25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Proximate Constraints Affecting the Reproductive Output and Mortality of Desert Tortoises

Shannon Collis and Harold W. Avery
U.S. Geological Survey, Canyon Crest Field Station, Dept. of Biology, University of California, Riverside CA 92521

Understanding the affects of resource availability on reproduction is critical to the study of life history and demography of animal populations. The desert tortoise (Gopherus agassizii), a long-lived species with delayed sexual maturity, is dependent on stored nutrients and water to minimize fluctuations in reproductive output. Tortoise populations are subject to lower fecundity and higher mortality in unpredictable desert ecosystems following extended periods of decreased resource availability. In an on-going study, we measured the reproductive output of female tortoises from a population at Ivanpah Valley, California, located within the Mojave National Preserve, from 1997 - 1999. We measured egg and clutch size, and clutch frequency in 42 female tortoises using a portable x-ray unit. Eleven rain gauges were used to monitor monthly precipitation across three study sites within Ivanpah valley to measure variance in rainfall. Perennial plant cover and annual plant biomass were also measured. Precipitation was significantly greater at higher elevations across a 10 km distance characterized by a 400 m increase in elevation. Availability of food plants increased with higher elevation as well. Tortoise reproductive output was greater and recent mortality was lower, at the higher elevation along this short elevational and rainfall gradient. Resource variability is a defining feature of desert ecosystems, yet the importance of micro-geographic variation of these resources to desert tortoise populations has not been previously determined. Our study shows that micrographic differences in rainfall and primary productivity of annual vegetation can result in significant differences in survivorship and mortality of the threatened desert tortoise. These findings have important implications to designing reserves, managing public lands, and other conservation issues relevant to the desert tortoise.