Half of grizzlies' prehibernation diet made up of pine nuts which are being attacked by fungus

Grizzly bears in Yellowstone National Park can weigh up to 600 pounds. The whitebark pine nut weighs in at a thin fraction of an ounce.

Yet the tiny seeds -- embedded in neat, brown cones -- can make up more than half of a grizzly's pre-hibernation diet, one Canadian Journal of Zoology study shows.

But the fate of the nut is now in doubt.

Whitebark pines across the West are getting clobbered by an alien fungus and native beetle.

The attack on the whitebarks -- by Eurasian blister rust and the mountain pine beetle -- comes at the same time the U.S. Fish and Wildlife Service is poised to remove the park's renowned bears from the endangered species list.

''The Yellowstone grizzly is the Endangered Species Act's best recovery story," said Chris Servheen, a University of Montana professor and head of the federal agency's grizzly bear recovery team.

''We need to show that with cooperation, this law can work," Servheen said.

Since 1983, the bear population has roughly tripled to 600 animals roaming the 5.8-million-acre recovery zone.

Bears are spilling out into the Owl Creek Mountains, the southern Wind River Range and places where they haven't been in 80 years, bear researchers say.

A number of biologists, however, worry that vital bear foods in the heart of the recovery area may already be in peril.

''My experience is that the things happening in Yellowstone are things we really have no control over," said Dave Mattson, a U.S. Geologic Survey research biologist.

''Some things are beneficial. Some pose a dire threat," Mattson said. ''We won't likely have any good options for those."

While the National Wildlife Federation is backing the delisting of the grizzly bear, other environmental groups including the Sierra Club and the Natural Resource Defense Council are opposed and threaten to sue the federal government to block the move.

Once Endangered Species Act protections are removed, a host of new safeguards will ensure grizzly populations will not decline, Servheen said.

For example, new roads will be banned in national forests around the park, Servheen said.

State and federal agencies will monitor the bears and their habitat and responsibility for managing bears will be turned over to Montana, Idaho and Wyoming.

If bear numbers drop, the population could be returned to the endangered species list, Servheen added.

The fact that the bears have rebounded, critics say, doesn't necessarily mean they are recovered.

Female grizzlies mate only every second or third year. Only the shaggy musk ox -- like the bear a relic of the Ice Age -- breeds more slowly.

That means if grizzly deaths spike, it could tip the species into an ''extinction vortex," said Michael Scott, director of the Greater Yellowstone Coalition.

''You cannot take the second-slowest reproducing species in North America, manage it on a knife-edge and expect that in the long term it will survive," Scott said. ''Nature will always throw you a curve."

The combination of blister rust and the beetle outbreak is such a curve.

About 24 percent of Yellowstone area whitebarks were found to be infected with rust in a 2004 Forest Service survey.

''The history of blister rust is clear: it spreads and intensifies," said Diana Tomback, a professor at the University of Colorado at Denver who directs the Whitebark Pine Ecosystem Foundation. ''I am very fearful about the future of the whitebark pine."

Some whitebarks have genetic resistance to blister rust. By identifying those trees and cultivating their seeds, researchers hope to grow new rust-resistant forests in the future.

Whitebarks, however, have no defense against the invading mountain pine beetles, which have exploded into Yellowstone.

Federal bear biologists contend the rust is moving more slowly in Yellowstone than in northwest Montana and that the trees have co-evolved with mountain pine beetles for thousands of years.

''We don't know if whitebark pine will decline," said Chuck Schwartz, a U.S. Geological Survey researcher leading the Interagency Grizzly Bear Study Team.

Forest pathologists take a much different view.

''A lot of trees are going to be killed," said John Schwandt, national whitebark pine coordinator for the Forest Service.

''A lot of the seeds that grizzly bears really like are going to be greatly reduced, and not over decades but in the next 10 years," Schwandt said.

Federal managers contend the bears eat just about anything around Yellowstone, including grasses, roots, ants, miller moths, trout and elk.

''We're not saying things will always be stable," said Servheen. ''We're saying we'll be able to respond to changes to make sure the bear's OK."

In late summer and fall, grizzly bears follow an urge to gorge in order to store enough fat to survive hibernation, a condition called hyperphagy.

A 350-pound female needs to average 18,000 kilocalories, or the equivalent of 30 Big Macs a day, according to Mattson. Big males and sows with cubs need even more food.

Given a chance, the bears will even eat the trash. Until the 1970s, watching dump-diving grizzlies was a popular attraction at Yellowstone.

When the dumps were closed, hungry bears invaded park campgrounds and more than 225 had to be killed.

That, critics of delisting say, serves as a cautionary tale.

There are already fewer native cutthroat trout swimming up spawning streams, where bears hunt them, because they're being gobbled up in Yellowstone Lake by illegally introduced lake trout.

In years when whitebarks naturally produce fewer nuts, more Yellowstone grizzlies move to the park's edges, where they find hunters' elk carcasses.

Foraging on the edge of the park is dangerous for bears because that's where people live. Humans are responsible for 85 percent of Yellowstone bear deaths, said Schwartz.

At least 25 bears died in the greater Yellowstone area last year, including eight breeding-age females killed by people.

In fact, the six-year average of female bear deaths exceeded the maximum allowed by the recovery plan in 2004 and 2005, according to federal data.

The deaths fall within new mortality limits that are based on more thorough population estimates now being substituted for the old ones, federal managers say.

Dealing with bears near the recovery zone boundary will be the toughest challenge for state and federal biologists, Schwartz said. Problem bears have to be removed quickly so the public will accept living with the rest.

''The Yellowstone ecosystem has a fixed amount of habitat," Schwartz said. ''Once bears fill that habitat, the excess bears will probably wind up where they don't belong and are going to die."

Grizzlies don't recognize recovery area boundaries, forest management plans or endangered species politics.

All they care about is finding food -- a lot of it.

And the reddish crowns of dying whitebark pines continue to advance across Yellowstone.

This summer, Tom Reed, a former Wyoming Game and Fish spokesman, rode horseback through dead whitebark stands in Yellowstone that were healthy just three years ago.

''As a guy on the ground, I would say it doesn't look too good," he said.