The polar bear is the most carnivorous and gigantic of the ursids. No grizzly documented in Yellowstone has weighed more than 400 kilograms. The largest male polar bears? In excess of 770 kilograms.
No wonder United States Geological Survey wildlife biologist Karyn Rode describes collecting samples and measurements from tranquilized polar bears as "surreal." And yet, it wasn't the bears per se that most captured her imagination when she arrived on the southern Beaufort Sea by helicopter in 2007 for her first field season. It was the fact that they manage to eke out their existence on such a vast, featureless white expanse of ice, floating on the ocean. "I was like: 'Seriously, you live here? How do you figure it out?'" she recalls now. "It's just phenomenal."
Eking out a polar bear-sized existence has gotten ever harder as that ice recedes thanks to global climate change. To sustain their bulk, the bears spend more than 50 per cent of their time hunting and consume a diet that is up to 70-per-cent fat — mostly seals coming up for air at holes in the ice, or basking on its surface. Less ice means less opportunity to hunt seals, and makes travel and mating more difficult; of the nine most-studied populations (there are 19 total), four are declining, National Geographic reported last fall. In 2008, the federal government declared polar bears "threatened" and extended them special protections under the Endangered Species Act.
Recent studies and corresponding headlines have suggested that bears are increasingly turning toward sources of food on land — everything from berries and snow goose eggs to musk oxen — perhaps making up for the expanding caloric gap in their diets. Some have even argued that these sources could help the bears weather the transition to a less wintry world.
But in a review paper published online in April in Frontiers in Ecology and the Environment, Rode and co-authors argue that existing data just don't support those hopeful conclusions. "It's so intriguing to see big polar bears raiding a nest," she says. "It's generated a lot of excitement. Our motivation for the study was to stop and put it into context."
The places where increased terrestrial feeding has been observed, such as around Canada's Southern Hudson Bay, are also some of those with the richest available onshore foods. Yet bear body condition, survival and population levels in those places continues to decline as time spent on land increases, the scientists write. Meanwhile, documented cases of terrestrial feeding are so far limited to a small number of individuals — no more than 30 of any given population have yet been shown to be feeding on bird eggs, for example.
The scientists also point out that polar bears coming ashore in the Arctic are likely to come into direct competition with grizzlies that currently occupy much of the coastal habitat and rely for their survival on the lower fat terrestrial foods that polar bears are beginning to exploit. Those sources are already marginal enough there that they are some of the smallest of their species — and just half the size of polar bears.
But even if land foods are unlikely to help polar bear populations in a meaningful way, the bruins' increased reliance on them could have profound effects on the ecosystems they move into, in part because their caloric needs are so great. Polar bear predation has already caused "catastrophic" nesting failures for some species of seabirds, according to the paper.
There are still some hopeful signs, though. The bears that Rode now spends her spring field seasons with — the population that splits time between Alaska and Russia on the Chukchi Sea — have so far fared well despite ice losses, possibly because their marine environment is more productive. "We thought things were going to be really ugly, but we were happily surprised," she says.
Some mother bears even rear three healthy cubs there, a sign that they've been getting enough to eat.
"I don't know if it's going to stay that way. We'll see."
Sarah Gilman is a contributing editor at High Country News, viewed at www.hcn.org.