A warm north pole poses complex issues for polar bears. Jon Aars from the Norwegian Polar Institute reveals all about the threats faced by polar bears (Text by Jon Aars. Photos courtesy of Norwegian Polar Institute)
(Text by Jon Aars. Photos courtesy of Norwegian Polar Institute)
A warm north pole poses complex issues for polar bears. Jon Aars from the Norwegian Polar Institute reveals all about the threats faced by polar bears.
POLAR BEAR PREY
- Ringed seals are the polar bears’ most common prey. Ringed seals are highly dependent on sea ice, where they rest and make lairs for their pups. Spring and early summer are the best months for polar bears to hunt ringed seals: In spring, they hunt the pups in the lairs, and often the mothers as well; in early summer, when larger ringed seals shed their hair, they bask in the sun on the ice, where the bears can ambush them. Polar bears will also hunt ringed seals by waiting at their breathing holes, and taking them when they surface for air. This technique can be used year-round. So, in areas with sea ice year-round, polar bears may also hunt for 12 months a year.
- Bearded seals are another important prey species. It is a big prey, and adults usually weigh more than a female polar bear! Even so, the adult female bears may take them. Bearded seals rest on drifting ice floes, and the bears will swim and dive between the floes, trying to catch the seals by surprise before they get into the water.
- Harp seals and hooded seals are also hunted by polar bears.
- Narwhals and belugas are also sometimes targeted by polar bears.
- The most important time for polar bears to hunt and build up their fat reserves is spring and early summer. With a large fat reserve, the bears can survive for many months without eating.
- Females need certain types of sea ice in which to make “dens” to give birth to their cubs.
- Ice is important to enable polar bears to travel between feeding areas, and to make it to land where they can rest in the summer to wait for the sea ice to form again in autumn
A mother may give birth in a den where she will stay for half a year, nursing the cubs for several months, without eating any food.
Fast increases in the temperature in the Arctic and loss of vast areas of sea ice in many parts is considered by far the greatest threat to polar bears.
SHIFTING ICE, SHIFTING HABITATS
The unique ability to survive for long periods without food means that polar bears can thrive and survive in many parts of the Arctic where sea ice is not accessible for significant parts of the year. But it is vitally important to them that sea ice is present in the critical period of spring and early summer. So, as the period of sea ice coverage gets shorter and shorter in many areas, we will see a shift in areas where polar bears can live.
NOT ALL BEARS ARE AFFECTED EQUALLY
How polar bears react to the changes in sea ice availability is more complicated than the picture one usually gets through the media:
- In some cases, bears may be able to cope with a substantial reduction in the total number of days of sea ice, as long as prey and sea ice is available in the critical parts of the year.
- Areas with a lot of “multi-year ice” (ice that forms over several years, and can measure several metres in thickness), actually makes it more difficult for bears to hunt. Thus, a milder climate may lead to more “annual sea ice”, a substrate where polar bears can hunt with larger success.
Temporarily, and in some areas, climate change may improve the conditions for the bears. A few areas in Canada and Greenland in the high Arctic may become places where polar bears could find a refuge, with conditions that may be better than today, or at least good enough to host a viable population.
The areas where we have seen the clearest effects on polar bear populations due to loss of sea ice are Western Hudson Bay in Canada and Southern Beaufort Sea in Alaska.
SVALBARD, NORWEGIAN ARCTIC
- Nowhere else in the Arctic has the reduction in sea ice been as severe
- In large parts of this area, we now on average have a three-to-four months’ longer sea ice-free period compared to a few decades ago
- The Russian western islands of Franz Josef Land still have good sea ice conditions most years, although in 2016 there was no ice until well into the winter months
- Sea ice has been unavailable in more southern areas of the archipelago
Impact on polar bears:
• Bears in Svalbard are part of the Barents Sea population, shared with Russia.
• Severe reduction in the number of dens on traditional denning islands in the eastern islands of the Norwegian Arctic archipelago – lack of sea ice in the autumn makes them inaccessible at the vital time of year.
• Likely more bears use Franz Josef Land in Russia as an alternative denning area, but there may be a cost for the bears that used to den in Svalbard to reach these areas.
• Large shift northward in the distribution of bears
• Male adult polar bears in Svalbard are leaner in spring in years with milder weather
• Despite the effects seen on denning areas and condition, polar bears still seem to be surviving in Svalbard: We found no evidence of a reduction in population size between 2004 and 2015
THE COMING DECADES WILL BE CRITICAL
There will be a critical point in the reduction of sea ice availability up to which polar bears can cope, and still be able to survive and reproduce. With the very fast reductions of sea ice availability seen around Svalbard, if the trend in sea ice reduction continues at the pace seen over the last decades, we may reach this point soon.
In some other Arctic areas, bears are struggling. In some, they are doing well, and may even thrive also in the coming decades. But for every area, there will be some critical level regarding how much sea ice bears will need, and for how long, to exist into the future.
For the rest of this article and other stories from this issue, see Asian Diver 2017 Issue 1 Volume 144