An Examination of Scale-Dependent Resource Use by ... - BioOne

Management and Conservation Article

An Examination of Scale-Dependent Resource Use by Eastern Hognose Snakes in Southcentral New Hampshire KIRK E. LAGORY,1 Environmental Science Division, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA LEROY J. WALSTON, Environmental Science Division, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA CELINE GOULET, Department of Natural Resources, University of New Hampshire, Durham, NH 03824, USA ROBERT A. VAN LONKHUYZEN, Environmental Science Division, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA STEPHEN NAJJAR, New Boston Air Force Station, 317 Chestnut Hill Road, New Boston Air Force Station, NH 03070, USA CHRISTIAN ANDREWS, Environmental Science Division, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA

ABSTRACT The decline of many snake populations is attributable to habitat loss, and knowledge of habitat use is critical to their conservation. Resource characteristics (e.g., relative availability of different habitat types, soils, and slopes) within a landscape are scaledependent and may not be equal across multiple spatial scales. Thus, it is important to identify the relevant spatial scales at which resource selection occurs. We conducted a radiotelemetry study of eastern hognose snake (Heterodon platirhinos) home range size and resource use at different hierarchical spatial scales. We present the results for 8 snakes radiotracked during a 2-year study at New Boston Air Force Station (NBAFS) in southern New Hampshire, USA, where the species is listed by the state as endangered. Mean home range size (minimum convex polygon) at NBAFS (51.7 6 14.7 ha) was similar to that reported in other parts of the species’ range. Radiotracked snakes exhibited different patterns of resource use at different spatial scales. At the landscape scale (selection of locations within the landscape), snakes overutilized oldfield and forest edge habitats and underutilized forested habitats and wetlands relative to availability. At this scale, snakes also overutilized areas containing sandy loam soils and areas with lower slope (mean slope 5 5.2% at snake locations vs. 6.7% at random locations). We failed to detect some of these patterns of resource use at the home range scale (i.e., within the home range). Our ability to detect resource selection by the snakes only at the landscape scale is likely the result of greater heterogeneity in macrohabitat features at the broader landscape scale. From a management perspective, future studies of habitat selection for rare species should include measurement of available habitat at spatial scales larger than the home range. We suggest that the maintenance of open early successional habitats as a component of forested landscapes will be critical for the persistence of eastern hognose snake populations in the northeastern United States. (JOURNAL OF WILDLIFE MANAGEMENT 73(8):1387–1393; 2009)

DOI: 10.2193/2008-422 KEY WORDS eastern hognose snake, habitat selection, Heterodon platirhinos, home range, management, New Hampshire, spatial scale.

The protection of rare species is largely dependent on reliable natural history information that can be used to develop appropriate conservation plans (Dodd 1987). This basic information may be difficult to obtain for species that are secretive and therefore difficult to study (Gibbons and Semlitsch 1987, Seigel and Collins 1993). Populations of many North American snake species have declined due to factors such as habitat loss or modification (Dodd 1987, Greene 1997, Gibbons et al. 2000). As a result, knowledge of movement patterns and habitat associations are critical to identify and maintain the habitats essential for successful snake conservation. Johnson (1980) suggested 3 primary hierarchical orders of resource selection, which reflect the resource use of wildlife species across different spatial scales. These hierarchical orders of selection were defined as selection within the geographical range of the species (first order), selection of a home range within the landscape (second order), and selection of specific locations within the home range (third order). Most resource use studies are limited to second- and third-order selection (Blouin-Demers and Weatherhead 2001, McLoughlin et al. 2004, Row and Blouin-Demers 2006a, Beasley et al. 2007). Anthropogenic habitat loss affects landscape composition at multiple spatial scales (Keitt et al. 1997, Fahrig 2003), and this 1

