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Planet Earth

Polar Bears Stubbornly Stick to Habitats, Even as Ice Melts

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"I don't know what you're talking about," said the polar bear. "Everything seems normal to me! Watch out for that puddle." Up in the Arctic, things are getting slushy. But some polar bears are refusing to change their ways. Instead of compromising on where they spend their time, they're clinging to the icy habitats they've always loved. As those habitats keep shrinking, though, the bears will eventually find things too crowded and uncomfortable to ignore.  Researchers divide polar bears into 19 subpopulations based on where they live. For a new study, scientists in Alaska were interested in just one of these groups: bears living on the Chukchi Sea, between Alaska and Russia. Sea ice in this region is disappearing fast. Each year, ice covers the largest area of the sea in March, and leaves the most water exposed in September. But what that means has changed dramatically over the past 30 years. Ice is now starting to melt about a week earlier in the spring, and forming about a week later in the fall. That's leaving polar bears with a lot less ice to live and hunt on. As sea ice becomes scarcer, you might expect polar bears to adjust their habits. They might have to compromise, spending more time in suboptimal areas that they would have spurned in the past. To find out whether that's true, the researchers looked at data from two time periods: 1986 to 1994, and 2008 to 2013. During both these periods, female polar bears had been captured and outfitted with tracking collars that showed scientists exactly where they spent their time. Using data from 89 of these bears, the researchers modeled where the animals liked to hang out. How much ice did they prefer? What depth of ocean water was best? And from the icy 1980s to the slushy present, the researchers saw—no change. "Polar bears are sticking to using the same type of habitat conditions even while sea ice disappears," says lead author Ryan Wilson of the U.S. Fish and Wildlife Service. They still love regions with shallow water, a high but variable concentration of sea ice, and not much land-bound ice. He thinks that's because—for now—they can still eat. The Chukchi Sea region includes a lot of shallow water over the continental shelf. Even with the ice disappearing, there's still enough ice on this water for polar bears to reach their preferred prey (ringed seals and bearded seals). And the seals probably haven't changed where they're swimming. So these polar bears seem to be doing fine without changing their habits. Scientists haven't seen the population struggling or individual bears shrinking, as they have with the neighboring Southern Beaufort Sea polar bears. That population has only a thin strip of shallow water to hunt in, Wilson explains, so the loss of sea ice has hit those bears harder. But the researchers did see that the Chukchi Sea bears' favorite kind of habitat—the sea ice that's prime polar bear real estate—is getting smaller and smaller during the summers. "If it's decreasing in availability, it will lead to greater densities of bears," Wilson says. As the iciest spots continue to shrink, more animals will have to either crowd into them or move onto solid land during the summer. They can't stay stuck in their ways forever. These carefree polar bears will soon experience the same negative effects that their neighbors have, Wilson says: "It's likely only a matter of time." Image: by Kathy Crane, NOAA Arctic Research Program

Wilson RR, Regehr EV, Rode KD, & St Martin M (2016). Invariant polar bear habitat selection during a period of sea ice loss. Proceedings. Biological sciences / The Royal Society, 283 (1836) PMID: 27534959

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