300 Bison Were Released In Oklahoma In 1993, And Now More Than 2,000 Roam The Prairie

Quick Take

- Bison and cattle both graze the same prairie grass, yet only one of them actually helps the ecosystem recover, and the difference isn't obvious. See how bison differ →

- A 30-year study tracked what happened when bison returned to a dying grassland, and the results surprised even the researchers. Explore the study results →

- Less than 4% of America's original tallgrass prairie still exists, a grim reality that has prompted a conservation strategy running counter to how most such efforts work. See what's at stake →

- The bison reintroduction wasn't just about the animals. It was about fixing something invisible the prairie had been missing for over a century. Discover what was missing →

In 1993, Oklahoma oilman Kenneth Adams donated 300 bison to a stretch of northeastern Oklahoma grassland that The Nature Conservancy had been piecing together since 1989. Today that herd numbers more than 2,000, roaming across tens of thousands of acres of the Joseph H. Williams Tallgrass Prairie Preserve, the largest remaining protected patch of tallgrass prairie left on Earth, according to The Nature Conservancy.

The bison weren’t brought in just to look impressive on the horizon. They were part of a plan to put back something the prairie had been missing for a century.

How Fire and Bison Work Together

Tallgrass prairie depends on disturbance to stay diverse. Left alone, it slowly chokes itself out with dense grass and woody brush. The Conservancy’s management combines prescribed, carefully controlled burns, roughly a third of the preserve each year, with bison grazing, mimicking the natural cycles of fire and large herbivores that shaped the region for thousands of years before settlement.

Bison help in a specific way cattle don’t: they eat almost entirely grass, leaving broadleaf wildflowers and other flowering plants room to grow back. Cattle, by contrast, graze more indiscriminately across both grasses and flowering plants, which tends to suppress the very diversity managers are trying to restore.

More Than 750 Plant Species on One Preserve

The preserve now supports more than 750 plant species, along with over 300 recorded bird species. That diversity matters because tallgrass prairie itself has nearly vanished: once stretching across 14 states and more than 142 million acres from Texas to Manitoba, less than 4% of the original ecosystem remains today, most of it plowed under for farmland generations ago. What’s left survives mainly in Oklahoma and Kansas’s Flint Hills.

What a 30-Year Kansas Study Found

The clearest evidence that bison themselves drive this kind of recovery comes from just across the state line. At Kansas State University’s Konza Prairie Biological Station, researchers tracked more than 30 years of data comparing plots with no large grazers, plots grazed by cattle, and plots grazed by bison. Published in the Proceedings of the National Academy of Sciences, the results were striking: reintroducing bison roughly doubled plant biodiversity compared to ungrazed prairie, a far bigger jump than cattle grazing alone produced.

The timing of the study caught something else, too. Partway through the three-decade span, the region went through one of the most severe droughts the central Great Plains has seen in decades. The bison-grazed plots held onto their added plant diversity through it, showing that the benefit wasn’t just a fair-weather effect. Researchers also found that cattle grazing helped plant diversity too, compared to prairie left with no large grazers at all, just not nearly as much as bison did. Lead researcher Zak Ratajczak put it simply: grasslands across the central Great Plains likely carry far less plant diversity today than they would have before bison were wiped out across the region, and bringing them back, where it’s practical, can help undo some of that loss.