Evaluation of Movement Patterns and Space Use in Reintroduced Eastern Indigo Snakes (Drymarchon couperi) in the Florida Panhandle
Type of DegreeMaster's Thesis
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The Eastern Indigo snake (Drymarchon couperi; EIS) is a federally threatened species of colubrid snake endemic to the U.S. Southeastern Coastal Plain for which reintroduction efforts are currently ongoing to reestablish populations where EIS are known to have been extirpated. To evaluate reintroduction success, post-release monitoring of movement patterns is considered essential. However, methods for quantifying animal movement patterns and space use are typically restricted to two-dimensional metrics, which ignore topographic variation in space use. Moreover, most monitoring studies do not attempt to account for the influence of demographics (i.e., sex and age class) on post-release movement behavior in reintroduced populations—despite the fact that interactions with conspecifics may influence spatial decision-making. In this study, we examined percent survival, home range size, rate of dispersal and habitat use of reintroduced EIS at Apalachicola Bluffs and Ravines Preserve (ABRP), a reintroduction site in the Florida Panhandle. Following two groups of released EIS in 2017 and 2018, a total of 32 snakes (19 males, 13 females) were radio-tracked 1-2 times a week. We recorded fate of all snakes up to one year post-release to estimate survival as a proximate, short term measure of success. We used location data to calculate two-dimensional 100% minimum convex polygon (MCPs) home range estimates and looked at the effect of sex on home range size to determine if patterns of space use aligned with those reported for wild populations of EIS, and we calculated three-dimensional 100% MCP home ranges to look at how inclusion of topographic variation of the unique landscape of ABRP might influence home range sizes of EIS. Kernel density estimates (KDEs) were calculated to describe areas of core use. We also calculated an index of dispersal to investigate the potential influence of demographic factors (i.e., age class, sex) and release year on post-release movements. The Kaplan-Meier survival estimates for EIS in 2017 was 45% after 49 weeks, and only 2 of the 12 snakes released were known to have survived (the fate of 4 additional EIS was not confirmed because transmitter signals were lost). In 2018, survival estimates for EIS were 46%, where 7 of the 20 snakes released were known to have survived after 42 weeks, and an additional 5 EIS could not be located (signals were lost). At the end of the two year study, 9 of the 32 snakes released were confirmed alive, resulting in a 28% estimate of survival. Two-dimensional MCP home ranges for 16 EIS (5 males and 11 females) using the first 11 relocations averaged 15.0 ha, but varied among individuals (1.6 ha to 34.8 ha). There was a significant effect of sex on home range size (F= 4.129; df= 9; p=0.01) with males, on average, maintaining larger home ranges (ranging from 9.2 to 34.8 ha and averaging 23.2 ha) than females (ranging from 1.6 to 28.1 ha and averaging 11.56 ha), respectively. Three-dimensional MCP home ranges for 28 EIS (15 males, 13 females) using all relocations from snakes tracked 1-2 times per week ranged from 1.6 ha to 243.82 ha with a mean of 61.78 ha (Appendix 1). Our paired t-test to compare whether 2D and 3D revealed no difference (df = 54; p = 0.9551). Average 95 and 50% kernel density estimates and areas of core use for 8 EIS (2 males, 6 females) were 96.1 m (ranging from 15.2 to 345.8 m) and 16.0 m (ranging from 3.0 to 46.7 m), respectively (Table 3). We found a three-way interaction of sex, age class and release year (F= 4.961; df = 9) where young males released in year 2 dispersed more quickly (32.3 m/day) than all other groups (p = 0.004). Snakes tracked throughout the study exhibited a seasonal shift in habitat use during the winter months, where EIS were exclusively found in upland sandhills habitat as opposed to the rest of the year when they were found using both upland sandhills and ravine/wetland habitat. These data align with season shifts in habitat use reported for wild populations of EIS, suggesting reintroduced EIS in our study exhibiting similar habitat use similar to wild snakes.