Bears are being seen, and killed, in places where they were absent for decades. Researchers suspect climate change is wiping out one of the bear's food sources and they worry the trend will continue as the animals roam farther in search of food.

Yellowstone's 600 grizzlies were removed from the endangered species list in 2007, following a recovery program that cost more than $20 million. If the death rate stays high for a second consecutive year, that would trigger a review of the bear's endangered status.

"Last year may have been one of those fluke years," said Chuck Schwartz, a bear biologist with the US Geological Survey. "Last year could be the beginning of a trend."

Federal officials say 48 bears were killed by humans last year, out of 71 total deaths. At least 20 of the bears died at the hands of hunters who shot in self-defense or after mistaking them for other animals.

"It's kind of a spur-of-the-moment thing. All you see is a big bear coming at you full speed," said Ron Leming, a Wyoming elk hunter who said he survived an attack from a 500-pound (225-kilogram) male grizzly after his father shot it dead with an arrow.

"If you play dead he might sit there and eat you," Leming said. Schwartz and other biologists who study grizzlies insist the population in the 15,000-square-mile (38,850-square-kilometer) Yellowstone region of Montana, Idaho and Wyoming remains strong for now, growing on average 4 to 5 percent a year.

Yet they acknowledge climate change could prove the wild card that puts that growth in check. An epidemic of beetles in Yellowstone's high country has laid waste to tens of thousands of acres of whitebark pine trees, which have seeds that some grizzlies rely on as a dietary staple.

Beetle epidemics are cyclical in the Northern Rockies. The latest one has been prolonged by several consecutive winters in which subfreezing temperatures did not last long enough to knock back the infestation.

If a warming world leads to less whitebark pine, environmentalists fear grizzlies will become more aggressive in challenging hunters, contests that bears usually lose.

"The prospect is that every year is going to be a bad food year because of what's happening to whitebark," said Doug Honnold, an attorney for the group Earthjustice.

Citing dying pine forests as a major threat, Honnold's group sued the federal government in an attempt to get Yellowstone grizzlies back on the endangered species list.

Christopher Servheen, bear recovery coordinator with the U.S. Fish and Wildlife Service, said his agency is closely monitoring the population and already crafting a plan to stem the death rate.

Meanwhile, conservationists are trying to encourage hunters to use mace-like bear spray as a non-lethal alternative to keeping them at bay.

Other measures being considered are stepped-up public education efforts and restrictions on livestock grazing, to prevent bear attacks on sheep and cattle.

Gregg Losinski, an education specialist with Idaho Fish and Game, said promoting the possibility of future grizzly bear hunts might convince more people to buy into bear conservation.

Even with those measures, researchers say bear deaths are inevitable as the animals return to a different landscape than that occupied by their ancestors.

Before early European settlers drove bears to near extinction, there were an estimated 50,000 grizzlies in the western half of the United States.

Yellowstone's bears are among about 1,500 that have since repopulated the Northern Rockies. They must compete for space with several million tourists, and property owners.

"Some people say, 'This is terrible, there's more bears killed now than in many years,"' Servheen said. "Well, there's more bears now."