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22001155 NORTHEASTERN NATURALIST 2V2(o3l). :2623,0 N–6o4. 23
Quantifying New York’s Diamondback Terrapin Habitat
James P. Browne1, Alexandra Kanonik1, John P. Vanek1,2, Crystal A. Crown1, and
Russell L. Burke2,*
Abstract - Spartina marshes (S. patens [Salt Meadow Cordgrass] and S. alterniflora
[Saltmarsh Cordgrass]) are critical foraging, nursery, and overwintering habitats for
Malaclemys terrapin (Diamondback Terrapin). However, the relationships between
Spartina marsh quality, quantity, and distribution and resulting Diamondback Terrapin
distribution, abundance, and movements are poorly understood. To develop a model for
predicting these relationships, we needed a way to prioritize the locations where data are
collected for model building. As an initial effort, we used available data on New York
Spartina salt marsh distribution and estimates of Diamondback Terrapin home range
to identify marshes for initial surveys and pilot work for studies of habitat quality. We
present GIS-model results showing New York locations with 50-, 100-, and 260-ha hypothetical
home ranges (consisting of 50%, 75%, and 100% Spartina marsh), and use this
information to identify New York locations most likely to harbor Diamondback Terrapins.
Our models indicated there should be relatively large populations of terrapins in western
Hempstead Bay and eastern Jamaica Bay, but failed to identify a known terrapin population
at Piermont Marsh on the Hudson River.
Introduction
Malaclemys terrapin (Schoepff) (Diamondback Terrapin, hereafter, Terrapin)
are medium-sized turtles that live in brackish water. Outside of southern mangrove
habitats, Terrapins appear to be restricted to western Atlantic coastal Spartina
alterniflora (Loisel) (Salt Marsh Cordgrass) and S. patens (Muhl.) (Salt Meadow
Cordgrass) marshes (Brennessel 2006), collectively known as Spartina marshes, or
salt marshes. These marshes are vital Terrapin feeding and overwintering habitats
(Latham 1971, Yearicks et al. 1981). Throughout their range, Terrapins eat a variety
of marsh-dwelling prey species (Erazmus 2012, King 2007, Petrochic 2009).
Terrapin-conservation efforts are motivated in part by the animals’ role as a keystone
species. Terrapins can significantly and selectively impact invertebrate prey
populations, which, in turn, greatly affects the immediate ecosystem. For example,
Terrapin predation dramatically reduces Littoraria irrorata (Say) (Salt Marsh Periwinkle
Snail) abundance (Levesque 2000). These snails are important predators and
vectors of fungal infection of Spartina (Silliman and Bertness 2002), another saltmarsh
keystone species (Bruno 2000, Seliskar et al. 2002). Thus, Terrapins exert
top-down control on Spartina herbivores. Another factor that makes Terrapins a
keystone species is their ability to move large quantities of nutrients and calories
1Department of Conservation and Waterways, Town of Hempstead, PO Box 180, Point
Lookout, NY 11569. 2Department of Biology, Hofstra University, Hempstead, NY 11549.
*Corresponding author - biorlb@hofstra.edu.
Manuscript Editor: Todd Rimkus
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from water to land in the form of eggs and emerging hatchlings, which are eaten
by a variety of predators. For example, on one small beach in New York, Procyon
lotor (L.) (Raccoon) ate Terrapin eggs containing 300,000 Kcal during a single year
(Feinberg and Burke 2003). Eggs missed by vertebrate predators can also benefit
plant roots, which absorb nutrients from the decaying shells important for growth
in this nutrient-poor environment (Feinberg and Burke 2003, Stegmann et al. 1988).
Additionally, Terrapins are probably important seed dispersers for marine plants,
such as Zostera marina (L.) (Eelgrass) (Sumoski and Orth 2012) and Spartina (R.L.
Burke, unpubl. data).
However, the broader implications of Terrapins as a keystone species are poorly
known because system-level aspects of Terrapin ecology have been poorly studied.
This situation has arisen at least partly because Terrapin populations range-wide
were decimated in the 18th and 19th centuries when they were hunted and eaten in
huge numbers, particularly in turtle soup (Brennessel 2006, Schaffer et al. 2008).
