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

STUDENT PAPER

Determining the Carrying Capacity of Desert Tortoises

Mary B. Saethre¹, Todd C. Esque^2,3, P.A. Medica⁴, R. Marlow², and C.R. Tracy^2
1University of Nevada, Department of Biological Sciences, Reno, Nevada
²University of Nevada, Program in Ecology, Evolution and Conservation Biology, Biological Resources Research Center, Reno, Nevada
³U.S. Geological Survey, Western Ecological Research Center, Henderson, Nevada 4 U.S. Fish and Wildlife Service, Las Vegas, Nevada

Translocation is being studied as a method of handling desert tortoises that have been displaced by urban development in Las Vegas, Nevada. Beginning in 1990, displaced animals were housed at a holding facility, the Desert Tortoise Conservation Center (DTCC), until a release site and protocols could be established. Between April 1997 and November 1999, 2000 desert tortoises were released at the Large-Scale Translocation Study (LSTS) site. This was the maximum number allowed in accordance with the Fish & Wildlife Service Endangered Species permit and the LSTS was considered to be "full" at a somewhat arbitrary density of 29 animals/km2 for animals greater than 180-mm midline carapace (MCL). To investigate the feasibility of releasing more tortoises at the LSTS, the Clark County Nevada Multi-Species Habitat Conservation Plan (MSHCP) funded this study to establish a maximum density below which desert tortoises showed no adverse effects. The parameters studied were mortality, behavior, body condition, disease symptoms, and reproductive output. In April 2000, animals greater than 180 mm MCL were stocked at densities ranging from 150 animals/km² to 1500 animals/km² into a set of nine pens at the DTCC. To reflect more realistic conditions, additional animals smaller than 180 mm MCL were added to each pen. Beginning in 2000, animals were weighed monthly and the pens were monitored daily for aboveground activity from May through October. In 2001, monthly weighing continued and a sample of females were monitored for egg production. There were no significant differences among pens in mean annual plant biomass in 2000 (a year of below average rainfall) and in 2001 (a year of adequate rainfall). In the first year of release, there was a greater incidence of fighting in the three highest density pens than in the three lowest density pens for all months except the release month (April). Animals in the three highest density pens were also more likely to share burrows (up to seven in one burrow) than animals in the three lowest density pens. There were no trends evident in body condition index (CI) among pens in 2000 or in 2001. There was no trend between density and presence of upper respiratory tract disease symptoms in 2000 or in 2001. Reproduction also did not vary among the densities. While not significant, there was a positive trend in mortality among densities. It also appears that, at least through the first two years, extremely large and extremely small desert tortoises are most "at risk" for dying regardless of density. Although this is intended to be a long-term study, the first two years of data indicate that at least the three highest densities, densities over 850 animals/km² (tortoises greater than 180 mm MCL), are likely to be detrimental to survival.