Bison grazing boosts grass nutrition without harming soil

Yellowstone’s bison are not just big animals grazing grass. They are busy engineers that shape how plants grow, how nutrients flow, and who else can thrive.

Fresh research shows that when these herds move freely, the park’s grasslands stay productive and become more nutritious, without losing soil health.

Bison grazing helps grasslands

A research team led by Chris Geremia, a wildlife biologist at the National Park Service, found that grazing bison stabilized plant production while speeding up nitrogen turnover across northern Yellowstone. The team also documented higher plant nitrogen, without a drop in soil carbon.

Between 2015 and 2021, researchers ran field experiments comparing grazed and ungrazed plots. They then scaled up their findings using satellite images and GPS collar tracks to map effects across the migration corridor.

The experts used movable exclosures – simple fenced frames that keep herbivores out – to measure how much plant matter bison consumed and how fast plants regrew.

Collectively, the results revealed a pattern that challenges old grazing rules. Productivity held steady, plant biodiversity rose along the route, and the nutritional quality of grasses improved – even where animals fed heavily.

Bison grazing boosts plant protein

The work focuses on the nitrogen cycle – the movement of nitrogen between plants, animals, soil, and air. The experts report that regrowing plants in heavily grazed patches contain about 150 percent more crude protein than plants in areas without bison.

Soil microbe populations increase after a graze. They quickly recycle organic matter into plant-ready nitrogen, and plants pull that nitrogen back into new tissue. “Just the animals and plants doing their thing,” said Bill Hamilton from Washington and Lee University (WLU).

By mid-summer, this feedback keeps some river valley meadows clipped, dense, and nitrogen rich. Other places remain taller and less grazed, which helps spread different plant types across the landscape.

Free-ranging bison in Yellowstone

Yellowstone is the only place in the lower 48 that has had a free-ranging bison population since prehistoric times.

According to the Park’s history, numbers had fallen to about two dozen individuals by 1902. In recent years, however, estimates have ranged roughly from 3,500 to nearly 6,000 animals within and near the park.

Herds gather in river valleys each spring and summer, then shift as seasons change. That movement matters because it spreads grazing pressure in pulses rather than as a constant, even bite.

Cultural significance of bison

For many Indigenous nations, bison are more than just wildlife. They hold spiritual, cultural, and economic significance that stretches back thousands of years.

The return of herds carries meaning that goes far beyond ecology. These communities view the animal’s recovery as a restoration of identity as well as ecosystems.

Partnerships between Indigenous groups and conservationists have already brought bison back to several reservations and protected areas. These initiatives add to a growing collection of evidence that restoration can support both cultural renewal and ecological health.

Keeping ecosystems balanced

Earlier work showed Yellowstone bison can alter the timing and intensity of spring greening, creating rich patches where new growth keeps coming.

The new study builds on that insight by tying plant chemistry, soil microbes, and herd movement together at landscape scale.

Wet, nutrient rich habitats showed the strongest nutritional boost under heavy grazing, while drier areas saw lighter use and weaker effects. That uneven pattern, called heterogeneity, is a feature to keep, not a flaw to fix.

Grazing aids climate resilience

Ecologists note that large herbivores like bison may also help buffer grasslands against climate variability.

By keeping plants shorter and nutrient rich, bison grazing could support faster regrowth after droughts or extreme weather events. This dynamic may become increasingly valuable as Yellowstone faces hotter summers and shifting precipitation patterns.

At the same time, warming climates could alter migration timing and plant growth cycles. Understanding how bison adapt to these shifts will be critical for long term conservation and for anticipating how ecosystems across the West will respond to environmental change.

Global lessons from Yellowstone

Similar grazing-driven nutrient cycles have been documented in Africa’s Serengeti, where wildebeest herds recovered in the mid 20th century and reshaped entire food webs. Ecologists point to those parallels to show how large, migratory herbivores can restore balance far beyond the boundaries of one park.

Across North America, efforts to return bison to tribal lands and conservation areas are expanding. The Yellowstone study highlights that allowing animals space to move at scale may be just as critical as reintroducing them in the first place.

“With the current large herds of bison, Yellowstone grasslands are functioning better than in their absence,” said Hamilton.

Ultimately, the message is simple: restoring the natural movement and scale of bison allows these large grazers to enhance food quality for countless species without sacrificing carbon storage or overall productivity.

For land managers, the lesson is to aim for flexible goals that respect the ecological limits of each habitat, ensuring both biodiversity and resilience can thrive.

The study is published in the journal Science.

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