DIFFERENT RATES AND CAUSES OF HIGH MORTALITY IN 2 POPULATIONS OF THE THREATENED DESERT TORTOISE GOPHERUS-AGASSIZII

The desert tortoise Gopherus agassizii is federally listed as Threaten ed because of documented declines in some populations, yet the proxima te causes of these declines are not well understood. With use of radio telemetry, I monitored a total of 55 individual tortoises at two Mojav e Desert sites over three years. Both populations suffered high adult mortality during an extreme drought period, but the temporal pattern a nd inferred proximate causes of mortality differed between sites. At t he eastern Mojave site, no telemetered tortoises died in 1988 or 1989, but 41% died in 1990. All nine carcasses were found and only one show ed any evidence of predation or scavenging. Tortoises that died had sy mptoms of dehydration (high plasma osmolality) and/or starvation (high blood urea, as compared to tortoises that survived). Mortality in the eastern Mojave population thus seems to have been due to physiologica l stress associated with drought and concomitant lack of forage. In co ntrast, western Mojave tortoises that died did not appeal physiologica lly stressed, and all carcasses found showed evidence of predation ol scavenging. Strong circumstantial evidence implicated predation by coy otes, but respiratory disease may also have been a factor. Annual adul t mortality in the western Mojave population was at least 5-25% in eac h of three rears. High mortality in both populations was attributable to 'natural' effects of drought, directly in the eastern Mojave throug h starvation and dehydration, and indirectly in the western Mojave thr ough functional responses of predators to a diminished prey base and, possibly, increased susceptibility of tortoises to disease. Episodic, drought-related high mortality has probably occurred repeatedly in the evolutionary history of G. agassizii, but human exploitation of the d esert may exacerbate natural stresses, and recovery of populations is likely to be slow, In the absence of long-term data sufficient to eval uate the relative importance of potential causes of population decline s, a conservative management strategy is warranted.