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A. Parlin, S. Dinkelacker, and A. McCall
22001155 SOUTHEASTERN NATURALIST Vo1l4.( 114):,3 N3–o4. 01
Do Habitat Characteristics Influence American Alligator
Occupancy of Barrier Islands in North Carolina?
Adam Parlin1, Steve Dinkelacker1,*, and Aaron McCall2
Abstract - The geographic range of Alligator mississippiensis (American Alligator) extends
to North Carolina, where information on populations is limited. In North Carolina, American
Alligators are found near the coast, but typically not on the extensive barrier-island
chain known as the Outer Banks. The goal of our study was to determine if habitat varied
among sites occupied by American Alligators on islands—the Outer Banks and Roanoke
Island—and sites on the adjacent mainland. Water depth, variance in water depth, turbidity,
salinity, conductance, and pH varied among sites on Roanoke Island from sites on the
mainland (P = 0.008) and the Outer Banks (P = 0.001). However, sites on the mainland and
the Outer Banks were similar (P = 0.536). Ultimately, American Alligators may access the
Outer Banks and find suitable habitat, but to date, little research has examined American Alligator
habitat use in this portion of its geographic range; long-term occupancy is probably
limited by of the effects of human disturbance and major storm events.
Introduction
Alligator mississippiensis (Daudin) (American Alligator, hereafter, Alligator)
ranges from eastern Texas to North Carolina (Newsom et al. 1987, Palmer
and Braswell 1995). Alligator habitats have been well documented in the core
of the species’ range (Goodwin and Marion 1979, Joanen and McNease 1970,
Subalusky et al. 2009); however, information on northern populations is limited.
Surveys in North Carolina have documented low densities throughout the state
(Birkhead and Bennett 1981, O’Brien and Doerr 1986). Although temperature
is likely the most important environmental constraint on the species’ geographic
range (Lance 2003), the distribution of populations at the edge of the range may
be influenced by habitat availability. For instance, Alligator populations in North
Carolina are distributed along the mainland coast, (Birkhead and Bennett 1981,
O’Brien and Doerr 1986), but are typically not on the barrier-island chain known
as the Outer Banks. Given that Alligators have the capability to swim long distances
(Campbell et al. 2010, Elsey 2005, Lance et al. 2011), they could readily
access the Outer Banks.
Abiotic pressures on barrier islands are different from those on the mainland
(Bourdeau and Oosting 1959, Conner et al. 2005, Oosting 1945) and may prevent
Alligator occupancy. For instance, the unstable soil substrate supports unique plant
communities that could influence Alligator nests and burrows (Campbell 1999,
Goodwin and Marion 1978, Palmer and Mazzoti 2004). In addition, freshwater
1Department of Biology, Framingham State University, Framingham, MA 01701. 2The
Nature Conservancy, 701 West Ocean Acres Drive, Kill Devil Hills, NC 27948. *Corresponding
author - sdinkelacker@framingham.edu.
Manuscript Editor: Ron Davis
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2015 Vol. 14, No. 1
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ponds are filled by precipitation and therefore susceptible to drought (Bellis 1995),
which can result in reduction of water levels and increased salinity. Foraging in saline
environments requires either ingesting freshwater to counter the increased salt
intake or an adaptation for excreting salt. Although the presence of lingual glands
in some crocodilians aids in excreting excess salt, the lingual glands in Alligators
lack the efficiency to prevent dehydration (Talpin et al. 1982, 1985).
The goal of our study was to determine if Alligator habitat varied between barrier
islands and the adjacent mainland. Specifically, we tested whether sites occupied
by Alligators on the mainland differed from unoccupied sites on the Outer Banks.
Insights from this study could be used to identify habitat characteristics that support
and limit Alligators at the northern edge of the species’ range.
Methods
Study sites
We conducted the study at sites on the mainland, Roanoke Island, and the
Outer Banks (Dare and Hyde counties, NC) between 14 May and 18 June 2012.