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LaGory et al. N Snake Resource Use

may lead to scale-dependent patterns of resource use by wildlife species (Johnson 1980, Lord and Norton 1990, Orians and Wittenberger 1991). For instance, habitat fragmentation often occurs at the broader landscape scale and isolates suitable habitat in a matrix of less-suitable habitat. This fragmentation can result in heterogeneously distributed populations across a landscape (Fahrig 2003). In such an environment, resource use may appear to be homogenous at a finer spatial scale than at a broader landscape scale. Therefore, it is important to identify the variables most important to snake resource selection as well as their relevant spatial scales. The eastern hognose snake (Heterodon platirhinos) is widely distributed in the eastern United States. Throughout much of its range, the eastern hognose snake can be locally common in dry open habitats with sandy well-drained soils (Platt 1969, Ernst and Ernst 2003). In the northeastern states near the edge of its range, however, the eastern hognose snake typically occurs in smaller, more disjunct populations (Platt 1969, Michener and Lazell 1989, Brookhaven National Laboratory 2008). The eastern hognose snake is considered a species of regional concern by the Northeast Endangered Species and Wildlife Diversity Technical Committee (Therres 1999) and is protected by conservation measures in the states of Rhode Island, Connecticut, and Massachusetts. This species is listed as endangered by the state of New Hampshire (New 1387

STUDY AREA

Figure 1. New Boston Air Force Station in southern New Hampshire, USA (inset), showing the home range minimum convex polygons and individual locations for radiotracked eastern hognose snakes (N 5 8) in 2006 and 2007. Numbers on the minimum convex polygon boundaries indicate snake identification number. Snake 1 was male; snakes 2–8 were female.

Hampshire Natural Heritage Bureau 2009), where it is known to exist only in disjunct populations in the southernmost counties (Michener and Lazell 1989). The destruction or modification of suitable dry sandy habitats is one of the main factors implicated in the loss of eastern hognose snake populations in New Hampshire (New Hampshire Fish and Game Department 2005). Eastern hognose snakes occur on New Boston Air Force Station (NBAFS) in southcentral New Hampshire (Fig. 1). Because there are few detailed studies of this species near the edge of its range in the northeastern United States (Michener and Lazell 1989), we conducted a study at NBAFS to investigate movement patterns and habitat use. This information was needed for natural resource management planning in compliance with the Sikes Act, which mandates the conservation and rehabilitation of natural resources on military installations and provides for cooperation by the Departments of the Interior and Defense with state agencies in planning, developing, and maintaining fish and wildlife resources on military installations. Most studies of scale-dependent snake resource selection equate macro- and microhabitat selection with second- and third-order selection, respectively (Blouin-Demers and Weatherhead 2001, Richardson et al. 2006). However, few studies have examined snake macrohabitat selection across hierarchical spatial scales (but see Row and Blouin-Demers 2006a). Therefore, we present the results of a radiotelemetry study to determine eastern hognose snake home range size and resource use at different hierarchical spatial scales. Our investigation includes second- and third-order selection of macrohabitat types and other physical landscape features by eastern hognose snakes on NBAFS. 1388

We conducted this study on NBAFS between 8 May and 26 November 2006 and 2 February and 22 October 2007. NBAFS is a 1,144-ha satellite tracking station located in Hillsborough County, New Hampshire (Fig. 1). The station was located in an area of hilly and mountainous terrain, with elevations ranging from 104 m to about 389 m mean sea level. The NBAFS site was mostly forested (86% of the site), and dominated by conifers such as white pine (Pinus strobus) and eastern hemlock (Tsuga canadensis). Deciduous trees on NBAFS included red oak (Quercus rubrum), black oak (Quercus velutina), American beech (Fagus grandifolia), white ash (Fraxinus americana), sugar maple (Acer saccharum), and red maple (Acer rubrum). Since 1985, most of the NBAFS forest areas were actively managed to implement habitat management goals and increase forest health. Forest management practices included clearcutting, strip clearcutting, patch clearcutting, selective cutting, and shelterwood cutting. Other landcover types on NBAFS included developed areas (buildings, roads, parking lots, campgrounds, and recreational areas; 5%), old-fields (4%), and water (ponds, streams, and wetlands; 5%). Old-fields were early successional areas (including former clear-cuts) usually dominated by grasses and forbs; some old-field areas were maintained in an early seral stage by periodical mowing or burning.

