Library

National Wetlands Reseuch Center

U. S. Fish and Wildlife Service

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Lafayette. La. 70506

BIOLOGICAL REPORT 82(10.141)

JUNE 1987

HABITAT SUITABILITY INDEX MODELS:

SNAPPING TURTLE

--sh and Wildlife Service

,S. Department of the Interior SK

361

.U54

no. 82-

10.141
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 avail abl e. 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 assi stance.

Speci es

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(10.141)

June 1987

HABITAT SUITABILITY INDEX MODELS: SNAPPING TURTLE

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:

snapping turtle. U.S. Fish Wildl. Servo Biol. Rep. 82(10.141). 18 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 suggest ions that may increase the ut i 1ity and effectiveness of thi s 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

PREFACE iii

FIGURES vi

ACKNOWLEDGMENTS vi i

HABITAT USE INFORMATION 1

Genera1 ....................................•.......•..........••...... 1

Food 1

Water 2

Cover 3

Reproduct ion 3

Interspersi on and Movements 3

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

HABITAT SUITABILITY INDEX (HSI) MODEL 4

Model Applicability................................................... 4

Model Description..................................................... 5

Application of the Model 13

SOURCES OF OTHER MODELS 13

REFERENCES 16

v
Number

1

2

3

4

5

6

7

8

FIGURES

Geographic range of Chelydra serpentina in North America

(after Stebbins 1966 and Conant 1975) .

The assumed relationship between mean water temperature and

food suitability of a wetland for snapping turtles .

The assumed relationships between current velocity and aquatic

vegetation, and their respective suitability indices .

The assumed relationship between the percent silt in the

substrate and the suitability of a wetland as winter cover

for snapping turtles .

Relationship between distance to small stream and reproductive suitability for snapping turtles .

The assumed relationship between distance to permanent water

and interspersion suitability for the snapping turtle .

Relationships of variables, components, and habitat suitability index for the snapping turtle .

Definitions of variables and suggested measurement

techniques .

vi

5

7

8

10

11

12

14

15
ACKNOWLEDGMENTS

We would like to thank Michael Armbruster, Carl Armour, Adrian Farmer,

and Mel Schamberger of the National Ecology Center, U.S. Fish and Wildlife

Service, for professional and critical aid in development of this model. We

al so thank Carl Ernst, David Duvall, and John Legler for reviewing an early

dra ft of thi s mode 1.

The cover of this document was illustrated by Susan Strawn. Word

processing was provided by Elizabeth Barstow, Patricia Gillis, Carolyn Gulzow,

and Dora Ibarra. Funding for the development of this model was provided by

the U.S. Fish and Wildlife Service and the Army Corps of Engineers.

vii
SNAPPING TURTLE (Chelydra serpentina)

HABITAT USE INFORMATION

General

Snapping turtles (Chelydra serpentina) are large aquatic turtles that can

be found in vi rtua lly any permanent or semi permanent 1enti c or s1ow-mov i ng

lotic body of water (Alexander 1943; Webb 1970; Feuer 1971). The species ' range is from southern Canada to the Gulf of Mexico, and from the Atlantic

Ocean to the Rocky Mountains (Conant 1975). A subspecies, Chelydra serpentina

osceola, inhabits peninsular Florida, but there are no detectable differences

in habitat preference between the subspecies (Feuer 1971). Perhaps, as a

consequence of such a widespread distribution, there is large variation in the

density of snapping turtle populations that different wetlands support (Froese

and Burghardt 1975). Habitat features associated with such differences in an

area1s ability to support snapping turtle populations are the basis for this

model.

Food

Much has been written about snapping turtle dietary preferences; however,

there also is much discrepancy in reported findings even when stomach content

analyses have been conducted (Pell 1941). Such disagreement is probably due

to the omnivorous habits of snapping turtles. Indeed many authors have noted

that snapping turtles will eat virtually anything organic that is available

(Alexander 1943; Lagler 1943a; Hammer 1969, 1971; Feuer 1971; Punzo 1975).

The above authors reported a variety of items within the general categories of

aquatic vegetation, terrestrial vegetation, insects, mollusks, crustaceans,

fish, amphibians, reptiles, birds, and mammals as food of snapping turtles.

Specific food requirements likely do not play an important role in determining

habitat quality and will not be discussed in detail here.

Pell (1941) stated that snapping turtles are mainly carnivorous in early

spring, when they are often found wandering on land, because there is not yet

sufficient aquatic vegetation in lakes and ponds. Later in spring and summer,

the turtles eat mostly aquatic vegetation.

Young snapping turtles, perhaps due to their preference for smaller

streams, are thought to be largely carnivorous, feeding on insects, small

fishes, crayfish, small frogs, and various other invertebrates (Pell 1941;

Lagler 1943a).