We selected sites based on accessibility and previously documented accounts of
Alligator presence in these areas. On the mainland, Alligators are routinely found
along roadside ditches and lakes throughout Alligator River National Wildlife
Refuge, as well as private hunting reserves and farms around Lake Mattamuskeet
National Wildlife Refuge (S. Dinkelacker and A. McCall, pers. observ.). We have
been conducting spotlight surveys and mark–recapture studies throughout the
area since 2011. Although population and density data are not reported here, we
directly confirmed Alligator occupancy at all the mainland sites (n = 8), which
were located near or in pocosin swamps with very poorly drained mineral soils,
organic mucks, and peats (Sharitz and Gibbons 1982). Although not previously
documented on Roanoke Island (O’Brien and Doerr 1986, Palmer and Braswell
1995), we visually confirmed the presence of Alligators there in 2011; however,
no Alligators were observed during this study in 2012. Alligators have not been
reported north of Oregon Inlet on the Outer Banks, and have been documented
only in southern areas near Bogue Banks (O’Brien and Doerr 1986, Palmer and
Braswell 1995). Sites on Roanoke Island (n = 7) and the Outer Banks (n = 6)
were maritime forests and coastal plains in sandy soils characteristic of coastal
environments (Bellis 1995).
Habitat assessment
We assessed habitat characteristics for each site using both mapping software
and ground surveys. For each site, we used DeLorme Topo USA 8.0 (DeLorme,
Yarmouth, ME) to determine total area (ha) and Web Soil Survey (Soil Survey Staff
2012) to determine soil type and hydrology. We measured additional physical characteristics
at each site via ground surveys along a stratified random-transect grid.
To construct the grid, we used mapping software and drew a longitudinal transect
along the maximum length of the lake, ditch, or pond. We then drew transverse
transects every 500 m if the longitudinal transect was <2000 m, or 1000 m if the
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A. Parlin, S. Dinkelacker, and A. McCall
2015 Vol. 14, No. 1
longitudinal transect was ≥2000 m. On each transverse transect, we chose and recorded
GPS coordinates of points every 30 m, beginning 5 m from one shore and
extending to the opposite one. In the field, we located each GPS point and measured
water clarity (cm), turbidity (cm), and water depth (cm) with a weighted secchi
disk. We also recorded canopy cover (%) with a convex spherical densiometer
(Forestry Suppliers Inc., Jackson, MS), and percent vegetative cover (emergent and
submerged vegetation) using a 1-m2 quadrat. For each site, we calculated means;
however, we also calculated variance for water depth in order to represent heterogeneity
of depth.
We used a similar sampling approach to measure water chemistry; however,
instead of locating points every 30 m, we recorded GPS locations at points 25%,
50%, and 75% the total length of the transverse transect. At each of these points,
we recorded salinity (ppt), conductance (μS/cm), and dissolved oxygen (mg/L)
using a YSI 55 Dissolved Oxygen Meter (Forestry Suppliers Inc.), and pH using a
YSI EcoSense pH10A (Forestry Suppliers Inc.) near the water surface. Means were
calculated for all water-chemistry data.
Habitat analysis
We used nonmetric multidimensional scaling (NMS) to analyze differences and
group similar sampling units (i.e., sites) among locations. Nonmetric multidimensional
scaling is an ordination technique that explores arbitrary or discontinuous
data, assesses the data as sample units, and places them in a simple graphical representation
as coordinates on axes. To determine the appropriate number of axes,
we used NMS with the Sørensen distance measure and used the autopilot-mode
thoroughness setting (500 iterations, instability criterion of 0.0000001, reduction
in dimensionality from six to one, 250 runs with real data, 250 randomized runs)
in PC-ORD version 6 (McCune and Grace 2002). Additionally, a random starting
configuration was used to avoid local minima. After determining the appropriate
number of axes, we performed a single analysis to observe groupings created by
NMS (200 iterations, random starting coordinates, instability criterion of 0.0005,
and 50 runs with the real data).
Subsequently, we used a multi-response permutation procedure (MRPP) to test
for any differences between or among groups. MRPP is a nonparametric procedure
that compares pre-existing sample units according to designated grouping (i.e.,
Outer Banks, Roanoke Island, or mainland). We used the Sørensen distance measure
for the MRPP analysis; the Bonferonni adjustment was made at α = 0.016 for
pairwise comparisons.
Results
We chose a two-dimensional NMS solution because of the relatively low final
stress (5.468), instability (0.00042), and Monte Carlo P-value (0.0040) compared
across several NMS solutions (Fig. 1). The solution was stable after reviewing the
instability criterion, stress vs. iteration plot, and low final stress, which indicated
a low risk of drawing false conclusions. Habitat was considered significant if the
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Pearson’s correlation value (r) was >0.60 on either axis (Table 1). Water depth,
variance in water depth, turbidity, salinity, conductance, and pH were correlated
with axis 1, and total area was correlated with axis 2.