METHODS We used a variety of techniques to locate eastern hognose snakes for capture, including searches along roads and in other open areas and in denser habitats away from roads and openings. We used visual and aural cues to detect snakes, and checked materials under which snakes could hide (e.g., logs, bark). Concern over the potential bias toward certain capture locations prompted increased searching in portions of NBAFS away from roads and in forested habitats in 2007. Upon capture, we measured the mass (61 g) and snout–vent length (61 cm) of each snake. We determined the sex of adult snakes (.100 g) by the presence or absence of a hemipenis using a metal probe inserted posteriorly into the cloaca. We transported adult nongravid snakes to a local animal hospital, where a veterinarian surgically implanted a transmitter into the snake’s abdominal cavity using methods similar to those described in Reinert and Cundall (1982). Radiotransmitters (Advanced Telemetry Systems, Isanti, MN) had either an internal coiled antenna (Model R1170, 4.0 g) or a whip antenna (Model R1680, 3.6 g) that we implanted subcutaneously. We implanted transmitters approximately one-third of the body distance from the tip of the tail. We held snakes up to 48 hours postsurgery before releasing them at the site of capture. Transmitters were typically about 1% of a snake’s mass (mean 1.2%, range 0.5–2.6%). We located radiotagged snakes using a portable receiver (Model R-1000, Communications Specialists, Orange, CA) equipped with a 3-element yagi antenna (Model RA-150, Communications Specialists) by following the signal to the snake. We located each radiotagged snake 2–3 times per The Journal of Wildlife Management N 73(8)

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to characterize a number of randomly selected locations equal to the number of unique locations. We used a hierarchical approach to determine eastern hognose snake resource use at 2 different spatial scales (Johnson 1980): resource selection within the landscape (second-order selection; availability based on random points within NBAFS) and resource selection within the home range (third-order selection; availability based on random points within MCPs). At each scale, we used compositional analyses (Aebischer et al. 1993) to determine if snakes utilized habitat types, aspects, and soil types (all categorical variables) in proportion to their availability. We interpreted overutilization relative to availability as preference for a habitat type or characteristic; we interpreted underutilization as avoidance. Compositional analysis uses a multivariate analysis of variance (MANOVA) that alleviates pseudoreplication problems by accounting for within-individual variability. We followed the MANOVA with paired, 1-sample t tests to determine which macrohabitats were preferred or avoided. We used SAS version 9.1 (SAS Institute 2002) and implemented the code provided by Ott and Hovey (1997) to perform the compositional analyses (MANOVA and t tests). For categories with no snake observations, we substituted a value of 0.01 as recommended by Aebischer et al. (1993). To account for pseudoreplication among continuous variables (slope, elevation, and distance to water), we used repeatedmeasures analyses of variance (ANOVA), using the variance among individuals as the error term in the calculation of the F statistic, to determine if these characteristics at snake locations differed from those at random locations. In addition to the GIS-based characterization of snake and random locations described above, we conducted field work to characterize additional microhabitat conditions thought to be potentially important to eastern hognose snakes. We used a quadrat-based linear transect approach to quantify microhabitat variables at known snake and random locations throughout NBAFS. We mapped snake locations for each year and selected a random sample (N 5 28) of these locations for characterization. We randomly selected locations where snakes had not been found (N 5 18) to characterize microhabitat conditions on NBAFS. At each sample location, we placed an 8-m linear transect oriented in a randomly determined direction. We collected data along each transect in 5 0.25-m2 quadrats located at 2-m intervals. For each quadrat, one of us (R. A. Van Lonkhuyzen) visually estimated the following data: 1) percentage of cover of bare ground, rock, log ( 4 cm in diam), and litter; 2) percentage of cover of graminoid, forb, woody, and total vegetation in the herbaceous layer (ground level to 1 m above ground level); 3) maximum height (cm) of vegetation in the herbaceous layer; 4) percentage of cover of all vegetation in the shrub layer (1–5 m above ground level); 5) maximum height (m) of all vegetation in the shrub layer; and 6) the presence or absence of a tree canopy above the quadrat. For each variable, we used repeated-measures ANOVA, using the variance among individuals as the error term in the calculation of F, to determine if the characteristics of snake locations differed from those of L

