Andrew Derocher is a biologist at the University of Alberta who has researched polar bears for more than three decades. He is also a volunteer adviser to Polar Bears International, a conservation group.
Q. Worldwide in the Arctic, there are roughly 25,000 polar bears in what scientists consider to be 19 subpopulations. Over all, how are they doing?
A. People assume that because we're concerned about polar bears from a conservation and management perspective, that all polar bears must be doing terribly. That's not the case. Polar bears are doing just fine in many parts of their distribution, and with 19 different populations around the Arctic, we have 19 different scenarios playing out.
What about the southern Beaufort Sea subpopulation, which includes the bears in the Arctic National Wildlife Refuge?
This is one of the 19 that is showing the very clear effects of climate change and the reduction in Arctic sea ice. Historically the subpopulation was much more abundant, with more than 1,500 bears. Now it’s down to 800 to 900.
It’s pretty clear, if we look through time, that this species responds to changes in ice conditions. With warming, they are just getting pushed farther and farther north.
Because of warming, sea ice coverage in the Arctic has declined by about 13 percent each decade since 1980. How does the loss of sea ice affect polar bears?
From an ecological perspective, polar bears are highly adapted to a very specialized ecological niche. They use sea ice to hunt seals. We expect that these changes in sea ice will have negative consequences for their prey.
One of best ways to think about sea ice is that it’s almost akin to the soil in a terrestrial ecosystem. If you take away the soil in a forested area you won’t get the forest back.
The same thing is true for Arctic ecosystems that have sea ice. Ice algae lives inside the ice. Bacteria, viruses, little invertebrates live there, too. There are these algae forests that live underneath the ice. It's a whole ecosystem.
When you remove the ice, it changes the ecosystem. It has these ripple effects that start at the bottom of the food chain and go right up through to the fish and then, of course, to the important thing for the bears — the seals.
A polar bear and cub investigating a bridge to an oil production platform.CreditU.S. Fish and Wildlife Service, via Associated Press
The reduction in sea ice also means that polar bears spend more time on land. Is this a concern?
Once they’re on land, they’re not eating much at all and they’re losing weight. They lose just a little over two pounds per day when they’re away from their prey.
If you increase the length of their fasting period — the ice breaks up sooner in the spring and forms later in the fall — you're asking the bears to do more with less. They’re coming ashore in poorer condition, and have to stretch out that reduced energy reserve for another week or two or longer.
At some point, you just cross a threshold where the bears don’t accumulate enough energy and starve on land, or perhaps a female just can’t reproduce anymore because she doesn’t have enough fat reserves in her body.
We’re jiggling all the parts of their life history at the same time. We're probably changing seal abundance, we’re affecting primary productivity, we're asking the bears to walk farther, we're asking them to fast for longer periods, we're cutting into the best times of year for feeding.
Collectively, the problem is that these effects accumulate in bears in poor body condition. That translates into lower reproductive rates and lower rates of survival. And once you put those two together, it's just a matter of time before the population abundance declines.
Does the decline of sea ice also affect where female bears make their dens?
Polar bears do den offshore, on ice that has to be relatively thick and have snow on top of it. So that tends to be ice that’s more than one year old. Multiyear ice is already disappearing, and if you look at the projections, it will continue to disappear. So there’s been this big shift from multiyear-ice denning to terrestrial denning.
The pregnant bears look for a place with enough snow. On land, usually those are on the lee side of a prevailing wind — on the bank of a lake or on small creek where the snow has accumulated.
When they are in their den, basically they are in slowdown mode. Their cubs are born somewhere around December. The mothers raise them from a tiny one-and-a-half-pound critter to an animal that is about 20 pounds over three to four months, and then they take them out on the ice to hunt.
What do scientists know about the impact of oil and gas exploration and production on polar bears?
One of the things that’s pretty cool about bears in general and polar bears in particular is each bear has an individual behavior pattern, or personality, if you want to call it that. Some bears just don’t seem to care — they are just not worried by people, not worried by snow machines or all-terrain vehicles or trucks going by. Yet others are extremely wary, don’t like it and will move away quickly from disturbance.
By and large, I think polar bears are fairly robust to disturbance, but once they have small cubs they tend to be quite timid.
One of the concerns we have is den abandonment. If you harass a bear around a den, there’s a greater likelihood that she will leave it with her cubs. And that is not a good thing. Young cubs are not that well developed and rely on the dens for protection. Moving them around is never a good idea.
We have to accept at least the basic premise that disturbance is not going to be beneficial for the bears. Then the question is, just how bad is it going to be? That’s difficult to say. But anything that adds on to the current impacts of sea ice loss is not going to be good for the population.