Terrapin populations started recovering in some areas subsequent to the collapse
of the soup industry, until large-scale coastal urban development caused massive
habitat losses in the 20th century (Ner and Burke 2008). Current low numbers make
it difficult to reconstruct the previous ecological roles played by this coastal species
(e.g., Jackson et al. 2001).
Despite some recovery, Terrapin numbers are declining throughout their
range (Butler et al 2006) due to past over-harvesting (De Sola 1931, Hay 1917,
Roosenburg et al. 2008), ongoing mortality as by-catch in commercial crab traps
(Roosenburg 2004), past and ongoing habitat loss (e.g., Ner and Burke 2008), and
subsidized predation (e.g., Feinberg and Burke 2003). As a result, the International
Union for the Conservation of Nature is currently considering raising the status of
Terrapins from near-threatened to vulnerable (P.P. van Dijk, Conservation International,
Washington, DC, pers. comm.). Additionally, Therres (1999) suggested that
M. t. terrapin (Schoepff) (Northern Diamondback Terrapin) be considered for listing
under the federal Endangered Species Act. Terrapins are considered threatened
in Massachusetts, endangered in Rhode Island, and special concern in New Jersey
(Watters 2004). They are identified in Connecticut (CDEP 2005) and New York’s
(NYDEC 2005, 2014a) state conservation plans as S3 (few occurrences) on a list of
species of greatest conservation need.
New York is home to roughly 2181 km of potential Terrapin habitat, more than
any other state in the northeast. According to our calculations, this area amounts
to 36% of the total Terrapin habitat for the region between the Delaware River and
Cape Cod. However, due to their restriction to Spartina salt marshes, Terrapins in
New York only occur around Long Island, the lower Hudson River (at least as far
north as Piermont Marsh, river-mile 25), and the Hudson River Bight (Burke 2006,
NYDEC 2014b). New York Terrapin populations were probably once enormous in
the historically large, Crassostrea virginica (Gmelin) (Eastern Oyster)-dominated
estuarine habitats of the Hudson River Bight (Morreale 1992, Ner and Burke 2008);
however, they were subject to heavy harvest in the marshes around New York
City (Burke and Francoeur 2014, Latham 1971, Murphy 1916). Although Terrapin
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populations have made a partial recovery, they are still subject to very high levels
of nest predation (Feinberg and Burke 2003), and populations remain vulnerable
(NYDEC 2014a).
Terrapins are currently difficult to census with precision or accuracy. Unlike
other turtle species, they do not consistently enter traps or bask where they can
be easily observed (but see Harden et al 2009). Indirect measures, such as using
abundance of the larvae of the Terrapin-obligate parasitic trematode Pleurogonius
malaclemys Hunter (Byers et al. 2011, Hunter 1961) as an indicator of Terrapin
abundance, are currently too imprecise for most uses unless data are collected in a
limited time frame (Chodkowski et al. 2015). Thus, it can be difficult to collect the
population-size and trend data needed to inform Terrapin management, especially
over a large spatial range.
There have been 2 previous attempts to estimate the sizes and distributions
of New York Terrapin populations. First, in 1991, Morreale (1992) surveyed 73
Long Island sites using trammel nets, hoop nets, otter trawls, snorkeling, scuba,
examination of basking sites, and evidence of nesting. He chose these sites as
representative of the available New York locations that met the criteria of brackish
water; presence of brackish marsh-plant species such as Spartina, Limonium
carolinianum (Walter) Britton (Sea Lavender), Salicornia europaea (auct. non L.)
(Glasswort), Phragmites australis (Cav.) Trin. ex Steud. (Common Reed), and Typha
latifolia, (L.) (Cattail); and available upland nesting habitat. Morreale (1992)
captured or directly observed 993 individual Terrapins. He was not able to estimate
population sizes reliably at any of these sites because recapture rates were low, but
Terrapins were clearly widespread across Long Island.
The second attempt was during the New York State Amphibian and Reptile
Atlas Project (NYDEC 2014b), which assembled data collected by citizen scientists
and professional biologists from 1990 to 2007. The atlas reported that Terrapins occurred
in 30 different USGS 7.5-minute topographic quadrangles scattered around
Long Island and on the Hudson River (NYDEC 2014c). The Atlas provided only
presence data and did not estimate population sizes or trends.
