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26th Annual Meeting and Symposium of the
Desert Tortoise Council, March 16-18, 2001
Abstracts
Small-Scale Application of Distance Sampling in a Sonoran Desert Tortoise Population
Don E. Swann1 and Roy C. Averill-Murray2
1Saguaro National Park, 3693 South Old Spanish Trail, Tucson, AZ 85730
2Arizona Game and Fish Department, 2221 W Greenway Road, Phoenix, AZ 85023
We used line transects and distance sampling in combination with radio telemetry to estimate desert tortoise density in Saguaro National Park and the adjacent Rocking K Ranch near Tucson, Arizona, as part of a long-term study evaluating the impact of urban development on tortoises. In 2000, 34 1-km transects were each sampled twice on the 368.5-ha Rocking K Ranch study area. Forty-six adult (>150 mm) and 7 juvenile tortoises were observed. Encounter rate for adults was 0.63/km with a percent coefficient of variation (CV) of 17.76 and a 95% confidence interval (CI) of 0.44-0.91. The mean number of observable adults during radio telemetry was 82% with one standard error (SE) of 0.115. Corrected mean density of adults based on line transects and radio telemetry was 0.52 tortoises/ha (CV=25.37, C.I.=0.28-0.77). Distance sampling appears to be a feasible method of estimating density of Sonoran Desert populations of the desert tortoise, at least in small-scale applications.