Dubois, Wyoming -- As ecological drivers go, you wouldn't think an insect roughly the size of a rice grain would be that significant in the Greater Yellowstone Ecosystem. And yet, the mountain pine beetle, aided by a warming climate, is poised to send quite a shudder through the ecosystem.

Spanning roughly 20 million acres, the ecosystem that is home to Yellowstone and Grand Teton national parks is renowned for its diversity of wildlife. Bison, elk, pronghorn antelope, moose, mule deer, bighorn sheep, mountain lions, wolverines, martens, wolves, black bears and grizzly bears all can be found in the ecosystem.

The mountain pine beetle long has been another player in the ecosystem, having evolved right along with the lodgepole pine forests. Indeed, forest ecologists view the beetle as a natural agent for thinning forests. But climate change is enabling this beetle to make longer and longer forays into the upper elevations of the ecosystem, inroads that are seriously jeopardizing another key species, one whose disappearance could have dire ramifications for the overall health of the Greater Yellowstone Ecosystem in general and the grizzly bear specifically.

"What's occurring here is the potential loss of an ecosystem," says Dr. Jesse Logan, a retired U.S. Forest Service entomologist who is a leading expert on the mountain pine beetle.

Dr. Logan's comments came during a recent trek into the Wind River Range above Dubois, Wyoming, to see the extent of beetle infestation into the mountains' whitebark pine forests. From the air the impact is easy to spot -- infested trees first turn red as their needles die, and then go a ghostly grey as the needles fall off.

Since lodgepole pine co-evolved with the beetle, that species has developed an on-board defense system that can slow the beetles -- thick, gooey resin that smothers the bugs, and even an odor that drives them away. Beyond that, the trees rapidly reforest themselves.

Whitebark pines, however, did not co-evolve with the beetles and have no similar defenses. They also can take 75 years of growth before they sprout their first pine cone. Plus, they rely largely on the Clark's nutcracker, which feasts on whitebark seeds, to, basically, plant new whitebark pine stands by caching seeds that later go on to germinate.

Barring a dramatic change from the current course of events, the beetles will wipe out the mature whitebark pines. And the ripples of that episode will be felt throughout the ecosystem.

The problem, you see, is that in the Greater Yellowstone Ecosystem the whitebark pine is a key component of the grizzly bear's fall diet. As the days grow shorter and the temperatures fall, grizzlies spend most of their waking hours trying to pack on the pounds. Whitebark pine nuts are particularly nutritious, and in the Greater Yellowstone Ecosystem grizzlies seek out squirrel middens piled high with the pine cones to feast on.

Studies in the ecosystem have shown that when there's a good whitebark pine nut crop, sows gorge on them and head into hibernation both fatter and healthier. A key result is that they have larger, and healthier, cub litters than sows who go into hibernation with depleted reserves, says Louisa Willcox, who has been the Natural Resource Defense Council's Wild Bears Project director since 2003.

"Whitebark pine is the engine that dries the health of this (grizzly) population, and it's in trouble," she said during the visit into the Wind River Range south of Yellowstone and east of Grand Teton to examine the health of the whitebark pine forests.

Unlike the Northern Continental Divide Ecosystem, which encompasses Glacier National Park, Ms. Willcox says Greater Yellowstone grizzlies have fewer choices to obtain their protein and calories from in fall. Glacier has much more extensive berry patches the bears can forage on. Too, the introduction of non-native lake trout into Yellowstone Lake has greatly impacted that body of water's native cutthroat trout population, and its shrinkage has limited that food source for the bears.

While there were extensive outbreaks of mountain pine beetles back in the mid-1930s and again in the 1970s, those were small compared to the one currently under way, according to Dr. Logan. Not only is much of the Intermountain West under siege, but so is a giant swath of British Columbia. When compared to the past infestations, he says, the current one is larger by "orders of magnitude."

Stopping the beetles in the past have been cold snaps. We're talking cold snaps in the range of 40 below zero for ten days in a row. In recent decades those have been very few and very far between, most recently because of the warming climate.

Dr. Steven Running is a terrestrial ecologist from the University of Montana. His primary research interest lies within climatology. A chapter lead author for the 4th Assessment of the Intergovernmental Panel on Climate Change, Dr. Running has more than a passing understanding of what's happening to the Earth's climate. In the Greater Yellowstone Ecosystem, he says, the warming temperatures that benefit the mountain pine beetle likely will doom the whitebark pine forests.

"If you take a regional look there may be some other ridges that are doing better, but to be realistic it's hard to image that these old-growth ones (will survive)," he says. "They're already dead. They're standing dead now. So there's no kind of miracle that can wake them up."

Even if the world's populations somehow managed overnight to vastly reduce the greenhouse gas contributions that are driving climate change, Dr. Running adds, it still would take decades for the warming to stop.

"If emissions dropped to 'stability point' tomorrow, we still have 20, 30, 40 years worth of warming already in the oceans," he says. "And so the trends we're on are going to continue more or less for decades."

Adding to the impact of the beetle's latest outbreak is the rampant spread of white pine blister rust, an invasive disease thought to have arrived in 1910 in British Columbia from France with imported eastern white pine seedlings. More than a few trees along Togowotee Pass between Dubois and Jackson and just south of Yellowstone already are infected with this disease.

While there are efforts under way to establish a strain of whitebark pine trees resistant to blister rust, there's no good solution to halting the mountain pine beetle, short of very, very cold winters. While a synthetic pheremone has been developed to ward off the beetles, its success has been limited.

"Blister rust can take years to kill a tree," says Dr. Diana Six, a forest entomologist at the University of Montana who specializes in bark beetles. "When you have a beetle mass attack, it can take three or four days. It's just amazing because it doesn't take much to kill these trees."

Down through the centuries whitebark pine forests have survived forest fires and droughts as well as previous beetle outbreaks. The current outbreak, however, seems poised to be the species' undoing.

High in the Winds, near 10,000 feet, Dr. Diana Tomback, a professor of biologist at the University of Colorado in Denver whose interests cover evolutionary and behavioral ecology as well as forest ecology, stares at a massive whitebark pine that probably germinated 1,000 years ago. Telltale beetle holes in its bark and "frass," a reddish, sawdust-like excrement beetles kick out of their holes, signal not just the tree's infestation with the pine beetles, but also its death.

Dr. Tomback refers to the tree as a "zombie," the walking dead because the infestation is an incommutable death sentence.

"This is a big boy here. It's another heart-breaker. It's really old," she says. "All the remaining old growth up here are really old. The survivors are not surviving."