Migrant Bison in Yellowstone Benefit Grassland Ecosystems

A recent study has found that the freely roaming bison herds in Yellowstone National Park play a significant role in enhancing the health of grasslands and promoting ecosystem recovery.

These findings challenge the conventional view on grazing. Researchers suggest that facilitating large-scale migration of bison could positively impact landscapes across North America, as noted in a study published Thursday.

“With larger bison herds today, the grasslands in Yellowstone are functioning better than they would without them,” said Bill Hamilton, a research science professor at Washington and Lee University.

Hamilton added, “This gives us insight into what was lost when bison nearly disappeared from North America in the late 1800s.”

Historically, millions of bison roamed the continent, and their seasonal migrations altered grassland areas, according to the researchers. Today, however, very few wild bison herds exist.

In the late 1800s, extensive hunting and military actions nearly drove them to extinction, reducing their population to about 400,000.

Evidence shows that bison can help diversify habitats, affect plant communities, and enhance nutritional productivity. Yet, the broader ecological impacts of their migration are still not fully understood due to their current limited lifestyles.

The restoration of bison in northern Yellowstone provided researchers with a unique chance to observe how these animals can reshape such a large landscape.

The current bison population in Yellowstone is roughly 5,000. It has grown significantly from around 23 in 1902 and remained stable since the mid-2010s. Today, bison typically travel about 1,000 miles each year along a 50-mile route.

Along their paths, bison graze actively, consuming young plants that emerge after the snow melts.

From 2015 to 2022, researchers monitored the grazing behavior of these animals across 16 sites, examining how it affects carbon and nitrogen dynamics, plant life, and soil microbiology.

They ultimately found that bison grazing accelerates the nitrogen cycle—a crucial process involving the movement of nitrogen between organisms and the environment. The bacteria from decomposed animals enhance nitrogen in the soil.

While plants grew as they do without grazing, the nutritional value increased by 150%, which benefits other herbivores who feed on them.

“As bison roam the landscape, they enhance the nutritional quality and abilities of Yellowstone,” Hamilton noted.

“Their grazing likely has significant effects on other herbivores and the entire food web,” he added.

Hamilton drew a comparison to changes observed in the Serengeti, where the wild population’s recovery benefited the ecosystem. However, the researchers pointed out that the Serengeti’s routes are now fully protected for wildebeests.

Despite this, the study authors concluded that bison are restructuring nutrient flows, producing varying effects in regulated environments.

Co-author Jerrod Markle, an associate professor focusing on ecology and conservation at the University of Wyoming, remarked that these results challenge traditional views linking overgrazing to diminished nutritional storage.

He stated that bison introduce a certain “non-uniformity” in the landscape, leading to areas of both short and untouched vegetation.

Globally, he and his colleagues advocate for allowing large mammal groups to roam freely.

“Reintroducing substantial bison populations would provide clear benefits to the ecosystem services in Yellowstone,” Merkle concluded.