Abstract
We compared spring-summer activity of adult female Agassiz’s Desert Tortoises (Gopherus agassizii) 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 (October-March = 114 mm) for this area in water years (WY) 1997, 1998, and 1999, respectively. The substantial precipitation caused by an El Ni?o Southern Oscillation (ENSO) event in WY 1998 produced a generous annual food plant supply (138.2 g dry biomass/ m2) in the spring. Primary production of winter annuals during below average 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 indices (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 different 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 environmental conditions in previous years.
Highlights
The term El Niño, has been applied to specific climatic conditions but a single universal definition for the phenomenon is elusive
For Agassiz’s Desert Tortoises (Gopherus agassizii) that inhabit arid landscapes where resource abundance is unpredictable both spatially and temporally, the species should benefit from increased forage resulting from increases in precipitation associated with El Niño Southern Oscillation (ENSO) events
Because ENSO events can have dramatic impacts on some organisms and very little is published on the inter-year variation in activity patterns of G. agassizii, we investigated the relationship between Desert Tortoise activity patterns, annual plant biomass quantity, and rainfall patterns
Summary
The term El Niño, has been applied to specific climatic conditions but a single universal definition for the phenomenon is elusive One such definition is that El Niño occurs when five month running means of sea surface temperature (SST) anomalies in a portion of the Pacific Ocean exceed 0.4 ̊C for 6 months or more [1]. When these conditions occur, along with an atmospheric component known as the Southern Oscillation, precipitation in the western United States can be significant. For Agassiz’s Desert Tortoises (Gopherus agassizii) that inhabit arid landscapes where resource abundance is unpredictable both spatially and temporally, the species should benefit from increased forage resulting from increases in precipitation associated with ENSO events
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