas

are available for bullfrog use in areas of higher velocities, and judgment

must be used when evaluating wetlands for this variable. Other assumptions

may be inappropriate and the model should be evaluated carefully in its

entirety before use.

SOURCES OF OTHER MODELS

No other published

catesbeiana) are known.

Harrisburg, PA 17105-1567)

model was not published.

REFERENCES

habitat models concerning the bullfrog (Rana

The Pennsylvania Game Commission (P.O. Box 1567;

developed a bullfrog habitat model in 1980, but the

Bachman, K. 1969. Temperature adaptations of amphibian embryos. Am. Nat.

103(1) : 115-130.

19
Blair, A.P. 1963. Notes on anuran behavior, especially Rana catesbeiana.

Herpetologica 19(1):151. ----

Bohnsack, K. 1952. Terrestrial hibernation of the bullfrog, Rana _catesbeiana

Shaw. Copeia 1952(1):114.

Brattstrom, B.H. 1962. Thermal control of aggregation behavior in tadpoles.

Herpetologica 18(1):38-46.

Brown, E.R. 1972. Size variation and food habits of larval bullfrogs, Rana

catesbei ana Shaw, in western Oregon. Ph. D. Dissertation. Oregon State

University, Corvallis. 83 pp.

Bury, R.B., and J.A. Whelan. 1984. Ecology and management of the bullfrog.

U.S. Fish Wildl. Servo Res. Publ. 155. Washington, DC. 23 pp.

Cecil, S.G., and J.J. Just. 1979. Survival rate, population density, and

development of a naturally occurring anuran larvae (Rana catesbeiana).

Copeia 1979(3):447-453.

Cohen, M.W., and W.E. Howard. 1958. Bullfrog food and growth at the San

Joaquin Experimental Range, California. Copeia 1958(2):223-225.

Conant, R. 1975. A field guide to reptiles and amphibians of eastern and

central North America. Houghton Mifflin Co., Boston. 429 pp.

Corse, W.A., and D.E. Metter. 1980. Economics, adult feeding, and larval

growth of Rana catesbeiana in a fish hatchery. J. Herpetol. 14(3):231-238.

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

of wetlands and deepwater habitats of the United States. U.S. Fish Wildl.

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

Currie, W., and E.D. Belles. 1969. Home range and movements of the bullfrog,

Rana catesbeiana Shaw, in an Ontario pond. Copeia 1969(4):688-692.

Dickerson, M.C. 1906. The frog book. Doubleday, Page and Co., Inc., Garden

City, NY. 253 pp.

Dunson, W.A., and J. Connell. 1982. Specific inhibition of hatching in

amphibian embryos by low pH. J. Herpetol. 16(3):314-316.

Emlen, S.T. 1968. Territoriality in the bullfrog, Rana catesbeiana. Copeia

1968(2):240-243.

1977. "Doubl e cl utcht nq'' and its possible significance in the

bullfrog. Copeia 1977(4):749-751.

Gosner, K.L., and I.H. Blacl. 1957. The effects of acidity on the development and hatching of New Jersey frogs. Ecology 38(2):256-262.

20
Howard, R.D. 1978. The evolution of mating strategies in bullfrogs, Rana

catesbeiana. Evolution 32(4):850-871.

Hutchison, V.H., and L.G. Hill. 1978. Thermal selection of bullfrog tadpoles

(Rana catesbeiana) at different stages of development and acclimation

temperatures. J. Therm. Biol. 3(1):57-60.

Korschgen, L.J., and T.S. Baskett. 1963. Foods of impoundment- and streamdwelling bullfrogs in Missouri. Herpetologica 19(2):89-99.

Korschgen, L.J., and-D.L. Moyle. 1955. Food habits of the bullfrog in central

Missouri farm ponds. Am. Midl. Nat. 54(2):332-341.

Lanza, B. 1962. On the introduction of Rana ridibunda Pallas and Rana

catesbeiana Shaw in Italy. Copeia 1962(3):642-643.

Lind, O.T. 1979. Handbook of common methods in limnology, 2nd ed. C.V.

Mosby Co., St. Louis. 199 pp.

Lucas, E.A., and W.A. Reynolds. 1967. Temperature selection by amphibian

larvae. Physiol. Zool. 40(1):159-171.

Mahon, R., and K. Aiken. 1977. The establishment of the North American

bullfrog, Rana catesbeiana (Amphibia, Anura, Ranidae) in Jamaica. J.

Herpetol. 11(2):197-199.

Manion, J.J., and L. Cory. 1952. Winter kill of Rana pipiens in shallow

ponds. Herpetologica 8(1):32.

McAuliffe, J.R. 1978. Biological

bullfrog populations in Nebraska.

Parks Commission. 72 pp.

survey and management of sport-hunted

Report prepared for the Nebraska Game and

Moore, J.A. 1940. Adaptive differences in the egg membranes of frogs. Am.

Nat. 74(1):89-93.

_..,....---;::-__. 1942. Embryonic temperature tolerance and rate of development

in Rana catesbeiana. Biol. Bull. 83(3):375-388.

Moyle, P.B. 1973. Effects of introduced bullfrogs, Rana catesbeiana, on the

native frogs of the San Joaquin Valley, California. Copeia 1973(1):18-22.