1
Water

Snapping turtles are not usually found out of the water, reflecting their

highly aquatic nature, and body temperature closely approximates water

temperature (Pell 1941; Punzo 1975). Mean preferred temperature is 28.1 °C

(Schuett and Gatten 1980) and mean critical thermal maxima is 39.5 °C (range

37.4 to 40.6 °C) (Hutchinson et al. 1966). Obbard and Brooks (1981) stated

that snappi ng turtles do not eat until the water temperature is at 1east

16°C.

Many authors have reported that snapping turtles rarely bask in the sun

(Pope 1939; Schmidt and Inger 1957; Ernst and Barbour 1972; Conant 1975);

however, detailed investigations by Ewert (1976) and Obbard and Brooks (1979)

indicated that basking out of the water is not exceptional. Basking significantly raises body temperature and, consequently, is most common in the

northern part of the snapping turtle's range. Basking usually occurs on

offshore logs and less frequently on offshore rocks and on shore.

Snapping turtles are most often found in shallow water (Cagle and Cheney

1950; Major 1975). Specific depths have been cited by Obbard and Brooks

(1981); almost all turtles were found at <2.5 m depth (mean = 0.99 m in day,

0.42 m night). Lagler (1943a) found turtles in water no deeper than the

length of head and neck extended for breathing while resting on the bottom.

Hammer (1971) found turtles at depths from 0.6 to 1.8 m, and Toner (1960)

seldom found turtles below 2.4 m. Pell (1941) stated that during hot weather

snapping turtles move to deeper, cooler water and are usually found in water

0.6 to 0.9 m deep or more.

Lagler (1943a), Anderson (1965), Webb (1970), and Froese and Burghardt

(1975) indicated that snapping turtles prefer turbid waters. This preference

may be associated with better concealment or the affinity of snapping turtles

for muddy substrates (Lagler 1943b; Anderson 1965; Feuer 1971; Minton 1972;

Froese and Burghardt 1975; Punzo 1975; Froese 1978). Many researchers have

noted that snapping turtles are most often found in waters with a slow current

(Pell 1941; Lagler 1943a; Cagle and Cheney 1950; Feuer 1971; Hammer 1971).

Although snapping turtles can spend considerable time out of water,

permanent bodies of water are required to maintain populations (Pell 1941;

Webb 1970; Feuer 1971; Minton 1972). Snapping turtles can migrate considerable distances overland, but if potential habitat dries up frequently,

alternative, more permanent sites, should be nearby (Cagle 1942; Klimstra

1951; Anderson 1965). Toner (1960) theorized that in the East snapping turtles

are found in lakes, ponds, and marshes since these are often permanent or

proximal to permanent water. In the more arid West, however, the species is

restricted to the larger rivers, because these are the only permanent water

bodies in the region. Such permanent bodies of water cannot be snapping

turtle habitat if the water is saline, although snapping turtles can survive

in saltwater for short periods (Feuer 1971). This may be important for

migration to coastal islands. Dunson (1984) reported that large snapping

turtles spend weeks at a time in coastal estuaries foraging on the abundant

2
bi ota of such areas. However, adul ts must return to freshwater peri odi ca lly

to rehydrate, and small turtles, because of thei r hi gh surface to volume

ratio, cannot survive in brackish waters for even short periods.

Cover

Snapping turtles are reputed to use aquatic vegetation, stumps, logs,

roots, holes, and other available obstructions as cover (Pell 1941; Cagle and

Cheney 1950; Minton 1972; Major 1975). Froese (1978) showed that hatchling

and juvenile snapping turtles prefer obstructed areas. Once a snapping turtle

reaches maturity, however, it has virtually no predators other than man (Abbott

1941; Hammer 1971). Thus, cover is probably utilized by adults as a means of

concealment from which to ambush prey, or because prey are found in such areas

(Major 1975; Hammer 1971; Froese 1978).

Reproduction

Mating can occur at any time when turtles are active (Carr 1952; Minton

1972), and activity varies with latitude. Snapping turtles are active in

Ontario from early May to early October (Obbard and Brooks 1981), in Ohio from

April to December (Conant 1938), and in Illinois from February to late December

(Smith 1961). Nesting occurs in early summer (Hamilton 1940; Hammer 1969;

Minton 1972; Petokas and Alexander 1980), and Hammer (1971) claims that night

air temperatures above 7°C are required for such activity. Minton (1972)

stated that some females may lay two clutches per season. Females often move

up small streams to lay eggs (Ewert 1976) at nesting sites where soil is moist

but well drained and loosely packed in unshaded areas (Norris-Elye 1949;

Hammer 1971; Minton 1972; Punzo 1975; Ewert 1976; Petokas and Alexander 1980).

Loncke and Obbard (1977) and Obbard and Brooks (1980) found that many females

migrated to a nesting site on a dam from other lakes as far as 13.8 km away,

and that individual females returned to the nest site year after year. This

suggests that suitable nest sites may be scarce. Dikes, muskrat (Ondatra

zibethica) houses, and beaver (Castor canadensis) lodges are frequently used

for nesting as well (Ewert 1976). Incubation period varies from 70 to 120 days

(Hammer 1971), and length of incubation is inversely proportional to

temperature (Yntema 1978). Cool summers probably do not thwart reproduction,

however, because eggs can survive through winter in the nest (Toner 1933,

1960; Minton 1972).

