In the 1870s, one observer saw a bison herd along the Arkansas River estimated at 3 million, blanketing nearly 1,000 square kilometers. Such massive movements of these animals through the heart of North America rivaled the vast migrations that swarm across Africa’s Serengeti today.
But mass slaughter sanctioned by the U.S. government reduced these vast herds to just a handful of creatures confined to private ranches and places like Yellowstone National Park.
Now, a resurgence of bison inside that park is giving scientists insights into the ecological power of these massive creatures. There, a herd of some 5,000 animals migrating back and forth within the confines of the park are transforming their grazing grounds, altering nutrient flows and microbial communities in ways that enhance plant productivity and nutritional value, according to a new paper in Science.
“With the current large herds of bison, Yellowstone grasslands are functioning better than in their absence,” said Bill Hamilton, an ecologist at Washington and Lee University in Virginia, and a co-author on the paper. It’s “a glimpse of what was lost when bison were nearly wiped out across North America in the late 1800s.”
While ecosystems like the ones in north Yellowstone evolved beneath the hooves of millions of bison, it wasn’t axiomatic that the animal’s return would bring ecological benefits. Excessive grazing by these shaggy lawnmowers could, in theory, damage a landscape by packing down the soil and sucking up nutrients more quickly than they can be replaced.
The herd inside the park has grown from just 23 at the start of the 20th century to thousands of animals today, spanning a roughly 100-kilometer migration route. Their presence is evident in river floodplains and grasslands there, where their grazing creates closely cropped “lawns.”
To see how this population increase was altering the inner workings of these ecosystems, Hamilton and collaborators with the National Park Service and the University of Wyoming fenced off small plots in places where the animals grazed, then from 2015 to 2022 they tracked how those un-touched places differed from places the bison could reach.
The research revealed that bison were consuming so much plant material in the river bottom lawns that, in theory, they should be exceeding the productive capacity of the plants there.
But they weren’t. When scientists dug deeper, they found that despite the intensive grazing, the amount of nitrogen—a key plant fertilizer—was similar in plants growing in these lawns and in places that were shielded from grazing. The secret was in the soil. Grazed areas had a greater mass of microbes in the dirt, and more of the bacteria that convert ammonia into a form of nitrogen that plants can use. This supercharged plant growth by making nitrogen more readily available to the plants, while overall levels of nitrogen in the soil remained comparable to ungrazed places.
The influx of nitrogen into the plants also made them more nutritious. The protein levels in the plants in the grazed areas was around 150% greater than in comparable un-grazed plots.
“What we’re witnessing is that as bison move across the landscape, they amplify the nutritional quality and capacity of Yellowstone,” Hamilton said. “Their grazing likely has important consequences for other herbivores and for the food web as a whole, similar to the changes that occurred in the Serengeti when the wildebeest population recovered.”
The bison revival isn’t always a boon for vegetation, however. A different group of scientists recently reported a resurgence of aspen groves in parts of northern Yellowstone, likely tied to the return of wolves. The arrival of these predators led to a decrease in elk herds that had gone unchecked for decades.
Before the return of wolves starting in the 1990s, aspen groves had been declining as grazing animals chomped emerging aspen sprouts, leaving only the oldest trees that grew before elk numbers soared. Surveys in the 1990s found no tall saplings in aspen stands. But when researchers returned in 2020 and 2021 to 87 groves, nearly two thirds of them had saplings.
“The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades,” said Oregon State University ecologist Luke Painter, the paper’s lead author. “About a third of the 87 aspen stands we examined had large numbers of tall saplings throughout, a remarkable change from the 1990s when surveys found none at all.”
But there were few signs of new growth in 38% of the stands. Painter points the finger at bison—animals too large to often fall prey to wolves—as a likely cause. In previous research, he has documented bison breaking aspen saplings.
“Increasing numbers of bison may be emerging as a new constraint to aspen in some areas,” he said.
Geremia, et. al. “Yellowstone’s free-moving large bison herds provide a glimpse of their past ecosystem function.” Science. Aug. 28, 2025.
Painter et. al. “Changing aspen stand structure following large carnivore restoration in Yellowstone.” Forest Ecology and Management. July 2025.
Image from Envato/BlackBox Guild
