BIOLOGICAL REPORT 82(10.15 n

MARCH 1988

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HABITAT SUITABILITY INDEX MODELS:

DIAMONDBACK TERRAPIN (NESTING)-

ATLANTIC COAST

SK h and Wildlife Service

361 • •U54 ••

n o . 82-

10 . 151

Department of the Interior
MODEL EVALUATION FORt1

Habitat models are designed for a wide variety of planning appl ications

where habitat information is an important consideration in the decision

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

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

updating and reprinting as new information becomes available. Assistance from

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

Please complete this form following appl ication 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 the following address.

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Thank you for YOIJr assistance.

Species _ Geographic

Location---------------------------------- Habitat or Cover Type(s)----------------------------------- Type of Application: Impact Analysis Management Action Analysis. _

Baseline Other----------------------------------------------- Va ri ab1es Measu red or Eva1uated---------------------------------------

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(10.151)

March 1988

HABITAT SUITABILITY INDEX MODELS: DIAMONDBACK

TERRAPIN (NESTING)--ATLANTIC COAST

by

William M. Palmer

North Carolina Museum of Natural History

Raleigh, NC 27611

and

Carroll L. Cordes

U.S. Fish and Wildlife Service

National Wetlands Research Center

1010 Gause Boulevard

Slidell, LA 70458

Performed for

U.S. Department of the Interior

Fish and Wildlife Service

Research and Development

National Wetlands Research Center

Washington, DC 20240
This report may be cited as:

Palmer, W.M., and C.L. Cordes. 1988. Habitat suitability index models:

Diamondback terrapin Cnesting)--Atlantic coast. U.S. Fish Wildl. Servo

Biol. Rep. 82(10.151). 18 pp.
PREFACE

The habitat suitability index (HSI) model in this report on the diamondback terrapin is intended for use in the U.S. Fish and Wildlife Service's

(1980) habitat evaluation procedures for impact assessment and habitat management. The model was developed from a revi ew "and synthesis of existi ng

information and is scaled to produce an index of habitat suitability between 0

(unsuitable habitat) and 1 (optimally suitable habitat). Assumptions used to

develop the model and guidelines for model applications, including methods for

measuring model variables, are described.

This model provides a framework for evaluating the quality of diamondback

terrapi n habi tat and is not necessari ly based upon proven cause and effect

relationships. The model has not been field-tested, and the U.S. Fish and

Wi 1dl ife Servi ce encourages model users to provi de comments and suggestions

that may help increase the utility and effectiveness of this habitat-based

approach to fi sh and wi 1dl i fe management. Pl ease send any comments or suggestions you may have to the following address.

Information Transfer Specialist

National Wetlands Research Center

U.S. Fish and Wildlife Service 1010

Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

Page

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

ACKNOWLEDGMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Distribution and Importance............................................ 1

LHe History Overvi ew. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SPECIFIC HABITAT REQUIREMENTS............................................ 4

Food and Foragi ng Habitat.............................................. 4

Reproduction and Nesting Habitat. 5

Special Considerations................................................. 6

HABITAT SUITABILITY INDEX (HSI) MODEL... 7

Model Applicability.................................................... 7

Model Description...................................................... 7

Suitability Index (SI) Graphs for Model Variables... 9

HSI Determination...................................................... 10

Field Application of the Model......................................... 12

Interpreting Model Output.............................................. 13

SOURCES OF OTHER HABITAT MODELS........... 13

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

v
ACKNOWLEDGMENTS

The narrative and habitat suitability index model for the diamondback

terrapin were reviewed by the following individuals who have studied the life

history and habitat requirements of the species on the Atlantic coast: J.

Burger, Rutgers-The State University, New Brunswick, New Jersey; and R.A.

Seigel, Savannah River Ecology Laboratory, Aiken, South Carolina. Evaluation

of model structure and functional relationships were provided by personnel in

the U.S. Fish and Wildlife Service Ecological Services Offices at Annapolis,

Maryland, and Charleston, South Carolina.

Fundi ng for the model development and pub1i cati on was provi ded by the

U.S. Fish and Wildlife Service. Patrick Lynch illustrated the cover.

vi
DIAMONDBACK TERRAPIN (Malaclemys terrapin)

INTRODUCTION

Distribution and Importance

The di amondback terrapi n occurs ina narrow strip of salt and bracki sh

water habi tats along the At 1ant i c and gulf coasts of the United States from

Cape Cod, Massachusetts, to Corpus Christi Bay, Texas. Early reports of these

turtles from more southern areas of the Texas coast and from Mexico (Strauch

1890; Hay 1904; Carr 1952) have not been verified (Smith and Smith 1979; Ernst

and Bury 1982).

Seven subspecies of the diamondback terrapin (Malaclemys terrapin) are

recognized (Ernst and Barbour 1972; Conant 1975; Behler and King 1979; Smith

and Brodie 1982), but geographic variation throughout the range of the species

has not been studied and the taxonomic status of some races is problematic

(Mount 1975; Ernst and Bury 1982). Three subspeci es have been reported from

the Atlantic coast: (1) northern diamondback (!1. !. terrapin) from

Massachusetts to the vicinity of Cape Hatteras, North Carolina; (2) Carolina

diamondback (M. t. centrata) from Capa Hatteras to northeastern Florida; and

(3) Florida east-coast diamondback (!1. !. tequesta) from most of the eastern

Florida coast. Geographic patterns of intergradation between the subspecies

have not been defined. Ernst and Bury (1982) provided a comprehensive list of

the literature treating Malaclemys.

