Home Range and Habitat Selection of an Inland Alligator
(Alligator mississippiensis) Population at the Northwestern
Edge of the Distribution Range
Joseph D. Lewis, James W. Cain III, and Robert Denkhaus
Southeastern Naturalist, Volume 13, Issue 2 (2014): 261–279
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22001144 SOUTHEASTERN NATURALIST 1V3o(2l.) :1236,1 N–2o7. 92
Home Range and Habitat Selection of an Inland Alligator
(Alligator mississippiensis) Population at the Northwestern
Edge of the Distribution Range
Joseph D. Lewis1,2, James W. Cain III1,3,*, and Robert Denkhaus4
Abstract - Although well studied in coastal ecosystems, comparatively little information
exists on the ecology of inland Alligator mississippiensis (American Alligator) populations,
particularly at the periphery of their range. Our specific objectives were to estimate homerange
area and assess diel (i.e., day vs. night) habitat-selection patterns of an urban, inland
American Alligator population at the northwestern edge of the species’ range. During 2010–
2011, we captured 14 (6 female, 5 male, 3 unknown sex) American Alligators, 9 (5 female
and 4 male) of which were fitted with VHF transmitters. Mean home range (95% kernel)
was 68.9 ha (SD = 31.6) and 40.9 ha (SD = 20.7) and the mean core area (50% kernel) was
20.6 ha (SD = 18.5) and 10.1 ha (SD = 6.6) for males and females, respectively. American
Alligators primarily selected river channels and open-canopy shorelines during both day
and night. The amount of emergent or floating vegetation and canopy cover in a particular
habitat influenced the probability of selection by American Alligators but this probability
was dependent on the diel time period. During the day, the probability of selection was
higher in areas with emergent or floating vegetation and more canopy cover, whereas at
night the probability of selection decreased with increasing canopy cover. American Alligators
did not select open water at either the study-area level or within the home range,
which may have been due at least in part to the presence of recreational boaters or differences
in food availability between open-water areas and other areas occupied by American
Alligators on the Fort Worth Nature Center and Refuge. Overall, the results of our study are
largely incongruent with patterns of home-range size and habitat selection reported for the
species elsewhere, suggesting that further study of other American Alligator populations at
the periphery of the distribution range is warranted.
Introduction
Research on behavior (Brisbin et al. 1982, Lang 1976), population ecology
(Goodwin and Marion 1979, Rootes and Chabreck 1993, Taylor 1984), growth rates
(Rootes et al. 1991, Wilkinson and Rhodes 1997), sex ratios (Rootes and Chabreck
1992), habitat use (Goodwin and Marion 1979; Joanen and McNease 1970, 1972),
and physiological ecology (Brandt and Mazzotti 1990, Spotila et al. 1972) has been
1Department of Biological and Environmental Sciences, Texas A&M University-Commerce,
Commerce, TX, 75428. 2Current address - Texas Parks and Wildlife Department,
State Parks and Facility Management, Wildland Fire Management Planning and Operations,
12016 FM 848, Tyler, TX 75707. 3Current address - US Geological Survey, New Mexico
Cooperative Fish and Wildlife Research Unit and Department of Fish, Wildlife, and Conservation
Ecology, New Mexico State University, PO Box 30003, MSC 4901, Las Cruces,
NM 88003. 4Fort Worth Nature Center and Refuge, Fort Worth, TX, 76135. *Corresponding
author - jwcain@nmsu.edu.
Manuscript Editor: David Steen
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conducted on Alligator mississippiensis Daudin (American Alligator; hereafter,
Alligator). However, the vast majority of these studies have been carried out on
Alligator populations in coastal regions, with little research on inland populations
(but see Saalfeld et al. 2008, 2011; Webb et al. 2009). Although they are important
to the ecology of wetlands (Craighead 1968, Rosenblatt and Heithaus 2011), comparatively
little is known about Alligator populations that inhabit inland wetlands,
habitats which are generally more heterogeneous than coastal wetlands (Ryberg et
al. 2002, Webb 2005). For example, water levels fluctuate to a greater degree in
inland wetlands than in coastal wetlands, which can lead to fragmentation of inland
Alligator populations (Ouboter and Nanhoe 1988). Resource availability, Alligator
density, growing-season length, and salinity differ between coastal and inland wetlands
(Saalfeld et al. 2011, Webb 2005). Alligator growth rates and body condition
may also differ between inland and coastal populations; for example, sub-adult
Alligators from an inland population grew faster and had lower estimated body
conditions than those reported for sub-adult Alligators from a coastal population
(Saalfeld et al. 2008).
Although there has been limited research on inland American Alligator populations,
information from populations at the edge of their known range is virtually
nonexistent. However, the abundance and distribution of a species are limited by
the combination of physical and biotic environmental characteristics (Brown 1984);
individuals at the edge of the species’ range tolerate environmental conditions different
from those experienced by most of the population. Differences in biotic and
abiotic parameters near range limits can make ecological processes more variable
than in the interior of the range. The magnitude of seasonal variability in climatic
conditions is often higher at the edge of a species’ range and is more likely to approach
physiological tolerance limits than in the center of the range (Sexton et al.
2009). Climatic variability can influence food availability, growth, survival, and
reproductive rates. For many species, population density tends to be greatest in
the center of the range and declines gradually toward the range boundaries (Brown
1984, but see Sexton et al. 2009).
With a few exceptions (e.g., Saalfeld et al. 2008, Webb 2005), the vast majority
of research on Alligators has been conducted in coastal systems; this bias potentially
limits our understanding of the species’ ecology across its range. More information
is needed on inland Alligator populations, particularly those that occur at
the edges of their geographic range. The goal of this study was to determine habitat
use by Alligators within an urban, inland population at the edge of the species’ geographic
range. Our specific objectives were to estimate Alligator home-range area
and assess diel patterns of habitat selection.
Methods
Study area
We conducted this study on the Fort Worth Nature Center and Refuge (FWNCR),
located in Tarrant County, TX (Fig. 1). The FWNCR is located just inside the Fort
Worth city limits on the north end of Lake Worth and encompasses 1465 ha of
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mixed forests, short-grass prairies, and wetlands (Fort Worth Nature Center and
Refuge 2008). The average annual precipitation for Fort Worth is 864 mm (National
Climatic Data Center 2000).
The north end of the FWNCR includes the West Fork of the Trinity River, which
flows into Lake Worth. Submerged vegetation is sparse throughout the FWNCR.
Aquatic vegetation communities include shallow marshes covered with Typha
domingensis Pers. (Southern Cattail) and other emergent species. The low, flat terrain
surrounding the river and lake is seasonally flooded due to heavy precipitation
during the winter and spring (Fort Worth Nature Center and Refuge 2008).
Figure 1. Known distribution range of the American Alligator (Alligator mississippiensis)
in Texas (Adapted from Distribution of American Alligators in Texas; TPWD 2003). Core
range = areas with optimal habitat conditions and higher quality of food sources resulting in
a higher density of Alligators; extended range = areas with less than optimal habitat conditions,
a lower quality and quantity of food sources, and a lower density of Alligators. Study
area is located on the northwest side of Fort Worth, TX.
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Capture and handling
We captured Alligators <1.2 m in length either by hand or using a pole snare
from a flat-bottomed jon boat. We captured Alligators >1.2 m using swim-in live
traps (Ryberg and Cathey 2004, Webb 2005) baited with decaying chicken carcasses
and small perforated tins of sardines wired to the edge of the traps. For each individual
captured, we measured total length (ventral tip of snout to tail), snout–vent
length (ventral tip of snout to proximal tip of vent), eye to naris length, total head
length (dorsal tip of snout to distal part of head scute), tail girth (circumference of
tail directly behind rear legs), and mass (Saalfeld et al. 2008). We determined sex
by cloacal examination (Chabreck 1963, Joanen and McNease 1978) for individuals
>50 cm (individuals <50 cm cannot be accurately identified to sex; Joanen and
McNease 1978). We uniquely marked all captured Alligators by removing a dorsal
tail-scute (Chabreck 1963, Saalfeld et al. 2008) and we inserted a passive integrated
transponder (PIT) tag into either of the hind legs or in the tail depending on the size
of the Alligator (Saalfeld et al. 2008, Webb 2005). For Alligators >1.5 m in length,
we attached a VHF radio transmitter (Telonics Inc., MOD-400; 205, Mesa, AZ) to
the dorsal scutes located behind the head (Kay 2004). We fastened transmitters by
steel cable woven through holes drilled into the keratinized portion of the dorsal
scutes and used crimps to secure cable ends. Marine epoxy was used to make the
transmitters more hydrodynamic and to decrease the chance of debris being caught
underneath, increasing transmitter retention time. We released all Alligators at the
site of capture as quickly as possible (i.e., ≤ 60 min.). For those Alligators that were
either captured by hand or with a pole snare, we used a handheld GPS to record
the location where the Alligator was initially observed and placed a numbered,
weighted buoy for habitat-data collection (see below). All capture and handling
procedures were approved by the Texas A&M University-Commerce Institutional
Animal Care and Use Committee (IACUC protocol #P10-01-01) and Texas Parks
and Wildlife Department (Scientific research permit # SPR-0310-028).
Home range and habitat selection
We relocated all radio-marked Alligators by homing using the VHF transmitter
until we obtained a visual observation of the Alligator. We attempted to relocate
each Alligator at least 6 times per week, at 3 day and 3 night locations at least 12
hours apart; however, we were unable to relocate each Alligator on every attempt.
Because each Alligator retained its transmitter for varying lengths of time, the total
duration of radio tracking for each animal also varied. We used Hawth’s Tools
(Beyer 2004) in ArcGIS 9.3 (Environmental Systems Research Institute, Redlands,
CA) to delineate 95% fixed-kernel home-range and 50% core areas (Worton 1989)
using least-squares-cross validation to select the smoothing parameter (Seaman and
Powell 1996, Seaman et al. 1999).
