Off the beaten path
Scientists find bison make their own way through life, not just on established roads
By MICHAEL MILSTEIN
Of The Gazette Staff
YELLOWSTONE NATIONAL PARK, Wyo. - Stomping on snowshoes up a steep, snow-covered slope above the Madison River, Montana State University graduate student Dan Bjornlie stops to point out a clear path cutting diagonally uphill.

It's not one of the many hiking trails that cross-cross Yellowstone National Park.

It's a bison trail, one of many that the shaggy beasts cut across Yellowstone's snow-clad landscape every winter.

And it illustrates a paradox of bison travel that raises questions about the traditional view that park roads channel bison out of Yellowstone, and often to their death, during the winter.

Down below is the paved park road from Madison Junction out of the park to West Yellowstone, its snow packed down by grooming and snowmobile traffic. Bison could simply follow the road, the most direct westward route out of the park. But they don't. Instead, they cross the frigid Madison River, clamber up this steep slope, and take a more circuitous route through deep snow out of the park to the north of West Yellowstone.

"If they wanted the easy way out, they'd take the road," Bjornlie says. "But that's not what they do. That's what makes it so interesting."

In the first-ever comprehensive study of bison movement through the national park, Bjornlie is finding that bison maintain their own extensive trail network and seem less dependent on roads as travel routes than other biologists have suggested. It's not that bison never travel on roads, because they do, but the idea that roads define their movements within the park may be too easy.

"The people arguing for the nice, clean interpretation that bison leave the park because of the roads - it's simply too simple," says Robert Garrott, a MSU professor of wildlife ecology overseeing Bjornlie's research. "Bison clearly use the roads sometimes, but the ecological consequences and the physical consequences are not as clear-cut as the one-liners coming out."

One clear consequence of bison leaving Yellowstone - no matter how they do it - is that many get killed under a State of Montana policy that they not wander beyond the park because some carry the disease brucellosis, which can infect domestic cattle.

A lawsuit by environmental groups has also forced Yellowstone managers to more closely examine the impact winter recreation in the park has on wildlife, especially bison.

Retired park biologist Mary Meagher, who has observed park bison for decades, was the first to suggest that the grooming of park roads for snowmobile traffic turned the roads into convenient walkways for bison that would rather not slog through deep snow. But no formal study had ever directly tested the hypothesis until Bjornlie began his research last winter.

Funded by federal money routed through the U.S. Geological Survey's Biological Resources Division, Bjornlie has walked hundreds of miles on snowshoes and covered thousands of miles on his snowmobile, observing bison throughout Yellowstone. He has also set up electronic counters along known bison trails to track how many bison move past and when they move.

His initial conclusions come from observations last winter, a mild winter that may have not pushed bison onto roads as much as past, harder winters. But Bjornlie hopes his work this winter will provide more conclusive results.

At the same time, USGS biologist Peter Gogan and others are monitoring the movements of about 30 bison they captured in the Hayden Valley last fall and fitted with radio collars.

Members of a research team capture a Yellowstone bison.

Yellowstone's roughly 2,200 bison are split into two populations: About 400 that dwell mainly on the park's northern range, and the rest, which summer in the Hayden Valley.

Gogan's team and Bjornlie have found that as snow builds in the Hayden Valley, bison move across Yellowstone's Central Plateau via a path known as the Mary Mountain Trail to the lower-elevation drainages of the Madison and Firehole Rivers, where hot spring basins and river bottoms provide forage even in the winter.

Such movements are evidence of an important bison migration corridor with no connection to the park road system, casting doubt on the idea that roads are a driving factor in bison movements.

"What drives the movement?," Garrott asks. "What it looks like is that what drives the movement has nothing to do with groomed roads."

Once the bison arrive in the Madison and Firehole drainages, some continue out of the park, prompting the next question.

"How do they get out of the park - on the roads?" Garrott continues.

The answer is no. In fact the bison may wander down the roads when narrow canyons cradling the lower Firehole and upper Madison River leave them no other choice, but they typically leave the straight and flat road at Gneiss Creek, cross the Madison River and hike up the steep slope where Bjornlie pointed out their well-worn path.

It has become clear to Bjornlie and Garrott that park roads are not the exclusive bison travel corridors some have suggested. In fact, bison spend much of their time traveling along a network of trails that their frequent traffic keep plowed through the snow. The Mary Mountain Trail between the Hayden Valley and the Madison-Firehole drainages is one example. Even in the deepest winter, it still remains clear of snow.

"The snow was up over their shoulders but they weren't pushing snow, because they were using a maintained trail," Garrott says. "It's probably a behavioral mechanism that also helps them avoid deep snow."

One of the main tenets of the argument that roads channel bison movements is that by following groomed roads, the bison expend less energy than they would plunging through deep drifts. But their own trail system may serve the same purpose, suggesting the roads are not so critical. Even when there's no trail where they want to go, groups of bison turn to another means of efficient travel - one bison leads the way, cutting a path through the snow for a time, and then falls back to let another animal take the lead.