Diamondback terrapins were collected in great numbers as food for humans

from around the turn of the century until the 1ate 1920 IS, and commerci a1

hunters seriously depleted some populations (Hay 1904, 1917; Coker 1906;

Babcock 1926; Hildebrand 1929; McCauley 1945; Finneran 1948). The "terrapin

fad" fortunately waned and, inmost places, these turtles have recovered from

earlier exploitation. The alteration of estuarine areas, however, poses an

imminent threat to many populations today.

Life History Overview

Terrapins live in coastal marshes, tidal creeks and channels, coves,

estuaries, and lagoons behind barrier beaches. During the spring and summer,

and occasionally on warm days in winter, terrapins forage in aquatic areas on

a variety of crustaceans, mollusks, and other invertebrates (Carr 1952; Ernst

and Barbour 1972; Spagnoli and Marganoff·1975). When not feeding, individuals

often bask on exposed mudflats (Ernst and Barbour 1972).

During the winter, diamondback terrapins burrow in the mud of tidal

creeks and ponds to hibernate (Coker 1906; Carr 1952; Ernst and Barbour 1972).

1
Along salt-marsh creeks on the Cape May Peninsula in southern New Jersey,

Yearicks et al. (1981) found adult terrapins hibernating singly and in small

groups below the surface in the sides of banks and in shallow depressions on

creek bottoms. Individuals may hibernate in dune areas above the mean high

tide level on rare occasions (Lawler and Musick 1972). Hatchlings enter

hibernation shortly after hatching in autumn and may remain buried in the mud

well into the next spring (Ernst and Barbour 1972).

Mating occurs in the water with the onset of spring (Hay 1904; Seigel

1980a). Females leave their aquatic habitats only during the nesting season,

which varies with latitude (Ernst and Barbour 1972). Oviposition has been

reported from April 28 to July 1 in Florida (Seigel 1980b), June 9 to July 23

in New Jersey (Burger and Montevecchi 1975; Burger 1977), and June 10 to July

20 in Massachusetts (Lazell and Auger 1981). Nearly all observations of

nesting terrapins were in the daytime, from 0700 to 1900, during high tides.

The eggs are deposited in triangular or flask-shaped nests dug and

covered by the females (Rei d 1955; Ernst and Barbour 1972; Burger 1977).

Depths of 43 New Jersey nests ranged from 10.80 to 20.30 cm (mean 14.98 cm;

Montevecchi and Burger 1975). The topmost eggs inmost nests usually hatch

first (Burger 1976a), and the hatching success of all eggs is greater in

moderately deep (average 18.2 cm) rather than in very deep or very shallow

nests (Burger 1976b, 1977).

Cl utch sizes vary from 4 to 18 eggs (Montevecchi and Burger 1975), and

females in the South tend to produce fewer but larger eggs than females in the

North (Montevecchi and Burger 1975; Seigel 1980b). Large females tend to lay

the most eggs (Montevecchi and Burger 1975; Burger 1976b), and some females

may deposit several clutches in a season (Barney 1922; Hildebrand 1932; Ernst

and Barbour 1972; Seigel 1980b; Lazell and Auger 1981). When freshly laid,

the ellipsoidal or oblong eggs are pinkish white with dimply, leathery shells

that fill out during early incubation (Ernst and Barbour 1972; Montevecchi and

Burger 1975). The length of the eggs shortly after oviposition ranges from

26.3 to 40.8 mm, the width from 15.9 to 24.0 mm, and the weight from 5.0 to

13.2 g (McCauley 1945; Montevecchi and Burger 1975; Seigel 1980b). Average

length, width, and weight of eggs produced by New Jersey females were 31.1 mm,

21.2 mm, and 7.7 g, respectively (Montevecchi and Burger 1975). Comparable

averages of eggs from Florida terrapins were 39.0 mm, 22.3 mm, and 12.48 g

(Seigel 1980b).

Incubation periods vary with temperature (Burger 1976b), and some

hatchl i ngs may overwi nter in the nests (Ernst and Barbour 1972). In New

Jersey, incubation of eggs in natural nests varied from 61 to 104 days, eggs

in individual nests hatched over a period of 1 to 4 days, and neonates emerged

from the nests 1 to 9 days after hatching and usually in the daytime (Burger

1976a, 1977). Carapace length of hatchlings ranges from 2.5 to 3.4 cm

(McCauley 1945; Reid 1955; Conant 1975; Burger 1977; Seigel 1980b). The

weight of 207 hatchling northern diamondbacks varied from 5 to 9 g (mean 6.8

g) (Burger 1977); weights of 29 Florida hatchlings varied from 6 to 10.8 g

(mean 8.8 g) (Seigel 1980b).

2
Growth rates of diamondback terrapins in captivity have been studied at

the U.S. Fisheries Biological Station (National Marine Fisheries Service) at

Beaufort, North Carolina (Coker 1906; Barney 1922; Hildebrand 1929, 1932).

Growth rates in wild populations may vary according to latitude or various

environmental factors, and there is even variation within local populations.

The plastron length of northern diamondbacks reportedly increases about 2.5 cm

duri ng each of the fi rst 2 years after hatchi ng, s 1i ght ly 1ess duri ng the

third year, and only about 1.3 cm in each of the fourth and fifth years

(Babcock 1919). Some North Carolina terrapins, hatched and reared in

captivity, attained plastron lengths of 14 to 15 cm in 6 years; others

required 12 or more years to reach such sizes (Hildebrand 1932). Two hundred

diamondbacks (mostly intergrades between the northern and Carolina

subspecies), held captive for two years, had an initial average plastron

length of 30.65 mm and an average weight of 6.485 g. At the end of the first

year, plastron lengths averaged 61.93 mm and weights averaged 81.83 g.

