Scientists track how polar bears in the Chukchi Sea are responding as their sea ice melts away.
For polar bears on the hunt for food, sea ice is a vital resource. Whether waiting for a seal to come up for air at a breathing hole, or snagging a seal hauled out on the ice before it slips back into the water, sea ice offers polar bears access to their prey. But with rising temperatures melting away Arctic sea ice, these hunting grounds are rapidly disappearing.
Some scientists are optimistic that ever-resourceful polar bears will adapt to the shifting ice sheets by changing up their usual land use patterns to boost hunting success. The polar bears living in the Chukchi Sea in the Arctic, for instance, do not appear to be suffering from retreating sea ice in their habitat, while neighboring polar bear groups have been hit hard.
“The adjacent population [in the Southern Beaufort Sea] has shown reduced reproductive rates and body size compared to the Chukchi Sea subpopulation,” Ryan Wilson, of the US Fish and Wildlife Service in Anchorage, Alaska, explains to The Science Explorer.
Wilson and his research team wanted to know if the Chukchi Sea bears were, in fact, finding new seal hunting sites in response to sea ice loss, so they retrieved data transmitted via GPS collars fitted to several of the bears to track how their site selection patterns had changed between two key periods: one when the sea ice was still plentiful (1986-1994), and the other after it had become scarce (2008-2013).
By looking at the space available for the polar bears to use, and comparing this to where the bears actually spent time, the researchers quantified how the bears were making their decisions on space use. For example, “If 90 percent of ice was over deep water, and 10 percent over shallow water, but 90 percent of bear locations occurred over shallow water, that would be an indication of selection for shallower water because their use of shallow water was much greater than its availability,” says Wilson.
Over the study period, the sea ice cover for the Chukchi Sea polar bears had indeed shrunk, but their habitat was only reduced in the warmer summer months, indicating “summer sea ice availability is likely to be a significant limiting factor to polar bear populations in the coming decades.”
Meanwhile, the data revealed that the Chukchi Sea bears were not changing their site selection patterns, despite the drastic cutback of summer sea ice. Rather, they tended to stick to their usual seal hunting spots, even as those became fewer and fewer.
“I think our results suggest that bears are not going to just adapt to continued loss of sea ice, as some have suggested,” says Wilson, who recently published the findings in the Proceedings of the Royal Society B.
So, how have the Chukchi Sea bears managed to maintain a relatively healthy population in the face of decreasing sea ice availability, while the bears in the Southern Beaufort Sea struggle?
Chukchi Sea bears can still spend most of the year, outside of the summer, on ice over the continental shelf, while the Southern Beaufort Sea bears “only have a very narrow strip of shallow water over which to hunt, so the effects of sea ice loss have been much quicker to manifest themselves,” Wilson suggests.
However, the researchers believe that the negative effects of sea ice loss will soon catch up to the Chukchi Sea polar bears as well, given the major reductions in their summer sea ice habitat observed during the study. “Continued sea ice loss is likely to further reduce habitat with population-level consequences for polar bears,” the authors write.
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