BOZEMAN, MONT.--Bison routinely travel along the groomed roads in Yellowstone National Park because it’s a heck of a lot easier than plowing through piles of snow, right? Not at all, according to a study by a former Montana State University graduate student who spent two winters documenting the shaggy beasts’ precise movements in the park’s western section. “Most of the travel is not taking place on groomed roads,” said Dan Bjornlie, who finished his master’s degree in ecology at MSU last spring. He currently works for the Wyoming Game and Fish Department.
The project was instigated by the mass exodus and shooting of bison during the 1996-97 winter, said Bob Garrott, an MSU ecology professor and project advisor. Some suspected the bison were exploiting the groomed roads to leave the park, a hypothesis repeated so frequently by the media as to appear as fact, he said.
“Dan’s the first person to directly address that question with field studies, and his intensive work does not support this hypothesis,” Garrott said.
The study, funded by the Biological Resources Division of the U.S. Geological Survey, has been accepted for publication in the Journal of Wildlife Management.
"There are people who believe that the groomed trails are making those animals change their patterns, but with the research that's being done...we just aren't seeing that," Yellowstone National Park spokeswoman Marsha Karle told the Billings Gazette.
Bjornlie monitored bison in the Madison, Firehole and Gibbon river drainages from November 1997 to May 1998 and from December 1998 to May 1999. In all, he and his team logged 28,293 bison observations. Only 8 percent of the time were bison travelling. Up to 20 percent of that travel time was on roads, but more often the animals followed natural corridors, streambanks and packed trails, Bjornlie found.
“The data show that of all activities, a really small part is travelling, and of that, a small part is travel on the roads,” Bjornlie said.
What’s more, bison road use peaked in the months before and after the roads were groomed, especially after mid-April when spring thaws opened up new foraging areas.
“It’s not even a fifty-fifty, on-road/off-road split. It’s so much lower than that,” Bjornlie said. “The majority of movement is off road.”
The results challenge several assumptions about the ecological role of groomed trails on bison survival and behavior.
For example, the study yielded no evidence that the animals use groomed roads for travelling long distances. Most--68 percent--travelled less than 1 kilometer while on groomed terrain. “I’ve seen them walk from a streambed up to a road, walk 500 to 600 meters along the road, then go off into another streambed,” Bjornlie said. “It’s part of their travel. They definitely use them [groomed roads], but they are part of a much larger travel network that includes off-road travel.”
The study instead documented heavy travel over the Mary Mountain Trail linking the Hayden Valley, where many of the animals in the study spend the summer, to the Firehole area, where the majority of them winter. Using infrared monitoring stations, Bjornlie recorded between 100 and 700 bison “events” in a two-week period along the trail.
Visitors to the park more than 100 years ago told of bison following the same trail, suggesting to Bjornlie that the nomadic animals re-established the migration pattern once their numbers began rebounding. When population control efforts ceased in 1967, there were about 400 bison in the park. Before the big die-off in 1996-97, there were about 3,500.
“So they’re moving because of range expansion, not because of the roads,” Bjornlie said. Park spokeswoman Cheryl Matthews said data collection on bison movement in the park will continue. Because the park hasn't yet experienced another winter as harsh as the 1996-97 one, when so many bison left the park and were shot, data from another tough winter will be critical to see if the trend of not using roads continues, she said.
Photos are available by calling 406-994-5607 or sending an e-mail to annettet@montana.edu.
Journal
Journal of Wildlife Management