Comparable averages of 100 individuals at the end of the second year indicated

increases to 89.63 mm and 143.08 g (Allen and Littleford 1955). At Merritt

Island National Wildlife Refuge, Seigel (1984) found that growth rates in

Flori da east coast terrapi ns were greatest duri ng the fi rst 2 years after

hatching and declined considerably as the animals reached maturity (at

plastron lengths of 9.0 to 9.5 cm in males and 13.5 to 14.0 cm in females).

The age at sexual maturity of terrapins along the Atlantic coast probably

varies. Most captive females from North Carolina deposited eggs for the first

time when they were 7 years old, but first clutches were produced by some

terrapins as young as 4 years and others as old as 8 years (Hildebrand 1932).

In a wild population at Dulac, Terrebonne Parish, Louisiana, males attained

maturity in their third year of growth; females did not become mature until

after their sixth year (Cable 1952). In the Florida population studied by

Seigel (1984), male terrapins attained sexual maturity at ages of 2 to 3 years

and females at 4 to 5 years. Maximum longevity of the species may exceed 40

years (Hildebrand 1932).

Sexual dimorphism is conspicuous in adult diamondback terrapins. Males

differ from females in that they are considerably smaller and have narrower

heads and longer, thicker tails, with the vent located posterior rather than

anterior to or even with the margin of the carapace. Carapace lengths of the

Atlantic sUbspecies range from 15 to 23 cm in mature females and 10 to 14 cm

in mature males (Ernst and Barbour 1972; Conant 1975). Among 221 gravid

females from New Jersey, plastron lengths ranged from 13.2 to 18.4 cm (mean

15.44 cm) (Montevecchi and Burger 1975). Males with plastron lengths of 8 to

9 cm usually have developed secondary sexual characteristics (Hildebrand

1932) .

Predation on eggs and hatchlings probably represents the major source of

mortality in most terrapin populations. Other sources include egg mortality

caused by rootlets of the grass Ammophi 1a brevi 1i gul ata i nvadi ng the nests

(Lazell and Auger 1981), entrapment of terrapins in crab pots and subsequent

drowning, and, to a limited extent, direct harvest (Bishop 1983). Although

the small openings of typical crab pots selectively exclude most adult

females, the great number of pots fished each year in diamondback terrapin

3
habitats "may account for more adult terrapin mortalities than any other

single factor" (Bishop 1983).

Harvest for human consumption, which has decreased considerably in recent

years, is virtually restricted to mature females, the largest individuals in

the population (Hay 1904; Coker 1906; Bishop 1983). Because of low recruitment rates caused by hi gh nest and hatch 1i ng mortality, it is doubtful that

diamondback terrapins can survive long-term direct harvesting (Bishop 1983).

Natural predators on the eggs and hatchlings include foxes (Vulpes

fu l va ) , raccoons (Procyon lotor), crows (Corvus brachyrhynchos), and gulls

(Larus atricilla) (Burger 1977). Raccoons killed an estimated 10% of the

female diamondback terrapins that nested over several years at Merritt Island,

Florida, where habitat alteration in the area apparently had resulted in an

increase in the raccoon popul ati on and forced the terrapi ns to use manmade

dikes rather than natural habitats for their nesting sites (Seigel 1980c). In

New Jersey, predators were responsible for egg losses of 51% and 71% in two

consecutive years; predators took 22% of the hatchlings in one year (Burger

1977). Barnacle infestations often occur on the carapaces and plastrons of

diamondback terrapins, and heavy fouling by these organisms may cause shell

erosion, mating and nesting interference, and occasionally death (Seigel

1983). Mortality of both adults and hatchlings resulting from motorized

vehicles (boats, automobiles, bikes, etc.) has not been quantified but may be

significant in some areas (J. Burger, Rutgers University , New Brunswick,

New Jersey, pers. comm.; R. A. Seigel, Savannah River Ecology Laboratory,

Aiken, South Carolina, pers. comm.).

SPECIFIC HABITAT REQUIREMENTS

Diamondback terrapins along the Atlantic coast have been reported in

brackish estuarine environments including salt marshes, tidal flats and

creeks, sounds behind barrier islands, and brackish lagoons and impoundments

(Hay 1904; Coker 1906; Carr 1952; Ernst and Barbour 1972; Burg$r and

Montevecchi 1975; Conant 1975; Martof et al. 1980; Seigel 1980b; Bishop 1983).

Carolina terrapins were caught in salinities of 4.3 to 22.0 parts per thousand

(ppt), with most captures in the 10.1 to 15.0 ppt range (Bishop 1983). Two

lagoons supporting populations of Florida east coast terrapins had mean

salinities of 27.6 and 31.6 ppt (Seigel 1983). Records of terrapins

inhabiting fresh water (Hay 1904) and the ocean (Carr 1952; Neill 1958) are

poorly documented.

