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An introduction to the successes and challenges of managing wild bison in the 21st century.
Near Extinction & Recovery
The American bison (Bison bison) once roamed across most of North America in numbers that reached into the tens of millions. Such abundance made the bison a critical part of Native American culture: every part of the bison provided something for their way of life. Before horses and gunpowder arrived in North America, native people hunted bison on foot. One technique involved stampeding dozens or hundreds of animals off cliffs where they would fall to their deaths. A single “jump” could sustain the members of a tribe for an entire year, providing food as well as materials for clothing, shelter, tools, and more.
As European Americans settled the west in the 1800s, the U.S. Army began a campaign to remove Native American tribes from the landscape by taking away their main food source: bison. Hundreds of thousands of bison were killed by U.S. troops and market hunters. By the late 1880s, the great herds of bison that once dominated the landscape were nearly gone. Some animals found protection on private ranches. In the Yellowstone area, their numbers dwindled to about two dozen bison that spent winter in Pelican Valley.
In one of the first efforts to preserve a wild species through protection and stewardship, Yellowstone’s managers set about recovering the bison population. In 1902, they purchased 21 bison from private owners and raised them in Mammoth and then at the historic Lamar Buffalo Ranch. Eventually, these animals began to mix with the park’s free-roaming population and by 1954, their numbers had grown to roughly 1,300 animals.
Population Growth & Conflict
Yellowstone bison reproduce and survive at relatively high rates compared to many other large, wild mammals, so even as the population recovered managers limited its growth with frequent culling. Hundreds of animals were removed to start or supplement herds on other public and tribal lands. Even more were killed and given to Native American tribes or relief agencies. But a moratorium on culling beginning in 1969 resulted in the bison population increasing dramatically: from 500 animals in 1970 to 3,000 in 1990. At the same time, elk numbers increased to more than 19,000 animals in the late 1980s. As bison and elk numbers increased, they began to leave the park in greater numbers. Only a few bull bison left the park prior to 1975, but as bison numbers increased, groups of bison began migrating across the north and west boundaries of Yellowstone to expand their winter range and pioneer new territory.
As they left the park, bison encountered a changed world. In the century since bison roamed a continent of open space, the grassy river valleys and plains they used to graze had been settled and developed by people, much of it for agriculture. The landscape had become a maze of fenced pastures, houses, and highways. Large groups of 1,000-pound animals searching for food create challenges for people sharing that landscape, both in terms of human safety and because bison want to eat the same grass ranchers grow to feed their livestock.
To further complicate things, bison and elk in the Greater Yellowstone Ecosystem carry brucellosis, a disease that can be transmitted to livestock and induce abortions or stillbirths in infected animals. Brucellosis has an economic impact on ranchers because it affects the reproductive rate and marketability of their animals, so park and state wildlife officials have gone to great lengths to prevent bison from mixing with cattle. Between 1985 and 2000, about 3,100 bison were killed as they tried to migrate out of the park: some were captured and shipped to slaughter, others were shot by hunters or state agents.
These actions generated a lot of controversy. Meanwhile, the bison population continued to grow, as did state and federal pressure to keep brucellosis out of livestock. In 1995, the state of Montana sued the National Park Service for allowing bison to leave the park. After five years of litigation and mediation, the state of Montana and the federal government developed the Interagency Bison Management Plan (IBMP) to guide the management of bison in and around Yellowstone. As part of this plan, five agencies and three tribal entities work to sustain a wild, wide-ranging bison population and reduce the risk of brucellosis transmission from bison to cattle.
In the decades since the IBMP was created, the bison population has ranged between 2,400 and 5,500 animals. There have been no cases of bison transmitting brucellosis directly to cattle, in part due to efforts by federal and state agencies to maintain separation between these animals. The state of Montana now allows bison to occupy some habitat adjacent to the park that was previously off-limits, including year-round in some areas, which is a major conservation advancement. However, lack of tolerance for wild bison in most areas outside Yellowstone continues to limit the restoration of this iconic species. Large parts of their historic winter ranges are no longer available due to human development, and because states only allow limited numbers of bison in areas near the park.
A Future for Bison
Many people don't like the fact that animals from a national park are sent to slaughter. We don't like it either. We’d like to see more tolerance for migrating bison on public lands in surrounding states; similar to deer, elk, and other ungulates. The park isn’t big enough to let bison numbers increase without more available habitat to sustain them, but we cannot force adjacent states to tolerate more migrating bison.
Last updated: February 14, 2023