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Greenland polar bears have learned to hunt without sea ice
A group of newly-discovered polar bears may reveal how the Arctic mammals could survive climate change.
Using glacial ice to hunt, rather than sea ice, these bears might inform how these animals can buffer themselves against the worst effects of rising temperatures.
As temperatures rise, Greenland glaciers are providing a lifeline for polar bears.
Polar bears living in southeast Greenland were able to hunt seals from chunks of freshwater ice breaking off the island's glaciers, rather than pack ice forming at sea. As pack ice becomes less common, this could allow the population to persist for the foreseeable future.
Dr Kristen Laidre, the lead author of a new study investigating this population, says, 'These bears provide a glimpse into how Greenland's bears may fare under future climate scenarios.'
'The sea ice conditions in southeast Greenland today resemble what's predicted for northeast Greenland by late this century. However, I don't think glacier habitat is going to support huge numbers of polar bears. There's just not enough of it.'
'We still expect to see large declines in polar bears across the Arctic under climate change.'
While the sea ice may allow these polar bears to persist longer than other populations, the group's isolation and small size could drive them towards extinction.
The study, published in the journal Science, calls for efforts to preserve the population to ensure their survival.
Polar bears hunt seals by using ice as a platform to attack from. Image © GTW/Shutterstock
Why do polar bears need ice?
The majority of the polar bear's diet is made up of seals. The bears generally lie in wait for a seal to surface when they come up to breathe before striking, but have also been known to charge slow-moving prey such as walruses.
This way of hunting relies on ice. Polar bears will typically wait by holes in the ice or break into seal birth lairs just under the surface to catch the seals. As global temperatures rise, however, shrinking sea ice reduces access to their preferred prey.
While there is some evidence that polar bears are adapting to other food sources, such as reindeer and seabirds, these are unlikely to be enough to maintain the species at its current population size.
The continuing decline of sea ice has seen the bears classified as Vulnerable to extinction, with a 70% chance that their populations will decline by a third in the next 30 years.
The combination of declining populations and a reduction in sea ice will also divide the populations of polar bears further still. This puts them at risk of inbreeding, which could also increase their chances of extinction.
As part of efforts to better understand how these populations are each being affected by climate change, researchers have been investigating east Greenland's polar bear population. They found that bears living in the far southeast of the country travelled smaller distances and never interacted with any in the northeast.
Co-author Prof Beth Shapiro says, 'They are the most genetically isolated population of polar bears anywhere on the planet. 'We now know that this population has been living separately from other polar bear populations for at least several hundred years, and that their population size throughout this time has remained small.'
A closer look at this population suggests they could represent one way polar bears could survive the coming decades.
The polar bears of southeast Greenland are less dependent on pack ice than other populations. Image © Kristin Laidre/University of Washington
Why are southeast Greenland polar bears unique?
Intrigued by the isolation of the southeast Greenland bears, the researchers took a look at the quality of ice which was available in that region. While pack ice could sometimes be available, it was very infrequent and only rarely washed into the fjords where these polar bears live.
Instead they found that the bears used what is known as fast ice, which is sea ice that is frozen directly to the coastline. While polar bears can survive for up to half a year without food, the fast ice here was on average only available for 89 days which would push the mammals beyond their limit.
In order to supplement this, the bears are also using glacial mélange as a surface to hunt from. This is a type of ice which forms as freshwater glaciers reach the ocean and breaks apart. While the Greenland Ice Sheet from which these glaciers come is declining, it is believed that it will provide enough ice to act as a hunting platform for many years to come.
Co-author Dr Twila Moon says, 'The marine-terminating glaciers in southeast Greenland are a fairly unique environment.'
'These types of glaciers do exist in other places in the Arctic, but the combination of the fjord shapes, the high production of glacier ice and the very big reservoir of ice that is available from the Greenland Ice Sheet is what currently provides a steady supply of glacier ice.'
Living on free floating ice, half of the bears in the study were washed out to sea. On every occasion, the researchers found the bears swam back to shore and returned to the fjord they originated it.
While these fjords offer a continuing supply of ice, they also contribute to these polar bears being much more genetically isolated than other populations. In general, adult females in southeast Greenland are smaller and have fewer cubs than those elsewhere, which may relate to the isolation of each fjord and the genetics of the group.
The researchers have called for further research to help identify how similar environments could provide sites of climate refuge for other polar bears, and protect this unique population.
'Preserving the genetic diversity of polar bears is crucial going forward under climate change,' Kristen said. 'Officially recognizing these bears as a separate population will be important for conservation and management.'
