Standing near the North Entrance to Yellowstone National Park on Wednesday, senior bison biologist Chris Geremia told the Facebook world in a live broadcast how park bison cleverly manage the landscape to suit their nutritional needs.
Some distance in the background during his online chat, a small herd of bison lounged.
“We spent 10 years studying how bison migrated across the park,” Geremia said. “It’s a pretty cool story. We figured out how and when the bison were moving across the park. We stumbled on the way that the bison change how spring comes to this park. They change how the park turns green.”
Geremia said park scientists put GPS collars on bison and tracked where the bison were going and when. They also “took poop samples” and fenced off tiny sections of real estate to prevent bison from grazing there to make comparisons.
Researchers guessed that bison, like other ungulates such as deer and elk, would “surf the green wave” as spring arrived at different elevations. Emerging grass and plants have the greatest nutritional value to the animals.
What they found, however, was that the herds of bison would follow the green wave for a time, then return back to previously cropped forage like a dutiful homeowner mowing his lawn every week or so to keep it at the same height. This strategy allows bison herds to catch the sprouting forage at its most nutritious.
“Bison return to graze the same areas repeatedly at such intensity that it turns back the clock on forage green-up, hitting reset on springtime,” Geremia said in his Yellowstone report. “Without several thousands of bison moving freely on the landscape in sync, the springtime season of plant growth would be shorter, the land would not be as green, and plants would not be as nutritious.”
He said as the population of bison in the park grew from a few dozen in the early 1900s to today’s number of more than 5,000, the park “greened-up earlier, faster, more intensely and for a longer duration. Their influence on the landscape affects the entire way that spring moves through the mountains and valleys of Yellowstone.”
Geremia hypothesized how the Great Plains must have been when millions of bison roamed across North America.
“If they can change how spring happens just to this park, you could just imagine what 30 million bison would do when they used to roam over the West,” he said.
Bison population in the park fluctuates between 3,000 and 5,000. It is estimated that the population grows 10 to 17 percent annually. The Interagency Bison Management Plan has set a population goal of 4,900. Officials set this year’s culling recommendation at 500 to 700 animals. This year only 170 bison were shot by hunters because winter weather failed to encourage the animals to leave the park.
In a normal year, several hundred would leave the park to find forage in the lower northern elevations. During the 2019/2021 season, hunters killed 200 bison and another 260 were shipped off to slaughter with Native American tribes distributing the meat and hides to their members.
With fewer bison culled this past season, the park’s herds could grow to about 5,100 to 5,200 animals by summer, Geremia said.
The National Park Service recently started the Bison Conservation Transfer Program to corral and identify bison that don’t have brucellosis and transfer them to the Fort Peck Tribes in Montana. The transfer plan received a funding boost this past year increasing its ability to process up to 250 animals over a three-year period. This allows the park to transfer about 80 animals per year. From the transfer corrals, the animals go to Fort Peck tribal lands and some are transferred to other tribes across the West, including the Shoshone-Bannock tribes at Fort Hall.
Some researchers say 4,900 bison in the park is too many, especially in certain areas.
One recent study, published by researchers at Oregon State University, said the record numbers of bison in Yellowstone’s Lamar Valley has begun to degrade the habitat as bison eat up the vegetation, trample and rub horns on woody plants, in a similar way that overgrazing cattle can degrade plant communities, stream and river channels and food webs.
“This system is on a trajectory that is not so good ecologically for everything except for bison,” said Bob Beschta, a professor emeritus of ecology and lead author of the study, which was published in the journal Food Webs.
Beschta and his colleagues have studied the park for many years and tracked how the reintroduced wolves caused Yellowstone’s streams to rebound to conditions prior to predator eradication leading to more willows, aspen, beaver and birds after the elk herds were put in check. He said conditions have improved throughout the park, except for the Lamar Valley.
“From a long-term perspective, they’re kind of the new kid on the block, with the capability to impact the ecosystem in a very big way,” Beschta told the Montana Free Press. “Wolves have seemingly taken care of the elk issue. Nobody has taken care of the bison issue.”
Geremia said the debate stems from beliefs on what plant communities are supposed to look like.
“Our research suggests that Yellowstone is better off for it,” he said in a park online post titled “Bison Engineering a Better Yellowstone.” “Soils are healthy and show long-term stability in their ability to let nutrients in and hold on to them. Soil textures and chemistries are within ranges that promote plant growth.”
Geremia said critics argue that bison grazing has changed the plant communities in Lamar Valley to the wrong ones.
“This belief stems from a long tradition in rangeland science for managing landscapes for a single plant community by limiting and spreading out grazing,” he writes. “Any deviation from this theorized community is taken as a sign of degradation. … We are learning that the same way that predators can influence the system from the top down, bison can push on the system from the bottom up. When we give bison room to roam, the influence of bison on plant communities in the park is as natural and important to the ecosystem as those of top predators like wolves.”