Long-term consistency of dispersal between two colonies of northern elephant seals

Dispersal drives extinction-recolonization dynamics of metapopulations and
is necessary for endangered species to recolonize former ranges. 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 full range after near extirpation by 19th-century hunting, and though
dispersal was observed it was not quantified. Here we enumerate lifetime
dispersal events among female pups given permanent marks during 1994–2010
at two colonies, then correct for detection biases in estimating
bidirectional dispersal rates. An average of 16% of females born at the
Piedras Blancas colony dispersed northward 200 km to breed at Año Nuevo,
while 8.0% of those born at Año Nuevo dispersed southward to Piedras
Blancas. The northward rate fluctuated considerably but was higher than
southward in 15 of 17 cohorts. The population at Piedras Blancas expanded
15-fold during the study, while Año Nuevo's changed only slightly,
but the expectation that seals would emigrate away from high-density
colonies was not supported. During the 1990s, dispersal was higher away
from the small colony toward the large. Moreover, cohorts born later at
Piedras Blancas, when the colony had grown, dispersed no more than early
cohorts. Consistently high natal dispersal in northern elephant seals
means the population must 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 expand their range further.