Yellowstone’s Lamar Valley has been called the American Serengeti, a landscape where people can see bison, elk, pronghorn, bighorn sheep, moose, wolves, coyotes, grizzly bears, black bears, otters and beavers.
But the habitat of Lamar Valley has been degraded in recent years, thanks in large part to record numbers of bison eating, trampling and rubbing their horns on woody plants. These behaviors drastically alter plant communities, stream and river channels and food webs, according to a new study published last week 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.
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, more aspen, more beavers and more 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 Lamar Valley.
“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.”
HOW MANY IS TOO MANY?
The study raises the question of whether the park’s current management plan of maintaining a population of 3,500 to 5,000 bison 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 than did then-Secretary of the Interior Ryan Zinke.
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.”
Chris Geremia, the bison biologist for Yellowstone National Park, said bison play a key role in the park’s history, as well as its ecological systems and processes. Park managers have to take a wider view of their role in preserving bison, Geremia said, while the study’s authors have a “very specific and narrow” view of what the park’s northern range should look like.
“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. Wolves have seemingly taken care of the elk issue. Nobody has taken care of the bison issue.”—study author Bob Beschta
“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.”
Instead, park managers are more focused on maintaining natural processes, Geremia said. And that means letting the number of bison in the park control plants through grazing.
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.”
HISTORY OF BISON IN YELLOWSTONE
Yellowstone National Park is the only place in the U.S. wild bison have been continuously present since prehistoric times, according to the park’s website.
As many as 60 million bison once roamed North America, but the species was hunted almost to extinction in the late 1800s, when the U.S. army campaigned to eliminate the species as a means of controlling Native American tribes, the website said.
By 1901, bison numbers in the United States had been reduced to as few as 300 individuals. Of those, 23 lived in Yellowstone National Park. The park bred those bison with a captive population at the park’s headquarters in Mammoth, and then at the Buffalo Ranch in Lamar Valley.
In recent years, Beschta and Geremia agree, there have likely 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.
Though bison are able to migrate throughout Yellowstone National Park and a small area outside of the park, the herds are not allowed to roam freely, because they carry brucellosis, a livestock disease transmissible to wildlife from cattle, and vice versa.
The bacterial disease, which causes ungulates to abort fetuses, is transferred through close contact between animals, particularly through interaction with an aborted fetus. Brucellosis is also transferable to humans through unpasteurized milk or undercooked meat.
Though elk are also infected with brucellosis, their movement is not limited. Though all known transmissions of brucellosis to livestock have come from elk, only bison are contained.
Management of the size of Yellowstone’s bison population is guided by the Interagency Bison Management Plan, which is produced by a coalition of federal, state and tribal officials. Population management is driven by conflict with agricultural interests outside the park, Geremia said, because as the herds get larger, there is an increased chance that bison will exit the park.
Each year, biologists and hunters, most associated with Native American tribes exercising treaty rights, kill hundreds of bison to contain the herd’s numbers and prevent expansion of the population beyond managed borders.
‘BISON FARM’ OR ‘REFUGE’
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, 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.