Each time we relocated an Alligator, we marked the location using a handheld
GPS and buoy, which served as the center for a 20-m-radius circular plot. We then
recorded the habitat type (as described below for second- and third-order habitat
selection), water depth, water temperature, distance from the center of the plot to
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nearest woody or aquatic vegetation, and visually estimated percent canopy-cover
(Webb et al. 2009). In addition, for each Alligator location, we collected the same
habitat data in 2 additional plots that we established 40 m from the relocation plot
along randomly selected compass bearings. These additional plots represented
available habitat. We recorded water temperature at the time the Alligators were observed;
however, to minimize disturbance to study animals, we collected all other
data the following day.
We assessed second-order (i.e., home range compared to study area) and thirdorder
(i.e., within home range) habitat-selection patterns (Johnson 1980). We
mapped the entire study area using a combination of existing geospatial data (i.e.,
park boundaries and roads; City of Fort Worth 2010) and satellite imagery (Tarrant
County 2005 NAIP 2M NC; TNRIS 2009) in ArcGIS 9.3. We mapped the study
area according to the following categories: open water (i.e., no vegetation above
the surface of the water), emergent vegetation (i.e., rooted vegetation breaking the
surface of the water), floating vegetation (i.e., vegetation floating on the surface of
the water), river channel, or open-canopy shoreline (Fig. 2). To incorporate periods
when seasonal flood conditions inundated portions of the shrubland and forest areas
near the lake and river, we assumed that terrestrial habitat ≤100 m from the water’s
edge was available to Alligators and we classified it as either shrubland or forested
area; we selected 100 m because it represented the maximum area we observed to
be influenced during spring floods.
Because our delineation of the habitat map was largely based on satellite imagery,
we were unable to incorporate changes in the water level throughout each study season.
Therefore, our demarcation between aquatic and terrestrial habitat classes was
based on the water level we observed during the majority of the study period. To assess
the accuracy of our habitat map, we overlaid the Alligator locations recorded in
the field and compared field-based classification of habitat types with those derived
from our map; 95.3% of our field-based locations corresponded with the map-based
classification. We defined the area available at the study-area level by pooling the
relocations of all study animals and calculating 95% fixed-kernel home-range as
described above. Using ArcGIS 9.3, we clipped the amalgamated home-range
to remove those areas >100 m from the water’s edge, then calculated the total area
(i.e., availability) of each habitat class within the study area (i.e., amalgamated
95% kernel home-range) in ArcGIS 9.3, overlaid the relocations of all radio-tagged
Alligators, and recorded the habitat type for each location. To assess third-order selection,
we calculated the total area (i.e., availability) of each habitat class within the
home range of each Alligator in ArcGIS 9.3.
Statistical analysis
Second-order habitat selection consisted of comparing the proportion of relocations
in each habitat type (i.e., used habitat) to the proportion of each habitat type
within the study area (i.e., available habitat). For each radio-marked Alligator, we
calculated selection ratios and associated 95% Bonferroni-adjusted confidence intervals
(Johnson 1980, Manly et al. 2002, McDonald et al. 2005). For third-order
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habitat selection, we calculated selection ratios (and 95% Bonferroni-adjusted confidence
intervals) using the proportion of the relocations in each habitat type and the
proportion of each habitat type available within the home range of each animal (i.e.,
availability differed for each animal). We used selection ratios and confidence intervals
to determine if the proportion of use differed from availability at the studyarea
and home-range scales. Selection ratios >1 with confidence intervals that did
not include 1 indicated habitat types that were used in a higher proportion than
Figure 2. Map of habitat types used to assess habitat selection of American Alligators (Alligator
mississippiensis) on the Fort Worth Nature Center and Refuge, TX, 2010–2011.
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available (i.e., selected for), whereas selection ratios <1 with confidence intervals
that did not include 1 indicated habitat types that were used in a lower proportion
than available (i.e., avoided) (Manly et al. 2002).
We also modeled habitat selection using logistic regression so that we could
incorporate both continuous and categorical predictor variables. We used mixedeffects
logistic regression including a random intercept for each Alligator to
determine the factors that influenced habitat selection (Breslow and Clayton 1993,
Gillies et al. 2006) with library lme4 (Bates et al. 2013) in program R version
2.12.2 (R Development Core Team 2013). Continuous explanatory variables were
those collected at the Alligator-use locations and the associated random locations
described above, and included water depth, distance to vegetation, percent canopycover,
and water temperature. We coded all habitat types into one categorical
variable and set open water as the reference category. We combined the terrestrial
habitat types into one category (other) due to low sample size. The binary responsevariable
was the location type coded as used or available.
