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By JOHNATHAN HETTINGER
Bison in Yellowstone National Park’s Lamar Valley are eating, trampling and rubbing their horns on woody plants, drastically altering plant communities, stream and river channels and food webs, according to a new study published by researchers at Oregon State University.
“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.
The researchers’ conclusions are not shared by Yellowstone’s bison biologist Chris Geremia. He said park managers have to take a wider view of their role in preserving bison, while the study’s authors have a “very specific and narrow” view of what the park’s northern range should look like.
“They view any deviation from a narrow description as a sign of degradation,” Geremia said. “We don’t believe there should be such a narrow view for the types of plants that exist in the park.”
The OSU study covers some of the same ground, and raises similar issues, as a 2018 Rangelands study conducted by Montana State University professor Jeff Mosely.
Cascade
For the past two decades, Beschta and his colleague Bill Ripple have published dozens of papers on one of the most heartening environmental stories of recent times: the trophic cascade caused by the reintroduction of wolves in Yellowstone National Park.
In the 1990s, wolves reintroduced from Canada started hunting Yellowstone’s overpopulated elk herds. Elk numbers quickly dwindled, and plants started growing in places they hadn’t grown for decades. Yellowstone’s streams rebounded to conditions that existed prior to predator eradication, leading to more willow, aspen, beavers and birds. In areas like Blacktail Deer Creek, plants like serviceberries and chokecherries have returned, Beschta said.
But that recovery hasn’t happened everywhere in the park, particularly in the Lamar Valley, the researchers claim.
“If you drive through Lamar Valley, and we’ve been telling people about everything getting better, you’d say, ‘You’ve got to be kidding me. Nothing is getting better here,’” Beschta said. “We’re finally now saying it’s not all roses. There’s a problem, and it happens to be bison.”
Too many?
The study raises the question of whether the park’s current management plan of maintaining a bison population of 3,500 to 5,000 is healthy for the overall ecosystem.
How many bison should be in Yellowstone is a contentious question. A disagreement over the appropriate size of the park’s bison herd contributed to the early retirement in 2018 of former Superintendent Dan Wenk, who believed the park could handle more bison.
Yellowstone National Park’s purpose statement says park managers should “preserve and protect the scenery, cultural heritage, wildlife, geologic and ecological systems and processes in their natural condition for the benefit and enjoyment of present and future generations.”
Consequently, park managers are more focused on maintaining natural processes, Geremia said. That means letting the number of bison in the park control plants through grazing.
Lamar Valley
Beschta said decisions about what role bison should play in Yellowstone taps into a larger question about whether Yellowstone is maintained for its pre-management character, or for the character that human management has implemented.
“Maybe bison are more important than any other species,” Beschta said. “But for a fish, a beaver, a small mammal, a bird, a bear or whatever to try to make a living in the Lamar Valley bottom, the habitat has been pretty well decimated.”
Growth
In recent years, Beschta and Geremia agree, there have been more bison in Yellowstone National Park than at any point in history, with the population hovering between 3,000 and 5,000 in two distinct herds. In August 2019, the northern herd was estimated to contain 3,667 bison, while the central herd had 1,162. The study focuses on the impacts of the northern herd.
Despite the current success of bison in Yellowstone, there is no evidence the species ever inhabited park boundaries in large numbers prior to recent years, Beschta said.
“We have a large number of bison in a place where they probably never existed. The park was not set aside to be a bison farm,” Beschta said. “If we dumped 4,000 bison in Yosemite, that would be unacceptable.”
The study found that bison exert 10 times as much pressure on the landscape as elk.
“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 said. “Wolves have seemingly taken care of the elk issue. Nobody has taken care of the bison issue.”
Geremia said Yellowstone’s role in restoring bison populations is significant, even if the animals weren’t previously present in large numbers. Geremia said historical trapping reports show that bison have been in the park continuously.
“Bison are survivors,” Geremia said. “There’s always been bison in Yellowstone, based on those reports. The number is debatable.”
Park scientists have demonstrated that the park can manage upwards of 5,500 bison, he said. The northern range of the park can support between 3 and 4 million pounds of herbivore biomass, an amount similar to what was present in the park in the 1980s. Only now, that biomass is more bison than elk.
The number of bison in the park is below the U.S. Department of Agriculture’s livestock stocking rates, Geremia said. Even in areas of high grazing soils are healthy and there is significant plant growth, he said.
“They may look short because they’re grazed, but they’re actually highly productive,” he said. “We don’t see (that bison grazing) is inhibiting plant growth.”
Geremia said people have a limited tolerance for bison because they’re big, can be dangerous, and, like elk, carry brucellosis, and that it’s important for bison to have a safe space in Yellowstone.
“It’s really the only preserve, the only refuge where they can thrive until the world is ready to accept bison like other ungulates,” he said.