YELLOWSTONE NATIONAL PARK, Wyoming- The bison here are fat and sleek right now. Absent are the jutting spines and protruding ribs sometimes seen at this time of year. And so far during this mild winter, the shaggy giants are staying in the park. Only one animal has left the boundaries. While the guns have been silent and the traps stand inactive, winter is far from over and things could change in the next couple of months if heavy snows arrive.

A Jan. 30 flight over the area counted 2,410 bison, nearly half of them in the western portion of the park. While an easy winter like this one means fewer dead bison outside the park, it doesn't mean much in the long term, according to Mary Meagher, a retired National Park Service biologist who has been studying bison for more than 40 years. Though there will be annual fluctuations, she predicts that bison numbers will continue to drop from the high point of 4,000 animals seen in 1994. "We're never going to see 4,000 bison in Yellowstone Park again," she said Friday. "A lot of times, things look pretty good until all of a sudden they go to hell," said Meagher.

She maintains that the system of groomed roads in the park has so altered the ecosystem that the park's bison herd can only decline over the long term. For the past decade, she's advocated shutting down the winter travel system in Yellowstone. "What you see is deceptive," she said. "The bison can look great, but that's not the ecosystem." Meagher, who is now working on a complex project that involves the mapping of bison densities and population analysis, calls the current situation "an ecosystem disaster."

The most recent bison count found 1,123 animals, nearly half the total herd, in the western portion of the park and almost 400 of them west of the Firehole River. In the early 1980s, when there were 2,000 bison in the park, they were never found west of the Firehole, Meagher said, and though cow/calf groups traditionally summered along the park's eastern boundary, they haven't done so in years. "We've driven the entire population westward," Meagher said. "That should be telling people something." The parkwide system of groomed roads, she said, "provides energy efficient linkages between places where bison want to be."

Though there are isolated pockets of food on the west side of the park and just outside its western boundary, there is no real winter range in the area because of the snow depth, she said. To find genuine winter range, bison would have to travel nearly to Ennis or Bozeman, she said. State policy won't allow that, because of fears the bison will spread brucellosis. Nor will a proposed new federal policy, which calls for only limited bison range on the west and north sides of the park. Plus, the areas inside the west portion of the park, where soils are poor, are suffering from too many bison during too much of the year, Meagher said. "That's why we'll drive the population downward," she said. "We have an ecosystem problem. It's not overgrazing. It's much more complex than that."

Elk herds often display what biologists call a "population density" reaction, which means the number of births drops when overall numbers are high. Bison don't do that. Rather, Meagher explained, they respond to high population density by moving to new areas. "Bison are trying to adapt to changes we've created in the system," she said. But when that means moving outside the park, the animals are shot, trapped or hazed.

A couple more big snowstorms this winter could get bison moving, Meagher said, but at least some of them are likely to move out of the park later this year anyway. In most years, bison head for the park's western boundary early in the spring because grasses turn green there sooner than they do in the park.

The Montana Department of Livestock in the past several years has shown more tolerance for springtime bison, choosing to haze them repeatedly until there is enough green grass inside the park to hold them here.

What will happen this year depends on the weather, said DOL spokeswoman Karen Cooper. "It's determined on a case by case basis," she said. "But hazing will be the first option."