We developed a set of a priori models to assess combinations of habitat variables
believed to potentially influence habitat selection by this Alligator population
(Table 1). We used an information-theoretic approach to select the most parsimonious
model using Akaike’s information criterion (AICc) corrected for small sample
size (Burnham and Anderson 2002). The number of parameters (k) was the sum of
the total number of parameters estimated for fixed effects plus 1 for each random
intercept (Skrondal and Rabe-Hesketh 2004). We accounted for model uncertainty
by calculating model-averaged parameter estimates (±SE) (Burnham and Anderson
2002). Odds ratios and 95% confidence intervals were then derived by exponentiation
of the model-averaged parameter estimates. The odds ratio indicated how
much more or less likely it was for the outcome of interest (i.e., selection) to occur
with a 1-unit change in the explanatory variable. Changes in explanatory variables
with odds-ratio confidence intervals not including 1 were deemed to result in a
change in the likelihood of selection.
Table 1. Model number and structure of the 11 a priori models relating the probability of habitat selection
by American Alligators (Alligator mississippiensis) to environmental characteristics at the Fort
Worth Nature Center and Refuge, TX, 2010–2011.
Model # Model structure
1 Habitat type
2 Canopy cover
3 Water depth
4 Water temperature
5 Distance to vegetation
6 Water temperature + water depth
7 Habitat type + canopy cover
8 Habitat type + water depth
9 Habitat type +water temperature
10 Habitat type + distance to vegetation
11 Habitat type + water temperature + water depth + distance to vegetation + canopy cover
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Habitat-selection patterns observed at a given point in time are often associated
with the behavioral state of the animal. For example, environmental characteristics
related to habitat selection often differ between animals that are actively foraging
compared with those that are resting (Smith 1975, Watanabe et al. 2013). Previous
research has demonstrated that Alligator activity patterns and movement rates differ
between daytime and nighttime periods (Rosenblatt et al. 2013, Smith 1975, Watanabe
et al. 2013). Thus, we expected that habitat-selection patterns would also differ
between day and night and we conducted separate analyses for data from diurnal and
nocturnal periods to assess differences in diel patterns of habitat selection.
Results
During the 2010 season (May–August), we captured 7 Alligators (3 males, 3
females, 1 unknown sex; Table 2) during 203 trap nights and fitted 5 (2 male and
3 female) with VHF transmitters; 2 Alligators were too small to be fitted with
transmitters. We were unable to locate 1 female 3 days after capture, likely due to
transmitter failure; therefore, we only collected relocation data on 2 males and 2
females in 2010.
In 2011 (April–September), we captured 7 Alligators (2 males, 3 females, 2 unknown
sex; Table 2) during 213 trap nights; 4 (2 males, 2 females) were fitted with
VHF transmitters. One of the males was a recapture from the previous year and had
a home range that was spatially distinct from the home range used during the first
Table 2. Sex, body-size characteristics, duration of radio monitoring, number of relocations, and
home-range (95% kernel) and core (50% kernel area) size (ha) for each radio-transmitted Alligator
captured at the Fort Worth Nature Center and Refuge, TX, 2010–2011. A = transmitter failed 3 days
after capture; B = female remained on nest after capture. U = unknown sex.
Eye to
nare Total 95% 50%
length length Duration of Number of home core
Alligator Sex (cm) (cm) radio monitoring relocations range (ha) area (ha)
2010
43 M 29 295 4 Jun–4 Aug 30 (16 night, 14 day) 64.9 15.5
53 F 15 160 11 Jun–4 Jul 21 (13 night, 8 day) 64.8 17.6
51 F 18 197 30 Jun–28 Jul 20 (10 night, 10 day) 27.9 7.4
46 F 26 232 A - -
- M 28 282 - -
38 M 24 275 19 Jun–10 Sep 17 (9 night, 8 day) 110.6 47.3
- U 2 24 - -
2011
38 M 23 281 8 Jul–10 Sep 22 (12 night, 10 day) 33.9 4.7
11 F 19 229 16 Jun–8 Oct 78 (37 night, 41 day) 30.1 5.3
39 M 27 335 26 Jun–8 Oct 66 (32 night, 34 day) 66.1 14.8
48 F 17 236 B - -
- F 13 142 - -
- U 1 23 - -
- U 2 23 - -
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season; therefore, we treated them as 2 unique samples. The second female was
tagged towards the end of the season while on a nest. Because she did not leave the
nest over the course of our study, she was excluded from further analyses.
Home range
Mean home range (95% kernel) for all Alligators combined for both years was
56.9 ha (range = 27.9–110.6 ha) and mean core area (50% kernel) was 16.1 ha
(range = 4.7–47.3 ha; Table 2). Overall mean home-range size was 67 ha in 2010
and 43.4 ha in 2011. Average female home-range (95% kernel; n = 3) was 40.9 ha
(range = 27.9–64.7 ha), and mean home range for males (n = 4) was 68.9 ha (range
= 33.9–110.6 ha). Mean core area (50% kernel) for female Alligators (n = 3) was
10.1 ha (range = 5.3–17.6 ha), whereas average core area for males (n = 4) was 20.6
ha (range = 4.7–47.3 ha).
