Along the tidal glacier fronts of Svalbard, an archipelago halfway between Norway and the North Pole, polar bears have changed their hunting practices. A recent study published in the Journal of Animal Ecology indicates the new behavior is a response to rapidly disappearing sea ice. Charmain Hamilton and other researchers from the Norwegian Polar Institute mapped changes in the spatial overlap between coastal polar bears and their primary prey, ringed seals, to better understand how the bears are responding to climate change. The results don’t bode well for the long-term survival of polar bear populations: as sea ice continues to shrink in area, ringed seals—calorie-rich prey that are high in fat— have become increasingly difficult to catch during the summer and autumn. The bears are now finding sources of sustenance elsewhere: in the archipelago’s thriving bird colonies.
The Arctic is warming at a rate three times the global average, and the sea ice in the Svalbard region is experiencing a faster rate of decline than in other Arctic areas. As Charmain Hamilton reported in an interview with GlacierHub, the findings could demonstrate what the future holds for the top predator elsewhere. “The changes that we are currently seeing in Svalbard are likely to spread to other Arctic areas over the coming decades,” she said.
Svalbard’s polar bears exhibit one of two annual movement patterns: some follow the sea ice as it retreats northward during the summer, while others stay local, inhabiting coastal areas throughout the year. Both groups of bears depend on sea ice as a platform to hunt ringed seals. Given a rapid decline of sea-ice levels that began in 2006, Hamilton and other researchers wanted to know if the coastal bears were still hunting ringed seals under the deteriorating conditions.
The researchers compared satellite tracking data for both polar bears and ringed seals from the periods 2002-2004 and 2010-2013 to assess whether the predator-prey dynamic had shifted. The data was analyzed according to season, with researchers paying careful attention to the dynamics of spring, summer and autumn.
In spring, access to fat-rich ringed seals is critical, particularly for mothers weakened from nourishing their young in winter dens. The study shows that coastal polar bears continued to spend the same amount of time near tidal glacier fronts in spring as they did when sea ice was more abundant. The authors conclude that the declines in sea ice in Svalbard have not yet reached the stage at which bears must find alternative hunting methods during the spring. This could help to explain why cub production is not currently declining.
However, during summer and autumn, bears are spending less time in the areas around tidal glacier fronts. The study shows a significant decrease in the amount of time bears spent within 5 km of glacier fronts and a sharp increase in the distances they traveled in search of food per day. The ringed seals, on the other hand, have remained near the glacier fronts. As Hamilton reported to GlacierHub, “The reduced spatial overlap between polar bears and ringed seals during the summer indicates that the reductions in sea ice have made it much more difficult for polar bears to hunt their primary prey during this season.”
As sea ice recedes, ringed seals are increasingly relying on calved pieces of glacier ice as shelters and resting places. Since these pieces of calved ice are no longer connected to land-fast ice, polar bears can no longer walk up to the seals or wait by their breathing holes, but have to attack from the water. This involves swimming surreptitiously up to seals resting on calved glacier ice and bursting onto the platform to make a kill. But this specialty hunting technique has only been observed in a minority of bears.
So where are the coastal bears getting their calories during summer and autumn? The study shows that along with the marked decline in sea ice, the coastal bears were spending more of their time around ground-nesting bird colonies. At present, these tactics seem to be working. The bears are benefiting from a large increase in the populations of several avian species in the region, which Hamilton attributes to ongoing international conservation efforts along migration routes. While an increase in the amount of time polar bears spend on land is considered a cause of deteriorating health in other bear populations, the adult bears and cubs of Svalbard have not shown marked signs of decline.
Have the bears found a lasting alternative? Jon Aars, a research scientist and one of the co-authors on the paper, doesn’t think so. In an interview with GlacierHub, Aars emphasized that while birds and eggs provide the bears with an alternative to burning fat reserves as they wait for the sea ice to return, the dynamic is not permanent. “It is not likely that switching to eating more birds and eggs is something that can save polar bears in the long run if sea ice is gone for the whole of, or most of, the year,” he said. “We do think the bears are still dependent on seals to build up sufficient fat reserves. And it is limited how many bears can utilize a restricted source of eggs and birds on the islands.”
The bears have adapted to the current change in their environment but may not be able to adapt as well in the future. The authors of the paper point out that the increased rates of movement required to hunt avian prey increases the bears’ energy needs. Additionally, as more bears rely on avian prey, their high rate of predation means that bird populations on the archipelago will likely decline, causing bears to alter their hunting strategies again. Ringed seals have not changed their own spatial practices, and the authors propose that more bears could learn, or be forced to learn, the aquatic hunting method.
However, ringed seal populations are in decline due to the loss of sea ice, according to Hamilton. Thus, the future of both species in the region is uncertain. In sensitive environments like the Arctic, predator-prey dynamics are fragile, particularly for species of such high trophic positions. In the future, Hamilton would like to include other Arctic marine top predators in similar studies to better understand how Arctic marine mammal communities are being impacted.