Using MRPP sample units varied (Table 2). Pairwise comparisons of Roanoke
Island to either the mainland or the Outer Banks yielded results comparable to the
overall comparison. However, pairwise comparisons of mainland vs. Outer Banks
were broadly overlapping (P = 0.536 ).
Figure 1. Nonmetric multidimensional scaling plot examining the proportion of variation
(%) between each habitat location surveyed. The correlated measures are labeled on their
respective axes. Mainland sites are denoted by a triangle, Outer Banks sites are denoted by
a diamond, and Roanoke Island sites are denoted by a square.
Table 1. The Pearson’s correlation (r) value for each ordination axis in the final NMS solution.
The |r| value was correlated with the respective axis if ≥0.60. Correlated values are denoted with
an asterisk.
Habitat characteristics Axis 1 Axis 2
Canopy cover (%) -0.502 -0.166
Turbidity (cm) -0.724* -0.239
Water depth (cm) -0.953* -0.337
Water depth Variance (cm) -0.878* -0.228
Submerged vegetation (%) -0.054 -0.161
Emergent vegetation (%) -0.393 -0.195
Salinity (ppt) -0.756* -0.014
Conductance (μS/cm) -0.763* -0.002
Dissolved oxygen (mg/L) -0.160 -0.318
pH -0.684* -0.065
Total area (ha) -0.041 -0.606*
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Discussion
Sites on the Outer Banks and the mainland were similar based on the habitat
characteristics measured, and Alligators could potentially occupy nearby barrier
islands. However, Roanoke Island differed in physical characteristics and water
chemistry when compared to the mainland and Outer Banks habitats. Habitat on
Roanoke Island was characterized by greater water depths, increased salinity, pH,
and conductivity relative to other sites. Although Roanoke Island had different
habitat characteristics, Alligator presence was verified on the island in 2011 during
our pilot study.
There are other possible variables that could influence Alligator occupancy of
barrier islands. For example, major storm events can modify physical characteristics
and water chemistry (Paerl et al. 2006, Sallenger 2000). Major storm events
have been previously documented to displace Alligators. Hurricane Ike displaced
a juvenile Alligator in Louisiana approximately 489 km from its original release
site (Elsey and Aldrich 2009). Hurricane Irene passed over the mainland and barrier
islands of North Carolina in 2011, inundating Roanoke Island and parts of the
mainland with seawater for several hours, which could have displaced Alligators
from the area. The lingering effects of seawater inundation, such as increased
salinity and conductance (Fig. 1), could reduce habitat and foraging suitability. Interestingly,
Alligators have been previously observed in some habitats on the Outer
Banks near Cape Hatteras. In Frisco, NC, local citizens reported an adult Alligator
frequently basking in the pond; state biologist Chris Turner (NC Wildlife Resources
Commission, Manteo, NC, pers. comm.) confirmed the Alligator’s presence. However,
there have been no sightings of the Alligator in the past 3 years (Douglas
Oberbeck, Dare County Sheriff’s Office Criminal Investigator, Manteo, NC, pers.
comm.). Human presence on the Outer Banks is significant during the summer—7
million people visit the Outer Banks each year (The Outer Banks 2010)—and could
influence the suitability of habitats and behavior of Alligators.
Limited information, especially pertaining to habitat, is available on Alligators
in their northern geographic range (Birkhead and Benett 1981, Fuller 1981, Hagan
1982, O’Brien and Doerr 1986). Most available information regarding habitat suitability
has been documented for the core of the population in Texas, Louisiana,
and Florida (Newsom et al. 1987, Rice et al. 2004). North Carolina may contain
Table 2. Multi-response permutation procedure for location of habitats surveyed. T describes the
separation between or among groups: the more negative the value, the stronger the separation. A, the
chance-corrected within-group agreement, describes the effect size: when A < 0, there is less agreement
with groups than expected; when A = 0, groups are no more or less different than expected by
chance; when A = 1, sample units within each group are identical. P is the likelihood that an observed
difference between or among groups is due to chance.
