Bison, Fire and the Tallgrass Prairie Preserve

Grazing by bison is essential to the maintenance of a healthy prairie ecosystem.

By Jay Harrod

Hanging on the wall in an historic ranch house at The Nature Conservancy’s Tallgrass Prairie Preserve in northeast Oklahoma is a map that shows the historical range of the bison.

During the 18th century, these amazing animals — North America’s largest land mammals — were found from northern Canada south into Mexico and from California east into the Carolinas and Florida. Scientists have estimated the bison population at the time to be somewhere between 30 and 60 million. Early settlers reported watching bison herds that took five days to pass them by.

The same map shows the bison’s range in the late 1800s, when their numbers fell to fewer than 1,000 animals. (The last bison on Oklahoma’s prairies was recorded in 1851.)

“The bison that were saved from extinction were generally saved by ranchers,” says Harvey Payne, who’s served as the Tallgrass Prairie Preserve director since 1990. “They kept them from being totally exterminated. Bison came about as close to extinction as possible.”

Today bison are no longer in danger of extinction; there are approximately 500,000 of them living in North America. While some roam public lands, such as Yellowstone National Park, most are part of privately owned herds.

Bison and the Tallgrass Prairie Preserve

The Conservancy reintroduced 300 donated bison at its 39,000-acre Tallgrass Prairie Preserve in 1993 not for the protection of the animals, but for the benefit of the prairie. It turns out the prairie needs the bison as much as the bison need the prairie.

With productive birth rates, the herd at the Tallgrass Prairie Preserve — the largest protected tallgrass prairie remnant in the world — topped 2,000 in 2003. Today the herd has reached its target population, approximately 2,500 animals.

The bison roam freely on 23,000 acres at the preserve, and outside of feed used to lure them closer to pastures where they’re rounded up each November, the bison are not fed. They eat what the prairie amply provides — grass, and lots of it. An adult bison can consume up to 30 pounds of grass each day.

While the vast majority of the rich soil at the preserve has never been tilled, the prairie has changed — in some cases dramatically — because of heavy cattle grazing that began in the early 1800s.

“Bison graze the prairie differently than the cattle,” Payne says. “They select grasses as their predominant food. They pretty well ignore the broadleaf plants. We have about 750 plant species here. Of those, about 150 to 160 are grasses and the rest are broadleaf plants.”

On the other hand, about 15 to 20 percent of a cow’s diet consists of broadleaf plants, also called forbs.

“Over the last 150 years, while the cattle have grazed here, you’ve seen a shift in plant communities away from a lot of forbs to — in some cases — not many forbs,” Payne says. “When you lose plant diversity, animal diversity typically follows.”

The ‘Red Buffalo’

Equally important to the health of the tallgrass prairie ecosystem is fire, which Native Americans referred to as the “red buffalo.”

“You watch a fire going across the landscape, and it does create a rumbling sound,” Payne says. “It can sound like a stampede of bison. And you see the flames jump up [to 40 feet], and it’s easy to see why the Indians called fire ‘red buffalo.’”

Since 1993, Payne and his staff have used prescribed burns to mimic the seasonal fires that have shaped the tallgrass prairie for thousands of years.

Using a “patch burn” approach, Conservancy fire crews burn one-third of the preserve each year in randomly selected sections, meaning that some sections may be burned several years in a row and some sections might not be burned for several years. The crews burn approximately 40 percent of the sections in the spring, 20 percent in the summer and 40 percent in the fall.

“What that leads to are some areas that are very closely grazed, because the bison favor the new growth following a burn,” Payne says.

“Areas not burned for several years develop mature grasses and thicker, thatch-like vegetation — habitat preferred by species like Henslow’s sparrows. And it creates everything in between. Every species that occurs naturally here has a home. Biodiversity is what it’s all about.”

Cattle at Home on the Range

While bison graze 23,000 acres at the preserve (which is slated to increase to about 25,000 acres in 2008), cattle graze the preserve’s remaining acres. And beyond the preserve’s fences are some of the most productive cattle ranches in the nation.

For decades ranchers in the region have burned from fence line to fence line, usually in April. Most ranchers are also methodical and consistent in their approach to managing the number and duration of steer grazing per acre.

“When combined, these widespread, common practices have created a homogenous landscape that does not favor wildlife and biodiversity,” Payne says.

To increase biodiversity on the portion of the preserve grazed by cattle, Payne and his staff applied the same burning and grazing techniques used for the bison pastures.

And we’re seeing similar results,” Payne says. “The cattle actually gain as much weight as they would if you did area-wide burns, and it greatly benefits wildlife.”

The Conservancy is now working with Oklahoma State University to better understand how these techniques can benefit cattle ranchers and restore or improve tallgrass prairie habitat outside the preserve.

Jay Harrod is senior media relations manager for The Nature Conservancy in Arkansas.