Time and funding for Terrapin population research are limited, and development
of an unbiased method for prioritizing survey efforts based on measurable
criteria would aid researchers. One way to prioritize an initial survey of a population
is to rank the likely habitat patches by their potential to support populations
of the target species. We sought to estimate the distribution and abundance of
Terrapins in New York by using information about their required habitat—Spartina
marshes—and home-range size, with the assumption that Terrapin should be
present where Spartina marsh occurrences were large enough to satisfy Terrapin
home-range size. Terrapins nest in a wide range of habitats up to 1 km from water
(R.L. Burke, unpubl. data), and nesting habitat is apparently abundant in New
York. We sought to determine locations most likely to harbor Terrapins as locations
for future surveys. We also attempted to quantify the amount of potential
Terrapin habitat in New York.
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Methods
We used ArcGIS 9.3.1, and all layers were in geodetic datum NAD83, projection
UTM Zone 18N, with meters as a linear unit. We obtained wetland-area data
from the US Fish and Wildlife Service’s (USFWS) National Wetlands Inventory
GIS data layer for New York State (USFWS 2014). We chose these data because
the classification system of salt marshes included an estuarine and marine wetlands
class, which we considered to be Diamondback Terrapin habitat (Dahl and Stedman
2013). We extracted the estuarine and marine wetland polygons from the USFWS
coastal wetlands ESRI shapefile polygon GIS layer, and removed sandy beaches,
leaving us with a layer that included only intertidal marsh habitat. We converted this
polygon layer to raster format with a pixel size of 20 m2 using the ArcGIS polygonto-
raster conversion tool. Hereafter, we refer to this raster as the salt marsh raster
layer (SMRL). For ease of later calculations, we rounded any pixel with >50% or
less than 50% of its area covered by salt marsh to 100% and 0%, respectively .
Terrapin home-range data from the Northeast were not available, so we used
relevant data from Butler’s (2002) and Spivey’s (1993) studies of Terrapins in
northeastern Florida and North Carolina, respectively. Their results were disparate;
home ranges estimated using a 95% kernel-density estimator averaged 52.62 ±
47.77 ha in Florida and 252.1 ± 38.9 ha in North Carolina. Therefore we conservatively,
and somewhat arbitrarily, rounded these values to 50 ha and 260 ha to
represent a wide range of hypothetical average home ranges to model relative Terrapin
home-range size. We also used 100 ha as an intermediate home-range size.
To determine the percentage of intertidal marsh within each home-range estimate,
we used the spatial analyst extension (SAE) focal-neighborhood tool with
each of the 3 home-range sizes (50 ha, 100 ha, and 260 ha), assuming a circular
neighborhood (home range). To do this, we entered the corresponding radii for
circles with areas 50 ha, 100 ha, and 260 ha, which are 398.94 m, 564.19 m, and
909.82 m, respectively, and instructed the program to calculate the mean. Thus, we
obtained the percentage of salt marsh within the given radius of each pixel on the
SMRL by taking the mean of the pixel values (0 or 100) within the radius of that
pixel. For each diameter, we assumed complete overlap of home ranges with no territoriality.
The results from this analysis were 3 different habitat percentage layers
(HPL50, HPL100, and HPL260).
To create our final habitat maps, we used the SAE contour list tool on the output
maps from the focal-neighborhood tool to create contours representing 50% marsh,
75% marsh, and 95% marsh for both the 100-ha and 50-ha territories within the entire
marine district of New York State. The larger 260-ha range-size estimate lacked
many 95% habitat locations; consequently for this layer we also created 30%, 50%,
70%, and 90% contours. These contours enclosed the percentages of salt marsh
corresponding at least to the resolution of the original habitat map, losing only very
small patches of marsh, small marsh creeks, and ponds that were too small to map.
We used presence of salt marsh as a proxy to interpret the higher-value contours in
the resulting maps as locations with higher relative likelihoods of finding Terrapins.
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Results
We assumed that Terrapins would occupy only those areas with 100% Spartina
cover. Our GIS maps showed locations of potential Terrapin habitat across the New
York State marine district, including the estuaries surrounding Long Island, and the
Hudson River and New York Harbor estuaries (Fig. 1). Both the 50-ha and 100-ha
territory ranges may underestimate the actual feeding and overwintering range, but
the 50-ha assumption clearly identified sites that should be prioritized for survey.
