Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

Survivorship was studied in an eastern Mojave Desert population of desert tortoises from 1988 to 1996. The most reliable calculation of maximum survival rates for tortoises >139 mm in carapace length, based on survival of transmittered tortoises, was 0.934-1.00 in most years . However, severe drought lowered survival of females >186 mm in length to 0.696. Losses of 18.4-60 % of adult desert tortoises during drought have also been observed in other rigorous studies. Incorporating the longterm patterns of rainfall for this site, I estimated that survivorship for females >188 mm was 0.907-0.964. Minimum nest depredation rates at this site ranged from 18.4-42.0% of the annual egg production. The annualized hatching rate based on three years study was 0.687, although sampling artifacts suggest that the actual rate is 0.58. Incorporating the most realistic population parameters of growth, survival by life stage, and reproduction resulted in a lamda of 1.057 and Ro of 4.35 for this population. Accurate estimates of survivorship are critical for population behavior analyses. Survival rates calculated from mark- recapture analyses, the lack of adequate survival data for juvenile tortoises, and the lack of integration of long-term variability in survivorship are discussed in the context of the predictive ability of current population models for the desert tortoise.