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27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts
Life History and Demographic Analysis of the Desert Tortoise at Fort Irwin and Reference Sites: Study Design and Early Findings
Harold W. Avery1, Justin D. Congdon2 and
James R. Spotila1
1School of Environmental Science, Engineering, and
Policy, Drexel University, Philadelphia, PA 19104;
2Savannah River Ecology Laboratory, Aiken, SC, 29802
Tortoise populations are thought to fluctuate as a result of spatial and temporal variations in environmental factors, such as rainfall and forage availability. However the ability to accurately predict how environmental variations affect demographic parameters of tortoise populations is currently lacking, because accurate data on birth rates, recruitment rates, death rates, and growth rates in response to environmental variations are not known for the desert tortoise. Conservation and management efforts have been encumbered as a result of the inability to connect environmental variation to demographic rates of tortoises. Demographic studies are underway at and near NTC Fort Irwin, and Ivanpah Valley, Mojave National Preserve, California, and physiological studies are being initiated, to elucidate the relationships between environmental fluctuations and tortoise demography. We report on the preliminary demographic findings from two study sites, at Ivanpah Valley, and the Fort Irwin Reference Site (FIRS), located near the southern border just outside of the current NTC Fort Irwin boundary. At Ivanpah Valley, where reproduction has been monitored since 1997, rainfall varies three- to four-fold along an approximately 10 km gradient, ranging from 850 m to 1150 m elevation. At lower elevation sites where annual rainfall is lower, mean clutch size of female tortoises is 3.77 eggs clutch-1, whereas at higher elevation sites, mean clutch size is 4.87 eggs clutch-1. Mean clutch frequency at the lower elevation sites is 1.30, compared to 1.54 at the higher elevation sites. Adult female survivorship during 1997 to 2000 has been 77.8% at the lower elevation sites, compared to 100% survivorship at the higher elevation sites. At the FIRS Site, our first year of data collection in 2000 indicated relatively low fecundity rates of 3.18 eggs clutch-1, and a mean clutch frequency of 0.92. Adult female survivorship rate at FIRS was 90.5% from May 2000 to November 2001. In a separate symposium paper, nesting success is reported for each site by Bryan Franks. We plan to continue the study of demography at these two sites, and establish new study sites this spring in NTC Fort Irwin expansion areas and surrounding reference areas, to further our understanding of the viability of tortoise populations in this important area of the Mojave Desert.