High genetic variability of vagrant polar bears illustrates importance of population connectivity in fragmented sea ice habitats

Abstract

Projections by the Intergovernmental Panel on Climate Change (IPCC) and sea ice forecasts suggest that Arctic sea ice will decline markedly in coming decades. Expected effects on the entire ecosystem include a contraction of suitable polar bear habitat into one or few refugia. Such large-scale habitat decline and fragmentation could lead to reduced genetic diversity. Here we compare genetic variability of four vagrant polar bears that reached Iceland with that in recognized subpopulations from across the range, examining 23 autosomal microsatellites, mitochondrial control region sequences and Y-chromosomal markers. The vagrants' genotypes grouped with different genetic clusters and showed similar genetic variability at autosomal microsatellites (expected heterozygosity, allelic richness, and individual heterozygosity) as individuals in recognized subpopulations. Each vagrant carried a different mitochondrial haplotype. A likely route for polar bears to reach Iceland is via Fram Strait, a major gateway for the physical exportation of sea ice from the Arctic basin. Vagrant polar bears on Iceland likely originated from more than one recognized subpopulation, and may have been caught in sea ice export during long-distance movements to the East Greenland area. Although their potentially diverse geographic origins might suggest that these vagrants encompass much higher genetic variability than vagrants or dispersers in other regions, the four Icelandic vagrants encompassed similar genetic variability as any four randomly picked individuals from a single subpopulation or from the entire sample. We suggest that this is a consequence of the low overall genetic variability and weak range-wide genetic structuring of polar bears -few dispersers can represent a large portion of the species' gene pool. As predicted by theory and our demographic simulations, continued gene flow will be necessary to counteract loss of genetic variability in increasingly fragmented Arctic habitats. Similar considerations will be important in the management of other taxa that utilize sea ice habitats.

The relevance of genetic structure in ecotype designation and conservation management

“…For some species, habitat suitability models from parts of their range are thus not reliably transferrable to other regions, with important consequences for management (Bain et al, 2015 ; Denryter et al, 2017 ). Ecological knowledge of the species under study can provide perspectives on whether observed animal movements are common or atypical; for example, regarding sex‐biased dispersal (Støen et al, 2006 ), and the occurrence of seasonal movements in species that are usually nonmigratory (Musiani et al, 2007 ) or exhibit vagrant behaviour (Kutschera et al, 2016 ). Ecological data can also be combined with information on the spatial distribution of genetic variation as indicators of local adaptations (Fitzpatrick et al, 2015 ) or responses to ongoing environmental change (Koen et al, 2014 ; Kutschera et al, 2016 ).…”

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

“…Future sea-ice loss is expected to fragment polar bear subpopulations, increase isolation, alter gene flow, and disrupt population boundaries (Derocher, Lunn, & Stirling, 2004;Kutschera et al, 2016;Sahanatian & Derocher, 2012). Genetic variation in polar bears is relatively low, and genetics is a relatively insensitive means of identifying, defining, or detecting change in subpopulation structure, especially when compared to individual-based telemetry and tag recoveries.…”

Evolutionary History of Polar and Brown Bears

“…Likely having persisted for several hundred thousand years, polar bears as a species have experienced several glacial cycles. However, the polar bear genome carries testimony of past population bottlenecks that were presumably linked to environmental changes such as interglacial warm phases -indicative of vulnerability to climatic changes (Durner et al 2009;Peacock et al 2015;Kutschera et al 2016). In addition, although polar bears have persisted through previous warm phases, multiple current human-mediated stressors (e.g., habitat conversion, persecution, and accumulation of toxic substances in the food chain) are magnifying the impacts of current climate change.…”