New research: polar bears can survive shrinking sea ice

Three adult male polar bears feed on the remains of a bull caribou on Keyask Island on the Cape Churchill Peninsula on Aug. 8, 2012. (PHOTO BY R.F. ROCKWELL)

Polar bears living with a longer, ice-free season in the Arctic will turn to land-based foods to fill their tummies — and they’ll thrive.

That’s according to researchers from the American Museum of Natural History, whose paper was recently published in the online journal, Plos One.

In the article, co-authors Robert F. Rockwell and Linda Gormezano say predictions — made repeatedly by other researchers — that Western Hudson Bay polar bears will starve to death when the ice-free period increases by 60 more days are “overestimated.”

For their research, the two used the same data and calculations used by researchers who maintain that many polar bears will starve if the ice-free season increases to 180 days in 2068.

But Rockwell and Gormezano found there is more than enough food available on land to offset that starvation problem‚ as long as the polar bears take advantage of the land-based foods.

That’s a totally different conclusion than that reached by another researcher, who recently said a land-based diet won’t sustain polar bears.

“It is odd that people who have never spent much time on the Hudson Bay coast during the ice free period could make such claims,” Rockwell told Nunatsiaq News.

For their research, Rockwell and Gormezano draw on traditional knowledge from Cree and Dene elders and Inuit.

While polar bears usually rely on seal pups to get them through the ice-free periods, by using the same “energy-saving, surprise hunting methods,” polar bears could instead hunt more geese and caribou, they said.

“We feel that polar bears are flexible foragers and will shift their diets opportunistically as the needs arise,” said Rockwell, who has conducted research on the Cape Churchill Peninsula, east of Churchill, Man., for nearly 50 years.

Over that time, the snow goose population has increased from 2,500 pairs to 75,000 pairs and the caribou herd from a few hundred to several thousand, he said.

And polar bears have already discovered this growing source of food.

“We have only worked with the polar bears on the Cape Churchill Peninsula,” Rockwell said, but others working in Hudson Strait and on the Svalbard Islands have found polar bears “doing the same things we have.”

This isn’t the first time that studies by Rockwell and Gormezano have tried to demonstrate polar bears’ adaptability in a warmer climate.

In this latest article they look more closely at how polar bears may be better set up to hunt land-based foods when the ice-free period increases by two months.

The two say the polar bears’ earlier arrival onshore could allow for a better overlap with the incubation period of snow geese and caribou calving periods.

They look at how hungry adult male polar bears could be supported by eggs, geese and caribou in 2068, when Hudson Bay’s ice-free season expands to 180 days.

Gormezano calculated that, to thrive during the ice-free season, a starving polar bear would have to consume five egg clutches every three days, one incubating female off the nest every three days, two goslings every three days, one flightless adult each day and one adult male caribou.

This kind of “combination diet” could keep polar bears from starving.

“One of the big unknowns is how much energy the bears would have to expend to obtain it,” said Rockwell. “We do know that eggs can be acquired with little energy expenditure, snow geese are chased as groups when they are flightless into willow bushes and killed, and caribou are successfully ambushed rather than chased.”

But to know the exact amount of energy used by polar bears to catch geese and caribou would require studies of polar bears on treadmills wearing bonnets to collect their breath, he said.

“While those may someday be done, it will be a while,” Rockwell said.

Still, “there is so much energy available in just these land based foods that, even if the bears are inefficient, the land based food could offset much of the predicted starvation.”

And there are other foods that polar bears will likely use to flesh out their diet.

Rockwell and Gormezano only calculated the energy available from snow goose eggs, snow geese and caribou because those were the most common foods found in the polar bears’ diet.

But polar bears are also catching and eating seals that haul out on the shoreline in increasing numbers, Rockwell said.

“There has also been an increase in the numbers of beluga whale calves being captured in shallow water as the tide goes out,” he said. “In some of those cases, the polar bears stand on large rocks and jump onto the beluga calves to kill them.”

In their article Rockwell and Gormezano also debunk studies suggesting that polar bears can’t thrive on a less-fatty diet than seals, blaming that conclusion on the impact of capture-related stress on metabolic blood test results.