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

Desert Tortoise Survivorship: Absence of Evidence Does Not Mean Evidence of Absence

C. D. Bjurlin¹, J. A. Bissonette¹, and P. L. Cutler^2
1Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Utah State University, Logan, Utah 84322
²Southwest Division, Naval Facilities Engineering Command, Natural Resources Management (5731), 1220 Pacific Coast Highway, San Diego, California 92132-5190

The desert tortoise has been an important symbol to the peoples of the arid Southwest for centuries. In 1990 this long-lived herbivore was designated as a federally threatened species. One of the driving forces for listing was an apparent scarcity of young animals, even in areas of high adult abundance. Two explanations are probable; either young tortoises suffer from high mortality, or their cryptic coloration and limited activity schedule prevent detection. We addressed this issue for a population of desert tortoises at the Marine Corps Air Ground Combat Center, Twentynine Palms, California. From 1997-1999 we monitored the survival of tortoise nests, eggs, and dispersing young. Contrary to previous belief, survival during these stages was quite high: 70% of nests survived incubation; 68% of eggs in surviving nests developed and produced healthy hatchlings; and 88% of dispersing young survived until winter hibernation. We conclude that absence of evidence does not mean evidence of absence. Nests and hatchling tortoises may be abundant in natural areas, but simply not observed by the casual observer. While it is well documented that juvenile tortoises are susceptible to predation for the first year of life, studies that quantify mortality rate over these early years are vital to interpreting current age class distribution of tortoise populations and allowing a scientific data-based underpinning for their management.