Scientists Say Bison Have Returned to Their Natural Movements After 100 Years With Incredible Results

Bill Hamilton spent his summers kneeling in Yellowstone’s river valleys with a pair of clippers, cutting grass by the handful and weighing it.

Around him, roughly five thousand bison. Heads down and unhurried, grinding up the valley the way the herds did before anyone thought to count them, cropping the ground with a thoroughness that would make a rancher flinch.

“Bison are true grazers,” Hamilton told Eos. “They put their heads down and just do their thing.”

Everything the textbooks say about grazing this heavy points one way. It is supposed to be a wound. Hamilton went in expecting to spend six summers documenting damage.

Then he weighed the grass, and the numbers came back facing the other direction.

What Sixteen Cages Were Really Measuring

The tools were almost comically humble for a finding this size. Wire cages, nine square meters each, dropped over patches of ground to keep the animals off. Grass inside, grass outside. Come back, clip both, weigh the difference.

Some cages stayed put from April to October. Others were lifted and moved about once a month, which is the move that makes the study work: a cage that travels catches what the grass does in the exact weeks the herd is standing on it.

The team buried reverse osmosis membranes in the soil to follow the nitrogen, and used cameras and GPS collars to confirm which animals had done the eating. Sixteen sites, six field seasons, published in Science in August 2025 by Hamilton, a Washington and Lee University biologist, with Yellowstone bison biologist Chris Geremia and University of Wyoming migration ecologist Jerod Merkle.

Hamilton’s description of the method is still the plainest. “It would be like if each time you mow your lawn and put it into a bag and then you weighed that pile at the end of the summer, you would know how much your grass you actually grew,” he said.

The Grass Grew Back the Same Size and Came Back Richer

Grazed plots produced about as much plant matter as the caged ones. Bite after bite, season after season, the grass kept pace.

The nutrition did not just keep pace. It climbed.

“It’s not biomass,” Hamilton told WyoFile. “It’s more productive, the nutritional quality is improved. There’s more nitrogen and crude protein in the forage.”

Measured against the sheltered plots, crude protein ran about 156% higher in the flat grazing lawns of the Lamar River valley, 155% higher in high elevation grassland, and 119% higher in dry country. The same quantity of grass, carrying in the richest spots roughly two and a half times the protein.

Where the Nitrogen Comes From Is Not Glamorous

“It’s mainly coming from urine and feces,” Merkle said of how the researchers believe it works. “What happens is it goes into the ground, and it lights up the microbial ecosystem in the soil.”

The grass pitches in too. Hamilton explained that grazed grasses push carbon into the soil, and that stimulates microbial populations for up to 48 hours. Graze, fertilize, regrow, graze again.

“It’s a circle,” Merkle said. “Bison graze, pee and poop. That facilitates the insects and the microbes, which then creates better soil.”

It is not a glamorous engine, and it was never meant to be. It is also one park across six field seasons, the researchers’ reading of what they measured rather than a law of grasslands everywhere. What the plots showed is sturdier: grazed ground carried more soil microbes and more nitrogen than the ground sheltered under the cages.

Three Thousand Tons of Protein Nobody Planted

Then the team scaled it up with satellite imagery and collar data.

“We took that 150% increase and we calculated it across the whole migration (area), and it provides over three million kilograms more crude protein if you have bison,” Merkle said.

Across the Northern Range, some 329 square miles, that works out to roughly 3,549 tons of extra crude protein, near 37 pounds an acre. Total available plant protein rose from about 3.45 million kilograms to 6.67 million.

Nobody planted it. Nobody fertilized it, irrigated it or paid a cent for it. And it does not belong to the bison alone. Elk, deer, pronghorn and bighorn sheep feed on the same enriched ground.

The Mess Is the Whole Point

The other surprise was how uneven all of it was.

“There are spots that bison hit hard, and places they don’t touch at all,” Merkle said, and the pattern shifts from year to year.

Ecologists call that patchwork heterogeneity, and Merkle thinks it is the real gift. “The cool thing is that allowing these bison to freely move across the landscape at a big scale, creates the heterogeneity,” he said.

The land seems to do better for being eaten unevenly, by something that cannot be told where to go.

“The closest relatable ecosystem is the Serengeti, where we have big herds of wildebeest and zebra moving,” he said.

A Century of Standing Still

By 1902, poachers had cut Yellowstone’s herd to about two dozen animals, according to the National Park Service. Two dozen. The park is the only place in the country where bison have lived continuously since prehistoric times, and for most of the century that followed, that continuity rested on those few survivors.

The population settled near 5,000 in the mid 2010s. Today the herds cover roughly 1,000 miles a year along a 50 mile route, and a 2019 study found they were not simply chasing spring north. They were making it, keeping their own forage young and protein rich deeper into the season. The effect is large enough that NASA satellites can read it from orbit, the same eyes that watch heat gather over the Pacific.

A century is a long time for a valley to wait.

What This Does Not Prove

One park. One unusually free herd. Of the roughly 400,000 bison alive today, about 95% are privately owned and commercially raised, and conservation herds average around 300 animals. Nothing here promises that a 300 animal pasture will behave like a 5,000 animal migration. If anything it suggests the opposite: the space and the freedom are ingredients, not the species alone. And the climate these grasslands sit in is not holding still.

The politics have not caught up either. Montana has gone to court to hold Yellowstone’s population at or below 3,000, a target conservationists say would mean killing more than 2,000 animals.

What a Reawakening Sounds Like

Strip out the tonnage and the percentages and what is left is almost embarrassingly simple. Let a very large animal walk a very long way, and it feeds the ground that feeds it.

Hamilton watched it from inside those cages, one clipped handful at a time.

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

Then, more plainly: “It truly is a reawakening of what had been there in the past.”