Land-based diet won’t sustain polar bears: study

A polar bear pulls himself out of the Chukchi Sea and onto land in Alaska in June 2014. (PHOTO BY BRIAN BATTAILE/USGS)

Melting Arctic sea ice may continue to send polar bears onto the land to eat, but new research says a land-based diet won’t sustain polar bears in the long-term.

A team of scientists led by the United States Geological Survey says that, although polar bears have been documented eating berries, birds and eggs in recent years, polar bears are unlikely to thrive on this new diet.

That’s because those land foods are short of the fat-rich nutrients found in the marine life that polar bears have traditionally preyed on.

“We know that polar bears have been raiding bird nests and even catching some adult geese, but the critical question is: how important is this?” said Steve Amstrup, one of the co-authors of the study, in a release.

“There is a difference between seeing an animal eat something and understand what the value of that food is.”

A polar bear’s traditional diet is very high in lipids, which helps to store fat and uses minimum energy, while a polar bear’s choice of land foods range between low-fat animals and vegetation.

“Polar bears are not physiologically suited to digest plants, and it would be difficult for them to ingest the volumes that would be required to support their large body size,” the study said.

The study also notes that there’s no evidence to suggest there’s enough of those food sources to support polar bears if the animals are forces to live on land for longer and longer periods.

“This paper establishes in no uncertain terms that polar bears are very unlikely to be able to make a living on land, and that if we don’t save the sea ice, polar bears will indeed be gone,” said Amstrup, chief scientist with Polar Bears International, which collaborated on the study.

The new study, published this month in a science journal called Frontiers in Ecology and the Environment, also found little evidence to suggest that eating land foods has become a widespread behaviour among polar bears.

In its study, USGS researchers found that fewer than 30 polar bears have been observed actually eating bird eggs — this out of a population of anywhere from 900 to 2,000 bears.

“There has been a fair bit of publicity about polar bears consuming bird eggs,” said Dr. Karyn Rode, lead author of the USGS study, in an April 1 release. “However, this behavior is not yet common, and is unlikely to have population-level impacts on trends in body condition and survival.

Recent studies have found that, when no sea ice is available for hunting, polar bears have become more flexible in what they eat in order to maintain their energy levels, by consuming land food like caribou, geese or goose eggs.

American Museum of Natural History researchers suggest this behaviour that could be driven by polar bears’ shared genetic heritage with brown bears.

At the USGS, Rode and her colleagues note that those same land habitats where polar bears may be looking to feed are already used by grizzly bears.

And grizzlies are among the smallest of their species due to low food quality and availability, the USGS study says, making their habitat a poor one for polar bears to compete for.

“The evidence thus far suggests that increased consumption of terrestrial foods by polar bears is unlikely to offset declines in body condition and survival resulting from sea ice loss,” Rode said.