Implications of the Circumpolar Genetic Structure of Polar Bears for Their Conservation in a Rapidly Warming Arctic

Abstract

We provide an expansive analysis of polar bear (Ursus maritimus) circumpolar genetic variation during the last two decades of decline in their sea-ice habitat. We sought to evaluate whether their genetic diversity and structure have changed over this period of habitat decline, how their current genetic patterns compare with past patterns, and how genetic demography changed with ancient fluctuations in climate. Characterizing their circumpolar genetic structure using microsatellite data, we defined four clusters that largely correspond to current ecological and oceanographic factors: Eastern Polar Basin, Western Polar Basin, Canadian Archipelago and Southern Canada. We document evidence for recent (ca. last 1–3 generations) directional gene flow from Southern Canada and the Eastern Polar Basin towards the Canadian Archipelago, an area hypothesized to be a future refugium for polar bears as climate-induced habitat decline continues. Our data provide empirical evidence in support of this hypothesis. The direction of current gene flow differs from earlier patterns of gene flow in the Holocene. From analyses of mitochondrial DNA, the Canadian Archipelago cluster and the Barents Sea subpopulation within the Eastern Polar Basin cluster did not show signals of population expansion, suggesting these areas may have served also as past interglacial refugia. Mismatch analyses of mitochondrial DNA data from polar and the paraphyletic brown bear (U. arctos) uncovered offset signals in timing of population expansion between the two species, that are attributed to differential demographic responses to past climate cycling. Mitogenomic structure of polar bears was shallow and developed recently, in contrast to the multiple clades of brown bears. We found no genetic signatures of recent hybridization between the species in our large, circumpolar sample, suggesting that recently observed hybrids represent localized events. Documenting changes in subpopulation connectivity will allow polar nations to proactively adjust conservation actions to continuing decline in sea-ice habitat.

Range contraction and increasing isolation of a polar bear subpopulation in an era of sea‐ice loss

“…Previous analysis of polar bear population structure using microsatellites showed no significant genetic differences between BB and neighboring KB (Paetkau et al., 1999), though studies have found that bears from BB‐KB differed genetically from the LS and DS subpopulations (Paetkau et al., 1999; Peacock et al., 2015; Malenfant et al., 2016). In contrast, we found low but significant F ST estimates between winter–spring samples from BB and KB.…”

“…While these samples were collected and analyzed primarily for a genetic mark–recapture (MR) assessment (SWG, 2016), the eight polymorphic microsatellites selected were used in population genetic analyses with the objective of assessing differentiation between BB and neighboring subpopulations. As the samples were recent and temporally congruent, these analyses provided an update to past population genetic studies (Paetkau et al., 1999; Peacock et al., 2015; Malenfant, Davis, Cullingham, & Coltman, 2016) that used samples from the 1990s. We used standardized population genetic analytical tools and methods to investigate genetic connectivity between subpopulations (ADEGENET package, Jombart, 2008; ARLEQUIN Version 3.5.1, Excoffier & Lischer, 2010; BA3‐3.0.3, Wilson & Rannala, 2003; DAPC, Jombart, Devillard, & Balloux, 2010; FSTAT, Goudet, 1995; GENECLASS2, Piry et al., 2004; GENELAND, Guillot, Mortier, & Estoup, 2005; Guillot, 2008; STRUCTURE, Pritchard, Stephens, & Donnelly, 2000).…”

Assessing polar bear (Ursus maritimus) population structure in the Hudson Bay region using SNPs

“…We used blood and skin samples from FB, SH, and WH polar bears as well as from the adjacent Davis Strait subpopulation (DS) because of its intermediate genetic relationship to the Hudson Bay region and the Canadian Archipelago (Malenfant et al., 2016; Obbard et al., 2010; Paetkau et al., 1999; Peacock et al., 2015). Samples were collected between 2004 and 2010 for SH ( n  = 112) and WH ( n  = 120) from capture–recapture studies conducted by the Ontario Ministry of Natural Resources and Forestry and Environment and Climate Change Canada, respectively.…”

“…Global‐scale studies generally supported the currently designated 19 subpopulations used for management purposes (Obbard, Thiemann, Peacock, & DeBruyn, 2010; Paetkau et al., 1999; Peacock et al., 2015), whereas regional studies show differentiation between adjacent subpopulations (e.g., Campagna et al., 2013). Although the first global‐scale studies defined the Hudson Bay region as a single unique genetic cluster (Paetkau et al., 1999), this study did not include samples from SH, and more recent fine‐scale studies that included samples from SH (Crompton et al., 2008, 2014; Malenfant et al., 2016; Peacock et al., 2015) identified substructure within Hudson Bay, specifically, a unique population in James Bay.…”

“…Previous studies of polar bear population genetics found genetic differentiation at both the global scale (Malenfant, Davis, Cullingham & Coltman, 2016; Paetkau et al., 1999; Peacock et al., 2015) and regional scale (Campagna et al., 2013; Crompton, Obbard, Petersen, & Wilson, 2008, 2014). Global‐scale studies generally supported the currently designated 19 subpopulations used for management purposes (Obbard, Thiemann, Peacock, & DeBruyn, 2010; Paetkau et al., 1999; Peacock et al., 2015), whereas regional studies show differentiation between adjacent subpopulations (e.g., Campagna et al., 2013).…”