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25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts
Effects of Geology and Cover Site Choice on Desert Tortoise Populations at the Tiefort Mountains, California
Kristin H. Berry1, Glenn Goodlett2,
and Tracy Goodlett3
1U.S. Geological Survey, Western Ecological
Research Center, Riverside, CA 92507; 2On-Track Consulting and
Research, Ridgecrest, CA 93555
We studied use of above and below ground cover sites by desert tortoises in the Tiefort Mountains, National Training Center, Fort Irwin (Central Mojave Desert) between 1997 and 1999. The study site is in a military training area. Desert tortoises used four types of below-ground cover sites (natural caves, rock shelters, burrows in soil, and pallets), and when above ground were under shrubs or in the open. The locations of the different cover site types varied according to geology. Natural caves and rock shelters were in old (Jurassic to Pleistocene) geologic formations, whereas burrows were common in both recent (Holocene) alluvium and old geologic formations. Selection of cover site type differed by size and sex of the tortoise. Adult females were more often found in burrows than in other types of cover sites, whereas adult males were more often found in natural caves (P<0.0001). Adult female choice of below-ground cover sites differed significantly by season (P<0.01) and may be related to nesting sites. Juvenile and immature tortoises were found primarily in soil burrows and rock shelters (P< 0.0165). Soil burrows and pallets were more vulnerable to collapse from natural and anthropogenic changes in the environment than were the natural caves. During the study, none of the natural caves (N = 85) and rock shelters (N = 22) collapsed, whereas 30% of burrows collapsed or were damaged. Differences in vulnerability to collapse were also related to age and type of geologic formation and topography. Because adult female tortoises spent more time in burrows, they may have been more vulnerable to injuries and early mortality. These findings have implications for research designs on sampling different types of habitats on local and landscape scales. They also have management applications for both military training and recreation vehicle use areas. The Department of the Army funded the research.