Polar bears face swimming to land or 'ecological trap' as sea ice diminishes
Changing sea ice conditions are forcing polar bears to adapt. New research shows that a growing percentage of polar bears are coming to land and becoming dependent on human provisions for food, while those that stay on the dwindling sea ice to continue natural polar bear behavior may be floating on an ecological trap.
Researchers with the USGS Alaska Science Center have noticed the behavioral changes in polar bears on the Southern Beaufort Sea over the last 15 years and more recently a team began studies to determine which behavior was better for the bears. The researchers used GPS collars with video cameras and an accelerometer to track the bears, calculate how much energy they used, and compare the energy requirements of coming to land during summer months versus staying on the sea ice.
"Going into it we thought it's surely going to be more energetically expensive to come to shore, because often times bears are staying on the sea ice until the last possible minute before they come to shore," Todd Atwood, a research wildlife biologists with USGS said. "In some cases bears are swimming 400, 500 kilometers to get to land. Swimming is a lot more energetically expensive than walking. So we expected them burn through a lot more energy to get to land, and that's what we found."
By pairing the GPS camera collar with a tri-axial accelerometer, the researchers were able to estimate how much energy bears used for different behaviors by calculating overall dynamic body acceleration.
The study found that in August, bears moving to land required bears to use an average of 22 percent more energy than staying on the pack ice.
"But we also found that they were more tolerant of highly deteriorating ice conditions. So they would stay on the bad ice longer than their counterparts that made the move to the pack ice, and by staying on the bad ice they were able to hunt a little bit longer. So they made up for some of that extra energy they burned," Atwood said.
Yet for the bears that come to land, there is a bigger buffet that doesn't require the energy required to hunt seals - bowhead whale carcasses.
Atwood says that during the summer both bears on the sea ice and those that come to land spend most of the time fasting, moving little in order to conserve energy. However, during whaling season, land-based bears become more active to move to areas like Cross Island north of Prudhoe Bay and outside of Kaktovik where the remains of subsistence harvested bowhead whales are discarded.
"What the land-based bears do is essentially they switch prey. Rather than feeding on seals, they switch over to these bowhead whale remains. It's still a marine mammal, but it's obviously a resource subsidy that's coming from humans," Atwood said.
Polar bears scavenge near a whale bone pile.
Currently, the proportion of the Southern Beaufort Sea polar bears that come to land in the summer is small - around 15 to 20 percent - but Atwood says the behavior is becoming more common. The amount of whale carcass discarded is sufficient for the number of bears currently choosing to come ashore. However, as more bears come to land and seek out discarded bowhead carcasses, the amount of food available per bear will decrease.
"It definitely exposes them to risk associated with being in close proximity to people. When polar bears come to these bone piles to food, they're coming into areas where there's a relatively high, by arctic standards, human footprint," Atwood said. "A lot of bears will come into the mainland, too, once the food at the bone pile has been exhausted because now they're looking for other human provision foods like garbage. In villages, because the Kaktovik bone pile is adjacent to the village of Kaktovik, and because those whale remains that the villager gets for subsistence are sometimes stored outside, the smell of that will bring bears into the community. So that presents a risk to people, a human safety risk associated with bears coming ashore whether it's in the industrial footprint or near the villages."
Although bears coming to shore expend significantly more energy to get to land than those who choose to spend the summer on the sea ice, the study shows that the benefits of having access to the scraps of subsistence harvest outweigh the cost, meaning that the land bears have an energetic advantage over those that stay on the ice.
"You can have a debate whether that advantage that they get is fraught with other cascading effects, like influencing risk of human bear interactions and conflict, and that's I think a legitimate debate to have," Atwood said. "But purely from the bear perspective, it's a benefit to them."
Although the majority of polar bears in the Southern Beaufort Sea sub-population continue to spend the summer months on the sea ice, the researchers say the diminishing ice pack may be functioning as an ecological trap.
"It's this idea that a habitat that looks good might actually be bad for you. And you in this case - the sea ice over the Arctic base - it looks good to polar bears because it's still sea ice habitat and that's what polar bears are hardwired to exploit. But it's not high quality habitat because it's not over shallow water. It's not over biologically productive areas where there's also high densities for prey," Atwood said. "So from the outside it looks really attractive, but underneath it's relatively depauperate. There's not a lot of food available for them."
Atwood says the concept of ecological traps has gained scientific interest in the last 15 years as the effects of climate change have forced animal populations to respond. Previous research established that ecological traps have been documents in both land-based and marine systems.
The study found that declines in abundance, survival and body condition of the bears combined with the lack of sea ice and the use of less than optimal sea ice are in line with conditions that could create ecological trap.
"So by not being able to subvert that hard-wiring to seek out sea ice habitat, they stick with a habitat that is not optimal for them and they can't meet their energetic demands as much as they could in other types of habitat," Atwood said.
Todd Atwood is a wildlife research biologist and project leader for the USGS Alaska Science Center.
The study suggests that since polar bears learn habits from their mother, some bears in the population may not be aware of that spending the summer on land is a possible alternative. Thus, their decision to continue living summer months on unproductive sea ice may be based on inherited habits regardless of the energy cost.
With more polar bears learning that they can survive better by coming to land, the potential challenges for the bears extend beyond just direct conflict with humans.
During the time that polar bears on land are fasting in the summer, they move very little until they have an opportunity to feed at the bowhead whale bone piles. On days when they are fasting, biologists have found they lose 2 to 3 kilograms of weight per day.
"As the Arctic opens up for more and more development and more and more commercial activities - tourism, shipping, oil and gas development and extractions - that's going to put a strain on some of the terrestrial resources that polar bears rely on," Atwood said. "If you do something that disturbs them, that causes them to get up and move, now you're kind of compounding the energetic deficit they are entering into."
Biologists have found that weight and fat content is an indicator of reproductive success for female polar bears. Mothers with less fat give birth to cubs with lower survival rates. Disturbing fasting polar bears causes them to move and burn more energy.
"I think it's really going to be really important to understand the kind of energetic landscape of terrestrial habitats for polar bears so we know what areas we should avoid so that we don't disturb them, cause them to incur any additional energetic deficits, and what areas they're okay being disturbed because they're close to food subsidies that they can make up that deficit from," Atwood said.
was published in the journal