SAMPLING CONSIDERATIONS FOR IMPROVING HOME RANGE ESTIMATES OF DESERT TORTOISES: EFFECTS OF ESTIMATOR, SAMPLING REGIME, AND SEX

Home range estimation as a measure of spatial utilization is an important tool in the management of wildlife. Operational methods of defining the spatial requirements of an animal differ in sampling regime and interpretation. The two most commonly used estimators, the minimum convex polygon (MCP) and the fixed kernel (FK), each provide a different measure of land use, yet, together allow for a better understanding of the spatial needs of a particular animal. Sampling frequency, the number of individuals, and other user-defined inputs differentially affect home range estimates using these two procedures. For the comparison of either MCP or FK estimates to be reliable, these variables need to be as similar as possible across studies. We conducted an intensive radio-telemetry study on a large number of Desert Tortoises (Gopherus agassizii) to determine an optimal sampling effort for home range estimation using both the MCP and FK estimates, and to identify factors important to space use by this species. Data were parsed into sampling regimes representative of previous home range studies in an effort to compare estimates across studies. Home range estimates using the MCP were over two times larger in this study when compared to previous studies on the Desert Tortoise in the Mojave Desert. Results indicated that an increased sampling frequency inflates MCP estimates, while providing more use-specific detail and decreasing area for FK estimates. Analyses demonstrated home range area to be greatly affected by choice of estimator (MCP or FK), sampling regime, and sex. We recommend an intensive and systematic sampling effort to better define home range estimates; as well as, to provide comparable data across studies for this and other species of herpetofauna. Both home range estimators provide valuable information on the biological needs of the Desert Tortoise and should be identified as a priority in land use investigations and conservation decisions for this species.