Density-dependent age of first reproduction as a buffer affecting persistence of small populations

Abstract

We documented a density-dependent age of first reproduction using a population of Spanish Imperial Eagles (Aquila adalberti) in Doñana National Park (southwestern Spain). Then, a stochastic individual-based simulation was designed, to analyze the effect of variations in the age of first reproduction on the fluctuation of small populations. This is a discrete-time simulation model that uses the age structure, mortality rates, reproductive rate, and density-dependent relationships from empirical studies. Age of breeding in the real population was positively related to population density. The probability of persistence of simulated populations was affected by both breeding population ceiling and age of first breeding. Our simulations were compared to the historical evolution of the Doñana population, assuming an age of first reproduction of three years. Density-dependent variation in age of first breeding increased the mean time of persistence, allowing the population to stay at carrying capacity longer. This buffer mechanism could be crucial for the persistence of small populations.