The Yellowstone National Park bison population is bigger thanks to man-made roads, according to retired national park biologist Mary Meagher.

“Yellowstone now produces bison in numbers that would not occur without the changes we humans have caused,” Meagher said during her presentation at the June 2 Draper Natural History Museum Lunchtime Expedition at the Buffalo Bill Center of the West.

Bison settled in what is now Yellowstone by migrating from adjacent valleys as Pleistocene ice melted, Meagher said. They arrived in the northern range about 10,000 years ago and 2,000 to 1,500 years ago in the park’s interior.

Although there have always been times and places small numbers would exit the park, past winters kept the population in check.

During winter, bison learned to occupy geothermal features where snow melts, Meagher said. Geothermal activity varies from vegetated areas to bare ground. Even barren patches aid survival because a bison can stay put without expending energy. If the extreme weather doesn’t persist, and/or the animal has good reserves from summer grazing, geothermal sites can provide a survival margin.

But, in recent decades that has changed with the development of roads. The unique geothermal-dependent adaptation by Yellowstone bison has declined significantly, Meagher said.   

Due to heavy snowfall and powerful winds, Pelican Valley is the severest winter climate in the United States for bison, Meagher said. And, bison are not adapted to traverse deep snow; it saps their strength.

Once the Pelican bison began to move to Hayden Valley via the snow-packed winter road, the land-use patterns altered, Meagher said. “The grazing system changed because the bison would not go back to the patterns of land use that were so harsh.”

Traveling snow-packed roads allowed bison to find forage they otherwise could not locate, Meagher said. The unique geothermal-dependent survival system has been greatly reduced, Meagher said.

More bison cross park boundaries and elk-bison relationships have changed, Meagher said. Elk numbers are decreasing and bison numbers are increasing.

Bison numbers have grown since the late 1960s.

Frequent culling by park managers curbed bison abundance through 1966, but the population rapidly increased after a moratorium on culling in 1969, according to the National Park Service.

There were 476 bison in 1970, then 2,000 in 1980 and nearly 3,000 by 1987.

Today’s population is around 4,500. Without the roads or “linkages,” the population would likely average around 2,000, Meagher said. “We changed Yellowstone into a bison factory.”

No stopping

In late winter, bison leave the park to find food.

Bison will gradually colonize more ground if they are free to roam, Meagher said. “If we left them alone they would be in Livingston, Montana, in 10 years.”

Livingston is 56 miles north of the North Entrance to Yellowstone.

One member of the audience asked if introducing birth control would control growth.

“How are you going to deliver it?” Meagher asked. “With a dart gun?”

Live capture is inhumane, but there are no easy answers when it comes to bison, Meagher said.

Some fear bison could infect cattle with brucellosis, a disease that causes hoofed animals to abort their young.

Brucellosis, or Brucella abortus, arrived with domestic cattle, Meagher said. Brucellosis was probably introduced to Yellowstone bison before 1917.

Biologically, bison could, in the right circumstances, transmit brucellosis to cattle, but to date there has never been a case traced to or circumstantially attributed to the Yellowstone bison, Meagher said.

“Brucellosis is a ‘smokescreen.’ Livestock interests just don’t want bison out there. News articles, especially this winter, are making it clear that elk are the brucellosis source,” she said.

The National Park Service was forced to euthanize a bison calf in May when a tourist caught it in an ill-conceived attempt to save the animal. The man pleaded guilty and was fined $735 on June 2.

Despite the ballyhoo the story spawned, Meagher said one lost calf will not adversely affect the bison population. Some young always die of natural causes.