The Dynamics of a Colonization Event in the European Shag: The Roles of Immigration and Demographic Stochasticity

Abstract

-We developed both deterministic and stochastic (through Monte Carlo simulations) matrix projection models to account for the relative roles of local recruitment and immigration in the formation and growth of a new colony of the European Shag (Phalacrocorax aristotelis) in the western Mediterranean (Columbretes Islands). The best fit of observed and simulated growth curves of the colony was obtained by introducing two immigration waves and taking into account demographic stochasticity. Immigration and local recruitment of the founder population alone cannot explain the observed growth of the colony. Immigrants might come from a large colony in Majorca Island, approximately 160 km. away, that declined markedly at the same time that the Columbretes colony was established. According to a sensitivity and elasticity analysis, changes in ?4 year old adult survival have the largest impact on the population growth rate (X). Received 31 March 2000, accepted 1OJune 2000.