Hidden Threats to Persistence: Changes in Population Structure Can Affect Well-Preserved Spur-Thighed Tortoise Populations

Abstract

[eng] Population structure and survival are key components of wildlife management. Long-term monitoring of long-lived species, particularly those with indeterminate growth, is crucial when studying demographic processes. Here, we examined a population of Spurthighed Tortoises, Testudo graeca, over a 7-yr period (17% of its life span), including changes in population structure, causes of mortality, and growth patterns. We found a change in population structure, as evidenced by lower young adult density (both males and females) and a more female-biased population compared to the start of the study. Juvenile mortality was high, and the main cause was predation by common ravens. For adults, mortality was relatively low and was mostly observed in winter or due to anthropogenic reasons (forestry or road mortality). We also modeled adult size-dependent survival and juvenile threshold survival (minimum number of juveniles needed to reach the adult stage to maintain population viability) using a Bayesian framework and matrix projection models, respectively. Adult survival was high (0.97), but with variation between the sexes. Female survival was not size dependent, but male survival decreased when size exceeded 150 mm carapace length. In this population, longer female life spans and climate change effects seemed to be the most likely reasons for our female-biased population. This study particularly pinpoints the importance of high survival in older females, which contributes to species credit, and stresses the negative potential of low juvenile and male densities in the population. Indeed, the annual juvenile threshold survival range was estimated between 0.32 and 0.49, not accounting for the predation exerted by common ravens in subadults. Therefore, if predation reduces juvenile survival rates below this threshold, population viability can be affected in the future. The study contributes to this species’ conservation by anticipating time-lagged demographic responses based on current climate trends (less annual rainfall and more days over 408C) and predation.