Bjornlie has watched bison do this, forging a trail immediately parallel to a nicely packed road.

Observations by National Park Service staff last winter concluded that bison in the Hayden and Pelican valleys did not use groomed roads as major travel corridors, although they noted that a tougher winter could force bison onto the roads more often.

Still, hundreds if not thousands of hours watching bison and noting their behavior in detailed logs has also led Bjornlie to wonder just how much difference it really makes to bison whether they travel through snow or on trails and roads where the snow is already packed down. Bison spend almost all their time - probably 90 percent or more - searching out food.

"Every single one of them is displacing snow when they're feeding," says Bjornlie, looking at bison scattered along the shore of the Madison River. "That's a much bigger energy cost than the small amount of traveling they do. They obviously use the roads sometimes, but whether or not that's a significant enough energy savings for them to use the roads exclusively is much more difficult to say."

When bison traveling on roads encounter snowmobiles and run from them, as often happens in winter, they may quickly burn up any energy they saved by not trudging through the snow. In fact, taking an occasional snowmobile encounter into account, travel by road might well end up costing bison more energy than it saves them. Although the narrow canyon at the lower end of the Firehole River funnels bison onto the road through the canyon, they do not seem to prefer the road.

"I think that's a matter of topography," Bjornlie says. "Usually as soon as they can get off the road, they do."

He has also been tracking bison use of two short loop roads, Firehole Lake Drive and Fountain Flat Drive, which are not groomed in the winter. Bison use those roads, casting doubt on the idea that grooming alone is what makes the roads more attractive. Even in the summer, when packed-down snow is a thing of the past, bison sometimes wander along roads.

It may be that bison simply use roads for the same reason people do: They are clear routes through the trees and they tend to wind along rivers and past hot springs, which to people represent scenic attractions and to wintering bison represent reliable warmth and food.

Bison may come to include roads as an additional element of the intricate network of trails they already maintain through the snow. They may travel along the roads when it suits their purposes and fits into the migration routes they have followed for centuries, Bjornlie and Garrott say, but the creatures are not dependent on the roads alone.

"For as much travel as I see on the roads, there's an equal amount off," Bjornlie says. "It's simplistic to think the roads are controlling the bison and that the bison wouldn't be moving without the roads. These are complex animals that know how to survive in an extreme environment, especially in winter. They're the ones who decide where they go and when."

CPR on a buffalo?

Two members of a research team capturing Yellowstone National Park bison last fall to fit them with radio collars used chest compressions in hopes of reviving a bison after it went into a seizure during its capture. It didn't work, though, and the female bison weighing nearly a ton became one of five bison accidentally killed during capture operations.

Scientists and park officials concerned about the unusually high number of deaths have since revised their policies on the capture of bison.

Three of the deaths took place in late September and early October when scientists captured 33 bison in Hayden Valley and the Lower Geyser Basin near Old Faithful by "net-gunning," where a net is fired over the animal to entangle and immobilize it. Some of the bison had been radio-collared earlier but had to be recaptured because many of the initial radio collars - which were untested on bison - didn't work, said Peter Gogan of the U.S. Geological Survey Biological Resources Division.

The collar manufacturer agreed to replace the collars at no cost and cover the extra costs of recapturing the bison, Gogan said.

One of the bison went into a brief spasm during handling and stopped breathing. Two members of the handling team, an emergency room physician and nurse, began chest compressions on the giant animal in hopes of restarting its heart and bringing it around, but the animal did not respond and was declared dead. Two other bison died when nets were fired over them and they stumbled, fell and broke their necks.

Net-gunning was used instead of tranquilizer darts in this instance so the animals could be released quickly with no drug after-effects and to avoid the danger posed by darts that miss their targets and remain out in the open where they could injure someone.

The three bison deaths out of 33 captures represents a mortality rate of about 10 percent, Gogan noted, generally accepted as the maximum mortality rate for wildlife captures.

"We were just unfortunate in getting to that level," he said.

Later last fall, another 50 bison were captured in the Madison and Firehole river drainages and on Yellowstone's northern range for a study of the disease brucellosis within Yellowstone's bison herds. Biologists captured 28 with the use of tranquilizer darts and 22 by net-gunning. One of the bison captured by net-gunning died when the animal stumbled and broke its neck. A second bison died after the capture team hit the animal with the tranquilizer dart but then lost sight of it. Biologists found the animal dead about two days later and concluded that it had died of an overdose of drugs without veterinary attention.

Reviews following both operations led to new policies governing the capture of bison. Officials now will not use net-gunning unless there is sufficient snow on the ground to cushion the fall of bison so they do not break their necks or any bones. They will also conduct more extensive searches for drugged animals.

Biologists also agreed that in the event of future mortalities, they will more quickly halt capture operations and make corrections to prevent additional wildlife deaths.