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Land-based food won't sustain polar bears in a low-ice Arctic, study says

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  • Updated: May 31, 2016
  • Published April 1, 2015

As summer and autumn sea ice diminishes in the Arctic Sea, polar bears spending more time on shore have been spotted eating eggs, hunting down the nesting birds that lay them, hunting other land animals and even chewing on edible plants growing onshore.

But is that enough to sustain them in an ice-scarce Arctic?

No, says a new study by scientists from the U.S. Geological Survey, Washington State University and Polar Bears International.

The study analyzes food needs of polar bears, including their genetic metabolic characteristics, and the characteristics of the onshore ecosystems and the foods in them.

The verdict? Terrestrial foods are, for the most part, the wrong kind for fat-dependent polar bears, and there is too little of them. What bear food does exist on land is already being used by Arctic grizzly bears, which -- appropriately -- are the smallest and most spread out of all the world's brown bears, said the study's lead author, USGS research biologist Karyn Rode.

The study, published Wednesday in the journal Frontiers of Ecology and the Environment, finds that even though polar bears are spending more time ashore and are affecting terrestrial ecosystems, "evidence suggests that the nutritional contribution of terrestrial foods to polar bear diets will probably remain negligible."

As the world's largest bears, and as denizens of the cold Arctic climate, polar bears need large amounts of fat in their diet to survive, said co-author Steve Amstrup, a former Alaska-based USGS biologist who is now chief scientist at Polar Bears International.

"All bears are not created equal, and if you're a really big bear, like a polar bear, you need to have a rich food source to sustain you," Amstrup said.

Ringed seals, the primary prey for polar bears, are precisely that rich food source, with fat accounting for up to half their body weight, Amstrup said. The food sources on land cannot compare in quality or quantity, he said.

Consumption of bird eggs, birds, shore plants and even seaweed is not necessarily widespread, Rode and Amstrup said. Only a small portion of any of the world's polar bear populations have been observed doing such foraging, Rode said. And an important indicator that it is not doing much to help overall polar bear survival is that in the places where such behavior has been most frequently documented, polar bears have declined in population and in body condition, she said.

In some cases, polar bears have gone to extraordinary lengths to get at their alternate food sources. The study includes a photograph from Svalbard showing a bear perched precariously on a ledge in Svalbard and trying to get at murre nests lodged in the steep rocky cliff there.

That might be a sign of desperation, Amstrup said. "Foraging on the high cliff is very, very high-risk and the bears have to be really hungry before they take on that risk," he said.

Even if snacking on eggs and other land-based food sources benefits individual bears stuck on land, far away from ideal ice-hunting areas, Rode and Amstrup said, there is a cost -- potential catastrophe for some shorebird populations.

In Canada's Hudson Bay area -- at some of the lowest latitudes of the polar bear's range and with a sea-ice season two months shorter than it was in the 1980s -- bear incursions into murre and eider nests have increased seven-fold, according to a study by scientists from Environment Canada and Ottawa's Carleton University and published last year in the Royal Society's Proceedings B.

At Canada's Baffin Island, researchers working on that project watched a single bear devour eggs from a 300-nest seabird colony, leaving only 24 of the nests intact, according to that study.

A newly published study by an international team of researchers from institutions in Norway, the Netherlands and elsewhere found that predation by polar bears in Svalbard and eastern Greenland is threatening geese, eiders and glaucous gulls nesting there.

At those sites, polar bears are arriving on land almost 30 days earlier than they did 10 years ago, the study says. In years of very early arrival onshore, more than 90 percent of bird nests were raided, said the study, published online March 25 in the journal Frontiers of Ecology and Evolution.

In Alaska, a different onshore-feeding phenomenon has emerged: Polar bears from the faltering Southern Beaufort Sea population have been gathering to feast on bowhead whale scraps left on the beach each fall by Alaska Native hunters.

In the past, that autumn hunt was conducted on ice farther out to sea, but now launch sites for the hunts -- and the butchering sites for the camps -- are right on the beach, a seasonal event that draws the area's polar bears.

Now a study by University of Alaska Anchorage and USGS scientists has found strong evidence that those scraps make up a significant part of some Southern Beaufort Sea polar bears' diet.

That study, published online in the journal Polar Biology, evaluated chemical diet signals left in polar bear hairs and used radio-collar data to map the bears' movements.

Bears with hair that held a significant chemical imprint of a bowhead-meat diet in fall have, according to radio-collar data, been staying within 20 kilometers of the whale-scrap piles for an average 25 days of the year, according to the study findings.

Those whale-scrap eaters are also staying within 50 kilometers of shore for most of the year, a contrast with other bears that venture much farther out into the ocean, the study found.

"They seem to be not just coming in and scavenging, but they're hugging the shore," said lead author Matt Rogers, manager and research analyst at UAA's Stable Isotope Laboratory, where the chemical analysis of hair samples was performed.

"The fact that they're staying so close to shore all year, that's something nobody has documented before," Rogers said. "It seems to really show that there might be what we call an `alternative foraging strategy.' "

As with the egg-eating behavior, the Southern Beaufort Sea polar bears' dependence on bowhead scraps raises other concerns -- potential garbage-bear-type problems, including safety risks to people living in villages near the bowhead scrap piles, the scientists said.

There could be big problems if the bowhead scraps fail to appear on the beach in fall, leaving the bears hungrier than normal, the scientists said. "What happens if the availability of the whale scraps isn't there, or gets reduced?" said Amstrup.

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