Bison Photo
WCS’s efforts to save the American bison started in the early 1900s, when Bronx Zoo staff sent 15 bison by train to Oklahoma’s Wichita Mountains Wildlife Preserve to help restore the West’s depleted bison population.
Julie Larsen Maher ©WCS
Bison and Calf Photo
Bison “cows,” or females, give birth in the spring to calves with orangey-brown fur. Mothers are very protective of their young. The big babies can weigh up to 66 pounds at birth and are able to run after three hours.
Julie Larsen Maher ©WCS

The bison, North America’s largest mammal, provides one of the last evolutionary links to the Pleistocene era, a period (1.8 million to 11,550 years ago) when huge mammals dominated the landscape. Just two centuries ago, between 30 and 60 million bison roamed the continent’s grassy, shrubby plains and prairies, and their range extended from Mexico to central Canada. In the late nineteenth century, sport-hunting and mass slaughters of this mammal brought the species to the brink of extinction. In 1906, only about 1,000 bison, wild and captive, remained in North America.

Since then—with much thanks to strong conservation efforts that mobilized in the early 1900s—the continent’s bison population was able to grow to about 450,000. But fewer than 20,000 of these animals range freely and many contain genes from cattle or other bison subspecies. The vast majority of today’s bison are raised as domestic livestock. When wild bison numbered in the millions, and large herds migrated across the open grasslands, their ability to travel freely across vast expanses also meant they grazed across the region, coexisting with prairie dogs, ferrets, burrowing owls, and other grassland animals. These iconic American herbivores shaped the vegetation and landscape as they fed on and dispersed the seeds of grasses, sedges, and forbs. Several bird species adapted to or co-evolved with types of grasses and vegetation structures that had been, for millennia, grazed by millions of free-ranging bison.

With further understanding of this giant mammal and through collaborative efforts to restore wild bison on private and public land, bison, native grasses, songbirds and other prairie species may thrive together once again. 

Fast Facts

Scientific NameBison bison
  • Bison can gallop at speeds of up to 35 miles per hour and are good swimmers.
  • The largest populations of wild, free-ranging bison are found in Yellowstone National Park in the U.S. and Canada’s Wood Buffalo National Park.
  • By their first birthday, a bison may weigh more than 400 pounds.


Over 90 percent of bison today are under private ownership, raised like cows for bison meat. In fact at the turn of last century, ranchers often interbred bison with cattle to improve their cattle herds. As a result, cattle genes are now present in many bison populations, and few genetically pure bison herds remain. Current policies and a tradition of fencing ranches—as well as issues that include disease and competition for grazing land—discourage free-ranging bison herds in the West. Besides visitors to Yellowstone, the public mostly views bison behind fences rather than as free-ranging, native wildlife. This perceived domestication fosters confusion about the need and opportunity to conserve this wild species.

Bison confined by fences can no longer extensively interact with other species or influence the composition of their habitats by contributing to grassland nutrient cycling regimes and impacting plant communities. While we may not regain the 30 to 60 million bison that roamed this continent only 200 years ago, we can save this species and the ecosystems of which it was a part.

WCS Responds

Pioneering conservationists Theodore Roosevelt and William Hornaday originally founded the American Bison Society (ABS) at the Bronx Zoo in 1905 to save the bison from extinction. In 1907, Bronx Zoo staff sent 15 bison by train to Oklahoma’s Wichita Mountains Wildlife Preserve to help restore the western Plains’ depleted bison population. Recognizing a century later that wild bison and their native ecosystems were still in trouble, WCS re-launched the American Bison Society in 2005. ABS has steadily built a network of bison experts, including ranchers, state, and provincial governments, Native American nations, scientists, and non-governmental organizations from western states, Mexico, and Canada, with the purpose of securing an ecological future of bison in North America over the next century. 

WCS is calling on the federal government to better coordinate management of bison across federal lands and working with Canada and Mexico on cross-border bison management. We are also researching and promoting scientific resolutions to obstacles facing bison restoration.

Our conservationists are also studying how the grazing behavior of the bison herds that dot the Great Plains can help keep grasslands and grassland bird populations healthy, control invasive species, and create seasonal variety in grass height. Over an area of 450,000 acres, we are working to restore grasslands with a broad range of partners, monitoring bird populations and bird-bison habitat associations, and developing science-based guidelines for how to manage bison grazing so that it creates more bird habitat. 

WCS Projects

The American Bison Society

In its second century of bison conservation, WCS is working to restore the wild landscape that enables bison to roam freely, and to support larger, free-ranging herds. The ABS was founded at the Bronx Zoo in 1905.

From the Newsroom

Slideshow: A Historic Journey for Bison, 100 Years LaterNovember 25, 2013

This Thanksgiving marked the 100-year-anniversary of one of the greatest milestones in the modern conservation movement: the transfer of 14 bison from the Bronx Zoo to Wind Cave National Park, South Dakota.

Three Strikes You're out in the Old... Great... Plains!August 2, 2013

Steve Zack, WCS's Coordinator of Bird Conservation, explains the ways in which oil and gas fracking efforts may reshape the American prairie, and the consequences for grassland birds and bison.

Celebrating the Historic Ties of Native Americans to the BisonMarch 1, 2013

More than 30 million bison roamed the United States at its founding, but the largest land mammal in the Western Hemisphere nearly went extinct as Americans expanded to the west. Writing for National Geographic NewsWatch, Executive Vice President of Public Affairs, John Calvelli, discusses the ways in which Native American tribes (and WCS) have helped restore bison populations. 

Ted Roosevelt V Supports National Mammal LegislationSeptember 5, 2012

Ted Roosevelt V, the great-great-grandson of our country's 26th President, has voiced his support for the bison as the country's national mammal. Writing for USA Today, Roosevelt recounts the mammal's storied history and explains the importance of the National Bison Legacy Act. 

The Bison: A National Symbol For All AmericansAugust 8, 2012

In his first blog celebrating bison, WCS Executive Vice President for Public Affairs John Calvelli discusses ongoing efforts to protect this iconic species. WCS has been protecting buffalo since the turn of the 20th century and is currently working with conservationists, sportsmen, Native American tribes, and lawmakers to advance the National Bison Legacy Act.


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