Joint effects of inverse density-dependence and extreme environmental variation on the viability of a social bird species

Abstract

Environmental stochasticity and density-dependence can have critical synergetic effects on population dynamics, especially when dealing with threatened species. In this study, we examine the effects of these interactions on the dynamics of a vulnerable bird species, the Dalmatian pelican (Pelicanus crispus). We conducted demographic analysis and population dynamics modelling of the Amvrakikos pelican population (western Greece) based on a 20-y dataset. Results indicated that annual juvenile survival probability is low (mean = 0.65) and varies according to both negative (regulation) and positive (Allee effect) density-dependent processes. In contrast, adult survival is relatively high (mean = 0.95) and less variable. Deterministic and stochastic population dynamics models based on means, inter-annual variances, and covariances between vital rates revealed that the population is increasing deterministically. Simulations of the extinction risk for the study population and also for various population sizes revealed that short-term extinction risk remains extremely low, even for very small populations of the species. However, when we considered the possibility of rare catastrophic events and their interactions with density-dependence patterns, the projected extinction risk increased dramatically, especially for small populations. Given that many European Dalmatian pelican populations are small, most of them could be at risk for local extinction. Our results illustrate the critical and general importance of considering interactions between all potential factors of extinction in population viability assessments.