Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

Comprehensive and accurate desert tortoise density estimates are a critical component of both the Washington County Habitat Conservation Plan and the Desert Tortoise (Mojave Population) Recovery Plan. Distance sampling monitoring was implemented within the Upper Virgin Recovery Unit, managed as the Red Cliffs Desert Reserve, in Washington County, Utah, to gather baseline regional population densities of desert tortoises. In 1997, a pilot study was completed to standardize field techniques, to provide preliminary estimates of encounter rates and the detection probability, and to determine the field effort necessary to achieve dependable regional density estimates within the Red Cliffs Desert Reserve. In the spring of 1998, 201.42 km (103 transects) of distance sampling was completed within Management Zone 3 of the Red Cliffs Desert Reserve.

One hundred and thirty-four adult and subadult tortoises were observed with an encounter rate of 0.67 tortoises per km (range = 0-10). Densities were estimated at 0.23 tortoise per hectare with a 95% confidence interval from 0.17 to 0.33 and a correlation of variation for density (cv(D)) of 16.93 %. When all 178 tortoises encountered were included in the analysis, the encounter rate was 0.88 tortoises per km (range = 0-12). Densities were estimated at 0.30 tortoises per hectare with a 95% confidence interval from 0.21 to 0.42 and a cv(D) of 17.65 %.

Although density estimates have been reported previously within the Reserve, they are not comparable to density estimates from this study due to limitations with the monitoring technique previously used or unrepresentative areas sampled. These baseline regional density estimates can be compared to future estimates to reveal regional density trends within the Red Cliffs Desert Reserve.