Severe mortality of a population of threatened Agassiz’s desert tortoises: the American badger as a potential predator

Pg Emblidge,Ad Walde,Cm Aiello,Tc Esque,Ke Nussear

doi:10.3354/esr00680

Abstract

In the Mojave Desert of the southwestern United States, adult Agassiz's desert tor- toises Gopherus agassizii typically experience high survival, but population declines associated with anthropogenic impacts led to their listing as a threatened Species under the US Endangered Species Act in 1990. Predation of adult tortoises is not often considered a significant threat as they are adapted to deter most predation attempts. Despite these adaptations, some populations have experienced elevated mortality attributed to predators, suggesting that predation pressure may occasionally increase. During the tortoise activity seasons of 2012 and 2013, we observed unsus- tainably high mortality in 1 of 4 populations of adult desert tortoises (22 and 84%, respectively) in the western Mojave Desert in the vicinity of Barstow, CA. Photographic evidence from trail cam- eras and examination of carcass condition suggest that American badgers Taxidea taxus —a sometimes cited but unconfirmed predator of adult tortoises — may have been responsible for some of the mortality observed. We discuss the American badger as a plausible predator of a local tortoise population, but recommend further investigation into these events and the impacts such mortality can have on tortoise persistence.

Highlights

Ensuring that populations of threatened and endangered species maintain high survivorship is of critical concern for wildlife managers
Agassiz’s desert tortoise Gopherus agassizii is a late-maturing, long-lived species protected under the US Endangered Species Act across its range in the Mojave Desert of the southwestern United States (USFWS 1994)
Emblidge et al.: Desert tortoise mortality (Fig. 1). This high mortality site was located about 55 km east-northeast of Barstow, on the southeast corner of the National Training Center (NTC) and adjacent lands administered by the Bureau of Land Management within the US Department of the Interior

Summary

Introduction

Ensuring that populations of threatened and endangered species maintain high survivorship is of critical concern for wildlife managers. Agassiz’s desert tortoise Gopherus agassizii is a late-maturing, long-lived species protected under the US Endangered Species Act across its range in the Mojave Desert of the southwestern United States (USFWS 1994). Endang Species Res 28: 109–116, 2015 ogy, behavior, and burrow use (Woodbury & Hardy 1948). Burrows are important features of the desert tortoises’ environment, providing shelter from extreme temperatures, aridity, and predators, and act as sites of conspecific interaction and nesting (Woodbury & Hardy 1948, Turner et al 1986, Bulova 1994, Rostal et al 1994). Tortoises spend the majority of the year inactive in burrows or dens and are surface active primarily in the spring and late summer when forage and occasional water are available and high ambient temperatures do not preclude activity (Nagy & Medica 1986). Tortoises occupy multiple burrows throughout the year and a single burrow may be used by multiple tortoises, either synchronously or asynchronously (Bulova 1994)

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Conclusion