In order to demonstrate that tortoise populations are recovering adequately, it is important to use the best available data from the best available survey methods to describe the long-term population trends throughout the Mojave and Colorado deserts. Line distance sampling has been adopted as the best technique by the group of federal, state, and local agency partners that are working toward recovery of the Mojave desert tortoise. Distance sampling generates a detection function based on the perpendicular distance of objects (tortoises) from the center of a transect. Since tortoises are cryptic and harder to see the farther they are from a person, the detection function provides a correction for the number of tortoises occurring per kilometer walked along the transect. Because tortoises may also be hidden in burrows or deep bushes, even when close to an observer, further correction is provided by estimating the proportion of tortoises that are visible during the monitoring period. Tortoises equipped with radio transmitters are monitored each day to assess the proportion of encounters when they would be deep in a burrow or thick vegetation and not visible. The encounter rate and the two correction factors provide an estimate of the overall population density within each monitoring area.
Long-term monitoring areas were selected because they are managed to recover the desert tortoise in the Mojave and Colorado deserts. These tortoise conservation areas are managed under designations such as critical habitat, Bureau of Land Management Areas of Critical Environmental Concern, National Park Service lands, or National Wildlife Refuges. Distance sampling and visibility data have been collected in every year since 2001, except in 2006, in California, Nevada, Arizona, and Utah. These data are used to develop annual density estimates, reported in publications of the USFWS Desert Tortoise Recovery Office and available in our Library.