Effects of Drought on Desert Tortoise Movement and Activity

Abstract

We monitored movement and activity patterns of 38 desert tortoises (Gopherus agassizii) at 2 locations in the southcentral Mojave Desert during 2 consecutive years differing markedly in winter rainfall. During the first year, winter precipitation was 225% greater than the long-term average for this region, whereas a drought the following year resulted in precipitation that was 25% of the long-term average. These winter rains produced 2 distinct patterns of annual plant productivity: a bloom of annuals the first year, followed by their complete absence the second year. As measured by radiotelemetry, home range size, the number of different burrows used, average distances traveled per day, and levels of surface versus burrow activity were significantly reduced in both populations during the drought year. The pooled distribution of between-observation distances traveled showed a shift favoring shorter-distance movements during the drought year. Differences in levels of winter precipitation between years and the resulting variation of winter annual biomass in the spring appear to play a significant role in desert tortoise movement and activity patterns. Future management and conservation plans for the desert tortoise should consider weather and productivity as important factors influencing annual home range size, number of burrows used, average distances traveled, and activity levels.