Predators are major factors influencing nest success. Petokas and

Alexander (1980) found a 94~~ predation rate on snappi ng turtle nests in New

York. Raccoons (Procyon lotor), skunks (Mephitis mephitis), foxes (Vulpes

fulva), and mink (Mustela vison), in order of importance, are cited as the

major predators of snapping turtle nests (Hamilton 1940; Hammer 1969; Wilhoft

et al. 1979; Petokas and Alexander 1980).

Interspersion and Movements

Two factors should be considered in regard to interspersion: habitat for

young and proximity of permanent habitat. A number of authors have stated

that immature snapping turtles [maturity is reached at approximately 145 mm

plastron length (White and Murphy 1973)J use different habitat than do adults

3
(Pell 1941; Toner 1960; Hammer 1971; Minton 1972). It is thought that

juveniles remain in small streams until shortly before maturity, when they

migrate to the ponds, ri vers, marshes, and sha 11 ow areas of 1arge 1akes

preferred by adul t snappi ng turtles. Hence, small streams should be near to,

and preferably empty into, the larger aquatic habitats used by adults. This

is probably why females often move up small streams to lay eggs. Additionally,

standing water is required for all life stages of the snapping turtle. Hence,

although individuals are often found in semipermanent waters, permanent water

should be nearby in the event of habitat desiccation (Cagle 1942; Klimstra

1951; Anderson 1965; Feuer 1971). Abbott (1941) kept a snapping turtle in the

trunk of a car for 2 weeks, and the turtle appeared healthy afterwards.

Apparently, turtles can spend considerable time out of water with no ill

effects, which may allow extensive terrestrial migrations.

Snapping turtles may be territorial, which would limit population

densities. Hammer (1969) and Raney and Josephson (1954) observed violent

fighting between males. Pell (1941:5) states that "In only one case have I

trapped more than one specimen in a single spot, and this only after a week's

tn te rva l ," and Feuer (1971) stated that only one or, at most, two snapping

turtles will be found in ponds of <0.2 ha surface area. In contrast, Hammer

(1969) expressed doubt that territoriality exists, based on his trapping data.

Obbard and Brooks (1981) gave the mean home range size as 3.44 ha (s =2.18;

n =10).

Special Considerations

During the winter, snapping turtles often aggregate in localized areas

for hibernation. Frequent hibernation sites include muskrat houses and

burrows, under logs, under banks, buried in the mud below ice level, and at

the mouths of tributary streams and spring inlets (Cahn 1937; Pell 1941;

Lagler 1943a; Anderson 1965; Hammer 1971; Froese 1978).

HABITAT SUITABILITY INDEX (HSI) MODELS

Model Applicability

Geographic area. This model can be used to assess habitat suitability

throughout the geographic range of snapping turtles in North America

(Figure 1).

Season. Variables included in this model should be measured in midsummer

or as described. The model output will describe suitability of the area as

year-round snapping turtle habitat.

Cover types. The model may be used to evaluate snapping turtle habitat

suitability in and around any permanently or semipermanently flooded (defined

as containing standing water year-round during a majority of years) riverine,

lacustrine, or palustrine wetlands as defined by Cowardin et al. (1979).

4
Figure 1. Geographic range of Chelydra serpentina in North America

(after Stebbins 1966 and Conant 1975).

Minimum habitat area. No empirical studies have been conducted to

investigate the minimum habitat area required by snapping turtles; hence, only

anecdotal and speculative information are available. Those familiar with

snapping turtles state that these animals (especially young) will live in even

the smallest bodies of water provided food is available (Pell 1941; Lagler

1943a). Therefore, thi s model assumes that any permanent or semipermanent

(see above) body of water will be large enough to support snapping turtles.

Verifi cat i on 1eve 1. The model is a set of hypotheses descri bi ng assumed

snapping turtle-habitat relationships, but no attempt has been made to address

all causal relationships affecting population densities. The standard of

comparison for this model is year-round snapping turtle use of a site as

reported in the literature and interpreted by the authors. Information is

limited in several areas of this species' biology, but we have attempted to

fill those voids with habitat characterizations that we assume will at least

explain the potential for presence or absence of snapping turtles at a

particular site within the species' current range. There are some indications

5
in t he literature that t err itor ia l beha vi or may limi t densities . Until more

defin iti ve informati on become s avai l able, t he potent i a l pr ese nce or absence of

snappi ng turtles at a particul ar wet land may be an appropri ate level of

r eso l ut i on f or most habi t a t a sse ssme nt s.