Habitat selection
Habitat selection within the study area (second order). For 4 Alligators, the
highest selection ratio was for open-canopy shoreline (selection ratio range = 28.7–
58.4; Table 3); 3 Alligators selected against open-canopy shoreline (selection ratio
= 0.36 for all 3 Alligators). Five Alligators selected for river channel (selection
ratio range = 4.2–8.9). Two Alligators selected for and 5 Alligators selected against
floating vegetation (selection ratio range = 1.7–8.8 and 0.01–0.30, respectively).
Three Alligators selected for emergent vegetation (selection ratio range = 5.0–8.7),
and 2 selected against that variable (selection ratio = 0.03 for both Alligators),
and 2 had no selection for or against emergent vegetation (selection ratio range =
0.82–1.14; Table 3). One of the Alligators that did not select for or against emergent
vegetation was the only Alligator to select for shrubland. All Alligators selected
against open water (selection ratio range = 0.01–0.53), and all but 1 selected against
forested areas (selection ratio range = 0.01–0.99; Table 3).
Habitat selection within home range (third order). Four Alligators selected for
open-canopy shoreline (selection ratio range = 3.5–10.7; Table 4), and 1 Alligator
selected against open-canopy shoreline. Four Alligators selected for river channel
(selection ratio range = 1.5–5.8), 3 Alligators had high selection ratios for emergent
vegetation (selection ratio range = 3.5–18.5), 2 Alligators selected against emergent
vegetation (selection ratio range = 0.14–0.30), and 2 neither selected for nor against
emergent vegetation (selection ratio range = 1.1–1.3; Table 4). Two Alligators selected
for floating vegetation (selection ratio range = 1.9–29.4), 1 selected against
floating vegetation, and 4 Alligators did not have floating vegetation within their
home range. Similar to second-order habitat selection, all animals with open water
within their home range selected against it (selection ratio range = 0.04–0.38); all
individuals also selected against forested areas (selection ratio range = 0.01–0.59;
Table 4).
Diel habitat selection
The highest-ranking model in both the day and night habitat-selection analysis
was the model with all covariates (Table 5). There were no competing models in the
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Table 3. Second-order selection ratios (i.e., within the study area) and 95% confidence limits for American Alligators (Alligator mississippiensis) on the
Fort Worth Nature Center and Refuge, TX, 2010–2011. + indicates habitat types that were selected for (i.e., ratios >1) and - indicates habitat types that
were selected against (i.e., ratios < 1). Shrublands and forest s were seasonally flooded. Negative lower confidence limits set t o zero.
Open-canopy Floating Emergent
Alligator ID shoreline River channel vegetation vegetation Open water Shrubland Forested area
11 57.01+ 8.99+ 0.01- 0.03- 0.01- 0.41- 0.01-
(55.79–58.23) (8.53–9.46) (0.00–0.03) (0.00–0.07) (0.00–0.03) (0.27–0.50) (0.00–0.03)
43 58.38+ 7.04+ 1.69+ 0.822 0.01- 0.40- 0.01-
(56.44–60.32) (6.31–7.76) (1.23–2.16) (0.513–1.13) (0.00–0.04) (0.18–0.63) (0.00–0.04)
53 0.36- 7.13+ 0.30- 1.14 0.01- 7.99+ 0.99
(0.00– .75) (5.77–8.49) (0.00– .66) (0.47–1.81) (0.00–0.07) (6.17–9.81) (0.49–1.51)
38 (2010) 0.36- 0.05- 0.30- 7.76+ 0.53 0.41- 0.01-
(0.00–0.87) (0.00–0.25) (0.00–0.77) (6.68–8.84) (0.00 – 1.09) (0.00–0.96) (0.00–0.96)
38 (2011) 28.67+ 4.18+ 0.30- 5.02+ 0.01- 0.41- 0.01-
(26.27–31.04) (3.27–5.08) (0.22–0.58) (4.23–5.81) (0.00–0.05) (0.08–0.73) (0.00–0.05)
39 41.92+ 7.13+ 8.82+ 0.03- 0.23- 0.41- 0.01-
(40.51–43.34) (6.57–7.68) (8.05–9.59) (0.00–0.08) (0.11–0.36) (0.23–0.58) (0.00–0.03)
51 0.36- 0.05- 0.30- 8.73+ 0.27- 0.40- 0.01-
(0.00–0.87) (0.00–0.25) (0.00–0.77) (7.92–9.53) (0.00–0.69) (0.00–0.96) (0.00–0.08)
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Table 4. Third-order (i.e., within the home range) selection ratios (and 95% confidence limits) for American Alligators (Alligator mississippiensis) on the
Fort Worth Nature Center and Refuge, TX, 2010–2011. + indicates habitat types that were selected for (i.e., ratios >1) and - indicates habitat types that
were selected against (i.e., ratios < 1). Shrublands and forest s were seasonally flooded. Negative lower confidence limits set t o zero.
