Research may help explain declines in cub production and body condition
By Summit Voice
SUMMIT COUNTY — A 10-year study of polar bears around western Hudson Bay offers new clues to how the predators will cope with global warming.
The research, published in the British Ecological Society’s Journal of Animal Ecology, focused on how sea ice conditions drive polar bears’ annual migration on and off the ice.
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.
“The data suggest that in recent years, polar bears are arriving on shore earlier in the summer and leaving later in the autumn,” said Dr. Seth Cherry, of the University of Alberta. “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.”
Dr Cherry and colleagues wanted to discover how earlier thawing and later freezing of sea ice affects the bears’ migration.
“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 said.
“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 said.
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.
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,”Cherry said.
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.