Abstract
Conservation genetic techniques and considerations of the evolutionary potential of a species are increasingly being applied to species conservation. For example, effective population size (N e) estimates are useful for determining the conservation status of species, yet accurate estimates of current N e 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 N e. 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 N e 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. In particular, estimates of N e 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 N e 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.
Highlights
The benefits of using genetic analyses to inform species conservation and recovery are well known (Allendorf, Luikart, & Aitken, 2012; Frankham, 2010); the application of molecular techniques and the implementation of genetic considerations into recovery plans remains limited (Frankham, 2010; Frankham et al, 2017; Pertoldi, Bijlsma, & Loeschcke, 2007; Ralls et al, 2017)
Our findings provide insight into the consistency of genetic estimators of Ne, the importance of direct monitoring of genetic diversity, and unanswered questions concerning southern sea otters
We found little evidence for population genetic structure, and sea otter genetic diversity remained consistent throughout the study period, despite a large increase in sea otter population size
Summary
The benefits of using genetic analyses to inform species conservation and recovery are well known (Allendorf, Luikart, & Aitken, 2012; Frankham, 2010); the application of molecular techniques and the implementation of genetic considerations into recovery plans remains limited (Frankham, 2010; Frankham et al, 2017; Pertoldi, Bijlsma, & Loeschcke, 2007; Ralls et al, 2017). The southern sea otter is one of the few populations for which the recovery plan considers Ne. The US Fish and Wildlife Service set a delisting criterion of 3,090 otters (as measured by an index consisting of the 3-year running average of annual range-wide censuses), a number which was thought to be sufficient to maintain an Ne of 500 even if large losses were to occur from a major oil spill (Ralls, Demaster, et al, 1996; US Fish and Wildlife Service, 2003). The US Fish and Wildlife Service set a delisting criterion of 3,090 otters (as measured by an index consisting of the 3-year running average of annual range-wide censuses), a number which was thought to be sufficient to maintain an Ne of 500 even if large losses were to occur from a major oil spill (Ralls, Demaster, et al, 1996; US Fish and Wildlife Service, 2003) We obtained this criterion using a multiplier of 3.7 to convert Ne to census size (N), so that an Ne of 500 corresponds to 1,850 individuals (Ralls, Demaster, et al, 1996). The comparison between genetic and demographic Ne estimates will guide a diverse group of stakeholders including conservation biologists, wildlife managers, and policymakers, to the potential difficulties of each method, the utility of conducting multiple independent analyses, and the issues with obtaining estimates across large geographic areas
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