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Do Habitat Characteristics Influence American Alligator Occupancy of Barrier Islands in North Carolina?
Adam Parlin, Steve Dinkelacker, and Aaron McCall

Southeastern Naturalist, Volume 14, Issue 1 (2015): 33–40

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Southeastern Naturalist 33 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 Southeastern Naturalist A. Parlin, S. Dinkelacker, and A. McCall 2015 Vol. 14, No. 1 34 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 Southeastern Naturalist 35 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 Southeastern Naturalist A. Parlin, S. Dinkelacker, and A. McCall 2015 Vol. 14, No. 1 36 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* Southeastern Naturalist 37 A. Parlin, S. Dinkelacker, and A. McCall 2015 Vol. 14, No. 1 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 Southeastern Naturalist A. Parlin, S. Dinkelacker, and A. McCall 2015 Vol. 14, No. 1 38 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. 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