Data from: Measures of effective population size in sea otters reveal special considerations for wide-ranging species

Conservation genetic techniques and considerations of the evolutionary
potential of a species are increasingly being applied to species
conservation. For example, effective population size (Ne) estimates are
useful for determining the conservation status of species, yet accurate
estimates of current Ne remain difficult to obtain. The effective
population size can contribute to setting federal delisting criteria, as
was done for the southern sea otter (Enhydra lutris nereis). After being
hunted to near extinction during the North Pacific fur trade, the southern
sea otter has recovered over part of its former range, but remains at
relatively low numbers, making it desirable to obtain accurate and
consistent estimates of Ne. Although theoretical papers have compared the
validity of several methods, comparisons of estimators using empirical
data in applied conservation settings are limited. We combined thirteen
years of demographic and genetic data from 1,006 sea otters to assess
multiple Ne estimators, as well as temporal trends in genetic diversity
and population genetic structure. Genetic diversity was low and did not
increase over time. There was no evidence for distinct genetic units, but
some evidence for genetic isolation by distance. Notably, estimates of Ne
based on demographic data were much larger than genetic estimates when
computed for the entire range of the population, but were similar at
smaller spatial scales. The discrepancy between estimates at large spatial
scales could be driven by cryptic population structure and/or individual
differences in reproductive success. We recommend the development of new
delisting criteria for the southern sea otter. We advise the use of
multiple estimates of Ne for other wide-ranging species, species with
overlapping generations, or with sex biased dispersal, as well as the
development of improved metrics of genetic assessments of populations.