Effective management of threatened and endangered species (TES) requires accurate assessment of vulnerability to environmental changes and of the adaptive capacity to weather those changes. The Mojave desert tortoise (Gopherus agassizii) provides an excellent system for studying population resilience to environmental change, as many populations have been monitored extensively across its range, allowing us to leverage a huge arsenal of demographic data across diverse environments. Beginning in the 1970s and continuing for decades, capture-mark-recapture (CMR) surveys were conducted repeatedly at dozens of 2.6 km2 ‘permanent study plots’. Although primarily used for estimating abundance trends, these permanent study plots represent an invaluable resource for estimating the environmental sensitivities of key tortoise demographic rates. We are currently leading an effort to re-analyze the permanent study plot data using modern spatial capture-recapture methods.
Results/Conclusions
Results from these re-analyses show that adult tortoise survival is sensitive to environmental gradients (e.g., vegetation cover and soil properties) but insensitive to all climate variables tested; however, adult recruitment appears to be more variable and sensitive to precipitation gradients. Furthermore, adult recruitment appears to be density dependent, which may reflect dispersal away from higher-density sites. These analyses represent a critical first step in our efforts to assess the climate resilience of Mojave Desert Tortoise populations and to identify critical habitat areas capable of supporting tortoise populations into the future.