M. Rebecca Bolt

Factors Influencing Home-Range Sizes of Eastern Indigo Snakes in Central Florida

Abstract Wide-ranging snake species are particularly sensitive to landscape fragmentation, and understanding area requirements is important for their conservation. We used radiotelemetry to quantify how Eastern Indigo Snake home-range sizes were influenced by sex, land cover, and the length of time (weeks) individuals were radio tracked. We found that Eastern Indigo Snakes had the largest home ranges among other snake species studied. Female home ranges averaged 44 and 76 ha, respectively, for kernel and minimum convex polygon estimators. Male home ranges averaged 156 and 202 ha, respectively, for kernel and minimum convex polygon estimators. Many animal species respond to habitat fragmentation by using larger areas than in unfragmented landscapes, but we found that Indigo Snakes in fragmented landscapes used much smaller areas. The length of time that snakes were tracked had almost no influence on home-range size compared to sex and land cover type. Our results suggest that maintaining populations of this large wide-ranging predator will require large conservation areas with minimum fragmentation.

The role of the bandwidth matrix in influencing kernel home range estimates for snakes using VHF telemetry data

Abstract Context. Despite the diversity of available home range estimators, no single method performs equally well in all circumstances. It is therefore important to understand how different estimators perform for data collected under diverse conditions. Kernel density estimation is a popular approach for home range estimation. While many studies have evaluated different kernel bandwidth selectors, few studies have compared different formulations of the bandwidth matrix using wildlife telemetry data. Additionally, few studies have compared the performance of kernel bandwidth selectors using VHF radio-telemetry data from small-bodied taxa. Aims. In this study, we used eight different combinations of bandwidth selectors and matrices to evaluate their ability to meet several criteria that could be potentially used to select a home range estimator. Methods. We used handheld VHF telemetry data from two species of snake displaying non-migratory and migratory movement patterns. We used subsampling to estimate each estimator’s sensitivity to sampling duration and fix rate and compared home range size, the number of disjunct volume contours and the proportion of telemetry fixes not included in those contours among estimators. Key Results. We found marked differences among bandwidth selectors with regards to our criteria but comparatively little difference among bandwidth matrices for a given bandwidth selector. Least-squares cross-validation bandwidths exhibited near-universal convergence failure whereas likelihood cross-validation bandwidths showed high sensitivity to sampling duration and fix rate. The reference, plug-in and smoothed cross-validation bandwidths were more robust to variation in sampling intensity, with the former consistently producing the largest estimates of home range size. Conclusions. Our study illustrates the performance of multiple kernel bandwidth estimators for estimating home ranges with datasets typical of many small-bodied taxa. The reference and plug-in bandwidths with an unconstrained bandwidth matrix generally had the best performance. However, our study concurs with earlier studies indicating that no single home range estimator performs equally well in all circumstances. Implications. Although we did not find strong differences between bandwidth matrices, we encourage the use of unconstrained matrices because of their greater flexibility in smoothing data not parallel to the coordinate axes. We also encourage researchers to select an estimator suited to their study objectives and the life history of their study organism.

The Influence of Sex and Season on Conspecific Spatial Overlap in a Large, Actively-Foraging Colubrid Snake.

Understanding the factors influencing the degree of spatial overlap among conspecifics is important for understanding multiple ecological processes. Compared to terrestrial carnivores, relatively little is known about the factors influencing conspecific spatial overlap in snakes, although across snake taxa there appears to be substantial variation in conspecific spatial overlap. In this study, we described conspecific spatial overlap of eastern indigo snakes (Drymarchon couperi) in peninsular Florida and examined how conspecific spatial overlap varied by sex and season (breeding season vs. non-breeding season). We calculated multiple indices of spatial overlap using 6- and 3-month utilization distributions (UD) of dyads of simultaneously adjacent telemetered snakes. We also measured conspecific UD density values at each telemetry fix and modeled the distribution of those values as a function of overlap type, sex, and season using generalized Pareto distributions. Home range overlap between males and females was significantly greater than overlap between individuals of the same sex and male home ranges often completely contained female home ranges. Male home ranges overlapped little during both seasons, whereas females had higher levels of overlap during the non-breeding season. The spatial patterns observed in our study are consistent with those seen in many mammalian carnivores, in which low male-male overlap and high inter-sexual overlap provides males with greater access to females. We encourage additional research on the influence of prey availability on conspecific spatial overlap in snakes as well as the behavioral mechanisms responsible for maintaining the low levels of overlap we observed.

