The Northern Elephant Seal (Mirounga angustirostris) Rookery at Año Nuevo: A Case Study in Colonization

Our aim was to examine the colonization process in a large mammal, documenting the development of a breeding colony from its inception to reaching equilibrium numbers. We describe the development of a colony of northern elephant seals (Mirounga angustirostris) at Año Nuevo in central California from initial colonization in 1961 to 2010, a period when the worldwide population was expanding rapidly. We recorded pup production and pup mortality prior to weaning from censuses of animals by sex and age categories. Colony growth was rapid initially, and pup production reached approximately 2,700 births during the period 1995 to 2006. Pups born declined subsequently. The pattern of colony growth and decline is explained by species-specific seal behavior; population variables that influence immigration rates; and local environmental factors such as breeding space, animal density, and tidal and surf conditions at peak season that increase pup mortality. Colony growth was driven primarily by external recruitment of young females from large southern rookeries rather than internal recruitment. Births on the island segment of the colony, where breeding space was limited, peaked in 1980 then declined subsequently by 50% in association with increased density and pup mortality. Births on the adjacent mainland stabilized from 1995 to 2006, despite ample breeding space and low pup mortality; cessation of growth here was associated with reduced external recruitment of females. Primiparous females pioneer the establishment of new colonies, settling new sites to avoid low weaning success in crowded natal rookeries where they are dominated by older females. We conclude that the long-term study of the development of a single colony provides vital information on colony and population processes that have wide applicability to other mammals. The colonization of Año Nuevo recapitulated the process employed at other colonies during the recent growth and expansion of the population and signals the pattern to be expected in future colonies. Key Words: northern elephant seals, Mirounga angustirostris, colonization process, density-dependence, dispersion, immigration, recruitment.

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