Open-canopy Floating Emergent
Alligator ID shoreline River channel vegetation vegetation Open water Shrubland Forested area
11 8.00+ 3.31+ 0.30- 0.06- 0.01-
(7.56–8.44) (3.04–3.58) (0.18–0.42) (0.01–0.13) (0.00–0.02)
43 7.45+ 0.90 1.93+ 1.25 0.14- 0.01-
(6.63–8.26) (0.61–0.19) (1.45–2.40) (0.87–1.62) (0.01–0.26) (0.00–0.03)
53 0.15- 5.80+ 1.08 0.04- 0.94 0.59-
(0.00–0.39) (4.62– 0.98) (0.45–1.70) (0.00–0.17) (0.34–1.55) (0.22–0.97)
38 (2010) 0.30- 4.68+ 0.38- 0.01-
(0.00–0.74) (3.89–5.47) (0.00–0.83) (0.00–0.09)
38 (2011) 3.45+ 1.55+ 0.02- 18.47+ 0.01-
(2.66–4.24) (1.02–2.08) (0.00–0.09) (17.01–19.94) (0.00–0.04)
39 10.70+ 3.85+ 29.41+ 0.14- 0.34- 0.01-
(9.99–11.40) (3.45–4.25) (28.03–30.79) (0.038–0.23) (0.19–0.49) (0.00–0.03)
51 3.49+ 0.18- 0.02-
(3.04–3.94) (0.00–0.48) (0.00–0.12)
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model sets for either day or night habitat-selection (all ΔAICc > 8.0). In each case,
the model with all covariates was the most highly supported model based on model
weights (day Wi = 0.97, night Wi = 0.99).
Nocturnal habitat selection. Based on model-averaged parameter estimates,
the probability of use was 1.5 times greater for emergent vegetation, 128 times
greater for river channel, and 33.7 times greater for open-canopy shoreline than
for open water. Terrestrial habitats combined were 87% less likely to be selected
than open water. After accounting for the influence of habitat type, the probability
of use decreased by approximately 21% for every 1% increase in canopy cover and
decreased by 77% for every 1 m increase in water depth (T able 6).
Diurnal habitat selection. The probability of use during the day was 3.7 times
greater for emergent vegetation, 11.6 times greater for river channel, 5.8 times greater
for floating vegetation, and 31 times greater for open-canopy shoreline than open
Table 5. A priori models for the probability of habitat use during day and night for American Alligators
(Alligator mississippiensis) relative to environmental characteristics on the Fort Worth Nature Center
and Refuge, TX, 2010–2011. Maximized log likelihoods, number of parameters (k), Akaike’s information
criterion adjusted for small sample size (AICc), ΔAICc, and Akaike weights. Models ranked from
best- to worst-approximating model.
Model Log likelihood k AICc ΔAICc AICc weight
Day
Habitat type + water temperature + -186.1 16 338.9 0.00 0.9751
water depth + distance to vegetation
+ canopy cover
Habitat type + canopy cover -187.4 13 347.9 9.07 0.0104
Habitat type + water depth -183.2 9 348.0 9.14 0.0101
Habitat type +water temperature -188.5 13 350.1 11.27 0.0035
Habitat type + distance to vegetation -190.6 13 354.3 15.47 0.0004
Habitat type -190.7 13 354.5 15.67 0.0004
Water temperature + water depth -191.0 12 357.3 18.41 0.0001
Canopy cover -268.2 8 520.1 181.23 0.0000
Water depth -284.6 8 552.9 214.03 0.0000
Water temperature -299.2 8 582.1 243.23 0.0000
Distance to vegetation -305.9 8 595.5 256.63 0.0000
Night
Habitat type + water temperature + -120.2 16 207.1 0.00 0.9997
water depth + distance to vegetation
+ canopy cover
Habitat type + canopy cover -125.2 13 223.6 16.46 0.0003
Habitat type + water depth -126.9 13 227.0 19.86 0.0000
Habitat type +water temperature -130.5 13 234.2 27.06 0.0000
Habitat type + distance to vegetation -130.5 13 234.2 27.06 0.0000
Habitat type -130.6 12 236.5 29.39 0.0000
Water temperature + water depth -183.2 9 348.0 140.91 0.0000
Canopy cover -181.2 8 346.1 139.00 0.0000
Water depth -183.5 8 350.7 143.60 0.0000
Water temperature -198.5 8 380.7 173.60 0.0000
Distance to vegetation -199.0 8 381.7 174.60 0.0000
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2014 Vol. 13, No. 2
water (Table 7). Combined terrestrial habitat types were 99% less likely to be used
than open water. After accounting for habitat type, the probability of use during the
daytime increased by 12% for every 1% increase in canopy cover.
Discussion
The habitat-selection and home-range patterns we observed in this study differed
from those reported previously for inland and coastal Alligator populations.
