The bison at Yellowstone National Park control springtime through their grazing and engineer the environment to create themselves the “best food,” scientists have found. Instead of following the “green wave” of plant growth at the start of spring, bison can reinvigorate the landscape they have already grazed, meaning they can “set the terms” of spring with such a great impact it can be seen from space.
It is estimated that before European settlers arrived in the U.S., between 30 and 60 million bison used to roam the land. According to the Fish and Wildlife Service, the mass slaughter of bison over the 18th century saw the total population fall to just over 300, with 25 remaining in Yellowstone. The park is now the only place in the country where bison have been continually present since prehistoric times, with a herd of around 5,000 now roaming the landscape.
Scientists led by Chris Geremia, the lead bison biologist for Yellowstone National Park, were looking at migration patterns to see whether this population follows the green wave for its migration. The green wave is the way spring green-up moves from lower to higher elevations as the season progresses. The Green Wave Hypothesis suggests herbivores around the world migrate at the same pace as the wave, following the grass as it shoots up, because younger vegetation is better for foraging. The pace and timing of migration is therefore controlled by changes to the climate and weather.
In a study published in the journal PNAS, Geremia and colleagues tracked the migration patterns of Yellowstone bison using 13 years of data taken from GPS collars fitted to animals in the park. They also carried out field experiments to establish how intensely the herd grazed the land and estimated the “greenness” of the land using NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite.
Findings showed that while bison started their migration in line with the green wave at the start of spring, they soon let this wave pass them by and instead started to manipulate and maintain the grasslands to create food for themselves. By continually mowing the grass through grazing, and trampling the ground with thousands of hooves, the grasslands “green up faster, more intensely, and for a longer duration,” the authors wrote. The bison-grazed grass was also more nutritious, with higher ratios of nitrogen and carbon, they found.
If the bison were not present, springtime in Yellowstone would be shorter, less green and the grass would not be as nutritious, the team say. “We’ve been studying bison in Yellowstone for over a century, but this idea that bison set the terms of springtime, through their movements and grazing, was something that’s never been confirmed before in Yellowstone—or anywhere else,” Geremia said in a statement.
Study co-author Jerod Merkle, from the University of Wyoming, added: “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. They are not just moving to find the best food; they are creating the best food. This happens because bison are aggregate grazers that graze in groups of hundreds, or more than a thousand animals.”
The team say the impact of the bison on spring in Yellowstone raises questions about what impact the mass slaughter of bison across the U.S. may have had on the landscape.
In 2016, former President Barack Obama signed the National Bison Legacy Act into law, establishing the bison as a national mammal. Conservation work across the country is now helping the population grow, with an estimated 500,000 in North America today. In October, part of the Badlands National Park opened up an additional 22,000 acres to bison there—the first time these animals would have had access to the land for 150 years.
“Today there is growing effort to restore bison to habitats they once roamed,” Geremia said. “As we seek to reestablish bison, this study shows us what large bison herds are capable of when they are allowed to seek out the best forage and move freely across large landscapes.”
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