A polar bear in the Hudson Bay population. Photo credit: Alex Berger /Flickr through a Creative Commons license.
Editor’s note: “What Does the Science Say?” is a new Cool Green Science feature looking at what scientists and the scientific literature really say about big questions regarding nature and its conservation.
By Lisa Feldkamp, senior coordinator, new science audiences, The Nature Conservancy
Everyone has heard that polar bears are threatened by climate change. But what does that mean, exactly? Will they eventually go extinct, morph into another species, or something else? What does the science really say?
The Topline: No Sea Ice, No Seals, No Polar Bears
As with most species that are or will be vulnerable to climate change, habitat is key in the case of the polar bear. Climate change is warming the Arctic along with the rest of the planet — and in the Arctic, that means sea ice is melting.
And sea ice is crucial for polar bears survival.
Why? Because polar bears’ main prey — seals — are best hunted on sea ice. Without sea ice, polar bears cannot catch enough seals to eat — and without enough seals to eat, they are unlikely to gain enough weight to survive their autumn to summer fast (Derocher 2013, Stirling 2012).
In addition, longer periods between successful hunts decrease polar bears’ reproductive success. A female polar bear who has not gained sufficient weight may not produce cubs; may produce less milk (leading to smaller, less viable cubs); or may abandon cubs — all behaviors that, if they become prevalent enough, will impact a population’s viability (Robbins 2012).
Most scientists agree that these trends spell likely trouble for polar bears.
There are 19 distinct subpopulations of polar bears living in the Arctic, and each population will be affected somewhat differently.
Without human intervention, most polar bear populations will be extinct in 30-40 years (Castro de la Guardia 2013, Stirling 2012)).
The populations of polar bears living farthest to the north (in the Canadian Arctic Islands, northern Greenland, and farther to the north) have actually seen population increases in recent years (Peacock 2013) and may survive indefinitely (Stirling 2012).
However, they will be at risk from continued climate fluctuation (Bechschoft 2011, Stirling 2012); contaminants in the Arctic (Bechshoft 2011, McKinney 2013); increased disease risk (Derocher 2013, Stirling 2012); and decreased genetic diversity (Stirling 2012).
But Can’t They Eat Something Else?
If nothing is done, lack of sea ice will force polar bears on land for longer periods of time. On land, bears “fast” eating some berries, eggs and birds (Iles 2013).
However, scientists’ opinions are mixed on whether these foods could provide enough added energy for the long-term survival of these large bears.
Bears would still need to get some food from hunting seals; the question is, how many calories could be offset by terrestrial sources?
A polar bear in the Hudson Bay. Photo Credit Alex Berger/Flickr through a Creative Commons license.
David Iles, a Ph.D. student at Utah State University, expressed some hope for polar bears eating snow geese eggs in a recent story in National Geographic Daily News.
“The nests don’t run away from them like a caribou might,” Iles told Nat Geo. “It’s a ton of calories just basically sitting there that the bears can take advantage of.”
Nevertheless, Iles cautioned that we don’t really know if the bears will take advantage of this bounty.
“We need to do more research to figure out how much benefit this will really be to polar bears, the effects that this will really have on snow goose populations, and how sustainable this type of predation will be, depending on how severe it is,” he told Nat Geo.
But in the same story, Steven Amstrup, chief scientist for Polar Bears International, warned that nothing might replace what polar bears’ current habitat provides them.
“Unfortunately, we have no evidence to suggest that eating geese, or eating char, or eating anything else has any ability to offset the loss of the sea ice and the [primary] foraging opportunities it presents for polar bears,” Amstrup said.
Though Iles is cautiously optimistic about the possibility that polar bears could survive on goose eggs, his recent publication in Polar Biology found that adult geese were an unlikely source of energy for bears (Iles 2013). Other studies have expressed doubt that polar bears could survive by foraging on land at all (Cherry 2013, Derocher 2013).
Northern populations of polar bears may stay in place and continue eating seals, but not the same species. The changes in Arctic temperatures have caused subarctic species of seals to move farther north, and the northern polar bears are now eating them. These seals carry more man-made contaminants, which may affect polar bears (McKinney 2013).
Migrating South Instead of North and Interbreeding with Grizzlies
You might expect that polar bears would move north to find colder climates and sea ice to hunt on — but actually, sea ice drifts south as it breaks up. For this reason, polar bears in some populations are more likely to move south to stay with the ice (Cherry 2013).
Some Hudson Bay population polar bears have already been spotted farther south of their normal territory, leading to an increased number of conflicts between humans and bears (Cherry 2013).
If southern polar bears cannot find a better food source on land, then they will die off as a result of starvation unless humans begin to feed them (Derocher 2013, Stirling 2012).
Furthermore, even if polar bears manage to find food as they move south, they may hybridize with populations of closely related grizzly bears — essentially causing the extinction of a distinct polar bear species (Albrecht 2013).
Possible Next Steps: Reducing Emissions and More Drastic Measures
One possibility: Do nothing, and watch to see if polar bears “sink or swim.” But since climate change is a human-caused problem, doing nothing to mitigate its effects or help affected animals is unpalatable to many (Albrecht 2013).
A polar bear cub at the Detroit Zoo. Photo credit: Dave Hogg/Flickr through a Creative Commons license.
One of the most important things that we can do — and that we should do anyway because of all the other impacts climate change will have on people and nature — is to start cutting our carbon emissions, right now. (Find out more about your carbon footprint and how you can reduce it.)
The science says that significant reductions in greenhouse gas emissions alone could be enough to protect northern populations of polar bears indefinitely and to give southern populations a chance of survival (Castro de la Guardia 2013).
But should we not reduce carbon emissions significantly enough to keep polar bear habitat intact, observers have suggested at least three other options for saving polar bears:
+ Polar bear populations could be fed and managed in or near the areas that they currently occupy. This would be costly, but it would keep the bears alive and maintain the tourist industry that has sprung up around these charismatic animals (Derocher 2013).
+ Existing zoos and new specially designed zoos could take in large numbers of polar bears. This option would preserve the species and would provide opportunities for education, but it would be difficult to return the bears to the wild if conditions ever became suitable for them in the distant future (Albrecht 2013, Derocher 2013).
+ People could move polar bears to the Antarctic, where sea ice is expected to remain indefinitely. This would have serious ramifications for Antarctic species like penguins, but if polar bears were located at a distance from penguin colonies and both populations were managed, it could preserve the species as a wild population. We could also move other Arctic species to the Antarctic to increase biodiversity, though such moves may have unforeseen negative effects on the current ecosystem (Albrecht 2013). While this was suggested more as an exercise in making people think about impacts, a smaller scale relocation has been considered (Derocher 2013).
I made some corrections to dates and attributions – notably changing Bechsoft 2013 to Bechsoft 2011 wherever it appeared. I also added a note that the Cherry 2013 paper focuses on the Hudson Bay population and a clarification that polar bears could not get all of their calories from terrestrial food sources.
Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.
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Bechshoft et al. 2011. Cortisol levels in hair of East Greenland polar bears. Science of the Total Environment, 409: 831-834.
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