Science Inventory

Modeling Agassiz's Desert Tortoise Population Response to Anthropogenic Stressors


Tuma, M., C. Millington, N. Schumaker, AND P. Burnett. Modeling Agassiz's Desert Tortoise Population Response to Anthropogenic Stressors. JOURNAL OF WILDLIFE MANAGEMENT. Wildlife Society, Bethesda, MD, 80(3):414-429, (2016).


Plants and animals are essential components of the ecosystems the EPA is tasked with protecting. They are sensitive indicators of overall ecosystem health, they provide key ecosystem services, and many legally protected species are impacted by pesticide applications and other activities that EPA regulates. This manuscript describes how an EPA computer simulation model (HexSim) was used to study the impact on wildlife populations of multiple interacting natural and anthropogenic stressors. This study is of particular value to EPA because many threatened and endangered species that are impacted by pesticides are also subjected to a variety of other complex disturbances that interact in complex ways.


Mojave Desert tortoise (Gopherus agassizii) populations are exposed to a variety of anthropogenic threats, which vary in nature, severity, and frequency. Tortoise management in conservation areas can be compromised when the relative importance of these threats is not well understood. We used HexSim to develop simulation models for desert tortoise populations occupying two distinct study areas in the western-central (Superior Cronese) and the eastern (Gold Butte-Pakoon) Mojave Desert. The threats impacting tortoises vary by study area, and we conducted independent simulations of for each region. Tortoises in both regions were impacted by threats associated with human presence, and by subsidized predators. Additional threats in the Superior Cronese region included disease and habitat degradation on land in-holdings, whereas Gold Butte-Pakoon tortoises were further exposed to wildfire, livestock grazing, feral burros. We used our two study area-specific simulation models to rank the threats’ relative importance to desert tortoise population viability. We found that threats more uniformly distributed in time and space tended to have more severe impact on tortoise populations compared to threats that were unevenly distributed or temporally dynamic. Our threat prioritization will inform and improve ongoing management efforts attempting to increase desert tortoise population viability by altering anthropogenic disturbance regimes.

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Record Details:

Product Published Date: 02/29/2016
Record Last Revised: 11/27/2017
OMB Category: Other
Record ID: 315390