As the Arctic sea ice dwindles, so will hope for the region’s most dramatic predator, its polar bears. A creature fashioned by evolution to fast a whole summer and gorge through the autumn and winter may not last, as the ice melts ever earlier and forms ever later.
That is because Ursus maritimus can find the food for the next generation of its cubs only by prowling the firm sea ice for a high-calorie diet of seal flesh and blubber.
And now a team of Canadian and US scientists has begun to establish the unknown of polar bear survival: how many days the creature can survive without food and still nourish its young and sustain life.
They call this the “fasting impact threshold” and the answer, they report in the journal Nature Climate Change, is not encouraging.
“Polar bears everywhere will face longer periods without food, and this will affect their ability to reproduce, survive and persist”
If warming continues at the present rate, then by the century’s end most of the sub-populations of this charismatic animal will not survive.
“The challenge is that the Arctic ice will keep disappearing as the world continues to warm,” said Péter Molnár, of the University of Toronto Scarborough, who led the research.
“This means polar bears everywhere will face longer periods without food, and this will affect their ability to reproduce, survive and persist as healthy populations.”
The researchers had to start with one big uncertainty: how much stored energy the bear has when the fasting season begins. Because the shelf ice has been thinning and shrinking for more than 40 years, hunting seasons have become shorter and bears now spend longer and longer on land.
Natural variability
That raised a second factor: some parts of the Arctic lose ice earlier than others. The third unknown is the health of the 19 sub-populations of Ursus maritimus, spread over four distinct eco-regions within the Arctic Circle, and how these separate populations would consider a “good” hunting season and a happy period of fasting.
In the southern Beaufort Sea, fewer than 127 ice-free days could be considered “good”, but even this seemingly assured number was based on only five years of systematic demographic data.
And then the researchers had to calculate the demands placed on individual bears: an adult male might be able to last 200 days; a solitary adult female up to 255 days. But a mother bear might begin to lose what it takes to get cubs through to maturity as early as 117 days, and certainly after 228 days.
But however incomplete, the scientists had data for about 80% of the polar bear populations, collected between 1979 and 2016, and report that what they politely call “recruitment and survival impact thresholds” may have already been exceeded in some populations.
Too hopeful?
That is, there are increasing numbers of bears in the Arctic no longer sure of having cubs or keeping them alive. That includes the polar bears of Hudson Bay and the Davis Strait in northern Canada: perhaps the most photographed bears in the world.
And if the world goes on warming, only a few creatures in the very high Arctic will see the next century.
“While our projections for the future of polar bears seem dire, the unfortunate thing is they might even be too optimistic. For example, we assumed that polar bears will use their available body energy in optimal ways when fasting. If that isn’t the case, the reality could be worse than our projections,” Dr Molnár said.
“What we do know is that becoming fat before a fasting season will be more difficult for polar bears as on-ice hunting seasons become shorter, so it’s likely that fasting impact thresholds will be crossed in the early years of our projected range.”
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