For Hudson Bay polar bears, the end is already in sight
The polar bear has long been a symbol of the damage wrought by global warming, but now biologist Andrew Derocher and his colleagues have calculated how long one southerly population can hold out. Their answer? No more than a few decades, as the bears’ decline closely tracks that of the Arctic’s disappearing sea ice. 
No polar bears have been more closely studied than Canada’s western Hudson Bay population. In recent decades, biologists such as Andrew E. Derocher of the University of Alberta have compiled an impressive store of data on everything from the weight of females at denning, to the body mass of bears of all sexes, to the length of time the bears spend annually on the shores of Hudson Bay, to the decline of sea ice in the bay itself. Now, Derocher, working with Peter K. Molnar and other colleagues from the University of Alberta, has marshaled that data to forecast how long it will be before western Hudson Bay’s polar bears disappear. The calculation is not overly complex, since the health of polar bears is directly tied to the amount of time they spend on sea ice hunting seals. The basic facts are as follows: The region’s polar bears have been forced to spend an extra week per decade onshore; the bears have been losing, on average, more than 20 pounds per decade; the body mass of the bears has been steadily declining; females have lost 10 percent of their body length; and the population has dropped from 1,200 to 900 in three decades, with much of the decline coming in the last 10 years. Looking at projected sea ice declines, Derocher and his colleagues estimated in a recent paper in Biological Conservation that western Hudson Bay’s polar bear population could well die out in 25 to 30 years. Indeed, in an interview with Yale Environment 360 senior editor Fen Montaigne, Derocher said that the population — one of 19 in the Arctic — could be gone within a decade. All it would take is several straight years of low sea ice conditions — such as the current year — which could force the bears onshore for more than five months a year, leading to a sharp decline in the bears’ physical condition and the inability of females to gestate cubs. “One of the things we found was that the changes in this population could happen very dramatically,” says Derocher. “And a lot of the change could come within a single year if you just ended up with an earlier melt of sea ice.” Yale Environment 360: You predict that the western Hudson Bay polar bear population, which is one of the most southerly, could reach a point within three decades where there are too few animals to sustain a breeding population. Could you summarize for our readers how you reached that conclusion? Andrew Derocher: Sure. If you look at polar bears in the global context we’ve actually got 19 different subpopulations. Now, they’re reasonably distinct, but the interesting thing about the western Hudson Bay population is that it’s actually one of the most accessible and it’s certainly by far the most studied population that we have. So that’s one of the reasons that we’re focusing our attention on the western Hudson Bay population. But more importantly, we have a lot of the background information on the workings of this population. So we understand very well things like how fat a bear has to be to produce a certain number of cubs, we know a lot about how much energy these bears are burning during the period of time over the summer that they’re forced ashore when the sea ice melts. We also have a very good understanding of how the sea ice has changed in this part of the world. So really, what this is is kind of a model system that’s giving us some early indications about what one of the more southern populations is doing relative to the issue of climate change. … e360: In the 1980s the bears were on the ice around eight months a year and ashore and fasting around four months a year. Is that true? Derocher: That’s exactly it. And what we’ve seen is that over the last three decades we’re getting about one week extra onshore time for this population every decade. That doesn’t sound like a lot, but you have to remember that these bears are burning almost two pounds of body fat for every day that they’re ashore. …