C. Ernst, D. Duvall, and J . Legler pro vided co ns t r uct i ve revi ews of an

earlie r draft of t hi s mode l . Modifi cat i ons suggested by t hese re viewers have

bee n i nco r porat ed in t o the mode l wh ere possi bl e . and their as sistan ce i s

gra t e f ul l y ac knowled ged . Use of a rev iewe r ·s name , however, does not

nece ssaril y imply that he concurs with each sec ti on of the mode l , or t he mode l

i n i t s entirety as pre sented here .

Model De scription

Overvi ew. Snappi ng tu rtl e s are genera l ists wi th re spect to diet a nd

cove r t ype ; howe ver, ce rta in vari able s ca n be used to a sses s hab i t a t suit- abi lity f or t hi s speci es . Se ven sui t abil ity cr iter i a a re organi zed i nt o f our

components t o characteri ze t he year - round habitat r equirement s fo r snappi ng

t urtle s. Rela t i onshi ps be t ween c riteria and sui t abil ity i ndi ce s are drawn

f rom empi ri ca l and anecdotal i nfo rmat ion i n the l i terat ur e r ev iew pr e sent ed

above and fr om t he aut hors ' genera l impre ssi ons and i nt erp r et ations of t hat

i nforma t ion.

The fo l lo win g sec t i ons provi de document atio n of the l ogi c and a ss ump t ions

emp l oyed to ex t r apol a te mo del r e lat ionships fr om lite rature in fo rmati on .

Specifi cal ly , t hese se cti ons cove r : (1) id en t ifi cation of habitat - r el a t ed

va r i abl e s ; ( 2) de f in iti ons and j ust ifi cat i ons of t he suitab ili t y leve ls of

ea ch vari ab l e ; and (3 ) descr i pt ions of t he a ss umed re l at i onsh ip s between

vari ab l e s.

Food comp onent . A1t hough rn at ure snappi ng tu rt1e s ap paa r t o consume a

wide range of f ood i tems, we have as sumed that the re are cri t eri a that ca n be

used t o c har acterize the suitab i l ity of per manentl y and semi permanen t l y fl ooded

wetlands 1" terms of potent ia l food ava i lab il ity . The se c r iteria t nc l ude

wate r t empera ture , current vel oc ity , and abundance of aquat ic veget ation .

Be cau se snap pi ng t urtle body temperature is cl ose l y associated with wat e r

temperature, and body tempe rat ur e directl y a f f ec t s me t abol ic rate (and hence ,

energy a ssim i lati on , abi lity t o captu r e pr ey , repr oduct ive pr ocesse s , et c . ) , a

co nsider at i on of water tempera ture is i nc l uded . Water temperature must be

above 16 °C for turtle s t o eat, and mean pr e f err ed temperature i s 28 . 1 °C.

For thi s model . cr iti ca l therma l maxima i s i dent ifi ed as 37°C . Ther e fore,

tempera tu re s within t hi s r ange may suppor t snappi ng t ur t le popul ati ons . The

shape of t he sui tabi lity inde x graph (S IV l ) i n Fi gure 2 1s deri ved by as suming

t hat t empera t ur es s O °C or >37 °C are letha l t o snapping t urtle s. Tempe ra t ures

between 1 and 16 °C have minima l va l ue, and optimum condi t ions occu r i n a

narrow band around 28.1 °C.

6
- 1.0 ....-l

:>

......

Vl 0.8 x

Q)

"0

c: 0.6 ......

c-,

+-' 0.4

..Cl

<U 0.2 +-'

Vl 0.0

0 15 30 45

Mean water temperature

at mid-depth during

summer (DC)

Figure 2. The assumed relationship between mean water temperature and

food suitability of a wetland for snapping turtles.

Mature snapping turtles are most commonly found in permanently and semipermanent ly flooded wetlands with sti 11 or s1ow-movi ng water. Although no

empirical data are available, it is assumed that stationary water has the

potential to supply optimum foraging conditions for a species characterized as

an aquatic omnivore. Under stationary water conditions, it is assumed that

turtles can maximize foraging efficiency by conserving energy that would

otherwise be expended moving against flowing water or pursuing immobile but

current-borne food items. We have assumed that suitability decreases as mean

current velocity increases until some constant low value is reached (SIV2,

Figure 3a). Inherent in this relationship is the assumption that even in

streams yielding high measures of mean current velocities, refugia exist that

can be used by turtles until more suitable conditions become available.

7
a b

1.0 1. 0

N -M

> >

- - 0.8 0.8 - x x

"0

"

0.6 "0

"

0.6

- - c-, 0.4 c-,

+' +' 0 .4

.c

'- 0.2 .a 0. 2 '"

+' +' '"

" "

0.0 0.0

0 15 30 45 60 0 25 50 75 100

Mean curr ent velocity Pe rce nt canopy cover of

at mid-depth during aquati c vegetation

surrme r ( em/ s) i n the littoral zone

Figure 3. The assumed relationsh ip s between cur rent vel ocity and aquat ic

vegeta t i on, and thei r respective sui tab ility i ndi ces.

