Bison are the largest mammals in Yellowstone National Park. They are strictly vegetarian, a grazer of grasslands and sedges in the meadows, the foothills, and even the high-elevation, forested plateaus of Yellowstone. Bison males, called bulls, can weigh upwards of 1,800 pounds. Females (cows) average about 1,000 pounds. Both stand approximately six feet tall at the shoulder, and can move with surprising speed to defend their young or when approached too closely by people. Bison breed from mid-July to mid-August, and bear one calf in April and May. Some wolf predation of bison is documented in Canada and has recently been observed in Yellowstone.
Yellowstone is the only place in the lower 48 states where a population of wild bison has persisted since prehistoric times, although fewer than 50 native bison remained here in 1902. Fearing extinction, the park imported 21 bison from two privately-owned herds, as foundation stock for a bison ranching project that spanned 50 years at the Buffalo Ranch in Yellowstone's Lamar Valley. Activities there included irrigation, hay-feeding, roundups, culling, and predator control, to artificially ensure herd survival. By the 1920s, some intermingling of the introduced and wild bison had begun. With protection from poaching, the native and transplanted populations increased. In 1936, bison were transplanted to historic habitats in the Firehole River and Hayden Valley. In 1954, the entire population numbered 1,477. Bison were trapped and herds periodically reduced until 1967, when only 397 bison were counted park wide. All bison herd reduction activities were phased out after 1966, again allowing natural ecological processes to determine bison numbers and distribution. Although winterkill takes a toll, by 1996 bison numbers had increased to about 3,500.
Bison are nomadic grazers, wandering high on Yellowstone’s grassy plateaus in summer. Despite their slow gait, bison are surprisingly fast for animals that weigh more than half a ton. In winter, they use their large heads like a plow to push aside snow and find winter food. In the park interior where snows are deep, they winter in thermally influenced areas and around the geyser basins. Bison also move to winter range in the northern part of Yellowstone.
Bison are enjoyed by visitors, celebrated by conservationists, and revered by Native Americans. Why are they a management challenge? One reason is that about half of Yellowstone's bison have been exposed to brucellosis, a bacterial disease that came to this continent with European cattle and may cause cattle to abort their first pregnancy after exposure to Brucella bacteria. The disease has few population level effects. Outside the park wild bison from the Yellowstone population have not been known to transmit brucellosis to a visitor or to domestic livestock. The State of Montana believes its "brucellosis-free" status may be jeopardized if bison commingle with cattle. The risk of Yellowstone bison transmitting brucellosis to nearby livestock is very low. However, if livestock are infected, ranchers can be prevented from shipping livestock out of state until stringent testing and quarantine requirements are met. Some elk in the Greater Yellowstone ecosystem also carry the disease.
Most Yellowstone wildlife move freely across administrative boundaries set a century ago. Bison however, are not always welcome outside the park. Managers have tried to limit bison use of lands outside the park through public hunting, hazing bison back inside park boundaries, capture, testing for exposure to brucellosis, and shipping them to slaughter.
The NPS, U.S.D.A. Forest Service, U.S.D.A. Animal and Plant Health Inspection Service, and the State of Montana completed an Environmental Impact Statement for the Interagency Bison Management Plan for the State of Montana and Yellowstone National Park in November 2000. Alternatives considered ranged from: allowing bison to freely range over a large portion of public land inside and outside the park; managing bison like elk and other wildlife through controlled hunting outside park boundaries; and attempting to eradicate brucellosis by capturing, testing, and slaughtering infected bison at numerous facilities constructed inside the park. Additional options included purchase of additional winter range; attacking brucellosis with a safe and effective vaccine for bison; and quarantine of animals at appropriate locations such as Indian Reservations or other suitable sites outside Yellowstone.
State and Federal Records of Decision were signed in December 2000. The purpose of the IBMP is to maintain a wild free-ranging bison population and to address the risk of brucellosis transmissions to protect the economic interest and viability of the livestock industry in Montana. The principles of the Interagency Bison Management Plan include the following concepts:
- The State of Montana is responsible for managing bison when they leave the park and is the lead agency when conducting capture operations in the western Special Management Area
- National Park Service is responsible for all actions conducted within the park and is responsible (currently) for keeping bison from leaving the park in the area of Reese Creek along the northern Special Management Area
- Abundance and distribution of bison are monitored throughout the year
- When bison move beyond an established tolerance area the following procedures are implemented to enforce spatial and temporal separation between bison and cattle: