Scientists claim polar bears will not thrive on land food

A group of researchers say polar bears forced off melting sea ice will not find enough food to replace their current diet of fat-laden marine mammals such as seals, a conclusion that contradicts studies indicating that bears may be benefiting from bird eggs, berries, and other land food sources.

Few bears are using land food and what they find can’t replace lipid-rich ringed or bearded seals, said Karyn Rode in a review paper published in the journal, Frontiers in Ecology and the Environment.

The paper addresses recent research documenting bears feeding on land and suggestions that those foods are becoming more important for them, which could increase their chance for survival as summer sea ice recedes, preventing seal hunting.

Not everyone agrees with the conclusions reached by Rode and the other authors at Washington State University and Polar Bear International.

Robert Rockwell, a population biologist and ecologist who is also affiliated with the American Museum of Natural History, has conducted research at western Hudson Bay for 47 years. He has watched polar bears change their behaviour to eat goose eggs and caribou calves. “I find it hard to believe they’re going to get nothing out of it,” he said.

Polar bears are marine mammals that spend most of their lives on sea ice. They use the frozen ocean to hunt, often catching ringed seals.

The world’s 19 polar bear populations are divided into four regions. In three, entire populations historically remained on sea ice year-round. The fourth has seasonal sea ice and includes Hudson Bay, where polar bears have always spent time on land.

The Bush administration in 2008 listed polar bears as threatened.

Land food such as berries can be abundant for polar bears but low quality, Rode said. Eggs from ground nests are higher quality but limited. Polar bears feeding on berries lose significant weight, Rode said.

“There is evidence that some bears are using terrestrial sources in a place such as Hudson Bay, but there’s been no evidence that it’s contributing a significant amount to their energy requirements,” Rode said.

Land food may benefit individuals, but the overall contribution to polar bear diets probably will be negligible, the authors said.

Polar bears on land face competition for food from Arctic foxes, birds and grizzly bears. Rode said. Arctic grizzlies are among the smallest of their species.

“Food limitations would be particularly problematic for the much larger polar bears, which often have a body mass double that of Arctic brown bears,” the authors said.