CASPER, Wyo. — Sensors on NASA satellites are able to detect whether areas in Yellowstone National Park are heavily or lightly grazed by bison.
“Where Yellowstone bison congregate, plant green-up is different, and it’s not some fluke of local weather — the bison and their intense grazing are the cause,” the University of Wyoming explained in a Monday, Nov. 18 release.
The university added that a research team led by Yellowstone scientists also included biologists from UW, the University of Montana and the United States Geological Survey.
That team published a paper titled “Migrating bison engineer the green wave” on Monday in “Proceedings of the National Academy of Sciences.”
Their research looked into bison grazing behavior and found that herds manipulate “the landscape to maintain the best forage for themselves.”
“They even change the way spring green-up occurs in Yellowstone’s vast grasslands,” UW says. “Without bison moving freely on the landscape, the springtime season of plant growth in Yellowstone would be shorter, the habitat would not be as green, and the grasses would not be as nutritious.”
Bison migration patterns are different from other species, the research found.
“When bison move to higher elevation by mid to late summer, the herd’s small army of hooves and hungry mouths causes the grasslands to revert to earlier stages of plant growth,” the release states. “In effect, the bison graze with such intensity that they turn back the clock on forage green-up, hitting reset on springtime.”
UW says it has been well documented that species like bison and wildebeest create “grazing lawns” with their herds. This refers to areas where they graze heavily, with urine and dung fertilizing the soil and stimulating “near-continual new plant growth.”
“Repeatedly grazing an area keeps it growing, like a mower clipping a golf course,” UW explains. “But most previous studies had only quantified this effect at relatively small scales.”
“The findings are a result of years of research on Yellowstone bison by Geremia and Wallen, who deployed GPS collars to track the bison migration, while also establishing field experiments to evaluate grazing intensity. For 13 years, the team tracked migrating bison across the park and quantified their foraging habits, taking detailed measurements on plants as the experiments played out. They also collected and analyzed many samples of bison dung.”
That data showed that areas where bison heavily graze tended to be more fertile than areas they couldn’t access.
“‘The mowed-down forage 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,’” said the U.S. Geological Survey’s Wyoming Cooperative Fish and and Wildlife Research at UW Unit leader Matthew Kaufman.
Research into species like mule deer and elk shows that hooved animals tend to “closely choreograph their spring movements so they are always in sync with spring green-up, allowing them to continuously access the most nutritious, newly emerging plants,” UW says. This is referred to as “surfing the green wave.”
The research shows that bison behavior is different.
“Instead of following spring up to higher elevations, bison stopped about two-thirds of the way along their migration and let the wave of spring green-up pass them by,” UW says. “But, remarkably, because their stopping over allowed them to create profitable grazing lawns as an alternative, they maintained high-quality diets despite being mismatched with the broader patterns of green-up.”
“That finding sets bison apart from other North American ungulates such as mule deer, pronghorn or even elk that must closely time their migrations up to higher elevations at the whim of the green wave, which is dictated by temperature, precipitation and snowmelt.”
Unlike bison, mule deer can be thought of as “slaves to the green wave,” having to closely follow spring growth, UW says. That may explain why mule deer can migrate up to 200 miles one way.
“Put another way, following the wave of green-up is really important to mule deer, but their grazing is not important to the wave itself,” UW says. “That changes when you have 1,000 bison all grazing across the same grassland. As the title of the paper suggests, bison grazing in large aggregations engineer the green wave.”
Jerod Merkle, the Knobloch Professor in Migration Ecology and Conservation at UW, explains it another way.
“’They are not just moving to find the best food; they are creating the best food,’ Merkle says. ‘This happens because bison are aggregate grazers that graze in groups of hundreds or more than a thousand animals.’”
The research may suggest that North America’s grassland dynamics could have been very different prior to European settlement when bison herds numbered in the tens of millions.
“’Today, there is growing effort to restore bison to habitats they once roamed. 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,” says Chris Geremia, lead bison biologist at Yellowstone.
Mark Hebblewhite with the University of Montana summarized the team’s findings.
“’A real strength of our work was the pairing of on-the-ground grassland plots with remote sensing data, as well as GPS collar data from bison,’ says Hebblewhite, professor of ungulate habitat ecology at the University of Montana. ‘The fact that we could see the effects of bison on grasslands from space and interpret these effects with a long-term, carefully designed grasslands experiment highlights the team’s skill set. Our work shows that bison are capable ecosystem engineers, able to modify grasslands in a way that enhances their own grazing.’”