week until the snake entered its hibernaculum in the fall. We determined the coordinates of snake locations using a handheld Global Positioning System receiver. We separated snake observations by a minimum of 24 hours. Using a Geographic Information System (GIS), we displayed and analyzed snake locations to determine distance traveled between points and home range size. We calculated home ranges as 100% minimum convex polygons (MCPs) and core areas as 50% fixed kernels. The MCP is a common approach to estimate home range size (Plummer and Mills 2000; Powell 2000; Row and Blouin-Demers 2006a, b), whereas the 50% kernel estimator is a more appropriate indicator of core habitat use (Worton 1989). We calculated the MCP and kernel estimates using Hawth’s Tools for ArcGIS (Beyer 2004). We considered all snake observations in the calculation of both estimators. Three snakes utilized relatively small areas outside of the NBAFS boundary (snakes 3, 5, and 8; Fig. 1). Because the number of observations can affect the estimate of home range size, we plotted estimated home range size against the number of observations for each snake (Odum and Kuenzler 1955). Using these observation area curves, we 15 telemetry observations determined that we needed (including observations that were not unique, i.e., that were within 2 m of the previous observations) for each snake to obtain a reasonable estimate of home range size. Consequently, we included only snakes with 15 or more observations in our analysis. We characterized habitat types on NBAFS and offsite areas used by snakes in a GIS based on aerial imagery, timber management polygons provided by the NBAFS, and field surveys conducted over the past 15 years. For this analysis, we classified the habitat types on NBAFS into 4 broad categories: forest, old-fields, developed areas, and water, as previously defined. In addition to these habitat types, a fifth category, forest edge, was identified as forest habitat within 15 m of either old-fields or developed areas. We also characterized the physical and topographic features of NBAFS. We obtained elevation data at NBAFS as a 30-m digital elevation model (DEM) from the United States Geological Survey. We used the Environmental Systems Research Institute Spatial Analyst Extension for ArcGIS to calculate slope and aspect for the entire site based on the DEM. We categorized aspect as northeast (0–89u), southeast (90–179u), southwest (180–269u), or northwest (270–359u). We obtained soil type classifications from Bond and Handler (1981) to characterize NBAFS soils into 4 broad categories: sandy loam, gravelly loam, exposed rock, or hydric soils. We used a GIS to display the snake locations and associate them with the habitat types and physical and topographic features at NBAFS. We used the GIS datasets to characterize each unique snake location (those that were .2 m apart) based on the habitat type and physical and topographical features for the location. We based the determination of unique locations on GIS measurements and validated them by field notes of snake movement. We then determined resource availability by using GIS datasets

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random locations. For all analyses a 5 0.05; means are presented 61 SE.

RESULTS We captured 35 eastern hognose snakes during this study. Of these captures, 17 were in old-field habitat, 9 in developed areas, 5 in forest edge, and 4 in forest. We captured 22 juvenile or subadult snakes. We captured 13 adults (12 F, 1 M) and implanted them with radiotransmitters to monitor movements. Mean mass of adults at initial capture was 455.2 6 45.5 g; mean snout–vent length was 69.1 6 4.0 cm. We determined 15 locations for 8 radiotagged snakes (7 F, 1 M) and tracked them for an average of 202 (range: 30–420) days, with a mean of 41 (range: 15–81) locations per snake. For all but one of the 5 radiotagged snakes with insufficient number of observations, we could not identify a reason for signal loss; signal loss may have been the result of the snake moving out of the area, inhabiting dense habitats that limited signal transmission, dying below the ground surface, or being carried away from the area by predators. We attempted to find these snakes via helicopter, but were not successful. We found the transmitter for 1 of these 5 snakes without any other remains. This could indicate that the snake was eaten or that the transmitter was expelled through the fresh incision. Of the 8 radiotagged eastern hognose snakes used in the analysis of home range and resource selection, we tracked 4 during both years of this study. Due to the limited sample size, we did not attempt to determine differences between years in snake habitat use. Therefore, we pooled data across years for those individuals tracked in both years. Overall, snake daily movements ranged from distances of 0 m to 584 m per day; mean daily distance traveled was 51 6 4 m. Mean home range size (MCP) of snakes was 51.7 6 14.7 ha; mean core area size (50% kernel) of all snakes was 6.5 6 1.4 ha. Both home range and core area size were normally distributed (Kolmogorov–Smirnov tests; P . 0.13). The home range size of the male was 91.6 ha, whereas mean home range size of the 7 females was 46.0 6 15.6 ha. Despite the difference in MCP between the male and females, core area size was similar for both sexes. The kernel estimate for the male’s core area was 6.4 ha; mean core area size of the 7 females was 6.6 6 1.7 ha. Overall, we positively correlated snake core area size with home range size (Pearson correlation; r 5 0.80; P 5 0.02). We found no relationship between snake mass (g) and home range size for the 8 radiotagged snakes (r 5 20.01; P 5 0.98) or among the 7 radiotagged females (r 5 0.09; P 5 0.85). We identified 264 unique (i.e., 2 m apart) locations for the 8 radiotagged snakes. Compositional analysis indicated that macrohabitat use was nonrandom at the landscape scale (l 5 0.10; F4,4 5 8.83; P 5 0.03), but could not be differentiated from random at the home range scale even though the overall pattern appeared to be similar (l 5 0.16; F4,4 5 5.33; P 5 0.07; Fig. 2). At the landscape scale, we observed greater than expected use of old-field and forest edge habitats, less than expected use of forest and wetland L

