As global warming causes Arctic ice to melt , archipelago-based polar bears are having more difficulty reaching each other, especially during mating season, resulting in what researchers have dubbed an “alarming” drop in gene flow and genetic diversity due to inbreeding.
A team at the Norwegian Polar Institute in Tromsø have been collecting small tissue samples from polar bears (Ursus maritimus) inhabiting Svalbard’s islands in the Barents Sea since 1995. Most of these bears usually roam across sea ice throughout the archipelago and often mate with bears in other regions.
In recent decades, however, global warming has led to a rapid drop in the extent and the thickness of sea ice cover – in particular on the westernmost island, Spitsbergen. It has also caused the winter ice to melt earlier in spring, encroaching on the animals’ breeding season.
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“If there’s still ice, bears will move across quite easily. But as soon as the ice melts, and it’s now vast ocean waters, that oceanic water is a barrier,” says at the Norwegian Institute of Bioeconomy Research in Svanvik.
To understand the effect of this on polar bear genetics, Maduna and his colleagues analysed the DNA of 622 bears, representing four geographical regions of the archipelago: north-west, north-east, south-west and south-east.
They found that genetic diversity has dropped since the mid-1990s, by as much as 10 per cent in the most affected region – the north-west – suggesting that the bears are mating more locally than before.
In addition, the bears have started to form genetic subpopulations, with a significant amount of genetic differentiation from one region to another. Again, it was the most affected region – the north-west – where the changes were most substantial, according to the researchers.
The modelling revealed that, as global warming continues, these genetic changes will intensify. Over time, that could threaten the bears’ survival by limiting their ability to cope with disease, say the researchers.
“The magnitude and rate of loss of genetic diversity and gene flow that we observed is alarming considering that polar bears have historically shown relatively little genetic differentiation even on a global scale,” the scientists report.
Proceedings of the Royal Society B
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