Food and Foraging Habitat

Marsh grass or cord grass (Spartina alterniflora) is the typical

vegetation associated with the aquatic habitats of diamondback terrapins along

the Atlantic coast (Coker 1906; Carr 1940; Burger and Montevecchi 1975; Hurd

et a l . 1979; Martof et a l . 1980). A Florida salt-marsh habitat continued to

support a thriving population of Malaclemys after being transformed by the

construction of dikes for mosquito control into a series of lagoons and

impoundments of brackish water (Seigel 1979). Vegetation in this area

4
consisted

filiforme,

1978).

of dense

Thalasia

growths of

testudinum,

rooted seagrasses, principally Cymodocea

and Halodule wrightii (Snelson and Bradley

Diamondback terrapins feed chiefly on a variety of crustaceans, mollusks,

and other invertebrates occurring in brackish- and salt-marsh habitats (Carr

1952; Ernst and Barbour 1972; Spagnoli and Marganoff 1975). Food items of 14

North Carolina diamondbacks caught in Beaufort Harbor included snails

(Littorina irrorata, Melampus lineatus), marine annelids (Nereis sp.),

fragments of crabs (including Gelasimus), a sargassum bulb, and pieces of

grass (Coker 1906). Northern terrapins from a Delaware salt marsh voided

shell fragments of blue mussels, Mytilus edulis (Hurd et al. 1979). Predation

by terrapins on adult Atlantic silversides (Menidia menidia) has been recorded

in South Carolina (Middaugh 1981). Quantitative studies of their

feeding-habitat relationships have not been conducted, but shallow tidal

creeks and subtidal mudflats probably represent the most important feeding

areas for terrapins. For example, near Charleston, South Carolina, terrapins

were caught in crab traps most often during April and May in water less than 3

m deep; it was thought that they were feeding extensively at this time of year

and tended to concentrate on the subtidal mudflats where the traps were placed

(Bishop 1983).

In Delaware, it was estimated that 1,655 diamondback terrapins occupied

the lower 0.9 km of a 6-km long tidal creek draining a 200-ha Spartina

alterniflora marsh (Hurd et al. 1979). The width of this apparently prime

habitat ranged from 8 to 12 m at spring low tides and from 17 to 25 m at high

tides. The channel depth during lower water ranged from 1.3 to 1.7 m, with

occasional holes 3-m deep. Maximum current velocities were 1.45 m/s on the

flood tides and 1.21 m/s on the ebb tides. Sizable catches of terrapins in

Georgia were made in stop nets placed over shell bottoms and near oyster banks

in tidal creeks 50 to 200 m wide (Carr 1952).

Reproduction and Nesting Habitat

Observations of Florida east coast diamondbacks revealed that individuals

moved from large, open lagoons into small canals and ditches for courtship and

mating. The mating process, initiated and completed in the water, seemed only

to require conditions calm enough to permit copulation (Seigel 1980a).

Nesting habitats probably vary throughout the range of the species. Most

nests have been reported near the aquatic habitats of the terrapins, although

females in New Jersey were observed moving 150 m overland to nesting places

(Burger and Montevecchi 1975). In Delaware, a large female was seen laying

eggs on a site some 8 km from the creek where she was originally tagged (Hurd

et al. 1979). At the Merritt Island National Wildlife Refuge, Brevard County,

Florida, most terrapins nested on diked roads (Seigel 1979, 1980b). All

accounts of nesting activities in areas subject to tidal influence agree that

nests are consistently constructed in sandy substrates above the levels of

normal high tides.

5
Although terrapi ns usually nested on hi gh dunes in New Jersey, they

se 1ected fl at 1ocations wi thi n these dune areas for thei r nests (Burger and

Montevecchi 1975). The mean slope of 35 nest locations was 7.2° (0° to 24°),

which was significantly different (p < .001) from the mean slope of 18.1°

recorded for random points in the high dune area.

The density of vegetation near nests and the percentage of cover also

vary. Twenty nests in Massachusetts were equally distributed in vegetated and

virtually bare areas (Lazell and Auger 1981). Nests in New Jersey averaged

11.2 cm from the nearest vegetation, principally dune- or beachgrass

(Ammophila breviligulata), in areas with a mean cover of 8.2% (0 to 36%)

(Burger and Montevecchi 1975). Nests of Flori da east coast terrapi ns were

constructed in areas having a mean vegetative cover of 20% (0 to 75%) (Seigel

1979) .

Vegetation provides terrapins protection from predators, but heavily

vegetated areas also provide habitat for predators. Predation of terrapin

eggs by crows and gulls in New Jersey was highest in open sand areas;

predation of hatchlings by red foxes (Vulpes fulva) and raccoons (Procyon

lotor) was highest in nesting areas surrounded by trees and shrubs (Burger

1977) .

Special Considerations

The habitat requirements and activity patterns of juveniles and

hatchl i ngs after 1eavi ng the nests have not been determi ned. One of the

earliest studies of the species (Coker 1906) noted an unexplained scarcity of

hatchlings and small juveniles in the marshes near Beaufort, North Carolina.

Although several collecting techniques were employed during a 2-year study of

a salt marsh in Delaware, no terrapins with carapace lengths of 30 to 90 mm

were found among 792 individuals collected (Hurd et al. 1979). The smallest

of 281 South Carolina diamondbacks caught in crab traps was 76 mm in plastron

length (Bishop 1983). From late May to October, over a 3-year period at

Barnegat Bay, New Jersey, Pitler (1985) found 12 juvenile diamondback

terrapins with shell lengths ranging from 2.5 to 7.5 cm. They were discovered

under mats of Spartina and various other kinds of sheltering objects at low

tides. All were on well-drained ground in a tidal mud flat about 91 m from

the water's edge.

