In 1993, a few bison were introduced to America's dying grasslands. Now, thousands of those beasts, once hunted to near extinction, are helping bring the prairie ecosystem back to life

The scenery presented to the early settlers as they ventured westward beyond the Appalachian Mountains had to be daunting. The terrain was boundless, broken up by flowering plants, livestock and massive herds of bison.

The grassland habitat covered an estimated 170 million acres of land in what is currently known as the United States. This represented around 10% of the total acreage of the country at that time.

An estimated 30 million bison roamed freely, affecting the vegetation, wildlife included.


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Much of that has changed.

Most of the prairie has been turned into agricultural land, roadways and urban centers, while the hunting pressure caused a drastic decline in bison population to less than 1,000 by the end of the 19th century.

Restoration activities have seen the habitat and animals revive. The current number of bison in the country is estimated at about 350,000, but only a small proportion of the habitat is still intact.

One of such restoration initiatives is the bison habitat at the Joseph H. Williams Tallgrass Prairie Preserve in Oklahoma.

Oklahoma’s tallgrass prairie is becoming a bison stronghold

The development of the Tallgrass Prairie Preserve started in 1989 as The Nature Conservancy acquired the Barnard Ranch in Oklahoma. Further purchases resulted in increasing the size of the reserve to over 39,000 acres in the Osage Reservation.

The preserve is operated using cooperation of conservationists, ranchers, and Osage Nation landowners. The goal of the preserve is not only the protection of open spaces, but also the maintenance of ecological interrelationships inherent to the tallgrass prairie.

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In 1993, about 300 bison were introduced in the preserve. Since then the number of the animals has increased significantly. Generally, the winter herd numbers 2,100 individuals whereas in summer, it includes about 2,700 bison after birth of calves.

Periodical round-ups help in checking the health condition of the herd and vaccinating the bison. Some part of the herd is annually sold to maintain balance of the population with grasslands.

Such management is required as an ecologically healthy prairie cannot provide unlimited grazing.

The introduction of the bison is already affecting the land management practices. As the bison eat grasses, walk across the ground and disturb patches of the soil, they create favorable environment for plants, insects and birds.

Why bison grazing helps wildflowers and wildlife return

Bison are often treated as symbols of the American West, but their ecological role is more practical. Their feeding habits help determine which plants survive and how different parts of the prairie develop.

According to Harvey Payne, community relations coordinator for the preserve, grass makes up about 99 percent of a bison’s diet. Unlike cattle, bison tend to avoid many forbs, the broadleaf flowering plants that grow among grasses.

That difference matters. When cattle graze heavily, they can reduce the number of flowering plants across a prairie. Bison’s grazing patterns leave more room for wildflowers to grow, which in turn provides food and habitat for pollinators, insects and birds.

The preserve now supports more than 750 plant species. Around 115 are grasses, while most of the remaining species are broadleaf plants, including wildflowers.

Plant diversity is closely linked to animal diversity. Flowers attract insects, seeds feed birds and small mammals, and a varied plant community creates more shelter and food sources throughout the year.

Bison also graze unevenly. Some areas are clipped closely, while others are left taller. This creates a patchwork of vegetation rather than a uniform field, giving different species the conditions they need to survive.

Their movements can therefore influence the prairie even when they are not directly eating plants.

Wallowing, dung and grazing create benefits beyond the herd

There are several surprising effects that bison have on the prairie environment. First of all, there is wallowing – when the animals roll on the ground to groom themselves, get rid of loose hair, and ward off parasites.

These small depressions retain water during the rain, thus providing a temporary habitat for insects and amphibians in an area where permanent lakes are not always present.

Rubbing against small trees to remove their winter coats is another surprising behavior pattern of the bison that hinders the expansion of woodlands into prairies.

Finally, their hunger leads to yet another effect on the prairie environment: an adult bison is capable of consuming up to 30 pounds of grass in a day, including the dried out plants which would otherwise serve as a potential fuel for a wildfire.

However, this process does not prevent any fires from taking place, especially after an unusually moist year which leads to heavy vegetation growth. Still, grazing can help to decrease the dry vegetation. Finally, their manure serves the needs of the soil, as it is transported underground by the dung beetles.

Controlled burns remain essential to prairie recovery

In addition to the bison, fire plays an equally important role in the maintenance of the Tallgrass Prairie Preserve. Conservationists burn up to a third of the preserve yearly, imitating the natural fire processes which have shaped the area for thousands of years.

Fire has been used by indigenous people as a method of land management for many generations prior to the advent of conservation programs.

Burning clears out old grasses, limits the expansion of tree and bush populations and stimulates new growth of grasses. Burning may help keep a balance between the grasses and wildflowers.

Every burn should be timed and localized properly. The fire will behave differently depending on such factors as weather, wind, humidity, amount of fuel and wildlife.

The intention is not to stop any fires from occurring. On the contrary, controlled use of fire will reduce the probability of wildfires and make the prairie a suitable habitat for native plants and animals.

Bison reintroduction complements this strategy in several ways. Bison grazing will limit some fuel, wallows will provide a site of disturbance and the bison dung will enrich the soil. Fire will complete the cycle of regeneration of the grasslands.

Frequently asked questions

1. What is the number of bison living in the Tallgrass Prairie Preserve?

The number of animals usually is 2,100 in winter and increases to about 2,700 in summer when calves are born.

2. What reasons caused bison being brought to the brink of extinction?

The animals were killed on an extensive scale for leather, meat, and economic benefits. The hunting disturbed Indigenous societies that used the bison, whereas colonization led to shrinking their habitats.

3. How can bison benefit the prairie ecosystem?

The animals keep the grasslands grazed, form water-collecting walls through wallowing, prevent some woody plants from growing, and contribute to soil fertility by droppings. Grazing helps to prevent burning of the dried-up grasses.

4. Are bison and buffaloes one and the same?

In North America, the animals referred to as buffaloes are in fact American bison. Buffaloes comprise African and water buffaloes that are representatives of different groups of animals.