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Polar bears lose genetic diversity as sea ice decline divides populations
Polar bears are becoming less diverse as sea ice declines, putting them at an increased risk of extinction.
As climate change causes less sea ice to be formed each year, populations of the bears are becoming increasingly fragmented. This means less mating between the different groups and therefore a decline in the genetic flow between them.
This is putting them at risk of inbreeding, and as a result potentially reducing their ability to adapt to climate change as their habitat changes.
The research, published in Proceedings of the Royal Society B, saw the bears lost up to 10% of their genetic diversity within the past 20 years, with separate populations now almost twice as different from each other as they were before.
Polar bears face a variety of threats, including a greater risk of starvation, as sea ice retreats. © Andreas Weith/Wikimedia Commons CC BY-SA 4.0
On thin ice
The Arctic is rapidly losing sea ice cover year-on-year. A 2018 review found that the rate of ice loss has picked up from a loss of 2.4% each decade to 3.4% by the millennium, with current Arctic ice now much younger and thinner than it used to be.
The timings of the ice's formation are also changing, with melt occurring three days earlier and freezing taking place a week later for every decade that passes. There could be a complete loss of ice cover in the Arctic during summer months within the next 20 years if current trends remain the same.
As well as having severe implications for the climate, the loss of sea ice will also have impacts on the species that call the Arctic home. At the bottom of the food chain, ice algae have seen their production decrease by 22% between 1980 and 2015 as sea ice has retreated.
At the other end of the food chain, species such as the polar bear will also take a hit. In addition to a loss of habitat, the bears are facing a greater risk of starvation and drowning as they are forced to travel further to find sea ice from which they can hunt for seals. But it is also making it harder for them to find mates.
New research from Norway suggests that impacts on their reproduction will go beyond just a difficulty in locating a mate. Focusing on the Svalbard archipelago, the team used DNA samples from polar bears collected over a 21-year period to assess if the loss of sea ice was having an impact on their genetics.
They found that across four different areas of the islands the bear populations were getting increasingly different over time. The bears previously mixed a lot resulting in lots of genetic diversity. But this diversity has declined over time alongside the decline in sea ice.
The bears are also changing their behaviours. Previously, a significant number of mostly male bears used the sea ice to move between groups on different islands. However, as the sea ice has declined more bears tend to stay put and only breed with other bears around the area of their birth.
This was most pronounced in north west Svalbard, where the highest levels of sea ice loss were observed.
There is now less breeding between bears from different areas of Svalbard than there was 21 years ago. © Sprok/Wikimedia Commons CC BY-SA 3.0
Bear with us?
With the different bears on Svalbard increasingly less able and less willing to mate with those from different areas, the researchers found that the groups of bears were becoming more and more distinct from each other.
Over the 21 years the researchers found each area studied began to take on its own distinct genetic signature, as bears with unique genetics died out without passing these genes on to their offspring.
The decreases in genetic diversity pose a challenge to polar bears as the Arctic continues to warm. An increase in warming will see new environmental pressures act on the animals as they attempt to adapt to the changing Arctic.
With lower genetic diversity, the chance of survival drops as there is less variation within the population meaning that the entire group becomes vulnerable to sudden changes.
This could be exacerbated by inbreeding amongst the bears. While its impact is currently low, the researchers warned that it may become more likely in future. Inbreeding often results in genetic combinations that are harmful to an individual's chance of survival, again making it more difficult for them to adapt to change.
The researchers also modelled a series of scenarios to assess how the polar bear would fare in the coming years, based on different amounts of gene flow and ice loss. If ice continues to decline and gene flow is restricted, then the populations will see further loss of genetic diversity.
Based on their findings, the team of scientists have called for the genetics of polar bears to be monitored to give them the best chance of survival and to preserve their diversity for the foreseeable future.