During our study, Alligators consistently selected for river channels and opencanopy
shoreline, which is consistent with the findings of Joanen and McNease
(1989) where the majority of locations were in deep-water canals; however, this
finding differed from a study in north-central Florida where the highest proportions
of male and female Alligators were located in lake or open-water habitat, and lower
proportions of Alligators were observed in swamp habitat (Goodwin and Marion
1979). Open water was not selected at the study-area or home-range levels in this
Table 7. Model-averaged coefficients, standard errors, odds ratios, and 95% confidence limits for
odds ratios included in the best-approximating models for the probability of daytime habitat use by
American Alligators (Alligator mississippiensis) on the Fort Worth Nature Center and Refuge, TX,
2010–2011. Other = combined terrestrial habitats.
95% Confidence limits
Variable β SE Odds ratio Lower Upper
Emergent vegetation 1.30 0.62 3.67 1.08 12.42
River channel 2.45 0.51 11.59 4.19 32.06
Floating vegetation 1.76 0.87 5.82 1.05 32.27
Open-canopy shoreline 3.44 0.79 31.23 6.52 149.57
Other -4.22 1.11 0.01 0.002 0.13
Distance to vegetation -0.03 0.02 0.97 0.92 1.02
Water temperature -0.03 0.07 0.97 0.85 1.10
Water depth -0.16 0.31 0.85 0.46 1.58
Canopy cover 0.11 0.05 1.12 1.01 1.25
Table 6. Model-averaged coefficients, standard errors, odds ratios, and 95% confidence limits for
odds ratios included in the best-approximating models for the probability of nighttime habitat use by
American Alligators (Alligator mississippiensis) on the Fort Worth Nature Center and Refuge, TX,
2010–2011. Other = combined terrestrial habitats.
95% Confidence limits
Variable β SE Odds ratio Lower Upper
Emergent vegetation 0.41 0.65 1.51 0.42 5.39
River channel 4.85 0.73 128.47 30.56 540.15
Floating vegetation 0.46 1.02 1.58 0.22 11.60
Open-canopy shoreline 3.52 0.93 33.68 5.50 206.45
Other -2.018 1.01 0.13 0.02 0.96
Distance to vegetation 0.01 0.01 1.01 0.98 1.03
Water temperature -0.08 0.07 0.93 0.81 1.06
Water depth -1.49 0.49 0.23 0.09 0.59
Canopy cover -0.22 0.06 0.79 0.71 0.91
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study, which was contrary to the results reported from a study of an inland population
in East Texas, where adults occupied deeper, less-vegetated open-water areas
(Webb et al. 2009). In our study, habitat-selection patterns also differed somewhat
between day and night periods. Compared to habitat selection at night, during the
day Alligators were more likely to use areas near aquatic (floating and emergent)
vegetation and selected for areas with more canopy cover. At night, Alligators had
higher selection for river channel than during the day. During both time periods,
Alligators were more likely to select for any of the other aquatic habitat types rather
than open water.
Some of the differences between the habitat-selection patterns we observed and
those reported for other populations may be due to differences in both the configuration
and relative availabilities of each of the habitat types. For example, similar to a
study in East Texas (Webb et al. 2009), our study area had higher (e.g., 70% of aquatic
habitat) levels of open water than the 20–40% reported as optimal for coastal populations
in Louisiana and Texas (Newsom et al. 1987). However, in our study, areas of
open water were not interspersed with vegetated wetlands as Newsome et al. (1987)
reported was optimal, but rather vegetated areas were largely concentrated at one
end of the refuge and along the periphery of some of the open-water areas. Emergent
vegetation was also more abundant in our study area than in others and represented
approximately 19% of the aquatic habitat types, a proportion 2–3 times as high as reported
for inland populations in East Texas (Webb et al. 2009).
It is unclear why Alligators did not select for open water at either spatial scale.
The avoidance of open water by our radio-marked Alligators could have been due to
the presence of larger, unmarked Alligators relegating our radio-marked Alligators
to other habitat types. However, even our radio-marked Alligators that exceeded
2.5–3.5 m in length did not select open-water areas. We also did not observe any
large, unmarked Alligators during our study, and we believe that it is unlikely that
Alligators larger than our radio-marked animals would have gone undetected during
the entire course of our study. Another possible reason for the avoidance of
open water is the high level of recreation (i.e., boating) on the FWNCR throughout
the day. Boats, both motorized and non-motorized, were commonly observed
approaching Alligators (J.D. Lewis, pers. observ.) which may have resulted in Alligators
avoiding the open water where detection by boaters would have been more
likely. A study on Caiman crocodilus L. (Spectacled Caiman) in the Tortuguero
region of Costa Rica found that increasing boat traffic associated with ecotourism,
recreation, and local human population-growth increased the likelihood of
boat-collision–related injuries; Spectacled Caiman were also frequently observed
avoiding boats (Grant and Lewis 2010).
Changes in the biotic and abiotic parameters at range edges make ecological
processes more variable than at the core of the distribution range (Sexton et al.