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Figure 2. Selection of habitat types by eastern hognose snakes (N 5 8) at New Boston Air Force Station, New Hampshire, USA, in 2006 and 2007. Availability within the landscape and home range was determined from random points in the study area and within the minimum convex polygons, respectively.

habitats, and use of developed areas as expected (Fig. 2). Rank order of habitat preference at the landscape scale was old-field ... forest edge . developed . forest ... wetland (... denotes a statistically significant difference between 2 consecutively ranked variables). Use of old-field habitats relative to availability was greater than that for all other habitat types (P , 0.05). Use of forest edge relative to availability was not different from that of developed areas (t7 5 1.12; P 5 0.29), but the use of forest edge relative to availability was greater than that of forest (t7 5 3.41; P 5 0.02) and wetland (t7 5 5.62; P 5 0.006; Fig. 2). Compositional analysis of soil types at snake and random locations indicated that snake use of soils was nonrandom at the landscape scale (l 5 0.21; F3,5 5 6.25; P 5 0.04) and home range scale (l 5 0.14; F3,5 5 10.05; P 5 0.01). At the landscape scale, we observed greater than expected use of sandy loam and less than expected use of exposed rocks and hydric soils (Fig. 3). The rank order of soil use at the landscape scale was sandy loam ... gravelly loam . exposed rock . hydric soil. Snakes preferred sandy loam soils over all other soil types (P , 0.05). Use of gravelly loam soils relative to availability was not different from that of exposed rocks (t7 5 1.76; P 5 0.14); however, use of gravelly loam soils relative to availability was greater than that of hydric soils (t7 5 3.21; P 5 0.03; Fig 3). At the home range scale, we observed greater than expected use of sandy loam soils, and we observed less than expected use of gravelly loam and hydric soils (Fig. 3). The rank order of soil utilization at the home range scale was sandy loam ... exposed rock . gravelly loam . hydric soils. Again, snakes preferred sandy loam soils over all other soil types (P , 0.05). Among the remaining soil types, use relative to availability was not significantly different from one another (P . 0.09; Fig. 3). Compositional analysis of aspect at snake and random locations indicated that snake utilization of slopes with different aspect did not differ from random at the landscape scale (l 5 0.547; F3,5 5 1.38; P 5 0.35) or home range scale (l 5 0.363; F3,5 5 2.93; P 5 0.139). Repeated-measures ANOVA indicated that elevations of snake locations did not differ from those of random locations at the landscape scale (F1,14 5 0.12; P 5 0.75) The Journal of Wildlife Management N 73(8)

percentage of log cover, and greater graminoid cover than did random locations (Table 1).

DISCUSSION

Figure 3. Selection of soil types by eastern hognose snakes (N 5 8) at New Boston Air Force Station, New Hampshire, USA, in 2006 and 2007. Availability within the landscape and home range was determined from random points in the study area and within the minimum convex polygons, respectively.

or home range scale (F1,14 5 0.01; P 5 0.98). The mean elevation of snake locations was 217.5 6 2.0 m, whereas the mean elevation of random locations at the landscape and home range scales were 211.3 6 2.5 m and 218.0 6 1.8 m, respectively. At the landscape scale, the slope at snake locations was significantly lower than at random locations (F1,14 5 5.24; P 5 0.04); however, there was no significant difference in slopes at the home range scale (F1,14 5 0.06; P 5 0.81). The mean slope at snake locations was 5.2 6 0.2 degrees, whereas the mean slope of random locations at the landscape and home range scales were 6.7 6 0.3 degrees and 5.1 6 0.2 degrees, respectively. There was no difference in distance to water between snake and random locations at the landscape scale (F1,14 5 3.81; P 5 0.07) or home range scale (F1,14 5 0.77; P 5 0.40). The mean distance to water at snake locations was 181.3 6 7.4 m, whereas the mean distance to water at random locations at the landscape and home range scales were 169.2 6 9.2 m and 193.0 6 7.5 m, respectively. We sampled habitat transects at 28 snake and 18 random locations throughout NBAFS. For most quadrat-level habitat characteristics, there was no statistical difference between snake and random locations (Table 1). However, snake locations had greater percentage of bare ground, lower