Little is known about the specific habitat requirements of diamondback

terrapins during cold weather. In North Carolina and Georgia, individuals

reportedly burrow "in the 'pluff mud' of tidal flats, often in or near little

trickles of water from the exposed marshes, in late October or early November,

although any warm day is sufficient to bring them out again and they may be

acti ve off and on all through the winter. From the fi rst of March they are

rarely in hibernation ... " (Carr 1952). One juvenile Virginia terrapin, 54

mm in carapace length, was found on November 7 buried about 0.3 m deep in

moist sand above the high tide mark along the lower York River at Gloucester

Point (Lawler and Musick 1972). Hibernating terrapins at Cape May, New

Jersey, were discovered on the bottoms of tidal creeks under water 1.5 to 2.5

m deep at low tides, beneath undercut banks in the intertidal zone, and buried

in banks near the upper tide limit (Yearicks et al. 1981).

6
Terrapins must drink fresh or brackish water to replenish body water

stores (Dunson 1970), but the optimum amount of water requi red to support

local populations is not known. Rainfall provides freshwater for hatchlings

that cannot grow at salinities found near most nest sites. However, a

quantitative description of coastal sites offering optimum access to rainwater

in the form of runoff was not found in the literature.

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. The HSI model for the diamondback terrapin was

developed for application along the Atlantic coast from New Jersey to Florida.

With some modification, the model should be useful for evaluating habitat in

other parts of the range of the species.

Season. The model was designed to produce an index of habitat

suitability for diamondback terrapin nesting areas on a year-round basis.

Cover types. The model was des i gned for app1i cation in the coastal

upland (U) cover type. This cover type includes dunes near estuarine areas,

sandy mounds withi n marshes, natural sand 1evees, di ked areas, and other

coastal elevations above normal high tides.

Minimum habitat area. Minimum habitat area is defined as the mi mmum

amount of contiguous suitable habitat required by a species to complete all or

a portion of its life requirements. Specific information on minimum nesting

areas required by diamondback terrapins was not found in the literature. If

local information is available to define the minimum area needs of terrapins,

and less than this amount of area is available, the HSI for the species will

be zero.

Verification level. The diamondback terrapin HSI model was developed as

a synthesis of existing information on habitat requirements of the species.

Model structure was based on the authors· interpretation of the existing data

base and the review comments of Dr. Joanna Burger, Rutgers University, New

Brunswick, New Jersey; and Dr. Richard A. Seigel, Savannah River Ecology

Laboratory, Aiken, South Carolina. The model has not been field-tested.

Model Description

Overview. Diamondback terrapins occur basic,ally in two cover types:

estuarine open waters (EOW) (Cowardin et al. 1979), where they feed, bask,

hibernate, and mate; and coastal uplands (U) where they nest. This model

assumes that it is the suitability of nesting areas that primarily limits

terrapin populations. For this reason, and because quantitative data were not

found for factors that may be limiting to terrapins in estuarine waters, the

model only incorporates variables associated with upland nesting habitat.

Terrapins tolerate a wide salinity range within their natural environment, and

this factor is also assumed to be non-limiting. The model further assumes

7
that the phys i 01ogi ca 1 needs of terrapi ns for freshwater can be met by

rainfall and runoff.

Optimum interspersion or juxtaposition of the two cover types for

terrapins is not known, although both must be present to sustain local

populations. The model assumes that the nearest boundary of a suitable

nesting area must be within 250 m of a tidal creek or other estuarine waters

to support a viable terrapin population. The relationship between the nesting

habitat variables and the HSI is illustrated in Figure 1.

The following sections provide documentation of the rationale, including

the logic and assumptions, used to translate information on diamondback

terrapi n habitat use to the vari ab1es and equations used in the HSI mode l .

Nesting habitat variables. Diamondback terrapins construct nests above

the level of normal high tides in substrates composed of sand or a mix of sand

and fi ne shell fragments. Suitabil ity of sandy uplands for nesti ng is

dependent on three variables: the percent canopy cover of shrubs (V1 ) ; the

percent canopy cover of grasses (V2 ) ; and the mean substrate slope (Vs).

Shrubs growing on uplands used by nesting terrapins may provide a

protective benefit to hatchlings by serving as a visual screen between

hatchlings and avian predators. The model, however, assumes that the absence

of shrubs does not result in any significant increase in avian predation,

since upland grasses can provide the same protection. On the contrary, shrubs

provide habitat for mammalian predators that may consume eggs, hatchlings, and

adult females. The model assumes, therefore, that optimum suitability of

sandy uplands for nesting terrapi ns occurs when the shrub canopy cover is

<25%. Cover values >75% are assumed to render an area unsuitable for nesting.

Cover values between-25% and 75% are assumed to affect suitability in a linear

fashion.

Grasses growing on open (nonshrub-covered), sandy uplands also affect the

suitability of these areas for nesting terrapins. The absence of grass cover

exposes hatchlings to increased predation by birds. Too much cover, on the

other hand, makes nest construction difficult or impossible, and dense grassroot systems can destroy existing nests. Optimum suitability is assumed to

occur when the grass canopy cover is in the range of 5% to 25%. As cover

Habitat variable Habitat

V1 Percent canopy cover of shrubs

V2 Percent canopy cover of grasse,~Coastal uplands HSI

Vs Mean slope -

Figure 1. Relationship between the nesting habitat variables and the HSI for

the diamondback terrapin.

8
values decrease below 5% or increase above 25%, the model further assumes that

an area's suitability for nesting decreases in a linear fashion. Sandy areas

devoi d of grass (0% cover) and those completely covered (100% cover) are

considered unsuitable for nesting terrapins.

