27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

STUDENT PAPER

Home Range and Movements of Desert Tortoises (Gopherus agassizii) in the California Mojave Desert in Response to Rainfall and Vegetation Parameters

Bryan R. Franks
School of Environmental Science, Engineering, and Policy, Drexel University, Philadelphia, PA 19104

Between May and November 2000, I monitored 58 adult tortoises weekly for movement and activity at 4 sites in the Mojave Desert. Of the 58 tortoises, 44 were female and 14 were male. The 4 sites studied were all predominately creosote-scrub communities but each differed in rainfall and amount of vegetational cover. The sites range from Ivanpah Valley in the Eastern Mojave Desert to Helendale in the Western Mojave Desert. Ivanpah Valley received the largest amount of rainfall and had more cover when compared to the other 3 sites. Also, Ivanpah Valley differed in rainfall and vegetational cover within the valley itself.

I tracked tortoises using radio telemetry with transmitter attachments that did not inhibit movement of animals. From these data, I calculated home range size and timing of movements with respect to rainfall events. Home ranges were calculated using the minimum convex polygon method within ArcView GIS. From preliminary data analysis it appeared that home range sizes differed both between the 4 sites studied as well as within the Ivanpah Valley site. Also, home range size differed considerably between sexes with males having a much larger home range than female tortoises. Both minor and major amounts of precipitation in rainfall events had a considerable effect on tortoise activity. The results of this study provide insight into how precipitation patterns and plant productivity affect the movements and activity of tortoises.