Female Agassiz’s desert tortoise activity at a wind energy facility in southern California: The influence of an El Niño event

Abstract

We compared spring-summer activity of adult female Agassiz’s Desert Tortoises (Gopherus aga- ssizii) among three consecutive years (1997, 1998, and 1999) that differed dramatically in winter rainfall and annual plant production at a wind energy facility in the Sonoran Desert of southern California. Winter rainfall was approximately 71%, 190%, and 17% of the long-term average (Octo- ber-March = 114 mm) for this area in water years (WY) 1997, 1998, and 1999, respectively. The sub- stantial precipitation caused by an El Niño Sou- thern Oscillation (ENSO) event in WY 1998 pro- duced a generous annual food plant supply (138.2 g dry biomass/m2) in the spring. Primary production of winter annuals during below aver- age rainfall years (WY 1997 and WY 1999) was reduced to 98.3 and 0.2 g/m2, respectively. Mean rates of movement and mean body condition in- dices (mass/length) did not differ significantly among the years. The drought year following ENSO (WY 1999) was statistically similar to ENSO in every other measured value, while WY 1997 (end of a two year drought) was statistically dif- ferent from ENSO using activity area, minimum number of burrows used, and percentage of non- movements. Our data suggest that female G. agassizii activity can be influenced by environ- mental conditions in previous years.