Data from: Crossing to safety: Dispersal, colonization and mate choice in evolutionarily distinct populations of Steller sea lions, Eumetopias jubatus.

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=1,106) 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
(st = 0.222, 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 (1)
assortative mating among eastern and western forms, and (2) 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.