Genetic rescue remains underused for aiding recovery of federally listed vertebrates in the United States

Abstract

Restoring gene flow among fragmented populations is discussed as a potentially powerful management strategy that could reduce inbreeding depression and cause genetic rescue. Yet, examples of assisted migration for genetic rescue remain sparse in conservation, prompting several outspoken calls for its increased use in genetic management of fragmented populations. We set out to evaluate the extent to which this strategy is underused and to determine how many imperiled species would realistically stand to benefit from genetic rescue, focusing on federally threatened or endangered vertebrate species in the United States. We developed a "genetic rescue suitability index (GR index)" based on concerns about small population problems relative to risks associated with outbreeding depression and surveyed the literature for 222 species. We found that two-thirds of these species were good candidates for consideration of assisted migration for the purpose of genetic rescue according to our suitability index. Good candidate species spanned all taxonomic groups and geographic regions, though species with more missing data tended to score lower on the suitability index. While we do not recommend a prescriptive interpretation of our GR index, we used it here to establish that assisted migration for genetic rescue is an underused strategy. For example, we found in total, "genetic rescue" was only mentioned in 11 recovery plans and has only been implemented in 3 of the species we surveyed. A potential way forward for implementation of this strategy is incorporating genetic rescue as a priority in USFWS recovery documentation. In general, our results suggest that although not appropriate for all imperiled species, many more species stand to benefit from a conservation strategy of assisted migration for genetic rescue than those for which it has previously been considered or implemented.

Keywords: assisted migration; gene flow; Endangered Species List; conservation; translocation.

Fig. 1.

Distribution of GR index across endangered and threatened vertebrates listed by the U.S. Fish & Wildlife Service. A) GR index across all vertebrates surveyed. Numbers and lighter shading represent species that had explicit mention of genetic rescue in recovery documents. B) GR index across taxonomic groups. Silhouettes were downloaded from PhyloPic; desert tortoise silhouette ©Andrew A. Farke (https://creativecommons.org/licenses/by/3.0/legalcode).

Fig. 2.

Map of contiguous United States and Alaska divided into Lead Regions according to the U.S. Fish & Wildlife Service. The distribution of GR index across species in each Lead Region and a featured vertebrate from each Lead Region are shown. Hellbender photograph taken by Brian Gratwicke (https://creativecommons.org/licenses/by/2.0/deed.en).

Fig. 3.

Distribution of translocation status and GR index across evaluated species. A) Stacked bar graph with translocation status colored by whether genetic rescue was mentioned in recovery documents. B) Distribution of GR index according to translocation status.