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The Desert Tortoise (Mojave Population) Recovery Plan (US Fish
and Wildlife Service, Portland, Oregon, 1994) requires monitoring of desert
tortoises to assess changes in status with the best available data.
Line distance sampling (Buckland et al., Introduction
to Distance Sampling, Oxford University Press, 2001) has been chosen as the
standard method for conducting range-wide monitoring of tortoises in the Mojave
Desert. Distance sampling generates a detection function based on the perpendicular distance of objects (tortoises) from the center
of a transect. The detection function provides
a correction for the number of tortoises occuring per kilometer walked along the transect. A 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 beacause they are managed to recover the desert tortoise in the Mojave and Colorado Deserts. These tortoise conservation areas are managed under designations such as Desert Wildlife Management Areas, USFWS critical habitat, and BLM Areas of Critical Environmental Concern, or they occur on national parklands or national wildlife refuges.
The following data were collected under the range-wide desert tortoise monitoring program 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. The data available for request include:
- spatial and non-spatial attributes of monitoring areas,
- spatial and non-spatial attributes of transects walked in each monitoring area,
- spatial and non-spatial data collected on live and dead tortoises encountered on
each transect
- characterization of focal tortoises with radio transmitters (GSub0), used to describe the proportion of tortoises visible for detection on transects.
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