- Only 400,000 bison remain in America and 95% are raised for consumption.
- Researchers studied the last remaining migrating bison herd's impact on grasslands.
- The findings show the Great Plains could be much healthier where bison roam.
Bison roamed across America in herds so massive that they’re hard to conceptualize today. Legend of their scale is passed down from before they were hunted to near extinction during the 19th century.
In 1839, an explorer claimed to have seen a herd along the Santa Fe Trail that covered an area nearly the size of Rhode Island. An Army colonel saw a herd of approximately 3 million next to the Arkansas River in 1871; that one stretched 24 miles long and 14 miles wide. In Lakota oral histories, they measure bison herd size by how many days it took for them to pass.
What benefit these animals provided in those broader ecosystems has been hard to determine as there’s only one migrating herd left in America.
There may be more wild herds, such as the one in Utah’s Antelope Island State Park, but the only remaining herd that migrates — and is “large” — lives in Yellowstone National Park.
A group of researchers published a study in Science last month that looked at the impact that migrating bison had on the ecosystems where they traveled over a five-year period between 2017 and 2022.
By investigating the effects of the grazing, movement and droppings on soil and vegetation, researchers were able to determine the benefits their presence provided to the overall nutritional quality of the landscapes.
The authors called it “Yellowstone’s free-moving large bison herds provide a glimpse of their past ecosystem function.”
“It’s pretty safe to say,” said Bill Hamilton, the co-author of the study and biology professor at Washington and Lee University, “that when there were 30 million bison, that’s what made the Great Plains so fertile when we first started putting in crops.”
Letting the bison roam
Today, there are roughly 400,000 bison left in America, 95% of which are privately owned and raised for consumption.
In Yellowstone, there are about 5,400 bison split into two herds. Only one of which migrates through a 50-mile corridor in and adjacent to the park. It travels approximately 1,000 miles every year. They graze through a number of different habitats within that range, with each smaller ecosystem getting more or less contact depending on the seasons and snowpack.
Hamilton along with Chris Geremia from Yellowstone National Park and Jerod Merkle, professor of zoology and physiology at the University of Wyoming, focused on how bison grazing and droppings affected nutrients in the soil such as carbon and nitrogen. Subsequently, the study also measured how rich the vegetation becomes with what the report refers to as “crude protein,” a metric for how nutritious that plant matter is for grazers.
Over those five years, samples were taken from different regions that the bison frequented, such as lawn-like grasslands near rivers, drier habitats with mixed vegetation and higher elevation habitats.
“The main conclusion is for grassland ecosystems, animals — all herbivores — need to be able to move freely," Hamilton said. “They’re changing the grasslands and for them to maintain that, the level of productivity that we observed, there had to be something taking place below ground with nutrient cycling.”
In some areas, the researchers found that the presence of the migratory grazers enriched the plant matter with crude protein by 119%, but in other areas that number was as high as 156%. That’s nearly double what the amounts that were found in control areas, where the researchers fenced off small parcels of land to prevent the bison from grazing.
“Our findings suggest that the return of freely moving bison is actively stimulating and transforming Yellowstone grasslands,” the authors wrote.
“Landscape heterogeneity (diversity) governs these dynamics. Wet, nutrient-rich soils foster intense grazing and nutrient acceleration, whereas drier areas receive less grazing and exhibit weaker nutrient effects.”
Where the grass grows greener
One of the most important findings wasn’t just that the vegetation was enriched, but that it was enriched in different areas and that the bison were able to travel to find where it was best.
“It’s not just these grazing lawns that are providing crude protein for consumption, (the bison) are finding it everywhere,” said Hamilton. “It’s because they are able to migrate across that landscape.”
The study didn’t necessarily find that there was more grass, either. The presence of the migrating bison did not increase the amount of vegetation, rather what grew was just of a healthier, more nutritious kind.
“Our data doesn’t say necessarily anything new, but you can have large numbers of animals moving across the large landscape and have it operate and function properly,” Hamilton said.
“It’s just further confirmation that having these animals move around is important. It’s not earth shattering — particularly for indigenous tribes as this has been their understanding of bison moving across the landscape forever — but science is necessary in modern times to demonstrate that.”
Comparing wildebeest in the Serengeti
While the findings from the Yellowstone bison study are significant, they have been proven out in other places before.
“A lot of this ecological and ecosystem level phenomenon have been done in smaller experiments, in greenhouses, growth chambers, or out in the field, whether it’s in North America or in the Serengeti,” Hamilton said.
The Serengeti grasslands of central Africa, for example, which stretch across Tanzania and into Kenya, is one of the few comparisons researchers could make.
In that region, there’s a protected migratory corridor that sustains a carrying capacity of millions of wildebeest — large, migratory herbivores — and all the subsequent creatures of that ecosystem.
“The claim that large herbivores have outsized effects on ecosystems is true. However, our findings reinforce that it is not just body size that matters. Large numbers, high densities and the freedom to move across expansive landscapes are all critical,” the authors of the study wrote.
“Whether it is 1.2 million wildebeest that shape the Serengeti ecosystem, or a half-million caribou migrating annually in the Alaskan tundra, the ecological function of migratory herbivores depends fundamentally on their ability to move across landscapes.”
Does bison grazing differ from cattle grazing?
Much of the benefits from bison’s migratory grazing on the soil is not recreated with cattle, Hamilton said.
The authors do not make the argument that all bison should necessarily be allowed to migrate across our grasslands, Hamilton made clear, but the study does suggest that supporting our national mammal’s natural way of life might be good for America’s grasslands.
“Bringing back that ecological function that is not replaced by cattle to our grasslands,” Hamilton said, “can have positive effects.”
Growing the scale and range of migratory herds of bison is unlikely.
In addition to issues inherent from private land ownership and grazing conflicts with ranchers and states, there are already efforts in place to limit their range as it is. For example, late last year, the state of Montana sued Yellowstone to limit the number of bison that are allowed to leave the park.
Yet, the evidence is clear that migrating bison would be overall beneficial to the health and restoration of America’s grasslands.
“Our findings emphasize that ecosystems with large native herbivores, such as bison, can function successfully in today’s world,” the authors wrote. “Although challenges remain because of habitat fragmentation and sociopolitical constraints, restoring bison at this scale is not only possible, it works.”