2009), which may explain some of the home-range and habitat-selection patterns
we observed. For example, sex, reproductive status, habitat characteristics, and
temperature can all influence home-range area and movements of adult Alligators
(Morea et al. 2000). The home ranges of females from our study were 2–4 times
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2014 Vol. 13, No. 2
(mean = 40.9 ha, range = 27.9–64.7 ha) higher than those reported in Florida
(22–35.9 ha; Morea et al. 2000, Phillips et al. 2002) and coastal Louisiana (8.4 ha;
Joanen and McNease 1970), whereas average home–range size for males in our
study (mean = 68.9 ha, range = 33.9–110.6 ha) were substantially smaller than
documented in Florida (144 ha; Phillips et al. 2002), with the exception of those
reported by Morea et al. (2000), and coastal Louisiana (e.g., 328 ha; Joanen and
McNease 1972) .
Home-range sizes we observed during our study may be related to the habitat
types within the home ranges and low Alligator density (0.014–0.065 Alligators/
ha; Lewis 2012). The Alligator densities estimated at our study area are lower than
the estimated densities (0.12–0.31 Alligators/ha) reported for other inland Alligator
populations in East Texas (Lutterschmidt and Wasko 2006, Webb 2005), and
substantially lower than densities reported in coastal Louisiana (1.29 Alligators/ha;
Taylor et al. 1991), or in Florida (0.22–0.7 Alligators/ha; Woodward et al. 1996).
The location of our study population at the northwestern edge of the range experienced
colder winter temperatures, shorter growing season, and likely lower food
abundance in comparison with coastal Alligator populations (Rootes et al. 1991),
which probably contributed to both the lower Alligator density and larger home
ranges we observed in females. However, it is unclear why males had smaller home
ranges than in coastal populations. One possibility is that the overall area with suitable
environmental conditions for Alligators is relatively small in our study area
compared to coastal populations, and this may limit the upper home-range size for
male Alligators.
During our study, we were unable to fit a large number of Alligators with VHF
transmitters and some Alligators that were fitted with transmitters failed to retain
the transmitter for an entire season. However, a separate study estimated that
there were only 7–31 Alligators in the study area (Lewis 2012). Therefore, although
our absolute sample size could be considered low, our sample size relative
to the total population size was between 19% (6 unique Alligators captured out
of a population of 31) and 86% (6 out of 7). Given that our inf erences are strictly
limited to this population, we believe that our sample provided representative estimates
of home range and habitat-selection patterns for the Alligator size-classes
we studied. Unfortunately, we were unable to assess sex or age-specific patterns
of habitat selection.
Alligator populations at the edge of their distributional range may display habitat-
use patterns that differ not only from those of coastal populations, but also from
other inland populations. During our study, Alligators selected for areas that included
the river channel, floating and emergent vegetation, and open-canopy shorelines,
but they avoided open water. With the exception of open-canopy shorelines, these
environmental conditions also tended to occur in the areas of the wetlands that are
most susceptible to changes in water levels associated with flood control during
high rainfall periods and declining water tables during drought periods. Hence, the
management of water levels in this lake has the potential to profoundly influence
the conditions in areas most frequently selected by this population of Alligators.
Southeastern Naturalist
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2014 Vol. 13, No. 2
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Furthermore, the management of water levels in urban reservoirs is likely to be
more intensively controlled given that the costs associated with flooding in terms
of both property damage and human safety are higher than near reservoirs in morerural
and less-densely populated areas. Reservoirs located closer to urban areas are
also more likely to experience higher levels of water withdrawal during dry periods,
which can influence the availability of habitat for Alligator populations.
The avoidance of open water in our study is perhaps the most notable difference
between the habitat-selection patterns we observed and those reported for
other inland populations (e.g., Webb et al. 2009). We believe that avoidance of
open water may be related to either reduced food availability in the open-water
areas or to high levels of human recreation on the refuge, particularly people in
motorized boats and canoes. To our knowledge, this is the first published study
based on data collected at the edge of the distribution range for the American
Alligator; therefore, further work is warranted to determine if the patterns we
observed are characteristic of other Alligator populations at the edge of the range
(Lutterschmidt and Wasko 2006).
Acknowledgments
We thank the Department of Biological and Environmental Sciences and the Graduate
College at Texas A&M University-Commerce; Fort Worth Nature Center and Refuge;
Friends of the Fort Worth Nature Center and Refuge; IUCN-SSC Crocodile Specialist
Group; Safari Club International Foundation; Dallas Safari Club-Dallas Ecological Foundation;
Houston Safari Club; the Dallas-Fort Worth Herpetological Society; the Arthur A.
Seeligson, Jr. Conservation Fund; and the Texas Outdoor Writers Association for funding
and support. We also thank R. Daughtery, C. Schmal, J. Dudko, R. Baldwin, A. Benson, and
T. Huff for field assistance. We would like to thank S. Tuttle, M. Villafranca, R. Lassiter,
and the rest of the staff at the Fort Worth Nature Center and Refuge for logistical support.
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