Perez, M.E. 1951. The food of Rana catesbeiana Shaw in Puerto Rico.

Herpetologica 7(1):102-104.

Rabeni, C.F. 1985. Resource partitioning by stream-dwelling crayfish: the

influence of body size. Am. Midl. Nat. 113(1):20-29.

Raney, E.C. 1940. Summer movements of the bull frog, Rana catesbei ana Shaw,

as determined by the jaw-tag method. Am. Midl. Nat. 23(4):733-745.

21
Ryan, M.J. 1978. A thermal property of the Rana catesbeiana (Amphibia,

Anura, Ranidae) egg mass. J. Herpetol. 12(2):247-248.

Schroeder, E.E., and T.S. Baskett. 1968. Age estimation, growth rates, and

population structure in Missouri bullfrogs. Copeia 1968(3):583-592.

Seale, D.B., and N. Beckvar. 1980. The comparative ability of anuran larvae

(Genera: Hyla, Bufo, and Rana) to ingest suspended blue-green algae.

Copeia 1980(3):495-503.

Smith, P.W. 1961. Amphibians and reptiles of Illinois. Bull. Illinois Nat.

Hist. Survey 28. 298 pp.

Stebbins, R.C. 1985. A field guide to western reptiles and amphibians.

Houghton Mifflin Co., Boston. 336 pp.

Steinwascher, K. 1975. The effect of coprophagy on the growth of Rana

catesbeiana tadpoles. Copeia 1975(1):130-134.

Stewart, M.M., and P. Sandison. 1972. Comparative food habits of sympatric

mink frogs, bullfrogs, and green frogs. J. Herpetol. 6(3-4):241-244.

Telford, S.R., Jr. 1960. The American bullfrog, Rana catesbeiana, in Japan.

Copeia 1960(1):155.

Thrall, J. 1971. Excavation of pits by juvenile Rana catesbeiana. Copeia

1971(4):751-752.

Tyler, J.G., and R.D. Hoestenbach, Jr. 1979. Differences in food of bullfrogs (Rana catesbeiana) from pond and stream habitats in southwestern

Gklahoma~outhwest. Nat. 24(1):33-38.

Viparina, S., and J.J. Just. 1975. The life period, growth, and differentiation of Rana catesbeiana larvae occurring in nature. Copeia 1975(1):103-109.

Wiewandt, T.A. 1969. Vocalization, aggressive behavior, and territoriality

in the bullfrog, Rana catesbeiana. Copeia 1969(2):276-285.

Willis, Y.L., D.L. Moyle, and T.S. Baskett. 1956". Emergence, breeding,

hibernation, movements, and transformation of the bullfrog, Rana catesbeiana,

in Missouri. Copeia 1956(1):30-41.

Wright, A.H. 1914. Life histories of the anura of Ithaca, New York. Carnegie

Inst. Publ. 197. Washington, DC. 98 pp.

1920. Frogs, their natural history and utilization. U.S. Bur.

Fish Doc. 888.

Wright, A.H., and A.A. Wright. 1949.

United States and Canada, 3rd ed.

640 pp.

Handbook of frogs and toads of the

Comstock Publising Co., Ithaca, NY.

22
50272 -101

REPORT DOCUMENTATION 11. REPORT NO. 12.

PAGE Bioloqical Report 82(10.138)

3. Recipient'. Acce••lon No.

•• Tlt'e .nd Subtitle

aHabitat Suitability Index Models:

7. Author<.)

B. M. Graves and S. H. Anderson

Bullfrog

5. Report O.te

June 1987

8. Performln. O,..nlzetlon R.pt. No.

9. Perform'n. O,..nlz.tlon N......nd Addre..

University of Wyoming

Laramie, WY 82071

12. SpOnsor'n. O,.anlutlon N.me .nd Add.... Na t 10na 1 Eco Jogy Ce nte r

Research and Development

Fish and Wildlife Service

Department of the Interior

Washington, DC 20240

15. Supplement.ry Note.

18. Abstract (Limit: 200 _rds)

10. Projec:l/T..k/Work Unit No.

11. Contract(C) or Grent(G) No.

(C)

(G)

13. Type of Report & Perlocl Covered

A review and synthesis of existing information were used to develop a Habitat

Suitability Index (HSI) model for the bullfrog (Rana catesbeiana). The model

consolidates habitat use information into a framework appropriate for field application,

and is scaled to produce an index between 0.0 (unsuitable habitat) and 1.0 (optimum

habitat). HSI models are designed to be used with Habitat Evaluation Procedures

previously developed by U.S. Fish and Wildlife Service.

17. Document AIl.,ysI. •. Descriptors

Frogs

Wildlife

Habitabil i ty

Mathematical models

Bullfrog

Rana catesbeiana

Habitat suitabi li ty

Co COSATI Field/Group

Availability Statement

Release unlimited

19. Security CI••• (Thl. Report)

Unc 1assifi ed

20. Security C,.~. (Thl.aP••e)

Unclasslfle

S.. 'n.tructlon. on Rever.e

21. No. of p....

22

22. Price

OfOTIONAL FORM 272 (4-77)

(Formerly NTIs-3!5)

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