Sea ice reduction drives genetic differentiation among Barents Sea polar bears.

Simo Njabulo Maduna,Øystein Wiig,Jon Aars,Eve M L Zeyl Fiskebeck,Snorre B Hagen,Andrew E Derocher,Dorothee Ehrich,Hans Geir Eiken,Cornelya F C Klütsch,Ida Fløystad,Magnus Andersen,Tommi Nyman,Lutz Bachmann

doi:10.1098/rspb.2021.1741

Abstract

Loss of Arctic sea ice owing to climate change is predicted to reduce both genetic diversity and gene flow in ice-dependent species, with potentially negative consequences for their long-term viability. Here, we tested for the population-genetic impacts of reduced sea ice cover on the polar bear (Ursus maritimus) sampled across two decades (1995–2016) from the Svalbard Archipelago, Norway, an area that is affected by rapid sea ice loss in the Arctic Barents Sea. We analysed genetic variation at 22 microsatellite loci for 626 polar bears from four sampling areas within the archipelago. Our results revealed a 3–10% loss of genetic diversity across the study period, accompanied by a near 200% increase in genetic differentiation across regions. These effects may best be explained by a decrease in gene flow caused by habitat fragmentation owing to the loss of sea ice coverage, resulting in increased inbreeding of local polar bears within the focal sampling areas in the Svalbard Archipelago. This study illustrates the importance of genetic monitoring for developing adaptive management strategies for polar bears and other ice-dependent species.

Highlights

Climate change is rapidly altering the structure, dynamics and functioning of ecosystems, leading to large-scale changes in the distribution, demography and phenology of species [1]
Population declines as well as contraction and fragmentation of geographical ranges have been documented in ice-dependent species such as the Adélie penguin (Pygoscelis adeliae) and chinstrap penguin (Pygoscelis antarcticus, [7,8]), Baltic ringed seal (Pusa hispida botnica, [9]) and polar bears (Ursus maritimus, [10,11,12,13])
Our analysis focused on the polar bears inhabiting the Svalbard Archipelago in the northwest Barents Sea, because there is long-term genetic monitoring data available for this subpopulation going back until 1995

Summary

Introduction

Climate change is rapidly altering the structure, dynamics and functioning of ecosystems, leading to large-scale changes in the distribution, demography and phenology of species [1]. 2015), the study covers at least two generations, in particular if 2 one takes into account that the individuals sampled in the early years of the interval were born up to 20 years earlier (year of birth ranged from 1975 to 2015) As both sea ice habitat and connectivity among areas used by pelagic and local bears are in a decline, our hypothesis was that ‘local’ bears would become increasingly isolated, and that this would be reflected as increasing differentiation among and decreasing genetic diversity within areas. To allow tests of a potential negative association between habitat fragmentation owing to loss of sea ice cover and genetic exchange among Svalbard polar bears, we estimated pairwise genetic differentiation indices ( pairwise fixation, G0S0T; and allelic differentiation, DEST) per sampling site and period (for a detailed description of parameters, see the electronic supplementary material, appendix S1). We inferred historic bottlenecks (up to 100 generations back) using coalescent analysis as implemented in the R-package DIYABCskylineplot v. 1.0.1 [43,44] (electronic supplementary material, appendix S1)

Results

Findings

Discussion