UW Researchers Find That Bison Have Major Ecosystem Effects

Nov 26, 2019

Bison create their own nutritious grass to feed on in the spring.
Credit Bear River State Park

Hooved animals, known as ungulates, are known to "surf the green wave" and follow the nutritious spring growth as they migrate. But researchers have found that bison in Yellowstone National Park are not only surfing the green wave, they're also creating their own nutritious lawns and reducing their reliance on the wave.

"We know many animals in Wyoming make long distance movement following the wave of spring when the landscape turns from white to green. And we figured bison track spring green up like other ungulates," bison biologist Chris Geremia said. "So we set out tracking bison migrations, and we found that they did at first, but they stopped tracking spring about halfway to two thirds of the way along their migration."

When the bison stop, they congregate in herds of several hundred to more than a thousand. This congregation creates a large lawn that can even be seen from space.

"Where these bison are in the summertime, it always looks like springtime. The grass is always a couple inches tall. And it's green as can be. It looks like grass is just emerging after winter," Geremia said.

So Geremia partnered with University of Wyoming researchers Jerod Merkle and Matthew Kauffman to figure out just why the bison aren't as reliant upon the green wave as other ungulates are.

It turns out that the very act of a bison chomping down on the grass encourages it to keep growing, similar to mowing a lawn. Add the nutrients from the bison's waste, and the grass is just as good for the bison as new spring grass.

This could change how some of the theories of animal migrations are thought about. Traditionally, animal migrations are thought of as animals following the best food. But bison create the very food they're seeking and aren't as reliant upon following the wave.

"In most places throughout North America, bison are managed in small areas of protected land, at set abundance and for generally low grazing intensity," Geremia said. "Our research suggests a challenge is that bison need to be able to have room around, be able to move in large groups across big landscapes, in order to be expressed in the ecosystem. In order for bison to be bison, they need to have room to roam."

Without bison migration, the landscape of Yellowstone would be fundamentally different, he said.

Have a question about this story? Contact the reporter, Ivy Engel, at iengel@uwyo.edu.