Factors affecting population assessments of desert tortoises

Desert tortoises have been a subject of controversy since their listing as threatened in 1990. With a wide geographic range and more living individual s than any other listed land animal, biologists have needed to detect popul ation trends against a "noisy" background of strong annual changes. We obta ined annual population estimates of desert tortoises over 6 consecutive yea rs at a 2.59-km(2) plot in Joshua Tree National Park, California. Our estim ates, based on weekly spring surveys varied substantially, particularly bet ween wet and dry years. Concurrently, we followed 10 radiotagged animals fo r 3 years to corroborate the surveys. Population density was determined sep arately for each year and for all years combined. Our best population estim ate 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 popula tion size were most strongly related to the animals' varying susceptibility to capture. In dry years, home ranges decreased, captures decreased, and e ffort required to find each tortoise nearly doubled. Our data confirm that tortoises are likely to be undercounted during dry years and call into ques tion earlier studies conducted during droughts.