Density dependence, climate and fires determine population fluctuations of the spur‐thighed tortoise Testudo graeca

Abstract

AbstractPopulation fluctuations are driven by a combination of multiple factors, which can be classified into endogenous and exogenous. Endogenous factors are directly related to density‐dependent forces, whereas exogenous factors are environmental drivers (e.g. climate). Several studies have reported the effects of endogenous and exogenous factors using exponential population models in mammals, birds or insects, but few works have explored the effects on reptiles, specifically in tortoises. In this study, we developed logistic population growth models to decipher the role of these factors on the population dynamics of the spur‐thighed tortoise Testudo graeca. The role of environmental drivers in the distribution, movement, survival, reproduction or individual growth has been described for this species in previous works, but no studies have examined the effects of both endogenous and exogenous factors in population growth rates. Using long‐term data of T. graeca in southeastern Iberian Peninsula (1999–2013), we fitted non‐linear logistic models with endogenous and exogenous effects. The results showed that endogenous processes are important drivers of the population dynamics of this species, and that exogenous factors, such as freezing hours in winter, also directly affect the population growth rate. Intolerance of extreme winter temperatures may especially affect hatchlings and juvenile tortoises. Our models also showed that perturbations (i.e. a fire occurred during the monitoring period) have a lasting impact by reducing the carrying capacity of the population. This study shows that simple quantitative approaches based on population dynamic theory are useful for deciphering the ecological mechanisms underlying dynamics of tortoise populations.