Terrestrial foods can’t replace polar bears’ energy-dense diet, says study

Apr 1, 2015

Polar bears forced ashore when the sea ice melts in summer may eat vegetation, berries, goose eggs, and even some adult geese. But, because of limited availability and or low nutritional quality, these foods cannot offset lost access to lipid-rich seals caused by melting sea ice.
Credit BJ Kirschhoffer/Polar Bears International

APRN:  As sea ice continues to retreat and polar bears spend more time on shore, one question lingers – can the world’s largest species of bears survive on land-based food? A new study says, “no.”

Arctic sea ice this year covered about half a million square miles less than average and started its retreat two weeks earlier than in past years. The earlier the ice retreats, the earlier polar bears will come ashore, which means they are spending more time on land.

“It’s changed by about a week a decade in Western Hudson Bay,” Karyn Rode, a research wildlife biologist for the U.S. Geological Survey and the lead author on the study, said.

A polar bear’s normal diet consists entirely of fatty, energy-dense foods like seals and occasionally a whale carcass. But, Rode says as bears spend more time on land, some have been observed eating terrestrial food.

“The observations of polar bears eating bird eggs have been limited to 30 or fewer individuals,” she said. “And polar bear populations are between 900 and 2,000 individuals, so it’s a really small proportion of any population that’s eating some of these higher quality terrestrial foods.”

Other foods like berries and plants are also available. But, according to Steven Amstrup – the chief scientist at Polar Bears International and a co-author of the study – those foods provide little nutritional value to the bears.

“We know that in the human case. You have a lot more nutritional benefit if you eat a big hunk of steak than if you eat a few sprigs of celery,” Amstrup said. “They may take up the same amount of room in your gut, but the nutritional contribution to your welfare is very different.”

Some Polar bears in the Arctic can swim in excess of 200 miles.
Credit Mike Lockhart/USGS

Another factor that comes into play is competition with other predators like arctic grizzly bears for a limited supply of food.

Amstrup says the grizzly bears would likely fare much better on these land based foods because they have spent millennia adapting to those conditions.

“And so they are poised to take advantage of the foods that are already there and are evolved to do so,” Amstrup said. “Polar bears on the other hand would be, if they were forced ashore and attempting to take advantage of the terrestrial foods, they would be learning how to do it.”

Karyn Rode says the difference in body size also puts polar bears at a disadvantage on shore. They are much larger and require more food than the smaller grizzlies.

“You have to keep in mind that for grizzly bears, those that live in the Arctic are the smallest of their species and they occur at the lowest density. And studies show that those populations are limited by food availability,” Rode said. “So polar bears are entering that kind of environment, where the bears that occupied the habitat are half their size.”

Rode says the Western Hudson Bay polar bear population is where the most terrestrial feeding has been observed. But, population declines and lower survival rates in years bears spend more time on shore continue, despite the consumption of land-based foods.