Here, we test the efficacy of a mitigation translocation involving the Mojave desert tortoise (Gopherus agassizii), a species that is listed as threatened under the U.S. Endangered Species Act due to several threats, including loss and degradation of habitat due to human activities (e.g., urbanization, military training, mining, and energy production), subsidized increased predation subsidized by human development (i.
e., ravens, coyotes that expand their ranges by taking advantage of food and water associated with humans), and disease (United States Fish and Wildlife Service 1990). In recent years, translocation of tortoises has been implemented in several locations in the Mojave Desert to mitigate potentially harmful effects of some of these activities, particularly military training and renewable energy development (Drake et al. 2012; Nussear et al. 2012; Farnsworth et al. 2015; Nafus et al. 2017). However, studies documenting these translocation events only monitored individuals for 1-3 years, and there remains a need for a robust, multi-year characterization of drivers of tortoise survival following translocation.
Mojave desert tortoises can live upwards of ~50 years in the wild (Germano 1992; Curtin et al. 2009; Medica et al. 2012) and long-term studies on desert tortoises have detected substantial annual variation in survival (Zylstra et al. 2013; Lovich et al. 2014), mostly driven by drought severity. Therefore, long-term monitoring is essential for understanding the efficacy of translocation in this group of testudines.We captured, quarantined, and relocated Mojave desert tortoises during construction of the Ivanpah Solar Electric Generating System (ISEGS), a 370-megawatt-capacity solar energy project in the Ivanpah Valley of California’s Mojave Desert, near the Nevada border (Fig.
1). Translocated tortoises were captured close to the project boundary and released < 500 m from their original home range (individuals were radio-tracked prior to moving them to determine the approximate location of their original home range). Previous translocations found that desert tortoises are capable of navigating back to their original location following relocation (Hinderle et al. 2015); but in the case of this study, tortoise-proof fencing around the ISEGS prevented individuals from doing so. Also, paved roads were not present between the capture location and the release location, minimizing the potential for road-related mortality.
All translocated individuals were equipped with radio-transmitters, recaptured on a biannual basis for health assessments, and regularly monitored to detect movements and mortality events. For comparison, the same was done for two groups of tortoises that were not subject to translocation: a control group, comprising individuals with home ranges in comparable habitat within the Ivanpah Valley (< 20 km of the ISEGS site (; control areas in Fig. 1); and a resident group, comprising individuals with home ranges within the release area (where translocated tortoises were moved to).
Studying resident and control tortoises allowed us to isolate the effects, if any, of translocation from other potentially confounding variables (e.g., precipitation, soil and vegetation characteristics). Farnsworth et al. (2015) and Brand et al. (2016) documented short-term effects on the movement and thermal conditions of tortoises that were translocated as part of this study. Their findings indicated that in the initial two months of the first active (i.
e., non-hibernation) season post-translocation, translocated tortoises had larger home ranges, exhibited lower space use intensity (i.e., movement behavior that was less concentrated in a particular location), and experienced higher ambient temperatures than did resident and control tortoises.
However, space use and thermal conditions of translocated tortoises were indistinguishable from those of control and resident tortoises thereafter (Farnsworth et al. 2015; Brand et al. 2016). Based on these findings, we hypothesized that there may have been similar short-term effects on the survival of translocated tortoises, although previous studies did not analyze survival.
