Effects of sea ice decline and summer land use on polar bear home range size in the Beaufort Sea

Animals responding to habitat loss and fragmentation may increase their home ranges to offset declines in localized resources or they may decrease their home ranges and switch to alternative resources. In many regions of the Arctic, polar bears (Ursus maritimus) exhibit some of the largest home ranges of any quadrupedal mammal. Polar bears are presently experiencing a rapid decline in Arctic sea ice extent and a change in sea ice composition. For the Southern Beaufort Sea subpopulation of polar bears, this has resulted in a divergent movement pattern where most of the subpopulation remains on the sea ice in the summer melt season while the remainder move to land. We evaluated the effects of summer land use and maternal denning on the annual and seasonal utilization distribution size (i.e., home range) of adult female polar bears in the Southern Beaufort Sea subpopulation over 30 yr (1986-2016) during a period of rapid sea ice decline. For bears that remained on the summer sea ice, model-derived mean annual utilization distributions were 64% larger in 1999-2016 (x = 176,000 km(2)) relative to 1986-1998 (x = 107,000 km(2)). This increase was primarily driven by increases in summer utilization distributions that encompassed increased amounts of open water and decreased amounts of preferred sea ice. The mean centroid of summer utilization distributions for bears that remained on the sea ice was 193 km further north-northeast in 1999-2016. In contrast, bears that summered on land during 1999-2016 exhibited 88% smaller mean annual utilization distribution sizes (x = 22,000 km(2)) relative to bears that remained on the summer sea ice during the same period. Our findings highlight the impacts of sea ice declines on polar bear space use and the increasing importance of land as an alternative summer refuge.

Automated summary

As a response to declining Arctic sea ice, polar bears are adjusting their home ranges, with those staying on the sea ice in summer experiencing larger utilization distributions, incorporating more open water and less sea ice, while those moving to land have smaller utilization distributions. This shift highlights the impact of sea ice declines on polar bear space use, emphasizing the increasing importance of land as an alternative summer habitat.