Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

Between 1987 and 1994, the desert tortoise population at the Maricopa Mountains declined from over 140 to approximately 20 individuals. An eight year drought, detected by the Palmer Drought Severity Index, coincided with the period when tortoises died. Mortality occurred disproportionately on hotter, drier and less vegetated S and SW exposures, while tortoises in the Maricopa Mts. preferentially utilized N and NE slopes. Ecological correlates including high saguaro mortality and changes in other indicator species during the study support the climate hypothesis. Adult tortoises, especially females, suffered the highest relative mortality rate. Tortoises with high-domed shells suffered higher mortality than flatter tortoises during the drought at the Maricopa Mts. The diet was composed mostly of woody perennials, in contrast to other Arizona populations, which have higher quantities of succulent and energy-rich annual plants in the diet.

Reproduction was investigated to assess drought effects on the potential for population recovery. Shelled eggs were first detectable in the Maricopas on June 5 and oviposition was completed by July 24, in 1994. The number of females reproducing at the site of population collapse was significantly lower than at a nearby control site (Espanto Mt.) without high mortality. At Espanto, environmental stressors appeared less severe. Other than proportion of females reproducing, fecundity of the Maricopa and Espanto females was comparable to previously studied Sonoran tortoise populations. Body weight gradually declined during the arid premonsoon season, then dropped substantially at oviposition, and was quickly restored after heavy precipitation. Recovery of this population will be slow due to the apparently low reproductive rate.