The Ecological Benefits Of Far-Ranging Bison Herds

Bison once moved across North America in herds that blackened the landscape in all directions, in numbers estimated not in the hundreds but in the tens of thousands.

"These last animals are now so numerous that from an eminence we discovered more than we had ever seen before at one time; and if it be not impossible to calculate the moving multitude, which darkened the whole Plains, we are convinced that twenty thousand would be no exaggerated number,” Lewis and Clark remarked during their cross-country journey in 1805-06 of a herd encountered near the White River in present-day South Dakota.

It's been estimated that prior to American colonization there were tens of million of bison roaming North America, perhaps as many as 60 million. They traveled in huge herds that challenged present-day imagination. So great were bison numbers, and so faithful were they to their migratory routes, that countless years of millions of trampling hooves had carved “traces” as much as 50 feet wide across the landscape, paths that humans later turned into roads.

Lewis and Clark, George Rogers Clark, and even Abraham Lincoln walked the “Buffalo trace” that runs between present-day Louisville, Kentucky, and Vincennes, Indiana.

Richard Irving Dodge, a Civil War veteran transformed into a frontiersman for the U.S. Army, recalled standing atop a sandstone outcrop in Kansas that offered him views covering 6 to 10 miles in every direction.

"This whole vast space was covered with buffalo, looking at a distance like one compact mass, the visual angle not permitting the ground to be seen,” noted the colonel. “I have seen such a sight a great number of times, but never on so large a scale.”

And as the herds moved, their impact carried wide-ranging benefits. Their hooves served as rototillers in places, and their dung and urine was fertilizer that supplied prairie grasses short and tall with nutrients. Bird species such as killdeer and lark sparrows found these grazed grasslands to their liking.

What's not so visible to the untrained eye is that these herds essentially kept the grasslands themselves healthy, according to a study released Thursday in the journal Science.

Vast herds of bison slowing moving across the landscape consume a lot of vegetation, but in doing so they also stimulate a protein-rich revegetation of grasslands, the seven-year study of Yellowstone National Park herds found. That's because the grazing actually causes grasses and forbs to store carbon in soil while also stimulating microbial biomass production, said Professor William Hamilton, one of the three authors of the paper.

"That organic matter serves as a source for decomposition that then recycles nitrogen back into primary producers, grasses and forbs in the case of Yellowstone grasslands," Hamilton said during a Zoom call this week from his office at Washington and Lee University in Virginia.

According to the research, bison grazing in Yellowstone produced 156 percent more crude protein in "grazing lawns," parts of Yellowstone that contained "short, nutritious vegetation that supports a diverse array of plant and animal species." Even on dry hillsides the animals' eating habits led to 119 percent more crude protein in the renewing vegetation.

And because the herds are constantly on the move, they don't overgraze areas, something that can lead to soil compaction and reduced root growth, he added.

"It's certainly possible for (bison) to be overgrazing," the professor continued. "But from looking at soil and plant productivity, there's no evidence. We found no differences in above-ground productivity for grazed and ungrazed over the period that we studied, and no difference in soil organic matter.

"But we did see differences in response to grazing for decomposition and the recycling of nitrogen that improved and actually enhanced the nutrition of the above-ground biomass for consumption," Hamilton said.

Today there are an estimated 400,000-500,000 bison in North America, the vast majority in commercial herds. A much smaller fraction, like those in Yellowstone and tribal herds, are in "conservation" herds managed in part to protect their gene pool.

The study performed by Hamilton, Chris Geremia of Yellowstone National Park and Jerod A. Merkle from the University of Wyoming's Department of of Zoology and Physiology, said a return of large migratory herds of bison on the landscape would be greatly beneficial to grasslands.

"The claim that large herbivores have outsized effects on ecosystems is true," they write. "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. ... To move forward with conserving migratory herbivores and grassland ecosystems, we must embrace landscape-scale heterogeneity — not at the scale of individual ranches or pastures, but at sizes that allow for thousands of migrating large herbivores to move freely across the landscape."

Does that available landscape exist today? The 19th century is far in the rearview mirror. Since then, westward expansion has transformed grasslands into cities, ag lands, highways, and surburbia. 

So while the massive bison herds of centuries past will no longer return, there possibly are areas that could support herds numbering in the thousands.

There are 20 national grasslands managed by the U.S. Forest Service, including the roughly 550,000-acre Thunder Basin National Grassland in Wyoming; the nearly 600,000-acre Buffalo Gap National Grassland in South Dakota that touches Wind Cave and Badlands national parks, both of which have bison herds, and; the million-acre Little Missouri National Grassland in North Dakota that encircles Theodore Roosevelt National Park, which also has bison.

Cattle operations utilize many, if not all, of the grasslands, and removing them in favor of free-roaming bison likely would encounter fierce pushback.

The Wind River Indian Reservation in Wyoming covers a wide expanse, roughly the same size as 2.2-million-acre Yellowstone, and the tribes there are nurturing their own bison herds.

Hamilton said he'd like to get other bison advocates together to see what opportunities there might be.

"I think it would be a great thing to get us all together and see where we are and piecing things together," he said. "Because there isn't one solution, I don't think. Yellowstone is a unique opportunity (for research) because of the amount of land and the fact that bison restoration was a focus starting back to the '30s in the park for the Park Service. But finding areas that we could do more of this, and figuring out the appropriate mechanisms to do that (is the challenge). But what our paper certainly argues is it needs to be as much space as possible with with free movement."

Download National Parks Traveler Episode 339 on Sunday to hear the entire conversation with Professor Hamilton.