Location T A P
Overall -4.95 0.198 0.001
Mainland vs. Roanoke Island -3.66 0.185 0.008
Mainland vs. Outer Banks 0.32 -0.011 0.536
Roanoke Island vs. Outer Banks -5.66 0.249 0.001
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suboptimal habitat for Alligators, and the species’ range is probably limited in
this area by the influence of temperature (Lance 2003). We acknowledge that the
study was conducted during a short time-frame in summer and did not account for
seasonality, which can influence Alligator habitat use and physiology (Goodwin
and Marion 1979, Lance 2003, Seebacher et al. 2003). For instance, depending
upon wind-driven tides and rainfall, salinity in certain locations in the sounds
separating the Outer Banks and Roanoke Island from the mainland occasionally
approach levels associated with freshwater (A. McCall, pers. observ.). Alligators
could easily traverse the sounds during this time and access Roanoke Island and
the Outer Banks. Therefore, it is possible that Alligators could use the Outer Banks
at different times of the year, or under certain environmental or climatic conditions.
Alligators periodically access the Outer Banks, but long-term occupancy is
likely limited by major storm events because the topography of the barrier islands
has been altered by hurricanes more than once in the past decade (APNEP 2012).
Further investigations into the population dynamics, possibly via satellite tracking,
would aid in understanding movement patterns, occupancy, and habitat use of Alligators
in the northern portion of their range.
Acknowledgments
This research was supported by IUCN Crocodile Specialist Group Undergraduate Research
Grant, Framingham State University, and The Nature Conservancy. Research was
conducted under permits 9-2012 issued by the NC National Estuarine Research Reserve,
permits 42530-11-009 and12-002 issued by the Fish and Wildlife Services, and permit 12-
SC00491 issued by North Carolina Wildlife Resources Commission Division of Wildlife
Management. We thank R. Hignite and A. Harmon for site access and M. Gosselin, R. Tibbert,
and C. Mettey for their field assistance.
Literature Cited
Albemarle-Pamlico National Estuary Partnership (APNEP). 2012. Albemarle-Pamlico Ecosystem
Assessment. Available online at http://portal.ncdenr.org/web/apnep/researchreports.
Accessed 10 December 2012.
Bellis, V.J. 1995. Ecology of maritime forests of the southern Atlantic coast: A community
profile. Biological Report 30. US Department of the Interior, National Biological Service,
Washington, DC. 95 pp.
Birkhead, W.S., and C.R. Bennett. 1981. Observations of a small population of estuarineinhabiting
alligators near Southport, North Carolina. Brimleyana 6:111–117.
Bourdeau, P.F., and H.J. Oosting. 1959. The maritime Live Oak forest in North Carolina.
Ecology 40:148–152.
Campbell, H.A., M.E. Watts, S. Sullivan, M.A. Read, S. Choukroun, S.R. Irwin, and C.E.
Franklin. 2010. Estuarine crocodiles ride surface currents to facilitate long-distance
travel. Journal of Animal Ecology 79:955–964.
Campbell, M.R. 1999. Everglades alligator holes: Distribution and ecology. M.Sc. Thesis.
University of Florida, Gainesville, FL. 81 pp.
Conner, W.H., W.D. Mixon II, and G.W. Wood. 2005. Maritime forest-habitat dynamics on
Bulls Island, Cape Romain National Wildlife Refuge, SC, following Hurricane Hugo.
Forest Ecology and Management 212:127–134.
Southeastern Naturalist
39
A. Parlin, S. Dinkelacker, and A. McCall
2015 Vol. 14, No. 1
Elsey, R.M. 2005. Unusual offshore occurrence of an American Alligator. Southeastern
Naturalist 4:533–536.
Elsey, R.M., and C. Aldrich. 2009. Long-distance displacement of a juvenile alligator by
Hurricane Ike. Southeastern Naturalist 8:746–749.
Fuller, M.K. 1981. Characteristics of an American Alligator (Alligator mississippiensis)
population in the vicinity of Lake Ellis Simon, North Carolina. M.Sc. Thesis. North
Carolina State University, Raleigh, NC. 136 pp.
Goodwin, T.M., and W.R. Marion. 1978. Aspects of the nesting ecology of American Alligators
(Alligator mississippiensis) in north-central Florida. Herpetologica 34:43–47.
Goodwin, T.M., and W.R. Marion. 1979. Seasonal activity ranges and habitat preferences
of adult alligators in a north-central Florida lake. Journal of Herpetology 13:157–164.
Hagan, J.M. 1982. Movement habits of the American Alligator (Alligator mississippiensis)
in North Carolina. Ph.D. Dissertation. North Carolina State University, Raleigh, NC.
