Relative contribution of local demography and immigration in the recovery of a geographically-isolated population of the endangered Egyptian vulture

Abstract

In a context of increasing concern for biodiversity conservation but decreasing resources devoted to species management, measuring the impact of actions is critically needed so as to optimise conservation practices. Long-term longitudinal data collected on marked individuals allow for an assessment of the demographic response of a population to conservation actions while accounting for potential confounding factors. This study focused on the recovery of a geographically-isolated population of the endangered Egyptian vulture in south-eastern France, which experienced food shortages induced by major changes in habitat and sanitary legislation limiting the abandonment of dead animals in the field. The survival rates and population dynamics of the Egyptian vulture were computed, following the implementation of supplementary feeding stations, while accounting for the potential variation in environmental conditions on wintering grounds, reproduction and immigration. An integrated population model (IPM) was used to combine capture–resighting data with recoveries of dead birds, breeding surveys and counts of territorial pairs over a 16-year period. Results showed that the implementation of vulture restaurants improved the local survival rates and successfully stabilised the local demography of Egyptian vultures; this despite a reduction in average fecundity due to compensatory density feedback. Moreover, throughout the study period, about two thirds of the new recruits into the breeding population were immigrants, suggesting immigration constantly sustained the Egyptian vulture population in south-eastern France. However, we did not detect any effect of vulture restaurants on the immigration rate, indicating that the improvement of local survival by vulture restaurants remained the main driver of the positive shift in the population dynamics. This study highlights the positive impact of vulture restaurants on Egyptian vulture population restoration by compensating for natural food limitations. It also points out the methodological importance of estimating immigration for understanding population dynamics and that large-scale monitoring and internationally-concerted conservation should be promoted.