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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.)
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.
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.
Last updated: May 13, 2022