Abstract

Two tortoise species native to the American southwest have experienced significant habitat loss from development and are vulnerable to ongoing threats associated with continued development. Mojave desert tortoises Gopherus agassizii are listed as threatened under the US Endangered Species Act, and Sonoran desert tortoises G. morafkai are protected in Arizona (USA) and Mexico. Substantial habitat for both species occurs on multiple-use public lands, where development associated with traditional and renewable energy production, recreation, and other activities is likely to continue. Our goal was to quantify development to inform and evaluate actions implemented to protect and manage desert tortoise habitat. We quantified a landscape-level index of development across the Mojave and Sonoran desert tortoise ranges using models of potential habitat for each species (152485 total observations). We used 13 years of Mojave desert tortoise monitoring data (4732 observations) to inform the levels and spatial scales at which tortoises may be affected by development. Most (66-70%) desert tortoise habitat has some development within 1 km. Development levels on desert tortoise habitat are lower inside versus outside areas protected by actions at national, state, and local levels, suggesting that protection efforts may be having the desired effects and providing a needed baseline for future effectiveness evaluations. Of the relatively undeveloped desert tortoise habitat, 43% (74030 km2) occurs outside of existing protections. These lands are managed by multiple federal, state, and local entities and private landowners, and may provide opportunities for future land acquisition or protection, including as mitigation for energy development on public lands.

Highlights

  • Traditional and renewable energy development is a prominent fixture in deserts of the American southwest, with significant negative impacts on the conser-Endang Species Res 42: 167–184, 2020 al. 2018)

  • Mojave desert tortoises Gopherus agassizii were listed as threatened under the US Endangered Species Act in 1990, and Sonoran desert tortoises G. morafkai are protected in both Arizona (USA) and Mexico (BLM 2008, Arizona Game and Fish Department 2012, US Fish and Wildlife Service 2015b)

  • We identified 90 269 and 78 228 km2 of habitat for Mojave and Sonoran desert tortoises, respectively, by leveraging previously compiled maps of habitat for each species (Edwards et al 2015, Inman et al 2019) and expanding the boundaries of the most widely used distribution model for desert tortoises (Nussear et al 2009) to include all known habitat in Arizona

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Summary

Introduction

Traditional and renewable energy development is a prominent fixture in deserts of the American southwest, with significant negative impacts on the conser-Endang Species Res 42: 167–184, 2020 al. 2018). Development in the Mojave and Sonoran Deserts of the American southwest poses a significant threat to 2 desert tortoise species. Mojave desert tortoises Gopherus agassizii were listed as threatened under the US Endangered Species Act in 1990, and Sonoran desert tortoises G. morafkai are protected in both Arizona (USA) and Mexico (BLM 2008, Arizona Game and Fish Department 2012, US Fish and Wildlife Service 2015b). Both species have experienced significant habitat loss due to development, and continue to experience ongoing threats associated with development. Decreasing population trends of the Mojave desert tortoise in most locations (Esque et al 2010, Allison & McLuckie 2018) highlight the need for a more comprehensive evaluation of threats, including the increasing footprint of development, across the species’ ranges to inform listing, recovery, and management decisions

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