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Polar bears; on their way to extinction
By: Ragnhild Hårvik, Vet. Med. 1st. year
The Polar bear, Ursus maritimus, is the largest bear and predator in the world. It is a part of the very vulnerable ecosystem of the arctic drift-ice. Because of the changing habitat, the polar bear annually migrates for long distances to find its primary food resource, the Ring seals, Phoca hispida, and the Bearded seals, Erignathus barbatus. And is highly dependent of the drift-ice for its living.
In a study done by the Norwegian polar institute, a group of scientists monitored the movements of† 74 female Polar bears in a period of 13 years.
The migration of the bears tells how much space one bear needs for its survival, and therefore also how much space the whole population needs.
They found that the movements of the Polar bears differs according to month, and is primarily done by the polar bear it self (active displacement). The individual movements of the bears,† indicated that migration often is due to search of better food resources.
The seasonally changing habitat is also an important process underlying the annual migrations.
The sea ice differ in extent during the season. It has its maximum in April, where it extends 800km south of what it is in September, at its minimum. According to this, the Polar bear migrates northwards during summer ice decrease, and wanders southwards as the winter ice enlarges the territory.
the drift-ice of the
Due to its
annual migration, the Polar bears of the
They are therefore depending on access to large amount of prey. They therefore navigate through the moving sea ice to find territories of preferred habitat.
The Polar bears tend to prefer closed annual ice in early summer, due to the Ring seals pupping season. The pups have large amount of fat, which the polar bear needs.
The rest of the season, they stay close to the open areas near the ice edge, where the ecosystem is more productive and therefore attracts the seals.
The female Polar bear often chose terrestrial ice when building her den, where she will stay for months during her pregnancy. When she retreats with her cubs, she has not eaten for up to 7 months and depend on good spring ice conditions to get to a habitat with large amount of seals.
As the Norwegian research team has shown, the Polar bears are highly adapted to the harsh environment of the drift-ice, of which it also is highly dependent on.
With the knowledge of the polar bears dependency of its environment; how does the melting of the arctic drift ice affect the Polar bears?
Since the 1960ís the thickness of the drift-ice has been reduced by over 40%. This is due to longer and warmer summers. The prospects for the future is even more disturbing, as analyses show reduction of the summer ice for up to 50%. And some analyses even prospect that there will be no summer ice.
A Shorter summer-hunting season will affect the Polar bears capability of surviving the harsh winter, Because the summer season is when the bear eat the most.
The melting ice is probably most critical for the female bears, which emerge from the dens with cubs and are in desperate need for good spring- and summer-ice conditions in order to find enough food to survive. Longer summers, means longer periods of fasting, and the mother will not be able to give her cubs the proper nutrition they need.
A warmer climate can also separate the dens from the summer-ice feeding spots, which forces the female and cubs to swim for longer distances, something the cubs are not likely to survive.
Another problem with the loss of its habitat, is that the Polar bear eventually will be isolated into smaller groups. This will give less genetic variation, and therefore weaker individuals with less chance of surviving.
What will happen with the Polar bear population in the future is highly uncertain, but the environmental changes has already started to show its fatal consequences.
bears, stranded on drifting ice flakes has become a common sight for fishers
living in the
- only time will show if the Polar bear is capable of surviving the environmental changes.
Mette Mauritzen, Andrew E.
Derocher, Olga Pavlova, Øystein Wiig.(2002) . Female polar bears, Ursus
maritimus, on the
|Notes (if any) by Peter Kabai:|