For polar bears that pad and paddle around Hudson Bay, the trend toward an earlier melt and later freeze of Arctic sea ice is altering the timing of their seasonal migration in ways that leave the animals less time to feed.
Ice floes on the open water serve as hunting platforms for the bears, whose wintry diet of seals, snagged as they come up for air through breaks in the ice, builds the fat reserves polar bears need to survive on land during the sea-ice melt season.
The migration changes likely bode ill for the ability of the population to reproduce and to survive over the long term as global warming continues to build, say researchers who conducted a study published this week on the impact of climate on the area's polar bear migration patterns. The study appeared in Tuesday’s issue of the Journal of Animal Ecology.
Moreover, because the bears appear to have a strong sense of home turf, the researchers say the animals are likely getting off the melting ice earlier in order to return to familiar turf. If they stay on the moving ice to feed longer, they risk disembarking where they will have to spend more time and energy either returning to their usual range or exploring the new location for the best places to hunker down for the melt season.
The research team, led by University of Alberta biologist Seth Cherry, points out that this pickiness about territory would ordinarily work to the benefit of females bearing cubs. The mothers-to-be would not have to spend extra energy wandering in search of dens, because they already know where the best dens are located.
Polar bears, which the US Environmental Protection Agency listed as an endangered species in May 2008, have become fuzzy poster children for the some of the ecological effects of global warming – with reason, notes Dr. Cherry.
"It's true the public and media alike may be disproportionately concerned about polar bears" versus other ecological players above the Arctic Circle, he says. "However, there is scientific merit to the hype over polar bears and climate change."
As apex predators in the Arctic, he explains, polar bears sit at the top of the marine food chain. Monitoring their range, movement, and physiology makes them useful barometers for gauging larger environmental changes – in this case, global warming. And because they rely so heavily on the integrity of the food chain that supports them, they represent an early-warning network for changes that may be taking place with links father down the chain.
A US Geological Survey analysis of polar-bear population studies, published in 2010, put the number of polar bears Arctic-wide at some 24,600 animals. Some 5,000 roam the islands making up the Canadian Archipelago. About 11,900 animals are spread along the continental coasts open to the Arctic Ocean. Some 7,700 live around bays and basins where all the sea-ice tends to be seasonal, instead of a mixture of seasonal and multi-year ice. Hudson Bay falls into this category.
The bay boasts high biological productivity, so polar bears can gorge themselves in anticipation of the melt season. But the bay's melt season also has experienced some of the fastest growth.
Between 1979 and 2009, the melt season Arctic-wide expanded by at least 20 days, according to a study published four years ago by scientists from NASA and the National Snow and Ice Data Center in Boulder, Colo. That's a pace of 6.6 days per decade.
Hudson Bay, by contrast, has seen the melt season grow by more than 10 days per decade – among the highest rates for any region in the Arctic. It's also one of the southernmost habitats for polar bears, so in effect they live on a climatological knife's edge compared with their northern kin. Any effects of warming are likely to show up there earlier than at higher latitudes, Cherry says.
Between 2004 and 2009 the team tagged 68 female polar bears with GPS collars. The collars were programmed to phone home every 4 hours. The researchers tagged only females, because unlike males, their necks are narrower than their skulls. Males lose collars with a nod.
They compared migration patterns on and off the ice with a similar study done in the area between 1991 and 1997. In comparing polar-bear migration patterns from one period to the next, the researcher found that the bears remained landlubbers for increasingly long periods during the melt season.
The bears showed a surprisingly consistent sense of timing from one study period to the next, based on ice extent. They returned to land an average of 28.3 days after sea-ice cover had shrunk to 30 percent of its winter extent. They headed back onto the ice an average of 2.5 days after sea-ice had grown to about 10 percent of its winter expanse – reluctant to leave the restaurant and raring to return.
After looking at previous studies of the food supply's effect on polar bear reproduction and survival, the team suggests that the increase in the bears' fasting period likely is taking a toll on both. The bears likely to be hit hardest are the bears still shy of adulthood, because they can't hold as much body fat as an adult, the scientists suggest.
Indeed, some studies have tied a post-fasting hunger among bears to increasing run-ins between bears and humans in the western Hudson Bay.
Over the long term, the prognosis for polar-bear populations isn't encouraging, other research suggests.
A study formally published in the journal Ecology in 2010 – work influential in the process leading to listing polar bears as endangered – projected that if carbon dioxide emissions from burning fossil fuel and from land-use changes take a business-as-usual path through the end of the century, global warming would lead "to drastic declines in the polar bear population by the end of the 21st century."
Different populations in regions around the Arctic would be affected at different rates, the researchers noted. But even by mid-century, "the effects on polar bears will be severe."