Study: Polar bears can’t thrive on land, amid record-breaking ice loss

Polar bears are increasingly feeding on land-based foods instead of their traditional marine prey as climate change reduces the sea ice they use as hunting platforms — a dietary change that has contributed to declining health and survival rates among the species, a team of scientists led by the U.S. Geological Survey (USGS) has found.

“While it’s tempting to think that polar bears could survive by switching to a terrestrial diet, this paper establishes in no uncertain terms that land-based foods do not offer any hope of polar bear salvation,” said Steven Amstrup, the chief scientist for conservation group Polar Bears International (PBI), who is a co-author of the study, which was published Wednesday in Frontiers in Ecology and the Environment. Scientists with the USGS, PBI and Washington State University participated in the research.  

“If we don’t save the sea ice, polar bears will indeed be gone,” Amstrup said in a press release.

Arctic sea ice reached a record low in 2014 due to climate change, with the maximum extent of ice before the melting season the smallest in the satellite record. The ice loss represents an existential threat for polar bears, which depend on the ice as platforms for hunting marine animals such as seals, advocacy groups say.

Two-thirds of the world's polar bear population could disappear by 2050 if Arctic sea ice loss continues to worsen, the USGS said.

The melting ice has increasingly forced the bears onto land for food, scientists behind the study said. Some of the bears may be eating terrestrial foods such as berries, birds and eggs, but the land diet cannot compensate for the loss of the bears’ traditional lipid-rich prey of seals, the study found.

“Although some polar bears may eat terrestrial foods, there is no evidence the behavior is widespread,” Karyn Rode, the lead author of the study and a scientist with the USGS, said in a press release. “In the regions where terrestrial feeding by polar bears has been documented, polar bear body condition and survival rates have declined.”

Fewer than 30 polar bears — in populations ranging from 900 to 2,000 bears — have been observed eating bird eggs, researchers said. “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?” Amstrup said in the PBI press release. “There’s a difference between seeing an animal eat something and understanding what the value of that food is.”

Marine prey is among the most energy-dense foods, researchers say. Some studies have suggested that polar bears’ diet has the highest lipid content of any species. Lipids, or fats, are a source of energy and help with the absorption of fat-soluble vitamins A, D, E and K.

But potential foods found on land in the polar bears’ habitat are mostly high-protein, low-fat animals and plants, and the bears are not physiologically suited to digest vegetation, scientists have said.

"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.