Blood on the ice. A torso ripped open. Bits of hide scattered to the wind.
On the frozen surface of the Arctic Ocean north of Alaska, these signs usually chronicle a quick meal by the Earth’s largest non-aquatic predator.
Photo by USFWS
Polar bears have always made their living by scarfing down ringed seals, what federal biologist Steve Amstrup likes to call “fat pills.” Over tens of thousands of years, the bears have evolved the curiosity and patience to find these subnivian lairs hidden to human eyes — and the lightning reflexes to bust in and seize the seal before it can escape into the sea below.
But this was no seal carcass. It was a female polar bear. And it had been stalked, killed and consumed by an adult male bear.
“This is not something that we had ever seen before,” said Amstrup, who has studied polar bears north of Alaska for the past 26 years. “Cannibalism is common in bears but not under these circumstances. … The animal was killed in classic polar bear fashion by bites to head and neck, resulting in canine penetration through the skull.”
Twice more in the spring of 2004, U.S. and Canadian scientists found carcasses of polar bears that had been preyed upon. These disturbing scenes marked one more indication that something fundamental has changed for polar bears across the Arctic, where retreating summer ice and thinning multi-year floes have shrunk the habitat that polar bears need to survive.
For the first time ever, scientists report spotting polar bears that drowned while struggling to swim across open ocean. In the spring 2006, Amstrup and others found three and possibly four bears that had apparently starved to death.
And that’s not all.
Fewer cubs have been surviving the summer. Studies have documented a decline in weight in bears of the Beaufort Sea. The bears are summering over deeper water as the ice edge pulls back, a locale with fewer seals. And more female bears have been digging winter dens to birth cubs on land in response to deteriorating ice out to sea. The bears appear to be finding it harder to find enough to eat and live in the face of changing ice.
Will the iconic white carnivores face slow elimination over the next century as their vast frozen habitat melts away?
“Are we just going to become polar bear historians?” Amstrup asked at one point. “Are just going to see continuing increases in temperatures and continuing declines in ice and nothing changing as far as we can see?”
Polar bears may be the “sentinels” of climate change, according to Amstrup, a lead scientist with the Alaska Science Center of the U.S. Geological Survey in Anchorage. He made the remarks during two presentations in mid-January in Anchorage. They came as the U.S. Fish and Wildlife Service considers a proposal to list polar bears as threatened under the Endangered Species Act. People have until April 9 to comment.
Photo by USFWS/Scott Schliebe
It’s about ice. Arctic sea ice cover in late summer has shrunk 7.7 percent per decade since satellite observations began in the late 1970s. The ice pack has thinned by almost one-third since the 1960s and 1970s, as more and more of the multi-year floes flush into the North Atlantic Ocean. A record for minimum ice extent was set in 2005, and nearly matched in 2006. An area the size of Texas has basically disapeared from the summer cap.
Since polar bears have evolved to live and hunt on ice, such a meltdown literally means their habitat is disappearing from beneath their paws. If the decline in ice continues as predicted by many climate models, the bears will be driven ashore or onto smaller and less stable floes in their endless feast-or-famine hunt for seals to eat.
Many animals will then sicken and starve. Populations will die out.
As many as 25,000 bears live across the Arctic in 19 separate populations. Where ice retreat has been most dramatic, polar bear numbers have already started to plunge. In western Hudson Bay off Manitoba, for instance, the bears must spend as much as three weeks more each season fasting on land because the ice melts sooner and forms later each year. The population has suffered a 22 percent drop in number as a result.
The fear is that Alaska’s bears, and other polar bear populations, will experience similar declines in step with retreating ice.
A new population estimate suggests that 1,500 bears may roam the Beaufort Sea in a population shared with Canada. Scientists are not sure whether that represents a decline from earlier counts conducted with different methods.
Other comments by Amstrup:
“In the spring 2006, we found four dead polar bears that had been previously known to be in good condition,” he said. One was too far gone to figure out the cause of its death. “But three of them we found in a very emaciated condition, and they apparently starved to death. … We’ve been studying the survival of polar bears for a long time and we hadn’t seen observations like this before.”
Scientists have documented an increase in cubs emerging from dens in the spring, but a decrease in yearlings. “Tremendously fewer females captured with cubs,” Amstrup said.
“What this seems to suggest is that polar bears are going into their dens, they’re having cubs, they’re coming out of their dens with cubs and the cubs aren’t surviving.”
Another study presented at the Marine Science Symposium in Anchorage reported that the number of polar bears denning on land has been rising fast since the mid-1990s. After using satellite tracking collars to pinpoint 383 “denning events” over over 20 years, the scientists found the proportion of dens on pack ice declined from 67 percent between 1985 and 1994 to only 37 percent between 1995 and 2005.
The shift, the scientists said, was likely due to ice that freezes later in the season, isn’t as thick and doesn’t build pressure ridges where bears can dig out dens to nurture cubs. Instead, the females were forced to excavate dens on land, often in drifts formed along shoreline bluffs.
“If these ice trends continue, as predicted, we expect the proportion of polar bears denning on coastal habitats will continue to increase, until such time as the autumn ice retreat precludes offshore pregnant females from reaching the Alaska coast in advance of denning,” wrote the three authors, Amstrup, David Douglas and Anthony Fischbach, of the USGS Alaska Science Center.