406 pp.
Joanen, T., and L. McNease. 1970. A telemetric study of nesting female alligators on Rockefeller
Refuge, Louisiana. Proceedings of the Southeastern Association of Game and
Fish Commissioners 24:175–193.
Lance, V.A. 2003. Alligator physiology and life history: The importance of temperature.
Experimental Gerontology 38:801–805.
Lance, V.A., R.M. Elsey, P.L. Trosclair III, and L.A. Nunez. 2011. Long-distance movement
by American Alligators in Southwest Louisiana. Southeastern Naturalist 10:389–398.
McCune, B., and J.B. Grace. 2002. Analysis of Ecological Communities. MjM Software
Design, Gleneden Beach, OR.
Newsom, J.D., T. Joanen, and R.J. Howard. 1987. Habitat suitability index models: American
Alligator. US Fish and Wildlife Service, Biological Report 82(10.136). 14 pp.
O’Brien, T.G., and P.D. Doerr. 1986. Night count-surveys for alligators in coastal counties
of North Carolina. Journal of Herpetology 20:444–448.
Oosting, H.J. 1945. Tolerance to salt spray of plants of coastal dunes. Ecology 26:85–89.
Paerl, H.W., L.M. Valdes, A.R. Joyner, B.L. Peierls, M.E. Piehler, S.R. Riggs, R.R.
Christian, L.A. Eby, L.B. Crowder, J.S. Ramus, E.J. Clesceri, C.P. Buzzelli, and R.A.
Luettich, Jr. 2006. Ecological response to hurricane events in the Pamlico Sound system,
North Carolina, and implications for assessment and management in a regime of
increased frequency. Estuaries and Coasts 29:1033–1045.
Palmer, M.L., and F.J. Mazzotti. 2004. Structure of everglades alligator holes. Wetlands
24:115–122.
Palmer, W.M., and A.L. Braswell. 1995. Reptiles of North Carolina. University of North
Carolina Press, Chapel Hill, NC. 412 pp.
Rice, K.G., F.J. Mazzotti, L.A. Brandt, and K.C. Tarboton. 2004. Alligator habitat suitability
index. Pp. 93–110, In K.C. Tarboton, M.M. Irizarry-Ortiz, D.P. Loucks, S.M. Davis,
and J.T. Obeysekera (Eds.). Habitat suitability indices for evaluating water management
alternatives. Office of Modeling Technical Report, South Florida Water Management
District, West Palm Beach, FL. 148 pp.
Sallenger, A.H., Jr. 2000. Storm-impact scale for barrier islands. Journal of Coastal Research
16:890–895.
Seebacher, F., R.M. Elsey, and P.L. Trosclair III. 2003. Body temperature null distributions
in reptiles with nonzero heat capacity: Seasonal thermoregulation of the American Alligator
(Alligator mississippiensis). Physiological Biochemical Zoology 76:348–359.
Sharitz, R.R., and J.W. Gibbons. 1982. The ecology of southeastern shrub bogs (pocosins)
and Carolina bays: A community profile. US Fish and Wildlife Service, Division of
Biological Services, Washington, DC. FWS/OBS-82/04. 93 pp.
Southeastern Naturalist
A. Parlin, S. Dinkelacker, and A. McCall
2015 Vol. 14, No. 1
40
Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. 2012. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.
gov/. Accessed 7 July 2012.
Subalusky, A.L., L.L. Smith, and L.A. Fitzgerald. 2009. Detection of American Alligators
in isolated, seasonal wetlands. Applied Herpetology 6:199–210.
Talpin, L.E., G.C. Grigg, P. Harlow, T.M. Ellis, and W.A. Dunson. 1982. Lingual salt glands
in Crocodylus acutus and C. johnstoni, and their absence from Alligator mississipiensis
and Caiman crocodilus. Journal of Comparative Physiology [B] 149:43–47.
Talpin, L.E., G.C. Grigg, and L. Beard. 1985. Salt-gland function in freshwater crocodiles:
Evidence for a marine phase in eusuchian evolution? Pp. 403–410, In G. Grigg, R. Shine,
and H. Ehmann (Eds.). Biology of Australasian Frogs and Reptiles. Royal Zoological
Society of New South Wales, Australia. 527 pp.
The Outer Banks. 2010. Visitors guide. Available online at http://www.vgnet.com/outerbanks/
index.php. Accessed 10 December 2012.