Recruitment of desert tortoises (Gopherus agassizii and G. morafkai): A synthesis of reproduction and first-year survival

Recruitment is integral to population persistence, therefore characterizing this process is essential for evaluating recovery actions for species in decline. We gathered all data available and used Bayesian analyses to quantify annual recruitment of Mojave Desert (Gopherus agassizii) and Sonoran Desert (G. morafkai) tortoises as the product of four components: proportion of females that reproduced, number of eggs produced per reproducing female, hatching success, and hatchling survival. For Mojave Desert Tortoises, the estimated proportion of females that reproduced (0.81 [95% Confidence Interval: 0.52–0.99]) and number of eggs produced per year (6.90 [5.51–8.16]) were higher than for Sonoran Desert Tortoises (0.52 [0.07–0.94] and 5.17 [3.05–7.60], respectively). For Mojave Desert Tortoises, hatching success averaged 0.61 (0.25–0.90). Data on hatching success for Sonoran Desert Tortoises and hatchling survival for both species were sparse, therefore we represented these components with a range of plausible values. When we combined components, average recruitment for Mojave Desert Tortoises ranged from 0.51 females/female/y assuming that hatchling survival was 0.30 to 1.18 females/female/y with hatchling survival assumed to be 0.70. For Sonoran Desert Tortoises, average recruitment ranged from 0.25 to 0.57 females/female/y for the same values of hatchling survival. Differences in recruitment between species likely reflect the evolution of different life-history strategies for tortoises in Mojave and Sonoran Deserts, perhaps in response to variation in precipitation regimes. To better inform conservation and recovery of desert tortoises, more information is needed for all recruitment components, but especially for hatching success and hatchling survival.