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Polar Bears Threatened By Climate Change

May 25, 2010

A new study has found that climate change will trigger a dramatic and sudden decline in the number of polar bears.

The research is the first to directly model how changing climate will affect polar bear reproduction and survival.

Based on what is understood about a polar bear’s physiology, behavior and ecology, the study predicts that pregnancy rates will fall and fewer bears will survive fasting during longer ice-free seasons.

These changes will take place suddenly as bears pass a “tipping point.”

The research was recently published in the journal Biological Conservation.

Most studies measured how a polar bear’s survival depends on a method called “market and recapture.”

This involves repeatedly catching polar bears in a population over several years, which is costly and time consuming.

The information that scientists have gathered using this method varies greatly.  For example, datasets span up to four decades in the best studied populations in Western Hudson Bay and Southern Beaufort Sea, but are almost non-existent for bears in some parts of Russia.

Another difficult task in this kind of research is measuring how survival and reproduction might change under future climatic conditions.

“Some populations are expected to go extinct with climate warming, while others are expected to persist, albeit at a reduced population size,” Dr Peter Molnar of the University of Alberta, Edmonton, Canada, told BBC News.

However, these types of predictions are simply educated guesses, based on experts judging or extrapolating how current population trends might continue as the climate changed.

“So we’ve looked at the underlying mechanisms of polar bear ecology to assist our understanding of what will happen in a warming world,” Molnar told the BBC.

Molnar, along with Professor Andrew Derocher and colleagues from the University of Alberts and York University, Toronto, focused on the physiology, behavior and ecology of polar bears, and how these might change as temperatures rise.

“We developed a model for the mating ecology of polar bears. The model estimates how many females in a population will be able to find a mate during the mating season, and thus get impregnated.”

Male polar bears find a female mate by wandering the ice and sniffing bear tracks as they come across them.  If a female has made the tracks in mating condition, the male follows the tracks to her.

The team of scientists modeled how this behavior would change as warming temperatures fragment sea ice.

They also modeled the impact on bears’ survival.

Southern populations of polar bears fast in summer, forced ashore as the sea ice melts.

As these ice-free seasons grow, fewer bears are expected to have enough fat and protein storage to help them survive.

The researchers were able to estimate how long it takes a bear to die of starvation by developing a physiological model that estimates how fast a bear uses up its fat and protein storage.

“In both cases, the expected changes in reproduction and survival were non-linear,” Molnar explained to BBC.

“That is, as the climate warms, we may not see any substantial effect on polar bear reproduction and survival for a while, up until some threshold is passed, at which point reproduction and survival will decline dramatically and very rapidly.”

The U.S. Endangered Species Act lists polar bears as a threatened species.

The latest U.S. assessment of the conservation status of polar bears included the only two previous studies to assess the impact of climate change.  However, these extrapolated population trends, instead of directly modeling how the ecology of polar bears may be altered.

The new study offers a way to improve these predictions, and it suggests the potential for even faster declines than those found by the U.S. assessment.

“Canada has about two-thirds of the world’s polar bears, but their conservation assessment of polar bears didn’t take climate change seriously,” said Molnar of a flaw noted by the IUCN/SSC Polar Bear Specialist Group last year.

“Our view is that the Canadian assessment should be redone, properly accounting for climate change effects.

“The status of polar bears is likely much more dire than suggested by the Canadian report,” he told BBC.

“For instance, for a while we will only see small changes in summer fasting season survival in Western Hudson Bay.

“[But] eventually mortality will dramatically increase when a certain threshold is passed; for example, while starvation mortality is currently negligible, up to one-half of the male population would starve if the fasting season in Western Hudson Bay was extended from currently four to about six months.”

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