The average home range size (MCP) for the 8 radiotracked eastern hognose snakes on NBAFS was 51.7 6 14.7 ha, which is similar to the home range sizes observed in other parts of the species’ range. For example, Plummer and Mills (2000) reported an average home range size of 50 ha among 8 radiotracked eastern hognose snakes in Arkansas. The only male tracked in our study had the largest home range of any radiotagged snake on NBAFS (91.6 ha). Although we were only able to track the movements of one male, our observations were consistent with other studies that have reported sex-based differences in home range size, with males generally having larger home ranges (Brito 2003, Marshall et al. 2006). Possible explanations for this sexbased difference are related to reproductive behavior. Males may travel greater distances during mate-searching (Brito 2003), which may lead to larger home range size. Females may restrict their movements to those areas of optimal thermal quality for egg development (Marshall et al. 2006, Row and Blouin-Demers 2006a), thus potentially leading to smaller home range sizes. Although the male’s MCP was larger than those of the females in our study, the core area size (50% kernel) was similar to those of the females. Overall, mean core area size among the 8 radiotracked eastern hognose snakes at NBAFS was 6.5 6 1.4 ha. As a measure of concentrated activity, the kernel estimate is not as influenced by extreme or irregular movements as is the MCP estimate for home range size. Thus, we were not surprised to find that core area size was less variable among the snakes observed in our study. Other studies have observed similar patterns in core habitat use among snakes (Richardson et al. 2006). It should be noted, however, that several authors have recently questioned the appropriateness of using kernel estimates for estimating home range size for autocorrelated datasets.

Table 1. Statistics for quadrat-level data collected along transects sampled at eastern hognose snake and random locations on New Boston Air Force Station, New Hampshire, USA, in 2006 and 2007. Snake locations (N = 28)

Random locations (N = 18)

Snake vs. random locations

¯x

SE

¯x

SE

P-valuea

% cover in ground layer Bare Rock Log Litter

6.7 5.3 3.6 81.4

2.7 1.6 0.8 4.3

0.9 6.3 6.9 84.3

0.4 2.2 1.4 2.2

0.02 0.69 0.02 0.10

% cover in herbaceous layer Graminoid Forb Woody

4.0 12.1 20.8

1.4 2.8 4.4

1.2 6.1 11.8

0.8 2.6 3.1

0.05 0.17 0.40

Total Max. ht of vegetation in herbaceous layer (cm) % in shrub layer Max. ht of vegetation in shrub layer (m) % quadrats with tree canopy

33.0 48.0 28.3 2.6 26.7

5.3 5.3 4.2 0.3 7.4

19.5 34.9 19.7 2.5 39.4

4.3 6.3 4.2 0.4 10.9

0.15 0.33 0.51 0.97 0.20

Variable

a

P-values for repeated-measures analysis of variance using the among-individual variance estimate as the error term in the calculation of F.

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Because snakes and other herpetofauna usually exhibit high site fidelity (Plummer and Mills 2000, Diffendorfer et al. 2005, Row and Blouin-Demers 2006b), it has been suggested that studies of habitat use for herpetofauna employ MCPs rather than kernel estimators for calculating home range size (Row and Blouin-Demers 2006b). For such autocorrelated data sets in particular, kernels tend to overestimate home ranges, because in large part of the effects of the smoothing parameter (Row and BlouinDemers 2006b, Downs and Horner 2008). The MCP is a simple method with no assumptions about the underlying statistical distribution of the data points that represents the maximum home range area (Powell 2000). Studies examining resource selection at one spatial scale often provide limited information because resource selection can be scale-dependent. The presence of suitable macro- and microhabitat features often varies at hierarchical spatial scales, which may lead to scale-dependent resource use by wildlife (Johnson 1980, Lord and Norton 1990, Orians and Wittenberger 1991). We found that eastern hognose snakes at NBAFS exhibited differential selection of some habitat features at different spatial scales. The radiotracked snakes in our study exhibited associations for habitat type and slope at the landscape scale (second order), but associations were generally indistinguishable from random at the home range scale (third order). Second-order habitat selection analyses indicated that eastern hognose snakes were primarily associated with old-field and forest edge habitat and avoided forest and wetland habitat (Fig. 2). At this spatial scale, we also found that snakes used locations with lower slope (mean slope 5 5.2% at snake locations vs. 6.7% at random locations). The only selectivity that could be detected at the home range scale was for sandy loam soil types (Fig. 3). This selectivity may reflect greater local- and landscape-level heterogeneity in soil composition and the importance of sandy loam soils for this fossorial species. There are many studies of scale-dependent habitat selection by vertebrate species (e.g., McLoughlin et al. 2004, Anadon et al. 2006, Beasley et al. 2007), but few studies have focused on hierarchical resource use among snakes. For example, Row and Blouin-Demers (2006a) found that eastern milksnakes (Lampropeltis triangulum) exhibited second- and third-order selection and preferred fields, edges, and rock outcrops at the landscape and home range scales. We were only able to statistically detect second-order habitat type selection by eastern hognose snakes at NBAFS, but this may have been an artifact of the statistical test or sample size rather than a real difference in hierarchical selection. Although old-field was represented in a higher proportion within snake home ranges than in the landscape, it still occurred in much lower proportion compared to snake locations (Figs. 1, 2). This suggests selection occurred at both scales, but could only be detected at one scale. At the landscape scale, we detected patterns of resource use because of the patchy distribution of old-field habitat; we failed to detect significant selection at the home range scale because of the slightly higher proportion of oldfield at that finer scale (Fig. 2). Some predictive models 1392

