2009 NORTHEASTERN NATURALIST 16(3):317–320
The Status of the Northern Population of the
Copper-bellied Watersnake, Nerodia erythrogaster neglecta
Omar Attum1,2,*, Yu Man Lee3, and Bruce A. Kingsbury1
Abstract - The Nerodia erythrogaster neglecta (Copper-bellied Watersnake)
population north of 40 degrees north latitude is categorized as threatened under
the US Endangered Species Act. Previous efforts to use mark-recapture to estimate
population size have been unsuccessful because there were so few initial captures.
We therefore evaluated the status of the largest presumed subpopulation of this
population by using strip transects to provide a conservative (maximum) estimate of
density and population size of adults. Our results indicate that the largest presumed
adult Copper-bellied Watersnake metapopulation north of 40 degrees north latitude is
extremely small, with a total size of 94 ± 22 adults, and is characterized by a population
density of 1.17 ± 0.27 adult snakes/ha (95 % CI = 0.73–1.87 snake/ha). This
Copper-bellied Watersnake population appears to be in danger of extirpation within
the foreseeable future. Recovery will require protecting existing wetland complexes
and restoring wetlands and the surrounding upland habitats.
Introduction
Nerodia erythrogaster neglecta Conant (Copper-bellied Watersnake) is
a subspecies of Nerodia erythrogaster (Forster in Bossu) (Plain-bellied Watersnake)
(Conant 1949). The Copper-bellied Watersnake has been in decline
across its range for some time (USFWS 1997), and populations now persist
only in isolated pockets. Populations in Indiana, Michigan, and Ohio north
of 40 degrees latitude are listed as threatened under the US Endangered Species
Act, and these occur mostly in a small area that borders northwestern
Ohio and adjacent southern Michigan. To the best of our knowledge, there
are no published size estimates for these metapopulations. Efforts to estimate
population size using mark-recapture have failed because of a paucity
of captured snakes (B.A. Kingsbury, unpubl. data). We therefore used strip
transects to attempt to provide a conservative estimate of the density and
population size of the largest Copper-bellied Watersnake (referred to only as
watersnake later herein) metapopulation, which is believed to contain most
of the individuals of this watersnake north of 40 degrees north latitude.
Materials and Methods
This study was conducted in northwestern Ohio and adjacent southern
Michigan on public and private lands that are believed to contain the largest
1Center for Reptile and Amphibian Conservation and Management, Indiana-Purdue
University Fort Wayne, 2101 E. Coliseum Boulevard, Fort Wayne, IN 46805. 2Current
address - Department of Biology, Indiana University Southeast, 4201 Grant Line
Road, New Albany, IN 47150. 3Michigan Natural Features Inventory, Michigan State
University Extension, PO Box 30444, Lansing, MI 48909-7944. *Corresponding
author - oattum@ius.edu.
318 Northeastern Naturalist Vol. 16, No. 3
metapopulation of watersnakes (B.A. Kingsbury, unpubl. data). The study
area contains a variety of wetland sizes and types, including both ephemeral
wetlands and larger, permanent wetlands that had often been artificially
created and have water levels controlled for game fishes and birds. These wetlands
are fed by spring, river, lake, and local runoff. The St. Joseph River flows
through the study site. The upland landscape consists of a matrix of forested,
shrub-scrub, old-field, agricultural, and residential habitats and roads.
We sampled 34 wetlands, which ranged in size from 0.02–5.26 ha
and totaled 34.99 ha surface area. We treated each wetland shoreline as a
transect by walking along the shoreline edge or wading through water when
necessary for the entire length of each wetland (shoreline length range =
57–2357 m; total shoreline length surveyed = 20,366 m). The transects were
2 m wide, with 0.5 m occurring on the shore and 1.5 m in the water. Each
wetland was surveyed four times between 26 April–13 June 2006, when
snakes were most likely to be observed because of little vegetation growth
and conspicuous basking. We only observed adults during the surveys and
therefore our estimates do not include neonates or juveniles.
We assumed that animals on the transect were always detected because
the observer was presumed to eventually flush snakes near the shoreline
while walking the transect. For one wetland, we only surveyed 83 m of
373 m of shoreline because of dense vegetation and deeper water.
Wetland size and shoreline length were obtained from past studies (Attum
et al. 2008; Roe et al. 2003, 2004). The surveyed wetlands included all
the areas known to have the highest watersnake densities, based on previous
surveys and telemetry studies (Attum et al. 2008; Roe et al. 2003, 2004).
