In Yellowstone, Restored Bison Replenish Grasslands

Bison have made a remarkable comeback in Yellowstone National Park, going from fewer than two dozen animals at the turn of the last century to roughly 5,000 today.

Their return, a study finds, has had a remarkable impact on grasslands in the region.

The Yellowstone bison travel around 1,000 miles each year, moving back and forth along a 50-mile route through verdant river valleys.

As bison graze, they also churn the earth and deposit manure, helping to boost microbes in the soil. These microbes, in turn, nourish grasses with nitrogen from decaying plants.

Comparing grazed and ungrazed areas, scientists were able to quantify the impact on grasslands.

They found that, even where bison grazed heavily, grasses grew as much as they would have in the absence of grazing, and, importantly, were 150 percent more nutritious. The study was published in the journal Science.

While restoration efforts have largely focused on maintaining small herds of bison in enclosed areas, the new study indicates that allowing bison to roam freely could help reinvigorate grasslands elsewhere, to the benefit of other animals along the food chain.

“With the current large herds of bison, Yellowstone grasslands are functioning better than in their absence,” said study coauthor Bill Hamilton, of Washington and Lee University. “And this version is a glimpse of what was lost when bison were nearly wiped out across North America in the late 1800s.”

This article was first published by Yale Environment 360 on 29 August 2025. Lead Image: A bison in Lamar Valley, Yellowstone National Park. Jacob W. Frank / NPS.