Quantifying dispersal between two colonies of northern elephant seals across 17 birth cohorts

Abstract

Dispersal allows the extinction-recolonization dynamics of metapopulations and is crucial to recolonization of former range in endangered species. Yet few studies quantify dispersal and even fewer examine consistency of dispersal over many years. The northern elephant seal (Mirounga angustirostris) provides an example of the importance of dispersal. It quickly recolonized its former range after near extirpation by 19th century hunting, and dispersal events restablishing its colonies were observed, but the rate of dispersal was never quantified. Here we provide 17 years of dispersal rates based on resightings of > 1000 permanently marked individuals observed from birth to breeding at two colonies 200 km apart in California. An average of 16.7% of the females born at the Piedras Blancas colony dispersed northward to breed at Ano Nuevo, while 7.3% of those born at Ano Nuevo dispersed southward to Piedras Blancas. There were no temporal trends in these rates. During the study, the population at Piedras Blancas expanded 15-fold, while Ano Nuevo abundance was stable. In the early years of the study, far more dispersal was directed away from the small colony toward the large, rejecting a prediction of density-driven dispersal. Consistently high natal dispersal in northern elephant seals means the population should be considered a single large unit in terms of response to environmental change. High dispersal was fortuitous to the past recovery of the species, and continued dispersal means elephant seals will likely continue to expand their range and colonize new breeding sites.