No abstract

Desert tortoises have been a subject of controversy since their listing as threatened in 1990. With a wide geographic range and more living individuals than any other listed land animal, biologists have needed to detect population trends against a “noisy” background of strong annual changes. We obtained annual population estimates of desert tortoises over 6 consecutive years at a 2.59‐km2 plot in Joshua Tree National Park, California. Our estimates, based on weekly spring surveys, varied substantially, particularly between wet and dry years. Concurrently, we followed 10 radiotagged animals for 3 years to corroborate the surveys. Population density was determined separately for each year and for all years combined. Our best population estimate was an average of 67 adult tortoises, three times more than the density reported in a 1978 survey of the same site. Annual mortality was low ( <10%), and the animals showed extreme site fidelity. Apparent changes in population size were most strongly related to the animals' varying susceptibility to capture. In dry years, home ranges decreased, captures decreased, and effort required to find each tortoise nearly doubled. Our data confirm that tortoises are likely to be undercounted during dry years and call into question earlier studies conducted during droughts.

This paper describes an interdisciplinary study of the habitat requirements of threatened desert tortoises (Gopherus agassizii) on eight 225‐ha study plots in a 14 000 ha study area near the southern boundary of the U.S. Army's National Training Center at Fort Irwin in the central Mojave Desert of southern California. The objective of the study was to produce an empirical, statistical, GIS‐based model of desert tortoise habitat use based on a combination of field data and data derived from various spatial databases, including satellite imagery. A total of 11 primary and secondary data layers constitute the spatial database used for this project. Vegetation and tortoise relative density data were obtained from field surveys. Regression‐tree methods were used to develop the statistical model. The tree has 11 terminal nodes and a residual mean deviance of 1.985. Out of 73 potential predictors in the model specification, only eight were selected by the algorithm to be used in construction of the tree. The model suggests that tortoises tend to occur on southwest exposures and loamy soils, and that they avoid stony soils, north exposure, and areas of very low plant cover. Our results imply that soil composition and parent materials can be important determinants of habitat suitability for desert tortoises, and for burrowing animals in general. Our study also provides an example of how the interdisciplinary integration of biology, earth sciences, GIS, and statistical modeling can reveal important aspects of the habitat requirements of endangered species.