Thousands of bison move from winter grounds on the outskirts of Yellowstone every spring, heading uphill into the park’s interior in search of newly grown, nutritious grasses and wildflowers.
The shaggy beasts — part of the largest free-roaming Plains bison herd in the world — are “surfing the green wave” along their migration, to borrow a phrase popularized by University of Wyoming migration researchers. Their locations and elevations are synced with springtime, when the landscape is at its most fertile.
But then, the half-ton herbivores ease to a stop at ostensibly less than ideal grazing grounds.
“As they reach about half or two-thirds the way into their migration, they stopped tracking the green wave and let it pass them by,” Yellowstone National Park bison biologist Chris Geremia said.
The pause, downhill from where the richest plant matter sprouts, was unexpected because it suggests that Yellowstone’s bison are skipping the most optimal foods.
“We were incredibly surprised,” Geremia said.
But bison weren’t making any obvious nutritional tradeoffs, he found by doing an analysis of bison dung.
“We found that even though they stopped following the new plant growth,” Geremia said, “their diet stayed at the same quality.”
Geremia and six fellow scientists laid out what was going on in a recent edition of the academic journal Proceedings of the National Academy of Sciences, in a study titled “Migrating bison engineer the green wave.”
The closely congregated herds of foraging bison, they found, were manipulating the landscape in a way that suited them, creating “grazing lawns” through eating, trampling and defecation. The result is that vegetation is boot high and nutritious on parts of the landscape at a time of year when plants would otherwise be knee-high, beginning to cure and dropping in nutritional value.
“Really, the thought that went through our head is, ‘Wow, it really looks like springtime here, even though it’s the middle of June,’ ” Geremia said. “These areas that they grazed intensely looked more like a lawn that you would step out onto in Jackson or Bozeman.”
Yellowstone’s bison linger on the user-created fertile grazing lawns from late May into early July, then move along into the highest-elevation parts of their range, he said.
The scientists quantified these phenomena by tracking 64 female bison with GPS collars between 2005 and 2015. They clipped plants, took soil samples and built bison exclosures within the grazing lawns. Greenness of the landscape was estimated using a technology called Moderate Resolution Imaging Spectroradiometer satellite imagery.
The influence of the native aggregating grazers, which were essentially prolonging the green wave, was visible from space. Geremia’s recent paper focused on lawns that were 50 to 75 acres, but there were some grazing lawns that stretched for hundreds of acres.
“When you recover a large free-ranging bison population, this was an example of how they act as ‘ecosystem engineers,’ ” he said. “Bison, through their grazing, are actually shaping how the landscape changes from white to green across Yellowstone.”
Bison’s natural role in modifying vegetation around their native range in the Great Plains and Rocky Mountains has been vastly reduced, simply because relatively few are left. Prior to European settlers marauding the West, there were an estimated 28 million to 30 million North American bison, but market hunting killed all but a few hundred. Yellowstone was ground zero for the species staging a comeback, and today the approximately 5,500 bison that roam the 2.2 million-acre national park constitute the last and largest wild herd left.
Perhaps for those reasons, Geremia’s paper about “nature’s lawn mowers” has attracted a wealth of attention ever since the University of Wyoming spread word of the study Monday. Headlines and radio spots have appeared in The Atlantic and Newsweek, on National Public Radio and ABC News, and elsewhere.
Past research of how wildebeest influence East Africa’s Serengeti ecosystem planted the idea of assessing how bison are engineering Yellowstone’s plant growth. Geremia tipped his hat to co-authors Jerod Merkle, Matt Kauffman and Mark Hebblewhite for assisting with the decadelong study.
Now 15 years into the project, Geremia is turning his attention toward other ungulates that may be affected by the bison’s ecological engineering, like elk, pronghorn, mule deer and bighorn sheep. It’s too early for results, he said, but the hypothesis is that the other large four-footed mammals are benefiting.
“I would guess those animals have a higher chance of moving in sync with bison,” Geremia said.
The Proceedings of the National Academy of Sciences paper is attached to the online version of this story at JHNewsAndGuide.com.