Multi-level, multi-scale habitat selection by a wide-ranging, federally threatened snake

ContextAlthough multi-scale approaches are commonly used to assess wildlife-habitat relationships, few studies have examined selection at multiple spatial scales within different hierarchical levels/orders of selection [sensu Johnson’s (1980) orders of selection]. Failure to account for multi-scale relationships within a single level of selection may lead to misleading inferences and predictions.ObjectivesWe examined habitat selection of the federally threatened eastern indigo snake (Drymarchon couperi) in peninsular Florida at the level of the home range (Level II selection) and individual telemetry location (Level III selection) to identify influential habitat covariates and predict relative probability of selection.MethodsWithin each level, we identified the characteristic scale for each habitat covariate to create multi-scale resource selection functions. We used home range selection functions to model Level II selection and paired logistic regression to model Level III selection.ResultsAt both levels, EIS selected undeveloped upland land covers and habitat edges while avoiding urban land covers. Selection was generally strongest at the finest scales with the exception of Level II urban edge which was avoided at a broad scale indicating avoidance of urbanized land covers rather than urban edge per se.ConclusionsOur study illustrates how characteristic scales may vary within a single level of selection and demonstrates the utility of multi-level, scale-optimized habitat selection analyses. We emphasize the importance of maintaining large mosaics of natural habitats for eastern indigo snake conservation.

On the Road Again: Assessing the Use of Roadsides as Wildlife Corridors for Gopher Tortoises (Gopherus Polyphemus)

Abstract Small populations resulting from the impacts of habitat fragmentation are prone to increased risks of extinction because of a lack of population connectivity. Roads increase habitat fragmentation, but properly managed roadsides may be able to function as wildlife corridors. Here we use radiotelemetry to observe movement patterns of Gopher Tortoises (Gopherus polyphemus) along potential roadside corridors at the John F. Kennedy Space Center (KSC) in Florida, USA, to determine if tortoises use roadsides as movement pathways between larger habitat patches or as residential habitat. Additionally, we translocated tortoises to study the feasibility of roadsides to function as movement corridors. We found that roadsides are not used as a movement pathway but rather as an apparent long-term residential habitat. Only one tortoise was observed exiting the roadside corridor, and minimum convex polygon (MCP) home range sizes and distances traveled remain similar to those exhibited by tortoises in larger habitat patches. Following translocation, we observed a failure to return home, either by direct paths or by corridor use, for all but one tortoise. Instead, most tortoises remained along roadsides after only a brief period of exploration. Overall, we find that roadsides act as independent, residential habitat instead of as a movement corridor. Future studies should focus on understanding the actual suitability of roadsides, as they may function as ecological traps given their attractiveness but high risk of mortality. While we urge caution, current management should treat roadsides as residential locations for Gopher Tortoises and focus on reducing road mortality.

Stopped Dead in Their Tracks: The Impact of Railways on Gopher Tortoise (Gopherus polyphemus) Movement and Behavior

Habitat fragmentation is one of the leading causes of biodiversity decline and most commonly results from urbanization and construction of transportation infrastructure. Roads are known to negatively impact species, but railways can often cause similar effects. Certain taxa, such as turtles and tortoises, are more vulnerable to railways than others due to limitations in mobility. We studied the impact of rails on the movement and behavior of Gopher Tortoises (Gopherus polyphemus), a threatened, highly terrestrial species likely in frequent contact with railways. First, we used radio-telemetry to determine the frequency of railway crossings and compared this to correlated random walk (CRW) simulations to assess if tortoises were crossing the rails less frequently than is expected by unconstrained movement. Second, we placed tortoises into the railway and measured behavior for one hour to assess crossing ability. Lastly, we tested whether trenches dug underneath the rails could allow safe passage for tortoises. We found that railways impacted the movement of Gopher Tortoises. Gopher Tortoises crossed the railway less often than what would be expected by unhindered movement for five of our ten tortoises tracked. During behavioral trials, 0 of 24 tortoises placed within the railways were capable of escaping from the rails. Using game cameras, we detected tortoises using trenches dug underneath the rails and between the ties 68 times over the course of a single summer. For minimal financial cost, the trenches facilitated tortoise movement across the railway, maintained full rail functionality, and created an escape route for individuals that were trapped between the rails, and thus should be implemented as a mitigation strategy. Given the thousands of km of railways around the world, we recommend future studies focus on the new field of rail ecology.