have also indicated that patterns of snake resource use are less obvious at finer spatial scales (i.e., third order), where physical landscape features are more homogeneous (Guisan and Hofer 2003). Like all ectotherms, a snake’s body temperature is determined by heat obtained directly from the physical environment. Accordingly, thermoregulation has been suggested as the primary driver for snake habitat selection (Seigel and Collins 1993). Our observations of eastern hognose snake behavior support this hypothesis, where the selection of oldfield and forest edge locations likely provided an optimal range of temperature gradients. Other studies have also correlated snake utilization of open areas and forest edges with thermoregulatory behaviors to maximize physiological characteristics (Plummer and Mills 2000, Blouin-Demers and Weatherhead 2001, Row and Blouin-Demers 2006a). Our transect data supported the GIS results, revealing that snakes chose habitats with relatively more bare ground, less log cover at the ground layer, and a greater percentage of graminoids in the herbaceous layer. The greater percentage of graminoids in habitats selected by eastern hognose snakes at NBAFS is characteristic of early successional plant communities associated with open old-fields. Platt (1969) and Plummer and Mills (2000) noted that, in other parts of the United States, eastern hognose snakes utilize a variety of dry upland sites with sandy soils, including open areas and forest edges. In locations such as NBAFS, with an abundance of rocky areas and gravelly soils, areas of sandy soils are limited and can be a strong determinant of the distribution of this fossorial species. Our observation of eastern hognose snakes in locations with relatively lower slope than in the overall landscape might indicate that they chose home ranges that minimized energy expenditure when traveling. We also found that snake proximity to water was not different than random at the landscape and home range scales, reflecting the similar distribution of available water at both spatial scales (Fig. 2). This observation may suggest that, in regions with relatively abundant wetlands (such as NBAFS), proximity to water may not have a significant influence on eastern hognose snake distribution.

MANAGEMENT IMPLICATIONS Our study has 2 important management implications—one related to the habitat preferences of the eastern hognose snake in the northeastern United States and one related to future habitat selection studies. Our findings are consistent with previous studies that have demonstrated the importance of open early successional habitats to the eastern hognose snake. We suggest that maintenance of this habitat type as a component of forested landscapes will be critical for the persistence of eastern hognose snake populations at NBAFS and in the northeastern United States. Maintenance of these habitats also will benefit other imperiled early successional wildlife species including regionally rare shrubland birds (DeGraaf and Yamasaki 2003) and insect populations (Weber et al. 2008). Our results indicate that future studies of habitat selection for this and other species The Journal of Wildlife Management N 73(8)

should include measurement of available habitat at spatial scales larger than the home range size. A better understanding of the variables that affect habitat selection will enable resource managers to identify and set priorities for management actions that promote species conservation.

ACKNOWLEDGMENTS We thank the natural resources staff of the NBAFS for logistical and field support. We also thank 2 anonymous reviewers for helpful comments on earlier drafts of this manuscript. This work was supported under a military interdepartmental purchase request from the United States Department of Defense, United States Air Force, through the United States Department of Energy contract DEAC02-06CH11357.

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