Ecosystem Engineers
Study reveals how Yellowstone bison help create spring green-up in the park
By Kasey Rahn
We often think of beavers as ecosystem engineers, but it turns out so are bison.
Instead of being constrained by what might be called more traditional grazing patterns, bison in Yellowstone National Park can maintain the best forage for themselves by manipulating the landscape as they graze, essentially restarting spring green-up in a way that’s big enough to see from space. So says a study published in the prestigious journal Proceedings of the National Academy of Sciences by a team of researchers that included UM Professor Mark Hebblewhite and alumnus Dan Eacker.
The study, “Migrating Bison Engineer the Green Wave,” was led by Yellowstone National Park scientists Chris Geremia and Rick Wallen in partnership with biologists at UM, the University of Wyoming and the U.S. Geological Survey.
As aggregate grazers, bison chow down in large groups that can number in the hundreds or thousands. As herds migrate to higher elevations during the summer months, they eat and trample plants below them, sort of like a lawn mower, keeping the plants in a perpetually growing, green state that’s more digestible and nutritious. Because bison graze in big groups and with such intensity, they essentially restart spring green-up as they move. In this way, bison can manipulate the length of the growing season for plants in the park and how nutritious and green those plants grow.
“Across the globe, ecologists have studied ungulates like bison and whether they affect their own ‘grazing lawns,’” Hebblewhite says. “Despite some evidence that African buffalo and Serengeti wildebeest can alter their ecosystems, this is the first evidence that bison can manipulate spring green-up in Yellowstone — or anywhere else in North America — through their movements and grazing.”
This ability to engineer grasslands also alters how bison migrate in surprising ways.
The Green Wave Hypothesis says that the green wave — the progression of spring green-up from low to high elevations or latitudes — determines where and how fast herbivores migrate. Come spring, many species, like elk and mule deer, “surf the green wave,” following the delicious siren’s call of new plant growth as it winds its way from the lowlands in the early spring to the mountaintops as the season progresses — or from southern latitudes to northern ones.
Most North American ungulates — hoofed mammals — have to closely match their migrations to the green wave, whose timing depends on temperature, precipitation and snow melt. It’s one reason why you might catch a glimpse of elk closer to campus in the bottoms of the Missoula Valley early in the season, only to have them disappear into the mountainous abyss as soon as it starts to warm — and green — up. And it’s one of the reasons Western mule deer migrations are so intense, sometimes 200 miles one way. When you’re dependent on spring green-up for healthy food, there’s not much else to do except follow it.
Bison, though, don’t surf very well. Instead, they build the wave.
Animals like bison and wildebeest, who gather and graze in very large groups, create grazing lawns — areas where heavy grazing and fertilization from urine and dung allow for constant new plant growth. But most previous studies have quantified this at only relatively small scales.
“The data showed that grasses heavily grazed by bison were more productive compared to exclosures where bison were not allowed to graze,” says UM alumnus Matthew Kauffman with the U.S. Geological Service Wyoming Cooperative Fish and Wildlife Research Unit. “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.”
The researchers also found that instead of following spring up to higher elevations, bison stopped about two-thirds of the way along their migration and let green-up pass them by. But by stopping, they created grazing lawns and maintained high-quality diets.
“We knew that bison migrated, and 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,” says co-author Jerod Merkle, the Knobloch Professor in Migration Ecology at the University of Wyoming and a UM alumnus. “They are not just moving to find the best food; they are creating the best food.”
FIELDWORK MEETS NASA DATA
The study pairs boots-on-the-ground field research with cutting-edge analysis of data from NASA satellites.
For 13 years, Yellowstone researchers Geremia and Wallen tracked migrating bison across the park using GPS collars and quantified their foraging habits, took detailed measurements on plants, and collected and analyzed samples of bison dung.
The UM team — consisting of Hebblewhite and Eacker — helped analyze all of that GPS data from hundreds of collared bison, using a habitat-use analysis they developed in-house. They also analyzed NASA data to determine if the changes NPS scientists saw in the field scaled up to the whole ecosystem.
They did. NASA satellite images showed that grasslands where Yellowstone bison graze heavily have different green-up patterns than elsewhere in the park.
