Bison in Yellowstone National Park, grazing green grass in the Lamar Valley in this August 2016 image, can control the green-up of spring/NPS, Neal Herbert

What does it mean to be an “ecological engineer”? In the case of Yellowstone National Park’s bison, it at times can connote a role in an agrarian society. And, it turns out, that might not be too far from the truth when spring arrives in the park.

“Bison essentially were changing how the new wave of plant growth was moving across very large areas of Yellowstone,” said Chris Geremia, the park’s senior bison biologist. “Bison started the year by moving to find food. But how they moved, and how they grazed actually created good food.

“So it makes you think about animal migration in an entirely new light, that animals actually have an active role, an active impact over the foods they’re thought to be solely at the whim of nature to go find.”

This was not a sudden epiphany for Geremia and his colleagues on the study from the universities of Wyoming and Montana as the U.S. Geological Survey. It was more than a decade in the making, years spent following park bison on their seasonal migrations, erecting “exclosures” so they could see what happen to grasses bison couldn’t graze, conducting nutritional analysis on bison dung, and analyzing satellite data to track the movement of spring green-up in Yellowstone.

“There’s not a better place to study how bison influence ecosystems left in North America than in Yellowstone,” Geremia said during a phone call earlier this week. “That’s why we spent more than a decade trying to figure out what is their role, how do they shape this ecosystem? We knew bison migrated, we know they migrated throughout the park. We had no idea when we started this work that they actually may be able to shape how spring comes and moves across the river valleys and the mountains of the park.”

There are in the park about 4,500 bison, on average, and these iconic animals migrate between 60-70 miles as they follow the seasons. Springtime and early summer is a particularly important time, according to the bison biologist, because of the rich nutrients in new vegetation that the bison consume.

“That’s true for mule deer, pronghorn, elk, so many taxa of animals,” said Geremia. “So we figured bison would just surf the ‘green wave,’ just follow the emerging grass as they migrated from their low to high elevation areas.”

Through satellite imagery, the researchers were able to see when various parts of Yellowstone would green-up in the spring. They noticed that bison would arrive at those areas just as they were greening up.

“And they start their migrations that way. Maybe the first half of their migration they’re tracking the green wave, but then they kind of just let it go by, let it pass them by. We had no idea why,” said Geremia.

What they learned, he continued, was that bison essentially grazed back the new vegetation enough to spur new growth, much as you might prune back an apple tree or a houseplant to induce new growth.

“They ate enough vegetation that they essentially froze it in that early spring form. Bison were essentially the lawn mower of Yellowstone,” the biologist explained. “The grasslands look much more like fairways on a golf course than grasslands that have tall, mature plants.”

While those freshly "mown" areas contained nutritious new growth, areas that weren't grazed held aging vegetation diminished in nutrition.

Matthew Kauffman, who heads the USGS's Wyoming Cooperative Fish and Wildlife Research Unit at the University of Wyoming, said that new vegetation “had higher ratios of nitrogen to carbon, a standard measure of nutritional quality. And the green-up was earlier, faster, more intense and lasted longer."

What helps make the impact on the green-up so great is the sheer number of individuals in the herds that shift back and forth across the park’s landscape.

"We knew that bison migrated, we figured they followed the green wave, but we didn't know that their influence on the landscape could affect the entire way that spring moves through the mountains and valleys of Yellowstone," said Jerod Merkle, the Knobloch Professor in Migration Ecology at UW.

"They are not just moving to find the best food; they are creating the best food," Merkle added in a release that announced the findings. "This happens because bison are aggregate grazers that graze in groups of hundreds, or more than a thousand animals."

It’s long been known that bison graze the landscape differently than domestic livestock. They constantly are on the move, and don’t congregate for long around riparian areas and so don't tromp down streambanks with their hooves or denude the vegetation. With this latest research in hand, Geremia and his colleagues believe it could play a role in the ongoing efforts by the Interior Department to find space on public lands for more bison herds.

“A take home of our study is that restoring bison and restoring grassland ecosystems really means to find a way to provide large numbers of bison room to roam,” he said. “To allow animals to naturally graze and to move with landscape patterns or plant phenology. When that happens, bison will act as ecosystem engineers.”