Movement and home range analyses
We utilized ArcMap 10.2 (ESRI, 2011) to create maps of the study sites with aerial photography images (1 meter resolution). We imported GPS data from each snake as separate map layers, and generated home ranges for each snake using the Home Range Tools (HRT) for ArcGIS 2.0.20 extension (Rodgers et al. 2015). We calculated home ranges using the kernel density estimate application in HRT. The kernel method is a non-parametric approach to estimating a Utilization Distribution (UD), a probability distribution of areas utilized by an animal using the locations it has previously used. It frees the UD from any parametric assumptions, making better use of locational data (Worton 1989).
We calculated annual home range estimates for each snake that had nine months or more data. Prior to analyses, we deleted duplicate GPS points (if an individual was found at the same location at consecutive recordings) to avoid skewing results. We generated a polygon representing the 90% isopleth and 50% isopleth for each snake (Fig. 3). For this study, we defined the total home range of an individual by the area encompassed within the 90% isopleth, and defined the activity core, or area most utilized by the individual, as the area encompassed within the 50% isopleth. We chose these values because most other studies use these criteria, allowing for easy comparisons among studies. We also generated seasonal home ranges for each snake using three-month intervals representing seasons (December–February, winter; March–May, spring; June–August, summer; September–November, fall). We only used data from individuals that were tracked throughout each three-month interval to calculate mean seasonal home range and activity core sizes.
We also used the HRT extension to generate the distance between successive relocations for each snake. We used these results and equation 1 to calculate the average distance moved between successive relocations in each month. We then calculated the average for each three month season.
\begin{equation} \overset{\overline{}}{D}=\frac{\sum_{i=x_{1}}^{x_{n}}d_{i}}{\text{\ n}}\nonumber \\ \end{equation}
Equation 1: Average distance moved between successive relocations.
1 \(\overset{\overline{}}{D}\) is the average distance moved between successive relocations in a month
2 \(d\) is the distance moved between each successive relocation
3 \(n\) is the total number of movements