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

Dispersion of rainfall is one of the important determinants of variation in primary productivity in desert ecosystems. Geographic, elevational and seasonal patterns of rainfall may affect populations of desert tortoises by affecting food availability, which in turn can affect patterns of growth, reproduction and survivorship in these and other desert animals. In Ivanpah Valley, California, rainfall may vary two- to threefold from lower elevation areas to higher elevation areas within a single rainfall event, regardless of season. We quantitatively analyzed the diets of tortoises in areas receiving significantly different rainfall in Ivanpah Valley, California, by analyzing contents of fresh tortoise scat soon after rainfall events. Tortoises in areas receiving significantly more rainfall primarily fed on green perennial grass (Hilaria rigida), whereas tortoises in lower rainfall areas fed primarily on dry cactus (Opuntia sp.). Differences in food and water availability have also resulted in a recent population die-off of tortoises in lower elevations, whereas populations in higher elevations within the same valley have not experienced this die off. Microgeographic differences in rainfall, caused by physiographic features of elevation, mountain ranges and other factors, may therefore at times affect nutrition and survivorship of tortoise populations. This variation in resource availability and survivorship cannot be evaluated with present monitoring programs that have unreplicated study sites for a given geographic area.