Federally ‘threatened’ for over three decades and recently uplisted to ‘endangered’ in California, the Mojave desert tortoise (Gopherus agassizii) continues to experience precipitous declines across its range. Multiple potential stressors on tortoise persistence and recovery have been identified through dozens of field studies over the years; however, limited funding for monitoring has resulted in many short-term studies that focus on the status of small populations.
Last month, CSP researchers joined scientists, land managers and other conservation professionals at The Desert Tortoise Council 50th Annual Symposium held in Las Vegas, Nevada. This four-day program highlighted a diversity of ongoing Mojave desert tortoise conservation efforts, including head starting and translocation projects, new monitoring and modeling methodologies, and habitat management objectives. In addition to learning about the important work happening across the Mojave Desert by various agencies and non-governmental organizations, the CSP team was able to share updates on current research efforts designed to fill critical knowledge gaps concerning survival of this imperiled species across its range.
Madeline Standen, a scientist in CSP’s T&E Lab, detailed the novel integrated modeling approach for assessment of Mojave desert tortoise survival developed by CSP staff as part of ongoing collaborations with the BLM, USGS, USFWS, DoD, NPS, UDWR, and University of Nevada Reno. This modeling framework integrates multiple types of information (e.g., GPS location data from individuals and tortoise capture-recapture data from study plots) to estimate survival probability and quantify additive and interactive effects of stressors on tortoise survival (Fig. 1). Through information sharing, this model is able to utilize data from dozens of field studies from across the range to generate improved estimates of tortoise survival across the entire range of the species in California, Nevada, Arizona and Utah over five decades (1970s–2020s).
Fig. 1. A component of the modeling framework that integrates multiple types of location data to estimate survival probability and quantify the additive and interactive effects of environmental stressors on tortoise survival over time and across the range of the species.
CSP scientists are currently using this modeling pipeline to quantify stressor impacts (e.g., solar development, subsidized predators, off-highway vehicle routes, drought) and to assess spatial and temporal patterns in population responses. Research outputs from this current work, and with support from the California office of the BLM and the DOD SERDP program, will provide important insight concerning Mojave desert tortoise response to multiple, interacting stressors and benefit land managers’ ability to design and implement effective conservation measures.