One of the most southerly populations of polar bears in the world – and the best studied – is struggling to cope with climate-induced changes to sea ice, new research reveals. Based on over 10 years' data the study, published in the British Ecological Society's Journal of Animal Ecology, sheds new light on how sea ice conditions drive polar bears' annual migration on and off the ice.
Lead by Dr Seth Cherry of the University of Alberta, the team studied polar bears in western Hudson Bay, where sea ice melts completely each summer and typically re-freezes from late November to early December. "This poses an interesting challenge for a species that has evolved as a highly efficient predator of ice-associated seals," he explains. "Because although polar bears are excellent swimmers compared with other bear species, they use the sea ice to travel, hunt, mate and rest."
Polar bears have adapted to the annual loss of sea ice by migrating onto land each summer. While there, they cannot hunt seals and must rely on fat reserves to see them through until the ice returns.
Dr Cherry and colleagues wanted to discover how earlier thawing and later freezing of sea ice affects the bears' migration. "At first glance, sea ice may look like a barren, uniform environment, but in reality, it's remarkably complex and polar bears manage to cope, and even thrive, in a habitat that moves beneath their feet and even disappears for part of the year. This is an extraordinary biological feat and biologist still don't fully understand it," he says.
From 1991-97 and 2004-09, they monitored movements of 109 female polar bears fitted with satellite tracking collars. They tagged only females because males' necks are wider than their heads, so they cannot wear a collar. During the same period, the team also monitored the position and concentration of sea ice using satellite images.
"Defining precisely what aspects of sea ice break-up and freeze-up affect polar bear migration, and when these conditions occur, is a vital part of monitoring how potential climate-induced changes to sea ice freeze-thaw cycles may affect the bears," he says.
The results reveal the timing of polar bears' migration can be predicted by how fast the sea ice melts and freezes, and by when specific sea ice concentrations occur within a given area of Hudson Bay.
According to Dr Cherry: "The data suggest that in recent years, polar bears are arriving on shore earlier in the summer and leaving later in the autumn. These are precisely the kind of changes one would expect to see as a result of a warming climate and may help explain some other studies that are showing declines in body condition and cub production."
Recent estimates put the western Hudson Bay polar bear population at around 900 individuals. The population has declined since the 1990s, as has the bears' body condition and the number of cubs surviving to adulthood.
Because polar bears' main food source is seals, and these are hunted almost exclusively on sea ice, the longer bears spend on land, the longer they must go without energy-rich seals. "Climate-induced changes that cause sea ice to melt earlier, form later, or both, likely affect the overall health of polar bears in the area. Ultimately, for polar bears, it's survival of the fattest," says Dr Cherry.
He hopes the results will enable other scientists and wildlife managers to predict how potential climate-induced changes to sea ice freeze-thaw cycles will affect the ecology, particularly the migration patterns, of this iconic species.
Seth Cherry et al (2013). 'Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics', doi: 10.1111/1365-2656.12050, is published in the Journal of Animal Ecology
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
Host Audie Cornish talks with Drew Gronewold, a hydrologist at the National Oceanic and Atmospheric Administration, about why water levels in lakes Superior, Michigan and Huron are rising.
Tornadoes in the United States are increasingly coming in swarms rather than as isolated twisters, according to a study by U.S. government meteorologists published on Thursday that illustrates another trend toward extreme weather emerging in recent years.
Just a few years ago, authors were predicting production would soon hit a peak and then decline. But since then, supplies have surged. So are the forecasters now slapping themselves in the head?
The presence of carnivores, which control herbivore numbers, helps plants without thorny defences thrive, a study of life on the savannah reveals.
The roll-out of cheap energy-saving street lights with a bluish glare has little regard for people or wildlife. There is a better way, says Jeff Hecht
In the Pacific Northwest, the salmon are running. They migrate from the ocean into rivers where they eventually spawn on gravel beds. But on the Washougal River near the border of Washington State and Oregon, Ben Tracy found the Salmon are hitching a ride.
Farmers will be able to plant types of corn and soybeans that can tolerate doses of two weedkillers. It may be one of the most significant developments the world of weedkilling in more than a decade.
From a floating house to a mobile city shaped like a giant lilypad, designers offer up some wild solutions for a wetter future
Italy will become the first country in Europe to legally require "advanced biofuels" in cars and trucks, the BBC has learned.
A US Department of Defense report says the impacts of climate change could fuel armed insurgency and challenge governments, and aims to prepare the US army for the worst