Bison Engineering a Better Yellowstone

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

NPS / Neal Herbert

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 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.

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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.)


It's time to let nature decide

There is no shortage of opinions about the number of bison in Yellowstone, their effects on the vegetation, and what defines healthy rangelands. Other scientists have claimed that bison are overgrazing and permanently damaging northern Yellowstone. The debate stems from beliefs on what plant communities are supposed to look like.

Northern Yellowstone produces 165-172 million pounds of plants, and supports 3-4 million pounds of bison, elk, pronghorn, mule deer, and bighorn sheep. Some people believe the best practice in managing rangeland for domestic livestock is to graze the land uniformly, at the right time, and using enough animals to eat 25% of the plant material produced. Yellowstone’s wild grazers eat about 30-35 million pounds of plants, which equates to only about 20% of the plant material produced, but free-ranging wildlife decide where and when they graze. They 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.

When you give bison room to roam, they intentionally don’t graze uniformly. This non-uniform pattern 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.

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

NPS / Neal Herbert

Our research suggests that Yellowstone is better off for it. Soils are healthy and show long-term stability in their ability to let nutrients in and hold on to them. Soil textures and chemistries are within ranges that promote plant growth. Bulk densities are less than levels that diminish root growth or water penetrance. Soils show the same levels of nutrients over more than 60 years of grazing, while short-term experiments show that grazing accelerates the recycling of nutrients back to soils. Soil organic matter, the decaying plant and animal material in soils that is responsible for most water and nutrient flow, is unaffected by grazing. The light-to-moderate grazing that occurs along migration routes increases the amount of plant material produced each year. Also, the intensely grazed short-dense lawns of plants on summer ranges are just as productive as they would be if bison weren’t there. Most critics argue that grazing has changed the plant communities in northern Yellowstone to the wrong ones. This belief stems from a long tradition in rangeland science for managing landscapes for a single plant community by limiting and spreading out grazing. Any deviation from this theorized community is taken as a sign of degradation.

The world endorsed the idea of trophic cascades in northern Yellowstone, which means that predators can affect their food's food. It is being tested with the restoration of wolves to see if their recovery changes plant communities by affecting elk behaviors and numbers. Such change would be championed as ecosystem recovery. There is something unique about Yellowstone that isn’t true anywhere else in North America. It has a large, healthy, valued bison population. We are learning that the same way that predators can influence the system from the top down, bison can push on the system from the bottom up. When we give bison room to roam, the influence of bison on plant communities in the park is as natural and important to the ecosystem as those of top predators like wolves.


Last updated: February 26, 2021

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