Polar Bears, Ringed Seals, and the Complex Consequences of Climate Change
It’s well known that climate change is bad news for polar bears, but they are just one part of Arctic ecosystems. Climate change is impacting different species at different rates, leading to alterations in how they interact. Understanding these delicate and changing relationships can help improve efforts to mitigate the negative effects of climate change on sea ice-dependent animals.
The Arctic is warming at a rate three times greater than the global average, and sea ice coverage is declining rapidly. Sea ice in the Svalbard region of Norway is declining at an especially precipitous rate.
Recently, researchers from the Norwegian Polar Institute and the Arctic University of Norway investigated the movement of polar bears and their traditionally most important prey, ringed seals, in coastal areas in Svalbard. In 2006, there was a sudden decline in sea ice in the region, so the researchers compared the space use patterns of the two species before and after the sea ice collapse.
“Before the change in sea ice conditions in Svalbard, ringed seals and polar bears both spent most of their time close to tidal glacier fronts in the summer,” says Charmain Hamilton, lead author of the study. “These are very profitable foraging areas for seals, and the land-fast ice that used to persist in coastal areas until late summer provided a platform for polar bears to hunt hauled-out seals.”
But after the change in sea ice conditions, which caused the land-fast ice to melt earlier in the year or to fail to form entirely, polar bears spent less time close to tidal glacier fronts. Ringed seals, meanwhile, spent the same amount of time near tidal glacier fronts before and after the sea ice change. Ringed seals can haul out and rest on very small pieces of glacier ice, but bears do not hunt them as readily on these “bergy bits.”
Deprived of their traditionally most important food source, polar bears moved greater distances daily and spent more time close to ground-nesting bird colonies.
Hamilton says polar bears can eat over 90% of the eggs in a ground-nesting bird colony, so this shift in prey items could have substantial effects on bird populations. It is also unclear at the moment whether eating more birds can energetically and nutritionally compensate for eating fewer ice-associated seals. In the long-term, birds may not be a viable option for replacing seals in polar bear diets.
This study demonstrates why it is necessary to consider multiple species and how they interact when investigating the impacts of climate change. Polar bears do not live in a vacuum. The sea ice decline has impacted the degree of spatial overlap between bears and seals, and this has consequences for the larger Arctic ecosystem.
Hamilton says research like this help tease apart what can be quite complicated changes in relationships between predators and prey and give scientists getter predictive capacity.
“By understanding how ice-adapted species, such as ringed seals and polar bears, are being impacted by climate change, society will be better able to direct management efforts for species conservation,” she says.
Hamilton, C. D., Kovacs, K. M., Ims, R. A., Aars, J. and Lydersen, C. (2017). An Arctic predator-prey system in flux: climate change impacts on coastal space use by polar bears and ringed seals. J Anim Ecol. doi:10.1111/1365-2656.12685.