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Effects of subsidized predators, resource variability, and human population density on desert tortoise populations in the Mojave Desert, USA

January 1, 2010

Understanding predator–prey relationships can be pivotal in the conservation of species. For 2 decades, desert tortoise Gopherus agassizii populations have declined, yet quantitative evidence regarding the causes of declines is scarce. In 2005, Ft. Irwin National Training Center, California, USA, implemented a translocation project including 2 yr of baseline monitoring of desert tortoises. Unusually high predation on tortoises was observed after translocation occurred. We conducted a retrospective analysis of predation and found that translocation did not affect the probability of predation: translocated, resident, and control tortoises all had similar levels of predation. However, predation rates were higher near human population concentrations, at lower elevation sites, and for smaller tortoises and females. Furthermore, high mortality rates were not limited to the National Training Center. In 2008, elevated mortality (as high as 43%) occurred throughout the listed range of the desert tortoise. Although no temporal prey base data are available for analysis from any of the study sites, we hypothesize that low population levels of typical coyote Canis latrans prey (i.e. jackrabbits Lepus californicus and other small animals) due to drought conditions influenced high predation rates in previous years. Predation may have been exacerbated in areas with high levels of subsidized predators. Many historical reports of increased predation, and our observation of a range-wide pattern, may indicate that high predation rates are more common than generally considered and may impact recovery of the desert tortoise throughout its range.

Citation Information

Publication Year 2010
Title Effects of subsidized predators, resource variability, and human population density on desert tortoise populations in the Mojave Desert, USA
DOI 10.3354/esr00298
Authors Todd C. Esque, Kenneth E. Nussear, K. Kristina Drake, Andrew D. Walde, Kristin H. Berry, Roy C. Averill-Murray, A. Peter Woodman, William I. Boarman, Phil A. Medica, Jeremy S. Mack, Jill S. Heaton
Publication Type Article
Publication Subtype Journal Article
Series Title Endangered Species Research
Series Number
Index ID 70176772
Record Source USGS Publications Warehouse
USGS Organization Western Ecological Research Center

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