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.
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.
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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.
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.
Last updated: February 26, 2021