Mean substrate slope is an additional limiting factor on upland nesting

sites. Flat substrates facilitate digging and egg laying by terrapins and are

less susceptible to erosion than sloping substrates. The model assumes that

an area has optimum suitability for nesting terrapins when the mean slope of

open, sandy substrates is <7°. An area with a mean slope of >25° is

considered unsuitable. The model further assumes that suitabil ity decreases

linearly as mean slope values increase from 7° to 25°.

Suitability Index (SI) Graphs for Model Variables

This section provides graphic representation of the relationship between

habitat variables and habitat suitability for the diamondback terrapin in

sandy upland (U) cover types. The SI values for each variable are read

directly from the graph (1.0 = opt-imum suitability, 0.0 = no suitability).

Major assumptions used in developing the SI graph for each variable are

summarized in Table 1. The SI graphs are based on the assumption that

suitability can be represented by a two-dimensional linear response surface.

Each vari ab1e is further assumed to operate independently over the range of

the others. Additional model assumptions are found in the text.

Habitat Variable Suitability Graph

1.0 U VI Percent canopy

cover of shrubs.

>C 0.8

G)

-e

.5 0.6 >-

,g- 0.4 as

..

UJ 0.2

0.0

0 25 50 75 100

%

9
Habitat Variable Suitabi 1ity Graph

U Y ~.u 2 Percent canopy

cover of grasses.

)( 0.8

Q)

'0

.5 0.6 >-

:!:

.a 0.4 as

.'!:

:I

en 0.2

0.0

0 25 50 75 100

%

1.

U Y3 Mean substrate

slope. )( 0.8

Q)

'0

C

- 0.6

>-

:S

.a 0.4 as

:I

en 0.2

0.0

0 5 10 15 20 25

Slope (0)

HSl Determination

Each of the three nesting habitat variables is assumed to be of equal

importance in determining the distribution and abundance of terrapin nests in

an area. A hi gh sui tabil i ty rating for one vari ab1e cannot compensate for or

offset a low rating in another. It is further assumed that female terrapins

construct their nests in open (nonshrub-covered), sandy areas.

The following equation is suggested for combining individual variable Sl

values for deriving the overall HSr. The HSl is set at 0 if the shortest

distance between the boundaries of estuarine aquatic and sandy upland areas

exceeds 250 m, as measured from the point of Mean High Water (MHW).

1/3

HSl = (SlY

1

x S1 Y2 x Sly

3

)

Examples of SI and HS1 values determined from three hypothetical data sets are

shown in Table 2.

10
Tab1e 1. Vari abl e sources and major assumptions for di amondback terrapi n

suitability indices. a

Variable and source

Burger (1977)

Seigel (1980a)

Burger and Montevecchi

(1975 )

Burger (1977)

Seigel (1979)

Lazell and Auger (1981)

Burger and Montevecchi

(1975)

Assumption

Suitability of nesting habitat is related

to the percent canopy cover of shrubs.

Optimum suitability occurs when <25% of the

upland area is shrub covered. Cover values

>75% are unsuitable.

Suitability of nesting habitat is related

to the percent canopy cover of grasses on

sandy substrates. Optimum suitability

occurs when cover values are 5%-25%.

Suitability of nesting habitat is related

to the mean slope of sandy substrates.

Optimum suitability occurs when the mean

slope is <7°. Slopes greater than 25° are

unsuitable.

aOther assumptions are found in the text.

Table 2. Calculations of the suitability index (SI) for each habitat variable

(V), and the habitat suitability index (HS1) for three sample data sets using

the diamondback terrapin HS1 model equation.

Model Data set 1 Data set 2 Data set 3

element Data SI Data SI Data SI

Variable

Vl 40% 0.80 75% 0.33 10% 1.0

V2 3% 0.60 40% 0.80 25% 1.0

V3 10° 0.83 18° 0.39 5° 1.0

HS1 0.74 0.47 1.0

11
Field Application of the Model

The diamondback terrapin HSI model is designed primarily for mitigation

planning purposes. It is not designed as a research or management tool for

increasing terrapin numbers. Variables included in the model can be estimated

to reduce costs of model application. Increased use of subjective estimates,

however, decreases model replicability. When estimates are used, they should

be accompani ed by appropri ate documentation to ensure that persons maki ng

decisions understand both the method of HSI determination and the quality of

the data used to arri ve at these determi nat ions. Techni ques for measuri ng

variables included in the model are suggested in Table 3.

Table 3. Suggested measurement techniques for habitat variables used in the

di amondback terrapi n HSI model..

Variable

Percent canopy cover

of shrubs.

Percent canopy cover of

grasses.

Mean substrate slope.

Suggested technique

Estimate canopy cover from aerial photos or

vegetation maps or make site visits to

measure the ground surface shaded by a

vertical projection of all shrubby

vegetation at random or selected sample

sites. Relate canopy cover for sample

areas to total upland area.

Refer to aerial photos or vegetation maps

or make site visits to measure the ground

surface shaded by a vertical projection of

grasses at random or selected sample sites.

Relate canopy cover for the sample sites to

total area of open sandy substrate.

Select sample sites in the habitat area

and measure the maximum slope angle at

various sampling points with a protractor

or surveyor1s transit. Sum angles and

divide by number of sample points to

determine the mean slope for the area.

or

Refer to U.S. Geological Survey quadrangle

maps and estimate slope angle for

individual sample sites by determining the

following: arctan (~h) where h is the

difference between two contour values being

considered, and d is the distance between

the two contours on the map for each sample

site. Sum slope angles and divide by

number of sample sites to obtain the

estimated mean slope for the area.

