Yellowstone National Park hasn’t just been shaped by simmering magma and ancient glaciers. Its famous landscape is kept green, in part, by its herds of bison.
A new study by scientists at the University of Montana, the University of Wyoming and the federal government have found that bison don’t follow “waves” of ripening grass, the way deer and elk do. Rather, they stay put — and the areas where they do become “grazing lawns” where the grasses keep regenerating.
“Bison are capable of, in essence, engineering a highly nutritious landscape for themselves,” said Mark Hebblewhite, a professor of ungulate habitat ecology at UM and one of the study’s co-authors. Its lead author was Chris Geremia with Yellowstone National Park; other co-authors were Daniel Eacker at the University of Montana, Jerod Merkle at the University of Wyoming, Rick Wallen and P.J. White with Yellowstone, and Matthew Kaufmann at UW and the U.S. Geological Survey. The research was published this week in the Proceedings of the National Academy of Sciences.
The data that led to this discovery were a long time coming. Yellowstone’s roughly 4,500 bison are the largest herd in North America. When the National Park Service started tracking their movements with GPS collars in the mid-2000s, they noticed something odd about their movements.
Every spring, a “green wave” spreads through Yellowstone, as grass ripens at higher and higher elevations. Deer and elk take heed, “surfing” the wave to keep up with the freshest grass.
“We paired the (bison) collar data with satellite imagery to assess how they surf the green wave,” explained co-author Jerod Merkle, Knobloch Professor of Migration Ecology and Conservation at the University of Wyoming. “They surf the green wave early in the spring,” he said, but “at some point, they stopped.” Tracking their migration routes from March through August, the paper’s authors found that “many bison did not reach their highest summer ranges until well after the green wave had passed.”
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Things got stranger when the researchers zoomed in. As the bison trailed the “green wave,” passing up what appeared to be the freshest, healthiest grass, their dung showed no signs of nutritional deficiencies.
Could the bison herds themselves be affecting the quality of the forage? To see if this was the case, from 2012 to 2017 the researchers fenced off plots of grass along the bison migration corridors, then areas they had and hadn’t grazed. They found that by staying in an area and trampling and nibbling away in such massive numbers, the bison herds kept the plants shorter and denser. They also forced them to keep growing and regrowing, giving the bison a steady supply of fresh, nutritious grass.
“Basically, they just start lawn mowing it … and keeping it in a state of perpetual spring,” Hebblewhite said. And in high-grazing areas, the “green wave” came earlier and more intensely.
Yellowstone’s grasslands are a vestige of the North American prairies prior to the 19th century, when tens of millions of bison thundered unimpeded across the landscape. This study, the UM biologist ventured, gives a window into the ecological niche bison once played.
“It makes us think a lot about how this grassland system worked at continental scales, when we had tens of millions of bison roaming around,” Hebblewhite said.
It could also, he added, bolster efforts to rebuild some of those herds. Hebblewhite’s currently involved with one of those projects, the reintroduction of bison to Banff National Park in Alberta. In western Montana, the Iinnii initiative is working to rebuild a bison herd on the Blackfeet Indian Reservation.
Now, Hebblewhite said, they have evidence that these ungulates can nurture a grassland — provided they’re able to gather and move in a way that will allow them to serve as nature’s lawn mower.
“Bison impacts in Yellowstone aren’t just bad,” he said. “Having too many bison isn’t just a bad thing.”