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27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts
Distance Sampling at Two Sonoran Desert Tortoise Populations, 2001
Roy C. Averill-Murray1, Annalaura
Averill-Murray2, and Don E. Swann3
1Arizona Game and Fish Department, 2221 W
Greenway Road, Phoenix, AZ 85023;
2Arizona Game and Fish Department, 555 N Greasewood Road,
Tucson, AZ 85745;
3Saguaro National Park, 3693 South Old Spanish Trail, Tucson,
AZ 85730
We used distance sampling to survey for desert tortoises at two sites in Arizona's Sonoran Desert during summer 2001. The surveys occurred at dramatically different scales: on a 450.8-ha parcel of Saguaro National Park (SAGU) and across the approximately 77,000-ha Ironwood Forest National Monument (IFNM). At SAGU we surveyed a grid of 34, 1-km transects systematically laid across the parcel. We surveyed each transect twice for a total of 68 km; transects were typically located on upland slopes of steep topography. We encountered 45 tortoises >150 mm MCL. Program DISTANCE estimated density at 0.41 tortoises/ha (%CV=24.27). Component percentages of density variance consisted of 55.2% for encounter rate, 40.5% for g0, and 4.3% for detection probability. At IFNM we surveyed 109, 1-km transects (108.25 km) randomly located across the landscape, but stratified according to BLM's tortoise habitat categorization. Sixty percent of transects occurred in combined categories 1 and 2, 30% in category 3, and 10% in uncategorized habitat. Many transects occurred in valleys and creosotebush flats, even in categories 1 and 2, due to the resolution of the habitat mapping. We encountered 39 tortoises >150 mm MCL. Stratification based on BLM categories did not improve results; unstratified analysis estimated density at 0.23 tortoises/ha (%CV=29.73). Component percentages of density variance were 63.2% for encounter rate, 23.3% for g0, and 13.5% for detection probability. Our results suggest that distance sampling can be effective at estimating tortoise density in the Sonoran Desert at both small and large scales, especially if effort is stratified based on topography.