Bison herds ‘reawaken’ Yellowstone’s prairies

There are few symbols of the American West more emblematic than bison – soft giants who once traveled tens of millions before being almost wiped out by European settlers.

A new study published Thursday in Science Award that bringing back the largest terrestrial mammals in North America to their old trampling land exudes new life in prairie ecosystems, with cascade advantages on the food network.

Scientists have long known bison as “Keystone species”, shaping the landscapes of meadows by pasture, wallowing and dispersion of seeds. Previous research has even suggested that their vast herds help lock the carbon in the soil.

But the latest discoveries, from Yellowstone National Park, reveal to what extent their influence can be dramatic for the health of ecosystems when they are allowed to wander freely in the desert.

By snacking through the herbs, the bison accelerates the nitrogen cycle, the overeating plants with nutrients.

The result: a fodder which is more than 150% richer in protein – a gift to each creature which feeds on the meadow, the elk and the deer in Pronghorn and the America mouflons.

“It is really a reappearance of what had been there in the past,” said Bill Hamilton, co-author of the newspaper and professor at Washington and Lee University.

Natural laboratory

The herds of bison were once vast in the west of the United States that their hooves rumbled like a distant thunder. Estimates from the beginning of the 19th century put their number between 30 and 60 million.

Then came the railways. The construction of the first transcontinental line was accompanied by a ruthless extermination campaign – fire in trains, carcasses left for rot – not only made skins, but to hungry Amerindians, for which Bison was a buoy of cultural and economic rescue.

At the beginning of the 20th century, the species fogted with the edge. Renaissance efforts have finally pushed their number to around 400,000, but almost all survive in small herds well managed on private ranchs or reserves.

It is only in Yellowstone National Park, which houses around 5,000 animals, bison always move with something like their old freedom, covering nearly 1,000 miles per year on a migration of 50 miles. For scientists, this makes the park a rare living laboratory.

Between 2015 and 2021, a study led by the Biologist of the National Park Service Chris Geremia followed animal movements and grazing patterns in their main habitats, measuring plant growth, the nutrient cycle, soil chemistry and more.

Restored balance

They compared the grazed and un loose patches, combining experiences in the field with satellite imagery and GPS necklace data to capture the impacts of the Bison in their migratory range.

The results show that itinerant bison keeps herbs and short, dense and protein -rich wild flowers and surprisingly diverse plant communities.

“Herbs remain carbon in the ground after being grazed, which actually stimulates microbial populations up to 48 hours,” said Hamilton.

This microbial burst has resulted in more ammonium and nitrates for new growth – a feedback loop that has stimulated both plants and animals. Bison dung and urine added another nitrogen shock, aggravating the fertilization effect.

“We have taken this increase by 150% and we calculated it throughout migration (zone), and it provides more than three million kilograms of raw protein more if you have bison,” said co-author Jerod Merkle of Wyoming University.

Even the areas that seemed “flat mowing” by the bison remained ecologically vibrant, with intact productivity and diversity.

The conclusions come at a difficult time for conservation, because the administration of President Donald Trump tilts strongly towards agricultural interests on wildlife.

The breeders argue that the bison authorized to browse the fences, would mix with cattle and the spread of diseases.

Merkle said there were social and political constraints, but ecological gain was undeniable.

“My vision when I see the data of our article is, let us simply remove from the thought of classic cattle with bison, and consider them as a species that creates heterogeneity, which needs a large space to move-and it is normal to sometimes have large groups,” he said.

© 2025 AFP