Snapping t ur tles not on ly feed on aquat i c vegetation, but also use it as

a hid in g place f r om wh i ch t o ambu sh prey . Aq uatic veget at ion, stumps , logs,

and ot he r debr i s may also serve as hab itat for prey spec ies. We have assumed

that t he abundance of aquat ic veget at i on in the l i t t or al zone can be used as a

measure of the food suitability of a wetland for snapp ing turtle s . A linear

relationship between pe rcent canopy cove r of aquat ic vege t at i on i n the l itt oral

zone and fo od su itability is assumed (SIV3, Fig ure 3b) .

We bel ieve t ha t the poten tial for opt imum food conditi ons f or snapping

t ur t l e s occur s 1n pe rmanen t l y and semi permanentl y flooded wetlands that can be

cha ra cte rized by wate r tempe ratures near t he speci e s mean preferred tempera -

ture , no current, and l Ore.; cove rage of aqua t ic vegetat i on within the littoral

zone . Va lues fo r any of these suitabil ity criteria t hat are l e ss than opt imum

should lower t he ove ra ll food sui tabil ity i ndex (SIF), but receive some

8
compensation from criteria with higher values. Because zero values for SIV1

and SIV3 woul d remove a 11 food value for a wetland, we selected a geometri c

mean to represent our interpretation of food suitability (equation 1).

SIF (SIV1 x SIV2 x SIV3)1/3 (1)

Wi nter cover component. Any cha racteri zat i on of year-round habitat for

snapping turtles requires an attempt to address wetland suitability in terms

of winter requirements. The snapping turtle literature is limited in its

descriptions of winter requirements, and we have elected to address this

aspect of the species' biology as winter cover needs. Although we assume that

the fi rst of the two cri teri a descri bed below wi 11 be most appropri ate to

habitat assessments in the more northern reaches of the species' range, its

use in areas where ice does not form in winter should not cause problems with

interpretation of model output.

Water depth sufficient to prevent a wetland from freezing completely to

its bottom is assumed to be necessary for winter survival of snapping turtles.

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 (SIV4), providing values of either 1

or 0, depending upon the following conditions:

If winter water depth is greater than maximum ice depth then SIV4 =1

If winter water depth is less than maximum ice depth then SIV4 = 0

Snappi ng turtles often burrow into the mud to hi bernate, and we have

assumed that composition of a wetland's substrate can influence suitability in

terms of wi nter cover. Compositi on of the substrate can be represented by

particle size, and, of all potential sized particles available, fine silt was

se 1ected to represent our i nterpretat i on of an idea 1 substrate for burrowi ng

(a 63 u-si eve allows passage of fine silt but not larger particles, such as

sand and gravel). The exact composition of the substrate required to provide

optimum burrowing composition is unknown. We have assumed a linear relationship between the percent of the substrate composed of silt (or finer particles)

and the suitability of the substrate to provide winter cover for snapping

turtles (SIV5, Figure 4).

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

snapping turtles can be expressed as the product of the suitability indices

for winter water depth (SIV4) and the percent silt in the substrate (SIV5).

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

equation 2.

SIWC = SIV4 x SIV5

9

(2)
-on 1.0 >

-'" 0.8

<U

-o

" 0.6 -'>,

.- 0. 4 -.c

0.2 .-

"

'"

0 .0

0 25 50 75 100

Percent silt in subst rate

Figure 4. The assumed r elationshi p betwee n t he percent si l t i n t he subst r at e

and t he sui t abi li t y of a we t land as wint er cover for snappi ng t urt l es.

Re pr odu c t ; on componen t . Fema 1e s na ppi n9 t urt 1e s often move up sma 11

st r eams t o egg-l ayi ng sites, and young snap pi ng turtles a re f r equentl y f ound

in such str eams . Female snappi ng t urtl es a r e known t o make l ong migrat ions t o

sui t abl e nesti ng sites and al so nest al ong l ar ge l ake s and i n ot her l ocati ons , a s well a s a l ong sma ll st reams. The r e f or e , unava il ab il ity of sma ll st r eams t s

not seen as t ot a lly l imiti ng . It is assumed t hat pe rmane nt ly and semipe rmanently fl ooded wetland s wi th nearby smal l streams r eprese nt optimal

snappi ng tu rtle nest i ng hab t t. at , but as distan ce fr om t he wetland t o smal l

streams in crea ses , habitat sui tabi l ity decrea ses in a l i near ma nner . The

sui t abi l i ty inde x fo r repr oduct ion (SIR) ; s equa l to SIV6, and can be obtai ned

from the r elat ionship depi cted in Fi gur e 5.

10
1.0

1.0

:>

......

V') 0.8

x

QJ

-=' 0.6 l::

......