“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,” Hebblewhite says. “The fact that we could see the effects of bison on grasslands from space and interpret these effects with a long-term, carefully designed grassland experiment highlights the team’s skillset. Our work shows that bison are capable ecosystem engineers, able to modify grasslands in a way that enhances their own grazing.”
Hebblewhite and Eacker used MODIS data for the study. MODIS (the Moderate Resolution Imaging Spectroradiometer) is a sensor housed on two satellites that view the entire Earth’s surface every one to two days. Launched about 20 years ago, MODIS sensors acquire data in 36 spectral channels — from near-infrared to invisible wavelengths up through thermal infrared channels. Information obtained from these bands are used for atmospheric, cryosphere, terrestrial and oceanic measurements. The data are used by scientists to understand numerous Earth processes, including changes in land cover, fire mapping and plant growth.
For this study, Hebblewhite and Eacker developed a new method to analyze these series of satellite photos over time to determine whether bison affect the growing season. But Hebblewhite says the work builds on decades of collected expertise of remote sensing of earth systems at UM, beginning with that of Regents Professor Emeritus Steve Running, a well-known climate scientist who shared in the 2007 Nobel Peace Prize.
“He had an immense role in developing the satellites that we used in this analysis,” Hebblewhite says. “My involvement builds on his mentorship of me when I first started as an assistant professor. In my Ph.D., before I started here, I was one of the first people to use MODIS data to test how it influenced animal movement and behavior. Early on, Steve and I got a NASA grant to continue that work and to help the state and federal agencies better understand why animals do what they do, using remotely sensed data.”
Originally hired to teach dendrology — tree identification — in 1979, Running became interested in some of the very first computer simulations in ecology. It’s a self-proclaimed quirk that quickly got him involved with NASA. Running was one of the principal investigators on the MODIS project when NASA began developing the sensor in the 1980s.
When he signed his MODIS science team contract in ’89, it was the biggest contract in the history of the Montana University System. Running is still on the NASA grant for two more years, until the satellites and sensors officially reach the end of their lifespan and are decommissioned.
“It was all quite a big deal back then,” Running says. “It was the biggest Earth observation mission that had ever been done, and I had a front-row seat. The MODIS dataset started in the year 2000, so when [Mark] started looking at it in 2004, it was still quite a new sensor, and very few biologists were paying much attention to it. Mark was one of the first wildlife bio guys to start thinking that could we learn by coupling our wildlife sensing data with these daily global satellite datasets coming from these sensors. It was certainly leading science back then, and it certainly still is if you’re getting papers in PNAS.”
“So much of previous land management — both wildlife and range management for grazing — has been, you might say, through the window of a pickup truck, driving around and seeing what things look like from the truck,” Running says. “It’s really a step into the future to be using satellites where you can see the entire landscape at once — not just the couple of miles on either side of the road you can see from your truck.”
SCIENCE WITH IMPACT
As a result of the study, the researchers say that the Green Wave Hypothesis needs revision to include group-forming grazers like bison, which don’t only migrate to find forge, but create forage as they go.
The findings also beg the question of what of what these dynamics might have looked like in the past, before European colonization, when 30 million plains bison roamed North America. Today, bison occupy less than 1% of their historic range. Yellowstone’s 5,500 bison, which still migrate up to 100 kilometers, are the last truly migratory herd.
Capable of engineering the ecosystem they inhabit, grasslands with wild bison would have looked radically different than they do today.
“Today there is growing effort to restore bison to habitats they once roamed,” Geremia says. “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.”
Hebblewhite says the research also has potential impact for hoofed mammals the world over.
“It’s very timely because there’s a big debate in Yellowstone coming about whether there’s too many bison,” he says. “That question really has a bunch of problems, but one of them is the assumption that if you have too many animals that eat grass, they’ll eat too much grass and lead to overgrazing. But our study shows that bison modify forage in a way that benefits them and probably lots of other species. They did so by basically acting like a giant lawn mower.”
Hebblewhite says bison modify their ecosystem much like beavers do, improving conditions for themselves and other species.
“This tells us that bison probably had this same effect over the whole continent,” he says. “Our work has global impact for the conservation of big, large-scale migration. Caribou in Canada. Wildebeest in Africa. We thought we’d see something, but we didn’t expect to see as strong of an effect as we did see.” •