12
The major assumption of the HSI model is that an area1s potential value

to terrapi ns is determi ned by its habi tat features. There are, however, at

least two non-habitat factors that may affect this potential: commercial

crabbing operations and recreational activities. Crab traps, outboard motors,

and all-terrain vehicles may be responsible for unusually high mortalities

among both adults and hatchlings in some localities. Field users of this HSI

model may wish to take these factors into account when assessing an area's

suitability for terrapins. Unfortunately, quantitative data on the impacts of

crab traps and motorized vehicles on terrapins are generally lacking.

Interpreting Model Output

The HSI value obtained by applying the terrapin model may have no

relationship to actual population levels. Terrapin population levels may be

determined by non-habitat factors such as competition, seasonal storms, and,

as mentioned above, commercial crabbing and the operation of motorized

vehicles in the animal I s habitat. Outputs for this model, however, can be

used (1) to compare the habitat potential of two areas to support terrapins at

a single point in time, or (2) to compare the potential of a single area to

support terrapins at future points in time.

SOURCES OF OTHER HABITAT MODELS

No other habitat models of the type developed here for the evaluation of

diamondback terrapin habitat were located in the literature.

13

REFERENCES

Allen, J. F., and R.A. Littleford. 1955. Observations on the feeding habits

and growth of immature diamondback terrapins. Herpetologica 11(1):77-80.

Babcock, H. L. 1919. Turtles of New England. Mem. Boston Soc. Nat. Hist.

8(4): 325-431.

Babcock, H.L. 1926. The diamond-back terrapin in Massachusetts. Copeia 1926

(150): 101-104.

Barney, R.L. 1922. Further notes on the natural history and artificial

propagation of the diamond-back terrapin. Bull. U.S. Fish. 38(917):91-111.

Behler, J. L., and F.W. King. 1979.

American reptiles and amphibians.

Audubon Soci ety fi e1d guide to North

Alfred A. Knopf, New York. 719 pp.

Bishop, J.M. 1983. Incidental capture of diamondback terrapin by crab pots.

Estuaries 6(4):426-430.

Burger, J. 1976a. Behavior of hatchling diamondback terrapins (Malaclemys

terrapin) in the field. Copeia 1976(4):742-748.

Burger, J. 1976b. Temperature relationships in nests of the northern

diamondback terrapin, Malaclemys terrapin terrapin. Herpetologica 32(4):

412-418.

Carr, A. F., Jr. 1940. A contribution to the herpetology of Florida. Univ.

Fla. Publ. Biol. Sci. Ser. 3(1):1-118.

Carr, A. F., Jr. 1952. Handbook of turtles. The turtles of the United

States, Canada, and Baja California. Cornell University Press, Ithaca, N.Y.

552 pp.

Coker, R. E. 1906. The natural history and cultivation of the diamond-back

terrapin, with notes on other forms of turtles. N.C. Geol. Surv. Bull.

14:1-67.

15
Conant, R. 1975. A field guide to reptiles and amphibians of eastern and

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

Cowardin, L.M., 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. 131 pp.

Dunson, W.A. 1970. Some aspects of electrolyte and water balance in three

estuarine reptiles, the diamondback terrapin, American and "salt water"

crocodiles. Compo Biochem. Physiol. 32:161-174.

Ernst, C.H., and R.W. Barbour. 1972.

University of Kentucky Press, Lexington.

Turtles of the United States.

347 pp.

Ernst, C.H., and R.B. Bury. 1982. Malaclemys. Cat. Am. Amphib. Reptiles

299.1-299.4.

Finneran, L.C.

1948(2):138.

1948. Diamond-back terrapin in Connecticut. Copeia

Hay, W. P. 1904.

Bur. Fish. 24:

A revision of Malaclemys, a genus of turtles.

1-20.

Bull. U. S.

Hay, W.P. 1917. Artificial propagation of the diamond-back terrapin. U.S.

Bur. Fish. Econ. Circ. 5:1-21.

Hildebrand, S.F. 1929.

diamond-back terrapin.

Review of experiments on artificial

Bull. U. S. Bur. Fish. 45: 25-70.

culture of

Hildebrand, S. F. 1932.

ratio, and longevity.

Growth of diamond-back terrapins, size attained, sex

Zoologica 9(15):551-563.

Hurd, L.E., G.W. Smedes, and LA. Dean. 1979. An ecological study of a

natural population of diamondback terrapins (Malaclemys !. terrapin) in a

Delaware salt marsh. Estuaries 2(1):28-33.

Lawl er, A. R., and J. A. Musick. 1972. Sand beach hibernation by a northern

diamondback terrapin, Malaclemys terrapin terrapin. Copeia 1972(2):389-390.

Lazell, J.D., Jr., and P.J. Auger. 1981. Predation on diamondback terrapin

(Malaclemys terrapin) eggs by dunegrass (Ammophila breviligulata). Copeia

1981(3):723-724.

Martof, B.S., W.M. Palmer, J.R. Bailey, and J.R. Harrison, III. 1980.

Amphibians and reptiles of the Carolinas and Virginia. University of North

Carolina Press, Chapel Hill. 264 pp.

McCauley, R.H. 1945.

Privately printed.

The reptiles of Maryland and the District of Columbia.

Hagerstown, Maryland. 194 pp.

16
Middaugh, D.P. 1981.

Atlantic silverside,

1981(4):766-776.