>,

+-' 0.4

.r-

..0 0.2 ttl

+-'

V') 0.0

0 2 4 6 8

Distance to small stream (km)

Figure 5. Relationship between distance to small stream and reproductive

suitability for snapping turtles.

Interspersion component. When a wetland is permanently flooded, and some

1evel of suitabi 1ity >0 exi sts for food (SIF), wi nter cover (SIWC), and

reproduction (SIR), that wetland can function as year-round habitat for

snappi ng turtles. In semi permanent ly flooded wetlands, however, snappi ng

turtles may be required to periodically migrate to other wetlands to avoid

desiccation. In such situations, permanently flooded wetlands should be

nearby to provide refuge until conditions improve. Quantitative information

describing travel distances or suitable interspersion of wetlands was not

located for snapping turtles, but it is assumed that permanent water should be

close by. For the purposes of this model, it is assumed that permanently

flooded wetlands should be located within 100 m of the evaluation site in

order to provide optimum interspersion suitability (SIV7, Figure 6). A linear

decrease in suitability is assumed as the distance from the wetland under

evaluation to permanent water increases.

11
1.0

".

-

V> 0.8 x

"0

'"

c 0.6 -

:;.

.~ 0.4

..0-

... 0. 2 .

V>

0.0

0 2 4 6 8

Distance to permanent water ( km)

Figure 6. The a ssumed rel at ions hi p between di sta nce t o perma nent water

and in terspersion suitabi l ity for t he snapping t ur t l e .

HSI de termination . We have a ssumed that year-round hab ita t su itab ility

for the sn ap ping turtle i s a r ef l ection of the characteristics of i ndivi dua l

permanent l y or semi pe rmanent ly fl ooded r i veri ne , 1acustri ne , or pa1ustri ne

wetlands {Cowa rdi n et a1. 1979 ) . Yea r- round habitat in th i s mode l i s defi ned

by criteri a characteri zi ng t he suitability of f ood ( SI F) , winter cove r (S IWC),

r eproduct ion ( SIR) , and i nterspersion (511) . Limi t atio ns i n one of t he f irst

t hree components a re ass umed to be compensa ted f or by hi ghe r in d i ce s in t he

other two components, but a zero va lue for fo od or win ter cove r in d i cat e s a

wet l and t hat i s uns uitable as ye ar-round habi t at f or snapp ing turtl es .

Permanen t water ( SI I) is assumed critica l not on ly in terms of habita t su itabil ity, but for ba sic surviva l of s napp i ng turt le popul at io ns . Because of

t his a ss umed importance , t he i nte r spers i on i ndex ( SI I) i s used to l owe r the

value of a semiperma nent ly floode d we t land that is not l oca t ed within 100 m of

a permanently flooded wetl and that exhi bits some habitat va lue fo r snappi ng

t ur t l e s. These r ela ti onsh ips a re desc r- I bed by equation 3.

J

HSI =(SI F x SIWC x SIR)I/ 3 x SII

12

(3)
Application of the Model

Summary of model variables. Seven habitat variables are employed in this

model to determine habitat suitability for snapping turtles. Relationships

between habitat variables, component indices, and HSI value are summarized in

Figure 7. Variable definitions and suggested measurement techniques are given

in Figure 8.

Model assumptions. This model has been designed to assess the suitability

of habitat for Chelydra se~ntina. The model is not intended to reflect

actual population densitTes, because many factors in addition to habitat

suitability influence population densities. Furthermore, model variables and

relationships are based on inferences drawn from the literature, much of which

is anecdotal. Therefore, refinements of this model should be made as necessary

to accommodate local conditions. It is important that sound judgment be used

in applying and interpreting this model.

Users may wish to make some modifications before using the model. For

example, in the more southern reaches of the snapping turtle's range, SIWC may

be inappropriate. This component can be deleted from the model; however, its

use in ice-free areas should not cause interpretational problems with model

output. SIV4 will always be 1.0 in ice-free areas, thus, SIWC = SIV5, or the

percent silt in the substrate, a criterion that appears related to t urt l e

habitat use regardless of the location. Both distance relationships (SIV6 and

SIV7) should be carefully scrutinized before use, as they are derived from

limited empirical data. Other assumptions also may be inappropriate and the

model should be carefully evaluated in its entirety before use.

SOURCES OF OTHER MODELS

No other model s are known that are designed to assess snapping turtle

habitat quality.

13
Y~r:lab~ Component

Mean water temperature at mid-depth during summer (OCIJ

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

1

- - - - - - - - Food

Percent canopy cover of aquatic vegetation --------

in the I ittoral zone

Maximum

depth

Pe rcent

water depth greater than maximum ice I .

.-------- Winter

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

cover

I HSI

.+» Di stance to sma II stream (km) Reproduct ion

Distance to permanent water (km) Interspersion

Figure 7. Relationships of variables. components. and habitat suitability index for

the snapping turtle.
Variable definition

Mean water temperature at

mid-depth during summer (OC).

