Compounding effects of drought on long-term demography of a threatened reptile

Abstract

Assessing the status and trends of at-risk species requires long-term data gathered from populations throughout a species' range. Often, these data can help to identify processes that govern spatiotemporal variation in demography, which can guide conservation efforts and inform predictions about a species’ response to future environmental change. We evaluated 34 years of capture-recapture data from 17 populations of Sonoran desert tortoises (Gopherus morafkai) in Arizona. We characterized spatial and temporal variation in adult and juvenile survival, rates at which individuals transitioned from juveniles to adults, and rates of population change. Annual survival was high for adults (females: 0.96 [95% credible interval = 0.94–0.98]; males: 0.95 [0.93–0.97]) and lower for juveniles (0.77 [0.70–0.82]); transition rates were low (0.09 [0.06–0.13]). Adult survival was lower in populations near cities with 10,000 or more residents and transition rates were higher in more mesic areas. These sources of spatial variation were small, however, compared to the negative effects of drought on both juvenile and adult survival, particularly in arid regions. Although our findings suggest that populations in Arizona are currently stable or increasing, there is a high probability of future population declines if the severity and duration of droughts increase as expected.