Reproductive ecology and spawning periodicity of the

Menidia menidia (Pisces: Atherinidae). Copeia

Montevecchi, W.A., and J. Burger. 1975. Aspects of the reproductive biology

of the northern diamondback terrapin, Malaclemys terrapin terrapin. Am.

Midl. Nat. 94(1):166-178.

Mount, R.H. 1975.

Agric. Exp. Stn.

The reptiles and amphibians of Alabama.

347 pp.

Auburn Univ.

Neill, W. T. 1958. The occurrence of amphibians and reptiles in saltwater

areas, and a bibliography. Bull. Mar. Sci. Gulf Caribb. 8(1):1-97.

Pitler, R. 1985. Life history notes:

Herpetol. Rev. 16(3):82.

Malaclemys terrapin terrapin.

Reid, G. K., Jr. 1955. Reproduction and development in the northern

diamondback terrapin, Malaclemys terrapin terrapin. Copeia 1955(4):310-311.

Seigel, R.A. 1979. The reproductive biology of the diamondback terrapin,

Malaclemys terrapin tequesta. M.S. Thesis. University of Central Florida,

Orlando. 40 pp.

Seigel, R.A. 1980a. Courtship and mating behavior of the diamondback

terrapin Malaclemys terrapin tequesta. J. Herpetol. 14(4):420-421.

Seigel, R·.A. 1980b. Nesting habits of diamondback terrapins (Malaclemys

terrapin) on the Atlantic coast of Florida. Trans. Kans. Acad. Sci. 83(4):

239-246.

Seigel, R.A. 1980c. Predation by raccoons on diamondback terrapins,

Malaclemys terrapin tequesta. J. Herpetol. 14(1):87-89.

Seigel, R.A. 1983. Occurrence and effects

diamondback terrapins (Malaclemys terrapin).

of barnacle infestations on

Am. Mi dl. Nat. 109(1): 34-39.

Seigel, R.A. 1984. Parameters of two populations of diamondback terrapins

(Malaclemys terrapin) on the Atlantic coast of Florida. Pages 77-87 in R.

A. Seigel, L. E. Hunt, J. L. Knight, L. Malaret, and N. L. Zuschlag, eds.

Vertebrate ecology and systematics. A tribute to Henry S. Fitch. Mus. Nat.

Hist. Univ. Kans. Spec. Publ. 10. 278 pp.

Smith, H.M., and E.D. Brodie, Jr. 1982. Reptiles of North America. Golden

Press, New York. 240 pp.

Smith, H.M., and R.B. Smith. 1979. Synopsis of the herpetofauna of Mexico.

Vol. 6. Guide to Mexican turtles, bibliographic addendum 3. John Johnson,

North Bennington, Vt. 1,044 pp.

Snelson, F.F., Jr., and W.K. Bradley, Jr. 1978. Mortality of fishes due to

cold on the east coast of Florida, January 1977. Fla. Sci. 41(1): 1-12.

17
Spagnoli, J.J., and B.I. Marganoff.

Conservationist (New York) 29:17-19.

1975. New York's marine turtle.

Strauch, A. 1980. Bermerkungen uber die Schildkrotensammlung in zoologischen

Museum der Kai serl i schen Akademi e der Wi ssenschaften zu St. Petersbourg.

Mem. Acad. Imp. Sci. St. Petersbourg, Ser. 7, 38(2):1-127.

Yearicks, E.F., R.C. Wood, and W.S. Johnson. 1981. Hibernation of the

northern diamondback terrapin, Malaclemys terrapin terrapin. Estuaries

4(1): 78-80.

18
snn-utl

REPORT DOCUMENTAnON II. 1ll11'01IT NO.

Report 82(10.151) 12.

I. lIIeclplenr. Accft_ ....

PAGE Biological

.. TItle ............ .. ItetIott tIMe

Habitat Suitability Index Models: Diamondback March 1988

Terrapin (Nesting)--Atlantic Coast I.

7._u I. ""rformI.. Orpnlutlon R.pt. N•• W.M. Palmer and C. L. Cordes

i. "",1ooIiiI.. OrpnlutiM N.me .nd Add.... 10. .....ectlT..../W_ Unit No.

North Carolina Museum of Natural Hi story

Raleigh, NC 27611 u. ContrKtlc) .. Grent(Q) No.

(C)

(G) 12. 1......-.. Orpn_1on N.m••nd Add....

National Wetlands Research Center II. T,.1I8 of R.port & _ Cow,,"

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240 101.

I" Supplement.,.,. Not"

·11._ IUmlt: 200 WDrda)

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

Suitabil ity Index (HSI) model for the diamondback terrapin (Malaclemys terrapin). The model 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. DDcu_ Anal,.... •. D....rtpt....

Turtles Habitabil ity

Wil dl ife Mathematical models

It. l-.tIera/Dpen·Ended T.nn.

Diamondback turtle

Malaclemys terrapin

Habitat suitability

Co COSATI Fi.ld/Group

II. Av.lI.bllltJ' St.t.ment It. Secum,.CI... (Thl. R.port) 21. No. of Papa

Release unlimited Unclassified 18

20. SecurltJ' CI... (Thl......) 22. PrIce

Unclassified

(lee ANII-l3t.11) OPTIONAL fO_ m (4-771 Dep._ (f.....-lJ' NTIs-J5)

01 CDmm.....
Take Pride in America

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l!"So . 'ISH.WILDLIFE ME.VlCt;

,, ' :\ .'

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