Mean current velocity

at mid-depth during

summer (cm/s).

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).

Maximum water depth greater

than maximum ice depth.

Percent silt in substrate

(silt is defined as material

0.004-0.06 mm in diameter).

Distance to small streams (km).

Distance to permanent water (km).

Suggested quantification technique

Drop a temperature sensitive probe

to the bottom then raise to middepth and read temperature from

readout in boat.

Speed of neutrally buoyant object

in midstream.

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.

Records or map depth of lake.

Monitor ice thickness throughout

several winters.

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.

Pacing or measurement of distances.

Records to determine permanence

of habitats. Pacing or measurement

of distances.

Figure 8. Definitions of variables and suggested measurement techniques.

15
RE FERENCES

Abbot t , R.L. 1941. The bi ography of Che lydra . Nat. Hist. 50(1):46-49 .

Alexander, M.M . 1943. Food habits of the snapping turtle i n Connecticut . J .

Wi ld l. Manage. 7(3 ) :276-262.

Anderson, P. 1965 . The reptiles of Misso uri . Un iversity of Missouri Press ,

Columbia . pp. 6-6 .

Cagle. F. R. 1942 . Turtle population s in southe rn Illi nois . Capeia 1942( 3) :

155- 162 .

Cagle . F. R. • and A.H. Cheney . 1950 . Turt le populations in Louisia na . Am .

Midl . Nat. 43(2) :363-366 .

Cahn, A.R. 1937 . The turtles of Il linois. l l l i nots B1 01 . Mon ogr. 16:4-218 .

Car r , A.F . 1952 . Handbook. of t urtl es. Comstock., Ithaca . 542 pp .

Conant, R. 1936. The rept il e s of Ohio . Am . Mi d I. Nat . 20: 1-200 .

1975 . A f ie ld guide to rept il e s and amphibian s of eastern and

centra l No rth America , 2nd ed . Houghton-M ifflin Co . • Boston. 429 pp .

Coward1 n, L.J . , V. Carter, F. C. Go let, and E.T. LaRoe. 1979 . Classificati on

of wetlands and deepwater habitats of t he United State s . U.S . Ftsh Wildl ,it.

Servo FWS/OBS-79/31. 103 pp. -.6

Dunson, W.A. 1984. Estuarine populat i ons of the turtle Chel ydra as a mode l

f or the evolution of marine adaptat ions in rept i l es. Pre sent ed at Society

for t he Study of Amph ib ian s and Rept iles Annual Mee t i ng. Norman, OK .

Ernst, C.H ., a nd R.W. Barbour. 1972 . Turtl es of the United States.

University of Ke ntucky Press, Lexin gt on. 347 pp.

Ewert, M.A. 1976 . Nests , ne sting and ae r i a l baski ng of Macroc lemys under

natural cond itions . and comparfsons with Chelydra (Te st udines: Chelydridae) . Herpetolog lca 32(2):150-156 .

Fe uer , R.C. 1971. Ecol ogi ca l factors in success and dispe rsal of the snapping

turtle Chel ydra serpentlna ( Linnaeus). Bul l . Phi l. Herpetol . Soc . Vol . 19.

12 pp.

Froese, A.D.

Chelydra s. 12(1) : 53-58 .

1978 . Habitat preferences of the

·ser pent i na (Rep t i lia, Testud fnes ,

common snapp i ng turtle,

Chelydrldae) . J . Herp.

Fr oese , A.D.• and G.M . Burghard t. 1975 . A den se natural population of the

common snapp in g turtle Chelydra ser pent i na ) . Herpe tologica 31(2) :204 -20 8 .

16
Hamilton, W.J., Jr. 1940. Observations on the reproductive behavior of the

snapping turtle. Copeia 1940(2):124-126.

Hammer, D.A. 1969. Parameters of a marsh snapping turtle population, Lacreek

Refuge, South Dakota. J. Wildl. Manage. 33(4):995-1005.

_______________ . 1971. The durable snapping turtle. Nat. Hist. 80(1):59-65.

Hutchinson, V.H., A. Vinegar, and R.J. Kosh. 1966. Critical thermal maxima

in turtles. Herpetologica 22(1):32-41.

Klimstra, W.O. 1951. Notes on late summer snapping turtle movements.

Herpetologica 7(2):140.

Lagler, K.F. 1943a. Food habits and economic relations of the turtles of

Michigan with special reference to fish management. Am. Midl. Nat.

29(2):257-312.

1943b. Methods of collecting freshwater turtles. Copeia

1943(1):21-25.

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

Mosby Co., St. Louis. 199 pp.

Loncke, D.J., and M.E. Obbard. 1977. Tag success, dimensions, clutch size,

and nesting site fidelity for the snapping turtle, Chelydra serpentina,

(Reptila, Testudines, Chelydridae) in Algonquin Park, Ontario, Canada. J.

Herp. 11(2):243-244.

