Published: February 4, 2014
There were more than 300 nests in the bird colony when the polar bear arrived.
When it meandered off with a belly full of eggs only 24 nests remained, say scientists who witnessed the “near total” destruction of nests on the bird colony off Baffin Island.
It was far from an isolated event, the team from Environment Canada and Carleton University reported Tuesday.
Hungry polar bears are becoming a bigger threat to seabirds in the Canadian Arctic than traditional nest robbers like foxes and gulls, the researchers say in a study that points to the “cascading ecological impacts” of climate change.
Polar bears are now seven times more common in bird colonies in the Hudson Strait area between Northern Quebec and Baffin Island than they were in the 1980s, the study found.
It says the increased predation is clearly linked to the shrinking Arctic ice season, which is almost two months shorter than was 30 years ago.
“Substantial open-water areas are now routinely encountered in May and the near shore seasonal ice environment upon which polar bears depend has been drastically altered,” the study says.
Polar bears use sea ice as a platform to hunt for seals, their favoured food, but resort to eating eggs and whatever else they can find when the ice melts and forces them on land.
Thick-billed murres, seabirds that nest on cliffs, are one target. But northern common eiders are even more vulnerable because the large seaducks lay three to four plump eggs in nests on the ground, says wildlife biologist Samuel Iverson at Carleton University, lead author of the study in the Proceedings of the Royal Society B.
“The polar bears just lumber around, press the eggs with their nose and lap up the contents,” Iverson says. He watched the carnage from boats that he and his colleagues used to survey nesting sites on 230 islands along 1,000 kilometres of Arctic coastline between 2010 and 2012, which were record low years for sea ice.
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They found bears — typically a single bear or a female with her cubs — or signs of bear activity on 34 per cent of eider colonies. Gulls, which normally can’t get past the parent birds, tend to join in when the bears start crushing eggs.
The researchers say the destruction is typically quick, and often leads to “near total reproductive failure” for the seabirds, which lay only one clutch of eggs per season.
At one eider colony near Cape Dorset on Baffin Island the field crew found 335 active nests. When they returned two days later they found a polar bear eating eggs, along with more than 50 gulls picking over the remains.“We resurveyed the colony after the bear left of its own accord and counted 24 active nests,” the team reports.
While not unique, the researchers report it “was among the most definitive cases of near total nest destruction on a colony.”
The study also found that bear visits have risen sharply at nesting sites that Environment Canada has been monitoring for up to 24 years. Fifteen to 24 years ago, bears wandered into the colonies for a day or two. “What we’re seeing now is close to 12 days in a season where might see bears on a colony,” Iverson said in an interview.
Murres and eiders are long-lived sea birds that can withstand reproductive failures in some years. “But if the frequency gets past certain tipping points then you’d expect them to decline,” Iverson says.
He says more study is needed to assess the “conservation” concern. “But we definitely to think that it is a concern,” he says.
And the scientists say the study clearly points to the cascading downstream effects of climate change.
Hudson Strait and Northern Hudson Bay Narrows, where the study was conducted, has seen a remarkable two-month reduction in annual ice cover over the past three decades. This means polar bears are on land for up to two months longer, with “unanticipated” effects on breeding sea birds, the study says.
“While the nutritional benefits to bears are not known, our results clearly demonstrate that nest depredation is not limited to a few bears or a handful of nests,” the scientists say. “Our results are consistent with assertions that polar bears are experiencing difficulty meeting their energetic demands in locations where ice-free seasons have grown significantly longer.”
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