There were an additional 29 ha of low-use or marginal wetland habitat that
we did not survey due to time-limit and additional man-power constraints
(Attum et al. 2008; Roe et al. 2003, 2004)
Several localities, of an unknown total number of wetland ha, where the
watersnake is known to have occurred north of 40 degrees latitude were not
surveyed as these localities are mostly represented by historic records, with
no recent observations (B.A. Kingsbury, unpubl. data). We believe including
these areas in our estimates would have increased population size by only a
few individuals and would have decreased density estimates.
We estimated population size and density from the strip-transect data
using DISTANCE 5.0 software (Olson et al. 2005, Thomas et al. 2006). In
order to use DISTANCE 5.0 to estimate density with strip-transect data, we
set the number of intervals to 1 and then set the interval to half the total width
(1 m) of the 2-m wide strip transect. We pooled the data for our analysis, but
used the multiplier function in DISTANCE to divide the estimates by four to
account for multiple surveys.
Results
There were 19 watersnake observations on our transects, with density
estimated at 1.17 ± 0.27 snakes/ha (95 % CI = 0.73–1.87 snake/ha), with the
percent coefficient of variation = 22.94%. The population size for the area
2009 O. Attum, Y.M. Lee, and B.A. Kingsbury 319
surveyed was estimated at 41 ± 9 individuals (95 % CI = 25–66). If we extrapolate
densities to the additional 29.31 ha of unsurveyed wetlands for a total of
64.3 ha, then the total adult watersnake population size would be 94 ± 22.
Discussion
Our density and population estimates can only be considered as the “best
case” scenario because of two main sources of bias that inflated our estimates.
First, the transects were not randomly placed, but instead occurred on
the wetland shoreline. Our transect placement violated an important assumption
of density estimation, namely, that transects must be randomly placed.
Watersnakes are more likely to be found near the shoreline, especially in
palustrine and lacustrine open wetlands (Laurent and Kingsbury 2003). This
violation will cause abundance and density to be overestimated because of
collecting data in the area of highest watersnake density, and application
of that density estimate to the entire wetland. The second main source of
bias is that we estimated population size by surveying the wetlands with the
presumed highest snake density and applied that density to wetlands that we
considered to be low-use watersnake habitat.
Despite the inflation biases in our analysis, the density estimates show
that this metapopulation is extremely small and characterized by a conspicuously
lower density than reported for other populations of Copper-bellied
Watersnake. For example, Lacki et al. (1994), in southern Indiana, reported
adult densities to be between 10–14 snakes/ha, depending upon locality, in
prime habitat, and Laurent (2000), in northwestern Kentucky, reported a
density of 11 snakes/ha.
According to the Endangered Species Act, a threatened species is one
likely to become endangered in the foreseeable future, while an endangered
species is "in danger of extinction within the foreseeable future throughout
all or a significant portion of its range.” We believe that the northern population
of the watersnake is already endangered based on the small size of
the largest metapopluation, extremely high extinction probability for small
populations, and the definition of endangered as specified by the Endangered
Species Act.
The small population size and low density of the northern population
is most likely the result of insufficient and degraded wetland habitat and
negative landscape effects on existing habitat (Attum et al., 2008; Roe et al.,
2003, 2004, 2006). The recovery of the watersnake will require protection
and restoration of wetland complexes embedded in a forest matrix, and not
just of individual wetlands (Roe et al. 2003, 2004). These complexes should
contain a mosaic of large, open wetlands surrounded by multiple, ephemeral,
and variable types of wetlands (Roe et al. 2003, 2004). To maintain connectivity
between wetlands and wetland complexes, forest areas within a 250-m
radius of each wetland should be protected (Attum et al. 2008). In addition,
the effects of roads as a source of mortality and habitat fragmentation also
need to be addressed (Kramer-Schadt et al. 2004, Roe et al. 2006).
320 Northeastern Naturalist Vol. 16, No. 3
Acknowledgments
This research was funded by the US Fish and Wildlife Service Section 6 Grant
Program, Michigan Department of Natural Resources, and the Center for Reptile and
Amphibian Conservation and Management, Indiana-Purdue University Fort Wayne.
We are indebted to Nathan Herbert, Kile Kucher, and Sean Zera, who provided field
assistance for this project. D. Soards provided logistical support. The following
landowners granted permission to survey private property: D. Brown, R. Neely, T.
Oberleiter, G. Richards, J. Roden, M. Schauer, D. and J. Wilson, and M. Wilson. We
thank the Michigan Nature Association and Toledo Area Council for permitting us to
survey their properties. All applicable Animal Care guidelines and required state and
federal permits were obtained for this study. Four reviewers made helpful comments
during manuscript review.
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