DESERT TORTOISE

Desert Tortoise Monitoring

The desert tortoise (Gopherus agassizii) is found within the Mojave and Sonoran Deserts of southern California, southern Nevada, western Arizona, southwestern Utah, and northwestern Mexico. The species is well-adapted for harsh desert environments, but populations have decreased from habitat loss, disease, predation, on- and off-road vehicle traffic, and livestock grazing. Climate change further threatens the continued existence of desert tortoises and suitable habitat. Such decreases resulted in the listing of the Mojave Desert population as threatened under the Endangered Species Act in 1990. Delisting may only proceed when population trends are stable or increasing for at least one tortoise generation (25 years). Long-term monitoring is necessary to determine population trends.

The U.S. Fish and Wildlife Service (USFWS) began its current range-wide monitoring program for the Mojave population of the desert tortoise in 2001. It is an intensive multi-faceted program occurring simultaneously in Mojave and Colorado deserts of California, Nevada, Arizona, and Utah. The Institute for Wildlife Studies is assisting USFWS by conducting monitoring and related activities in Nevada and California. Monitoring involves obtaining radio-telemetry locations from tagged tortoises and line distance sampling (counting tortoises found by walking 12 km transects each day). IWS hired 16 biologists who began field work in late April, after three weeks of training. Field work had to be completed in May before the tortoises returned to their burrows to avoid the summer heat; desert tortoises are most active in the spring.

In addition to our participation in the USFWS monitoring program, we are also partnering with the National Park Service to study tortoises in Zion National Park.  Recently, Zion National Park expanded its boundaries to include land donated by the Bureau of Land Management. This area is connected to the part of Zion National Park where tortoises are found; however, there is no information on desert tortoises for this area. Our partnership will work to obtain information on tortoise population size, and to learn more about their habitat use and movement patterns.

 Accomplishing these objectives will provide a better understanding of what features are important for the continued existence of desert tortoises in and around Zion National Park.  
Photo by Beth Jackson, US Fish and Wildlife Service