February 14, 2011
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In what may be another dismal harbinger for the fate of polar bears off Alaska and across the Arctic, scientists at the University of Alberta now predict that sea ice loss can trigger a stunning decrease in the number of cubs birthed by polar bear moms.
Based on detailed studies of 28 females and their body condition when they entered dens along Hudson Bay, the scientists calculated that 40 to 73 percent of pregnant females could fail to reproduce if sea ice breaks up one month earlier. During the 1990s, up to 28 percent of low-weight bears didn't reproduce offspring.
So what if ice breaks up two months earlier, a prediction made by some climate models? That scenario could mean no new cubs will be born in Hudson Bay at all.
The Hudson Bay bears face the same climate-triggered problem that has led the United States to list Alaskan polar bears as threatened under the Endangered Species Act: the record shrinkage of summer sea ice.
"Climate warming affects ecosystems worldwide and is a major conservation threat to Arctic species," the authors wrote in the study. "Sea ice-obligate species, such as polar bears (Ursus maritimus), are particularly vulnerable because their habitat is disappearing. Polar bear body condition, reproduction, survival and abundance are already declining in some populations, and further declines are expected with continued warming."
Hudson Bay ice has been breaking up a week earlier every 10 years for decades, scientists say. That means the bears end up spending less and less time on the frozen habitat where they munch seals to fatten up. When they arrive on shore in late summer without sufficient fat stores, they then cannot carry pregnancies to term or nurture newborn cubs.
"This is another piece of the puzzle of looking at how climate change could affect polar bears," co-author Andrew Derocher told the Calgary Herald. "There's no indication this population is sustaining itself."
The study, appearing last week in Nature Communication, is among the fist to decisively demonstrate the link between sea ice shrinkage and cub survival -- and it raises disturbing questions about what might happen in other areas of the Arctic if summer sea ice continues its multi-decade meltdown.
Last month, Anchorage polar bear biologist George Durner and other scientists shared details of how a polar bear mom was forced to swim about 426 miles over nine days in 2008 to find floating sea ice in the Beaufort Sea. Along the way, the bear lost her yearling cub and 22 percent of her body weight -- more than 100 pounds.
In the past, polar bears didn't have to swim as far to reach the floating pack ice, Durner told Alaska Dispatch, adding that the mother bear probably had no idea what she was getting herself into.
"The bear probably went swimming back in 2008 and probably didn't say to itself, 'I've got 687 kilometers to swim.' It probably just said, 'I'll go swimming and pretty soon I'll come to the sea ice habitat where I want to be," he said in January.
The specter of Alaskan polar bears undertaking epic swims to find sea ice where they can find food has become more common, according to these reports by the World Wildlife Fund.
Scientists have long warned that polar bears may not survive as a species in an Arctic that loses its ice every summer. But a study that appeared in Nature last year found that it might not be too late to halt the decline and mitigate the impact on bears.
If people significantly reduce greenhouse gas emissions during the next two decades, enough sea ice will remain to ensure the survival of polar bears, said former federal biologist Steve Amstrup. "Conserving polar bears appears to be largely a matter of minimizing temperature rise," Amstrup said at the time.
In Hudson Bay, home to the most southern range of polar bears, population numbers have been falling for decades. Scientists say this region shows what could happen elsewhere.
"With little to no terrestrial food available, bears rely on their energy stores for survival and reproduction while on land," the authors noted. "How fat and well-fed the females are when they crawl into their dens directly impacts how many cubs they can birth and nurse to health."