According to the 50-ha range estimate, the location predicted to have the most Terrapin
habitat in New York is Lawrence Marsh (Fig. 2). This marsh is located in the
far western end of Hempstead Bay and is occupied largely by Hicks Beach. The
95%-marsh contour for this location encompasses over 139 ha. The nearby Green
Sedge and Cedar Island Marsh complex also has a 95% contour, which encloses
Figure 1. Overview of tidal marshlands that provide potential Diamondback Terrapin habitat
in New York.
Figure 2. Hempstead Bay, using the 50-ha hypothetical home range and showing the 95%
contours on Lawrence Marsh, Green Sedge Complex, Parsonage Island, Crow Island Complex,
Seamans Island, and South Line Island along with extensive 75% and 50% contours.
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another 30 ha. We compiled a full list of the sites in New York with 95% contours
(Table 1).
The combined area enclosed by the 95% contour for the 50-ha territory layer for
the entirety of New York State is 232 ha, of which 169 ha are located in the West
Bay section of Hempstead Bay. Thus, West Bay contains 72.8% of the high-density
salt marsh within New York State, and the entirety of Hempstead Bay contains an
additional 11.2% (total = 84%). Because of the high proportion of potential habitat
at these locations, we predict this area to support the highest density of Terrapins in
New York State (Table 1, Fig. 2).
Significant terrapin habitat was identified elsewhere in Great South Bay, Werheim
Preserve, and Stony Brook Harbor (Figs. 3–5). None of the other parts of
Long Island, including Peconic Bay, Moriches Bay, Shinnecock Bay, or the North
Shore harbors facing Long Island Sound other than Stony Brook had any 95% contours
based on a 50-ha Terrapin territory range. Staten Island and the Hudson River
marshes also contained no 95% contours, although extensive areas of 75% and 50%
contours exist. Oyster Bay, which has long had a Terrapin population (King 2007,
Marganoff 1970), did not even have a small 50% contour, and thus, appeared to
represent poor Terrapin habitat (Fig. 6).
When we applied 260-ha home-range contours to Hempstead Bay (Fig. 7), we
observed only small shifts in the estimates of highest density of potential Terrapin
locations. Additionally, the use of the larger range contours was less likely to follow
particular islands or to stay within marshlands. This effect was particularly prominent
when we compared maps using the 100-ha contours and the 50-ha contours on
Jo Co Marsh (Fig. 8).
Discussion
In general, as we increased the size of the assumed home range, it was increasingly
likely that habitats other than salt marsh would be included. Therefore, as the
Table 1. Locations in New York State where 50-ha territories can have 95% salt marsh, and the area
of the contours enclosing the 95% centroids.
Location Area (ha)
Hicks Beach, Lawrence Marsh 139.12
Green Sedge and Cedar Islands Complex, West Bay 29.80
Jo Co Marsh, Jamaica Bay 18.17
Crow Island Complex, East Bay 18.19
Gilgo Island, Great South Bay 8.37
Parsonage Island, Middle Bay 5.31
Cedar Island, Great South Bay 4.33
Wertheim National Wildlife Refuge, Great South Bay 2.80
South Line Island, East Bay 2.01
Stony Brook Harbor 1.77
Seamans Island, East Bay 1.12
Smith Point Park 0.81
Total 231.80
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size of the hypothetical home range increased, the area enclosed by a contour that
represented a high percentage of salt marsh decreased. The presence of a greater
proportion of lower-quality habitat within the larger home-range contours caused
the results to be less specific and less useful for locating Terrapins. When ranking
locations for survey effort, a key factor in choosing a particular percent habitat in
combination with a particular estimated home-range size would be the practical
consideration of the total area and number of separate locations that could be covered
during a reasonable anticipated survey effort.
Initial home-range sizes also seemed relatively unimportant, given that simply
reducing the threshold-minimum percentage of habitat for inclusion in a survey
effort would produce similar results. To focus on specific sections of habitat, and
exclude surrounding poor-quality habitat, it may even be beneficial to focus on the
Figure 3. The Great South Bay assuming 50-ha territories, including Gilgo Island and Cedar
Island 95% contours plus extensive areas of 75% and 50% contours.
