Bison Engineering a Better Yellowstone

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19 minutes, 18 seconds

Yellowstone is the only place in the United States where bison (Bison bison) have lived continuously since prehistoric times. They exhibit wild behavior like their ancient ancestors: congregating during the breeding season, migrating, and exploring that results in the use of new habitat areas. Yellowstone's Lead Bison Biologist Chris Geremia answers questions about how we study the natural role of bison in the park. (This video was a Facebook live from June 5, 2019.)


Yellowstone National Park recently conducted a 10-year study of bison to better understand bison impacts on the park ecosystem. Data collection techniques included putting GPS collars on bison, setting up field experiments to evaluate plant growth and grazing intensity, and collecting dung and plant samples. We also measured soil health, plant growth, grazing intensity, and plant community composition. As we seek to reestablish bison, this study shows us what large bison herds are capable of when they can seek out the best forage and move freely across large landscapes. Restoring grassland ecosystems with bison means finding a way to provide large numbers of animals room to roam.

Read more below about our findings below, in our 2020 Bison Conservation Update, and in the upcoming issue of Yellowstone Science.

Bison grazing in a wide, green valley
Bison grazing near Rose Creek in Lamar Valley

NPS / Neal Herbert

Bison do not just aimlessly eat grass

We discovered that bison change the way spring happens across Yellowstone’s vast grasslands. On a typical June day in Yellowstone, it’s not unusual to see thousands of bison grazing in the Lamar Valley. The groups appear aimlessly roaming back and forth across the historic valley. But, as it turns out, that’s far from the full picture. Bison return to graze the same areas repeatedly at such intensity that it turns back the clock on forage green-up, hitting reset on springtime. Without several thousands of bison moving freely on the landscape in sync, the springtime season of plant growth would be shorter, the land would not be as green, and plants would not be as nutritious.

For many years, scientists around the globe recognized that species like bison and wildebeest aggregate in large groups and intensely graze the same places, which creates grazing lawns. The behavior keeps plants growing, although the plants never appear more than a few inches tall. Short, young plants provide the best foods for migrating animals. Their grazing allows bison to migrate differently than other species. Evidence over the last decade supports the notion that migrating ungulates surf the green wave. The green wave is the progression of plants emerging in spring from river valleys to mountaintops. Many mule deer and elk, for example, have been shown to be in sync with spring green-up, which lets them eat high quality foods as they migrate. But bison and their intense grazing lets them fall behind the wave of spring because they create grazing lawns as an alternative. That finding sets bison apart from other North American ungulates. Bison are not just moving to find the best food; they are creating the best food by how they move.

The wave of spring across the park changed as the bison population increased to as many as 5,500 animals over the last decade. Images from NASA satellites of the same areas showed that when grazed more intensely by larger groups of bison, they greened-up earlier, faster, more intensely, and for a longer duration. Their influence on the landscape affects the entire way that spring moves through the mountains and valleys of Yellowstone.

A large, wide valley green during summer
Summer in Lamar Valley.

NPS / Neal Herbert

Bison make a better Yellowstone

Yellowstone can sustain 8,000-10,000 bison based on the 165-172 million pounds of plant matter that grow each year. While bison don’t graze the land uniformly, as managed livestock would, bison have developed finely tuned migrations to move and graze across the land to get the essential foods they need to raise young, survive harsh winters, and avoid predators. Nature decides who lives and dies, and the best moving and grazing strategies are passed on among generations. This non-uniform pattern of grazing makes northern Yellowstone a relic of North America prior to European settlement when bison dominated the continent. It is not waist-high grasses and wildflowers backdropped by snowcapped mountains but instead mosaics of dense mats of short-statured plants with dung piles peppering the land and wallows pitting the dense mats of grazed plants.

We used short- and long-term grazing exclosures to measure the effects of bison on grassland health. Our research suggests that Yellowstone benefits from the large roaming bison herds. Soils are nutrient-rich, uncompacted, and have the necessary biological, chemical, and physical properties to support the ecosystem. Grazing increases the rates that nutrients are recycled back to plants. Grazing also preserves soil moisture. Nutrients and water are the two ingredients that plants need to grow. As a result, grazing stimulates plants to regrow making plants more productive than they would be if bison weren’t grazing the park. More plant matter can mean more bison and other herbivores, more predators relying on the herbivores for food, and an overall healthier ecosystem.



More Information

A bull bison grazing in tall grass on the Blacktail Deer Plateau.
Bison Ecology

Learn more about North America's largest land-dwelling mammal.

Last updated: August 21, 2023

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