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