“For this particular population of polar bears, there’s been very little study of land use,” said Karyn Rode, a research wildlife biologist with the U.S. Geological Survey and lead author of the new paper. There are 19 polar bear subpopulations, and only a few have been studied for the land use patterns, including bears in Canada’s Foxe Basin and Hudson Bay region. “So really, the only work [for the Chukchi Sea bears] is some unpublished reports by Russian scientists who have been on the island in the summertime and observed polar bears there,” Rode said.
The researchers chose to examine the Chukchi Sea bears for two reasons, said Rode. First, these bears are one of just two subpopulations that the U.S. is charged with managing (the other being the Beaufort Sea bears). Second, the Chukchi Sea is recognized as one of the Arctic regions experiencing the most dramatic recent sea ice declines.
It’s a problem that’s not just exclusive to bears, either. The Chukchi Sea has garnered international attention the past few summers when hordes of walruses were forced to haul out on the Alaskan shore after their preferred sea ice habitat melted away.
For this paper, the researchers examined data from two study periods: 1986 to 1995 and 2008 to 2013. In both periods, scientists radio-collared bears (103 of them in the first study period and 47 in the second) and observed their movements during the summer months. Only female bears were included in the study, largely thanks to a quirk in male polar bear biology: male bears’ necks are wider than their heads, meaning the males are frequently able to slip loose of their radio collars.
In the first study period, the researchers found that 20 percent of all the bears spent more than seven days on land, while in the second period this proportion rose to 38.9 percent. The bears also increased the average amount of time they spent on land by 30 days. Most of these bears spent their time on either Wrangel Island or nearby Herald Island off the Russian coast, although a few also summered on the Russian or Alaskan coasts, and they included denning bears, or bears having cubs.
The study also suggests that changes in sea ice are, in fact, what’s driving the bears’ behavior. Data indicate that ice is retreating, or melting and shrinking northward, earlier in the season than it used to. In northern parts of the study region, the researchers found that ice retreated 20 to 40 days earlier between 2008 and 2013 than it did between 1986 and 1995. In more southern areas, it’s retreating up to 15 days earlier.
“I think the article is quite interesting to see how they use the sea ice data and the temporal information they have to start looking at correlations over time,” said Vicki Sahanatien, a recent doctoral program graduate of the University of Alberta, who was not involved with this paper, but whose research has focused on polar bears in Canada’s Foxe Basin.
One of the biggest concerns, though, is how much longer the bears are staying on land once they come off the ice. Thirty days is an impressive amount of time for the bears to extend their time on land, because many scientists believe bears don’t feed as well on land as they do on sea ice.
Polar bears use sea ice as their hunting grounds, perching on the ice to snatch up tasty seals as they surface. On land, bears are often forced to pursue smaller prey or scavenge carcasses of already dead animals, so scientists worry that their nutrition could suffer the more time they spend on land.
Interestingly, the bears in this study didn’t seem to be suffering any negative nutritional consequences as a result of their time on land, as there was no change in the bears’ body conditions between the two study periods.
This could be largely a regional phenomenon, Sahanatien said. There’s evidence to suggest that the Chukchi Sea habitat is highly productive and has a lot of food available in general, so the bears may just be coming off the ice in excellent condition, she said. She’s observed a similar phenomenon in the Foxe Basin as well.
Timing is also important, she added. The later the bears come off the ice and onto the land, the more time they’ve been able to spend feeding and fattening up. And in fact, Rode believes the reason the bears in this study seemed to favor Wrangel and Herald Islands so much is because they’re so far north, meaning sea ice retreats later in the season and the bears have more time to feed before they’re forced onto the land.
So in some ways, the results are “more optimistic than we anticipated,” Rode said, although it’s unclear whether the bears will remain so lucky in the future. Projections for the Arctic indicate that sea ice will continue to dwindle as the Arctic continues to heat up. This means bears in the Chukchi Sea — and elsewhere — may have to haul out on land earlier and earlier, and eventually their health could begin to suffer.
And nutrition isn’t the only worry, either. Increased land use by polar bears could also increase the risk of encounters with humans, and that puts both bear and human safety at risk. While there have been very few scientific papers studying trends in human-bear interactions — only one that Sahanatien knows of — she said “there is this feeling that definitely this is on the rise, and lots of organizations are starting to monitor more closely these kinds of encounters.”
Keeping track of land use trends can help scientists make more effective management decisions for polar bears.
“I think it’s really important to take a regional approach and consider what land is available to bears,” Sahanatien said. “One would consider enhanced protection in regions where there is limited habitat for bears to retreat to in the summer.” In regions where there’s plenty of land for bears to spread out, management might focus more on cooperating with local communities and promoting strategies to minimize human interactions with the bears.
The thing to remember is that the Chukchi Sea bears are just one subpopulation — the results don’t necessarily mean that all bears throughout the Arctic are experiencing the same patterns in sea ice declines, nor that the bears would react the same way in other places. So continued monitoring, both in the Chukchi Sea region and throughout the Arctic, is key.
“These are really important studies because we’re really still understanding how bears are linked to their sea ice habitat, and how it really drives them and their movements,” Sahanatien said. “So every one of these detailed studies is really helpful, and of course raises more questions and gets us all thinking more about how we can enhance our research and monitoring of polar bears.”