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25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts
1999 Baseline Desert Tortoise Density and Abundance in the Upper Virgin River Recovery Unit
Ann M. McLuckie, Deborah H. Harstad, and Rick A.
Fridell
Utah Division of Wildlife Resources, Washington
County Field Office,
344 E. Sunland Dr. #8, St. George, Utah 84790
Comprehensive and accurate desert tortoise density estimates are recognized as critical in both the Washington County Habitat Conservation Plan and the Desert Tortoise (Mojave Population) Recovery Plan. Distance sampling monitoring was implemented within the Red Cliffs Desert Reserve, 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 precise regional density estimates within the Red Cliffs Desert Reserve. In 1998, 201.42 km (103) transects) of distance sampling was completed within Management Zone 3 of the Red Cliffs Desert Reserve.
In the spring of 1999, 306.5 km (158 transects) of distance sampling was completed within Management Zones 2, 3, and 5 of the Red Cliffs Desert Reserve. Two hundred and twenty eight tortoises were encountered during the spring sampling period between March 30 to June 10, 1999. A total of 187 reproductive tortoises (³ 180 mm) were observed with an encounter rate of 0.61 tortoises per km (range = 0.46 to 0.81) and a coefficient of variation (cv(D)) of 14.45%. The density of reproductive tortoises within the Reserve was estimated at 0.23 tortoises per hectare with a 95% confidence interval from 0.17 to 0.32 and a cv(D) of 16.21%. The abundance of reproductive tortoises was estimated at 5,036 tortoises per area sampled (11,457 ha) within Zone 2, 3, and 5 of the Reserve with a 95% confidence interval from 3,666 to 6,920 and a cv(A) of 16.21%. Density and abundance estimates calculated for reproductive tortoises in 1999 within Management zone 3 are within the 95% confidence interval of 1998 estimates (0.23 tortoises/ha, 2,361 tortoises per area sampled). The precision level of measured estimates (i.e., ˆPw, ˆ, ˆD, ˆA) will be refined as additional years of monitoring data are collected. Baseline regional density and abundance estimates can be compared to future estimates to reveal regional trends within the Red Cliffs Desert Reserve.