June 3, 2010 | Volume 47 Issue 19

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Concern for polar bears heightened by math

Brian Murphy

Ongoing concerns about the plight of polar bears becoming victims of climate change are now heightened with the release of a paper that has three University of Alberta researchers among its authors.

Peter Molnar, of the U of A’s Centre for Mathematical Biology, and university professors Andrew Derocher and Mark Lewis, used mathematical modeling to show polar bear populations could plummet if ice-free periods in the Arctic continue to increase.

“Mathematical modeling is a quantum leap forward in our understanding of how climate change will affect these animals,” said Derocher. The research team assigned numerical values to study how long male polar bears located on land near Churchill, Manitoba can go without food when they’re cut off from their hunting grounds on the sea ice of Hudson Bay. 

Thanks to previous studies, researchers know the weight- and energy-storing capacity of polar bears and the warming trend in the North is well documented. “We know climate change has increased the ice-free period in Hudson Bay by three weeks over the last three decades,” said Derocher.

Records also show that since 1990 ice-free periods in Hudson Bay have varied from 90 to 135 days and that the current polar bear mortality rate is around three per cent.

With climate change, predictions indicate that the ice-free period will increase.

Derocher has real concerns for the 900 polar bears in the study area. “If the ice-free period extends to 180 days our modeling shows that upward of half of those animals will die.” An important finding is that the changes could happen very quickly, and this contrasts with the slow decline in polar populations that have been found to date.

Derocher says long ice-free periods raise concerns for the town of Churchill, which already has problems with landlocked and hungry polar bears wandering its streets. “They see a lot of bears now,” said Derocher, “but they are not really prepared to deal with several hundred bears stuck in their town for weeks and weeks.”

The mathematical-modeling study also includes the polar bear mating process and birth rates. Derocher says that when the sea ice is in place, male polar bears set out across Hudson Bay to hunt and search for a female to mate with. He says the male’s age-old system of finding a mate is quite simple. The male heads out on the ice and walks a fairly straight line until it crosses the tracks of a female. If the tracks “smell right” and indicate the female is ready to mate, the male simply follows her tracks. “But when the sea ice breaks into pieces, the female can wind up on a drifting floe and the male will never find her,” said Derocher. “The ice becomes a jigsaw puzzle but there’s no way to put the tracks back together.”

Accounting for the changing efficiency of males finding females, Derocher says the team concluded the pregnancy rate could drop by 30 per cent.

He says this research gives Ottawa a longitudinal window into the effects of climate change on an Arctic species, and he’s hoping something will be done.

“We have two thirds of the world’s polar bears and collectively scientists around the world and people are looking to Canada for leadership, but we’ve been very slow to recognize polar bears as a threatened species,” he said. “That has to change.”

The work of Molnar, Derocher and their colleagues is published in the recent issue of Biological Conservation.