Different rates and causes of high mortality in two populations of the threatened desert tortoise Gopherus agassizii
Citation
Charles C Peterson, Different rates and causes of high mortality in two populations of the threatened desert tortoise Gopherus agassizii: .
Summary
The desert tortoise Gopherus agassizii is federally listed as Threatened because of documented declines in some populations, yet the proximate causes of these declines are not well understood. With use of radiotelemetry, I monitored a total of 55 individual tortoises at two Mojave Desert sites over three years. Both populations suffered high adult mortality during an extreme drought period, but the temporal pattern and inferred proximate causes of mortality differed between sites. At the eastern Mojave site, no telemetered tortoises died in 1988 or 1989, but 41% died in 1990. All nine carcasses were found and only one showed any evidence of predation or scavenging. Tortoises that died had symptoms of dehydration (high plasma osmolality) [...]
Summary
The desert tortoise Gopherus agassizii is federally listed as Threatened because of documented declines in some populations, yet the proximate causes of these declines are not well understood. With use of radiotelemetry, I monitored a total of 55 individual tortoises at two Mojave Desert sites over three years. Both populations suffered high adult mortality during an extreme drought period, but the temporal pattern and inferred proximate causes of mortality differed between sites. At the eastern Mojave site, no telemetered tortoises died in 1988 or 1989, but 41% died in 1990. All nine carcasses were found and only one showed any evidence of predation or scavenging. Tortoises that died had symptoms 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 physiological stress associated with drought and concomitant lack of forage. In contrast, western Mojave tortoises that died did not appear physiologically stressed, and all carcasses found showed evidence of predation or scavenging. Strong circumstantial evidence implicated predation by coyotes, but respiratory disease may also have been a factor. Annual adult mortality in the western Mojave population was at least 5–25% in each of three years. High mortality in both populations was attributable to ‘natural’ effects of drought, directly in the eastern Mojave through starvation and dehydration, and indirectly in the western Mojave through 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 desert may exacerbate natural stresses, and recovery of populations is likely to be slow. In the absence of long-term data sufficient to evaluate the relative importance of potential causes of population declines, a conservative management strategy is warranted.
Published in Biological Conservation, volume 70, issue 2, on pages 101 - 108, in 1994.
