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Endangered Species Research

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ESR 12:167-177 (2010)  -  DOI:

Contribution to the Theme Section 'Responses of animals to habitat alteration'

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

Todd C. Esque1,*, Ken E. Nussear1, K. Kristina Drake1, Andrew D. Walde2, Kristin H. Berry3, Roy C. Averill-Murray4, A. Peter Woodman5, William I. Boarman6, Phil A. Medica1, Jeremy Mack3,8, Jill S. Heaton7

1US Geological Survey, Western Ecological Research Center, 160 North Stephanie Street, Henderson, Nevada 89074, USA
2ITS - A QinetiQ North American Company, 8000 San Gregorio Rd., Atascadero, California 93422, USA
3US Geological Survey, Western Ecological Research Center, 22835 Calle San Juan de Los Lagos, Moreno Valley, California 92553, USA
4US Fish and Wildlife Service, Desert Tortoise Recovery Office, 1340 Financial Blvd, #234, Reno, Nevada 89502, USA
5Kiva Biological Consulting, PO Box 1210, Inyokern, California 93527, USA
6Conservation Science Research and Consulting, 2522 Ledgeview Place, Spring Valley, California 91977, USA
7Department of Geography, University of Nevada, Reno, 1664 N. Virginia Street, MS 154, Reno, Nevada 89557, USA
8Present address: Department of Zoology, Miami University, Oxford, Ohio 45056, USA

ABSTRACT: 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.

KEY WORDS: Gopherus agassizii · Coyote · Predation · Translocation · Mojave Desert · Prey

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Cite this article as: Esque TC, Nussear KE, Drake KK, Walde AD and others (2010) Effects of subsidized predators, resource variability, and human population density on desert tortoise populations in the Mojave Desert, USA. Endang Species Res 12:167-177.

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