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ABSTRACT.-Survival of adult Desert Tortoises (Gopherus agassizii) appears related to site-specific variation in precipitation and productivity of annual plants. We studied adult tortoise survival rates at two closely situated, but physiographically different, sites in the eastern Mojave Desert over a nine-year period (spring 1992 to spring 2001). Survival rates were initially derived from population surveys conducted over a three-year period and by radio-telemetry monitoring over a seven-year period beginning in 1994. After a period of initial stability, survival rates on the two sites diverged over the study period, and seven-year survival rates estimated from radio-telemetry monitoring were 0.900 and 0.269, respectively. A die-off in 1996 on the latter site appears to have been triggered by a period of drought, which began in the summer of 1995, coupled with a failure of annual vegetation production in 1996. Depressed survival rates on this site were associated with drought conditions during three of four years. Although the decline had the appearance of an epizootic, there were no clinical signs of disease. Relatively short-term drought, combined with little or no annual biomass, appears to have caused severe reductions in tortoise survival. If periods of drought-induced low survival are common over relatively small areas, then source-sink population dynamics may be an important factor determining tortoise population densities.
Desert Tortoises Gopherus agassizii exhibit a remarkable capacity to respond behaviorally and physiologically to drought conditions (Nagy and Medica, 1986; Henen et al., 1998; Duda et al., 1999). During unfavorable periods, desert tortoises decrease surface activity and generally remain inactive or dormant within their burrows (Duda et al., 1999), reducing water loss and minimizing energy expenditures (Nagy and Medica, 1986). Desert tortoises can tolerate extreme variations in energy, nutrient, and water balance on seasonal, annual, and potentially longer time frames (Nagy and Medica, 1986; Peterson, 1996a,b; Henen et al., 1998). The ability of desert tortoises to tolerate drought conditions, however, has limitations. Their primary springtime diet of succulent annual vegetation appears to be osmotically stressful because of high potassium levels (Nagy and Medica, 1986; Oftedal and Allen, 1996). Tortoises must void these ions as urinary waste, but to minimize water loss, tortoises may accumulate ions and nitrogenous wastes in their large urinary bladders for months or longer (Nagy and Medica, 1986). Seasonal...