The last Ice Age peaked between about 27,000 and 19,000 years ago. At this time the Arctic was buried under kilometers of glacial ice sheets, and so marine mammals were pushed southwards to areas of ice floes and more open water. Walrus survived in some areas of the Atlantic located further to the south, and as soon as climates warmed again, the ice edge retreated and walrus populations pushed quickly northwards again. This combination of warming and climate-driven dispersal led to local walrus populations becoming more genetically differentiated.
In addition, during the last thousand years, human hunting and commercial exploitation has led to numerous local extinction events. These include the expansion of Norse settlers into Iceland and the North Atlantic in pursuit of walrus ivory, which was a valuable trade good, and in more recent times, with industrial scale culling of walrus populations.
Currently, the genetic diversity of walrus stocks is a fragment of what existed earlier, making them even more vulnerable to pressures such as accelerating ice loss, disturbance by Arctic shipping, resource extraction and mass tourism, according to the researchers.
"As Arctic sea ice retreats, the depleted walrus stocks will disperse further into smaller and more isolated pockets, where the genetic isolation and reduced connectivity makes them ever more vulnerable to other stressors such as Arctic shipping, resource extraction and large-scale tourism", says Peter Jordan.
"Our results underscore the urgency of rethinking conservation goals for species in rapidly changing Arctic marine environments", he concludes.