28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

POSTER

The Desert Tortoise Density Experiment: Does Carrying Capacity Exist for a Species Living in a Stochastic Desert Environment?

Kimberley A. Goodwin¹, Mary B. Saethre², Todd C. Esque^1,2, P. A. Medica³, R. Marlow², and C. R. Tracy^2
1U.S. Geological Survey, Western Ecological Research Center, Henderson, Nevada
²University of Nevada, Program in Ecology, Evolution and Conservation Biology, Biological Resources Research Center, Reno, Nevada 3U.S. Fish and Wildlife Service, Las Vegas, Nevada

From 2000 through 2002, Clark County Nevada Multiple Species Habitat Conservation Plan (MSHCP) funded research to determine if tortoise health, behavior, reproductive output and mortality changed with increasing tortoise density. The results of this study will be used by the U.S. Fish and Wildlife Service to determine whether additional tortoises may be placed at the large scale translocation site. Research was conducted at the Desert Tortoise Conservation Center (DTCC) located in the central Mojave Desert, south of Las Vegas, Nevada. Tortoises were placed in nine pens (approximately 4 ha each) at densities ranging from 337 to 3204 tortoise/km2. Tortoises were monitored monthly during the spring and summer months for condition index (the ratio of body mass relative to body volume), signs of upper respiratory tract disease (URTD) and shell lesions, behavior, reproductive output of female tortoises, and mortality. Condition index was not significantly different with respect to the density of tortoises. Only tortoises that tested negative for URTD and showed no overt signs of disease were placed into the pens in 2000. By 2002, there were tortoises with signs of URTD in every pen, but the percentage of tortoises found to have signs of URTD and/or shell disease were not correlated to the density of tortoises in the pens. There were no significant differences in behaviors in relation to the density of the experimental plots in 2000 and 2001. However, in 2002, all tortoises spent more time below ground than in previous years, and tortoises in higher density pens spent significantly less time eating than tortoises in lower density pens. In 2001, there was no significant difference in the sizes of the first or second clutch in relation to tortoise density. The total average annual egg production of tortoises in high-density pens was significantly greater than that in lower density pens. Mortality did not vary significantly in relation to tortoise density, but the lowest density pen was the only pen that did not have any deaths. While there were very few statistically significant results, trends in the data suggest that if tortoise responses follow the same trajectory, differences between low and high density populations will begin to emerge in the future. Because the desert tortoise is a long-lived species the effects of population densities maintained above carrying capacity may not be measurable after only 3 years.