Figure 4. Wertheim Preserve has a small section of 50-ha home ranges with 95% or more
salt marsh, and a larger extent of a 75% contour.
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Figure 5. Stony Brook Harbor contains a section of salt marsh that is also likely to contain
Terrapins, and includes 1.77 ha of the 95% contour using the 50-ha range estimate.
Figure 6. Lloyd Neck and Oyster Bay are not represented, although Terrapin populations
have been reported there (Bauer 2004, Morreale 1992, Petrochic 2009)..
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core ranges that represent the 50% isopleth. Using this criterion, even the larger
home-range estimates of Spivey (1998) produced core-range estimates of 30.82
ha (adaptive kernel method) or 42.95 ha (MCP) that were both smaller than the
50-ha estimate tested here.
When designing an efficient survey, it may be prudent to weight the effort
by the habitat density. In this example, most of the effort would go to locations
with at least one 95% contour, with less effort spread through locations with 50%
contours but no 95% contours, and spot-check locations with only 50% contours.
However, even the 50% contours with the more conservative and detailed 50-ha
Figure 8. A comparison of the size of 3 levels of percentage salt-marsh contours produced
assuming 100-ha and 50-ha home-range sizes on Jo Co Marsh in Jamaica Bay.
Figure 7. Hempstead Bay, using the 260-ha hypothetical territory for comparison and showing
the 90% contours on Lawrence Marsh, Green Sedge Complex, Parsonage Island, and the
Crow Island Complex along with extensive 70%, 50% and 30% contours.
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assumption for home-range size failed to identify as quality habitat the site of the
known population in Oyster Bay. Our approach could be used as a tool for making
a limited Terrapin survey more efficient. Although head counts (e.g., Harden
et al. 2009), and/or Pleurogonius malaclemys surveys (e.g., Chodkowski et al.,
in press) could improve efficiency, only an exhaustive survey would have a high
probability of finding every population.
Our approach to identifying locations of New York Terrapin populations indicated
the presence of far fewer populations than either Morreale (1992) or NYDEC
(2014b), indicating that our approach is conservative. However, we point out that
our technique may identify sites where long-term Terrapin populations persist,
whereas Morreale (1992) or NYDEC (2014b) mostly indicated mere presence of
Terrapins. We believe our results are conservative and habitat-focused. Terrapin
distribution is presumably a function not of only habitat availability but other
historical processes, such as Terrapin harvest, road mortality, and predation. A comparison
of the locations used in each study suggests that some larger populations
were not adequately surveyed in the past, but that our approach may fail to identify
small populations.
Our approach indicates that there should be relatively large patches of Terrapin
habitat in western Hempstead Bay and Jo Co Marsh, in Jamaica Bay. Terrapins in
the former site have not been investigated, but recent data suggests that a relatively
large Terrapin population inhabits Jo Co marsh (Burke and Francoeur 2014). In
contrast, our analysis does not indicate the Terrapin populations known to exist at
Piermont Marsh on the Hudson River (Simoes and Chambers 1999) or Oyster Bay
(King 2007, Marganoff 1970).
We suggest several uses for our results. Our predictions of locations where New
York Terrapin populations might occur based on available habitat should be field
verified. Our results predict both Terrapin presence and relative population size, and
these predictions could be tested by measuring P. malaclemys abundance at a variety
of sites within a short time frame (Chodkowski et al., in press) or by head-count surveys
(e.g., Harden et al. 2009). Four New York Terrapin populations have been well
studied (central Jamaica Bay [R.L. Burke and A. Kanonik, unpubl. data]; eastern
Jamaica Bay [L.C. Francoeur, Port Authority of New York and New Jersey, Jamaica,
NY, unpubl. data]; Oceanside Marine Nature Study Area in northern Middle Bay
[M.A. Farina, Town of Hempstead, Hempstead, NY, unpubl. data]; and Oyster Bay
in the Long Island Sound, [M. Draud, Armstrond State University, Savannah, GA,
unpubl. data). Investigations into marsh use by these Terrapins would lead to a better
understanding of the relationship between marsh size and Terrapin populations.
Acknowledgments
We are grateful for excellent suggestions made by two anonymous reviewers.
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