Major, P.O. 1975. Density of snapping turtles, Chelydra serpentina, in

western West Virginia. Herpetologica 31(3):332-335.

Minton, S.A., Jr. 1972. Amphibians and reptiles of Indiana. Indiana Academy

of Science, Indianapolis. 346 pp.

Norris-Elye, L.T.S. 1949. The common snapping turtle (Chelydra serpentina)

in Manitoba. Can. Field-Nat. 63(4):145-147.

Obbard, M.E., and R.J. Brooks. 1979. Factors affecting basking in a northern

population of the common snapping turtle, Chelydra serpentina. Can. J.

Zool.57(3):435-440.

1980. Nesting migrations of the snapping turtle (Chelydra

serpentina). Herpetologica 36(2):158-162.

1981. A radio-telemetry and mark-recapture study of activity

in the common snapping turtle, Chelydra serpentina. Copeia 1981(3):630-637.

Pell, S.N. 1941. Notes on the habits of the common snapping turtle, Chelydra

serpentina (Linn.) in central New York. M.S. Thesis. Cornell University,

Ithaca, NY. 77 pp.

17
Petokas, P.J., and M.M. Alexander. 1980. The nesting of Chelydra serpentina

in northern New York. J. Herp. 14(3):239-244.

Pope, C.H.

New York.

1939. Turtles of the United States and Canada.

343 pp.

A.A. Knopf, Inc.,

Punzo, F. 1975. Studies on the feeding behavior, diet, nesting habits, and

temperature relationships of Chelydra serpentina osceola (Chelonia:

Chelydridae). J. Herp. 9(2):207-210.

Raney, E.C., and R.A. Josephson. 1954. Record of combat in the snapping

turtle, Chelydra serpentina. Copeia 1954(3):228.

Schmidt, K.P., and R.F. Inger. 1957. Living reptiles of the world. Doubleday

and Co., Garden City, NY. 287 pp.

Schuett, G.W., and R.E. Gatten, Jr. 1980. Thermal preference in snapping

turtles (Chelydra serpentina). Copeia 1980(1):149-152.

Smith, P.W. 1961. The amphibians and reptiles of Illinois. Illinois Nat.

Hist. Sur-vey Bull. 28:1-298.

Stebbins, R.C. 1966. A field guide to western reptiles and amphibians.

Houghton Mi ffl in Co., Boston. 279 pp.

Toner, G.C. 1933. Over winter eggs of the snapping turtle.

1933(4):221-222.

1960. The snapping turtle. Can. Audubon 22(1):97-99.

Copeia

Webb, R.G. 1970. Reptiles of Oklahoma. University of Oklahoma Press, Norman.

370 pp.

White, J.B., and G.G. Murphy. 1973. The reproductive cycle and sexual

dimorphi sm of the common snappi ng turtle, Che lydra serpent ina serpent ina.

Herpetologica 29(3):240-246.

Wilhoft, D.C., M.G. Del Baglivo, and M.D. Del Baglivo.

mammalian predation of snapping turtle nests

Chelydridae). J. Herp. 13(4):435-438.

1979. Observations on

(Reptilia, Testudines,

Yntema, C.L. 1978. Incubation times for eggs of the turtle Chelydra

serpentina (Testudines: Chelydridae) at various temperatures. Herpetologica

34(3):274-277.

18
50272-101

REPORT DOCUMENTATION 11. REPORT NO. 12-

PAGE Biological Report 82(10.141)

.. Title and Subtitle

abitat Suitability Index Models: Snapping Turtle

7. Author(.)

B. M. Graves and S. H. Anderson

9. PerformIn. O,.anlzetlon Name and Addre••

University of Wyoming

Laramie, WY 82071

12. Sponsorln. O,.anizetlon Name and Addr..s Na ti ona1 Ecology Center

Fish and Wildlife Service

Department of the Interior

Washington, DC 20240

15. Supplementary Not..

lL AbatrKt (Limit: 200 _rda)

3. Recipient'. Acce ••lon No.

50 Report Oate

June 1987

8. Performtn. O,.anlzetlon Repr. No.

10. Project/Ta.k/Work Unit No.

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

(C)

(G)

13. Type of Report & Pariod Covered

14.

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

Suitability Index (HSI) model for the snapping turtle (Chelydra serpentina). The

model consolidates habitat use information into a framework appropriate for field

pplication, 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 the U.S. Fish and Wildlife Service.

17. Document Analysl. a. Dnc:r1ptors

Turtles

Wildlife

Habitabil ity

Mathematical models

b. Identlfiers/Open·Ended Terms

Snapping turtle

Chelydra serpentina

Habitat suitability

Co COSATI FIeld/Group

Availability Statement

Release unlimited

(5..ANSI-Z39.IS)

19. Security Class (This Report)

Unclassified

20. Security Class (This Pa.e,

Unclassified

See 'nltruction. on Rever.e

21. No. of Pa...

18

22. Price

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(Formerly NTIl>-35)

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