Spatial Ecology of the Desert Tortoise: Sampling Frequency and Biological Influences

Abstract

Understanding the spatial ecology of an animal is crucial for making positive efforts to provide for its recovery. As a part of this understanding, home range estimates are used to answer a variety of questions in ecological studies. However, home range estimates based on a collection of radio-telemetry locations are sensitive to methodological variables, such as sample size, sampling frequency, and the choice of estimator. Further confounding these estimates are a number of physical, social, and ecological factors. Identifying the main determinants of space use patterns by a species may aid conservation efforts. The Desert Tortoise (Gopherus agassizii)of the Mojave Desert inhabits an extreme environment where a number of factors likely influence its land use patterns. Prior home range estimates of the Desert Tortoise are wide ranging from different portions of the desert, due in part to the use of a variety of sampling methodologies. My goal was to determine how different facets of sampling methodology affect home range estimates of the Desert Tortoise using two widely-used home range estimators, the minimum convex polygon and the fixed kernel density estimator. In addition, investigated physical, social, and ecological variables to examine the dominant factor(s) influencing the spatial ecology of the Desert Tortoise. Results suggested that previous home range estimates were highly influenced by the sampling regime utilized. Home range estimates in this study were much greater than those in the literature, possibly due to an intensive sampling regime. This suggests that tortoises may require more land than previously thought. Males and females demonstrated very different patterns of space and burrow use, suggesting these variables affect estimates for each sex differently. I conclude that a combination of these variables determines space use in tortoises. By adopting a uniform sampling methodology, researchers can better provide comparable data across studies in a holistic effort to understand the spatial ecology of a species