Crossing to safety: dispersal, colonization and mate choice in evolutionarily distinct populations ofSteller sea lions,Eumetopias jubatus

Abstract

Population growth typically involves range expansion and establishment of new breeding sites, while the opposite occurs during declines. Although density dependence is widely invoked in theoretical studies of emigration and colonization in expanding populations, few empirical studies have documented the mechanisms. Still fewer have documented the direction and mechanisms of individual transfer in declining populations. Here, we screen large numbers of pups sampled on their natal rookeries for variation in mtDNA (n = 1106) and 16 microsatellite loci (n = 588) and show that new Steller sea lion breeding sites did not follow the typical paradigm and were instead colonized by sea lions from both a declining (Endangered) population and an increasing population. Dispersing individuals colonized rookeries in the distributional hiatus between two evolutionarily distinct (Φ¯(st) = 0.222, R¯(st) = 0.053, K = 2) metapopulations recently described as separate subspecies. Hardy-Weinberg, mixed-stock and relatedness analysis revealed levels of interbreeding on the new rookeries that exclude (i) assortative mating among eastern and western forms, and (ii) inbreeding avoidance as primary motivations for dispersal. Positive and negative density dependence is implicated in both cases of individual transfer. Migration distance limits, and conspecific attraction and performance likely influenced the sequence of rookery colonizations. This study demonstrates that resource limitation may trigger an exodus of breeding animals from declining populations, with substantial impacts on distribution and patterns of genetic variation. It also revealed that this event is rare because colonists dispersed across an evolutionary boundary, suggesting that the causative factors behind recent declines are unusual or of larger magnitude than normally occur.

Natal and breeding philopatry of female Steller sea lions in southeastern Alaska

“…The eastern stock (SEAK through California) has grown moderately during the same period, including the stabilization of numbers at the largest and previously northernmost rookery, Forrester Island Complex (F, ~4000 pups produced annually [31]), and the appearance of four new breeding rookeries to the north of F [32]. The two northernmost of these new rookeries were formed by immigration of breeding females from both the eastern and western stocks [28, 33]. …”

Short‐term survival of Steller sea lion (Eumetopias jubatus) pups: Investigating the effect of health status on survival

“…, O'Corry‐Crowe et al . ). That is, some SSL females, or one of their direct‐line female ancestors, immigrated from the WDPS range to breed in the EDPS range.…”

“…Using genetic samples from pups branded at GR, we determined mitochondrial DNA haplotypes and assigned maternal stock of origin as described by O'Corry‐Crowe et al . (, ).…”

A re-examination of the relationship between Steller sea lion (Eumetopias jubatus) diet and population trend using data from the Aleutian Islands

“…Samalga Pass is a major biogeographical boundary delineating the eastern boundary of the Aleutian marine ecosystem , which is less productive and diverse than that of the Gulf of Alaska to the east (Mueter and Norcross 2002). For Steller sea lions, Samalga Pass also separates "stocks" on the basis of mtDNA haplotype frequencies (O'Corry-Crowe et al 2014) and diets (Sinclair and Zeppelin 2002;Sinclair et al 2013). Within the Aleutian Islands, however, there is a strong longitudinal gradient in Steller sea lion population trends, with steep declines in and at-sea habitat (cross-hatched area; Fritz et al 2014) of the western distinct population segment (DPS) of Steller sea lion (Eumetopias jubatus) in Alaska and Russia.…”