Image: courtesy of DAN BJORNLIE THOUGH HARDLY DAINTY, bison in Yellowstone National Park travel away from deep snow, which makes it harder for them to forage.

The winter of 1996-97 was a bad time for bison in Yellowstone National Park. The season was particularly harsh and snowy--and for some reason, scores of the massive animals began to leave their normal stomping grounds. As they crossed into non-parkland, they were killed by the Montana Department of Livestock. The agency needed to protect the state's livestock from the infectious diseases bison sometimes carry--but they ultimately shot dead more than 1,000 of the estimated 3,500 bison living in Yellowstone at the time. The tragic event led several scientists to ask the same question: why did so many bison decide to leave the park?

Mary Meagher, then an animal ecologist at Yellowstone National Park, had been working in the park and with the bison for almost 40 years. When she looked into the mysterious mass exodus, she decided to place at least part of the blame on groomed roads. These cleared paths, she concluded, made it easier for the bison to move around within the park and, on occasion, led them on mass migrations out of the park as well.

"Pieces of roads--suddenly available in the harsh conditions of winter--became easier, energy-saving travel routes," she explains. "Life is 50-50. It's what you take in, that's the calories, but the other half, especially in a harsh environment like this, is the energy it costs. Energy saving is really crucial in this environment. That means that any energy that an animal saves--especially if, in saving that energy, it gets to slightly more forage--is going to tip the balance of survival.¿

Meagher says the bison eventually left the park due to a domino effect: populations of bison from one valley moved westward, pushing the population there farther west and so on--until some bison were ultimately pushed out of the park altogether. "The first time I know that I had bison use a section of road to move to a new foraging site was in February of 1980," Meagher adds. "The next year they had moved another quarter of a mile further west. Animals learn too."

Image: courtesy of DAN BJORNLIE TRAVEL PATHS the bison take include river corridors and thermal features--natural trails that man-made roads also frequently follow.

But not everyone reached the same conclusion. In 1997 Dan Bjornlie, then a graduate student in ecology at Montana State University, also set out to determine exactly how and why the bison had migrated. Over the course of two winters, Bjornlie and his graduate advisor Robert Garrott--working with several technicians and other graduate students--monitored Yellowstone's bison populations. They recorded where the animals were and if they were feeding, traveling or resting. The group came up with some surprising results, which put many traditional ideas about bison behavior in question.

Indeed, earlier observations indicated that the bison population lived primarily in two regions: one at the northern end of the park, and one in the center. For the most part, the animals had not moved from these neighborhoods. In recent years, though, researchers had noticed an increased migration of the bison to different pastures during different seasons. Meagher and others attributed this movement to the bisons' natural migratory drive, and to man-made changes in the ecosystem of the park, including groomed roads.

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