Tens of millions of bison once roamed North America, grazing on grasslands, forests and plains, from the Great Basin of Nevada to the Atlantic Coast. It is difficult today to imagine the size of herds that are no longer seen; Lakota oral histories gauged herd size by the number of days it took them to pass.

Likewise, studying the role that migratory bison played in those ecosystems has become nearly impossible. After being driven nearly to extinction in the 1800s, the animals exist only in small herds.

But a new study conducted in Yellowstone National Park, where the last migratory herd still roams, offers a glimpse into the crucial role that these animals once played in restoring their ecosystem, and perhaps still could. “If we value a system, we need to allow them to operate as close to naturally as possible,” said Bill Hamilton, an ecologist at Washington and Lee University and an author of the study. “And this was a great case in point of how that can work.”

Of the roughly 400,000 extant bison, more than 5,000 live in Yellowstone’s 3,500 square miles; there are two herds in the park, one of which is migratory. (Most of the other bison outside the park are held in privately owned herds.) The migratory bison of Yellowstone travel more than 1,000 miles in a year, grazing different habitats along a 50-mile migratory route in the northern ecosystem.

The study, published last Thursday in the journal Science, examined how bison changed the soil and vegetation along their migratory route. Outwardly the effect can look like overgrazing. But the researchers found that bison essentially allow plants to keep growing: By grazing and moving on, the animals increase the density of microbes and nitrogen, an essential chemical for plant growth, in the soil, improving the nutrition for herbivores by up to 150 percent in some areas.

Troy Heinert, a member of the Rosebud Sioux tribe and chief of the branch of bison management for the Bureau of Indian Affairs, said that such research validated what Indigenous peoples have known for generations. “Buffalo helped shape this continent,” Mr. Heinert said. “And the more buffalo that are out there, the ecosystems are improved for all other animals as well.”