Yellowstone was created by an act of Congress in 1872 as a public park for the benefit and enjoyment of the people and for the preservation and retention of its resources in their natural condition. Yellowstone’s mandate, creating a dual mission to preserve natural resources while providing for the public’s enjoyment of them, has served as a model for the rest of the park system and for parks around the world. Yellowstone is at the center of approximately 20 million acres of land, commonly called the Greater Yellowstone Area or ecosystem. These lands are managed by four different federal agencies—the National Park Service, the Forest Service, the Fish and Wildlife Service, and the Bureau of Land Management (BLM); three different states—Idaho, Montana, and Wyoming; and numerous private land holders. The Park Service manages bison and elk only within Yellowstone. Outside the park, the neighboring states of Idaho, Montana, and Wyoming manage wildlife not only on their own lands but also on BLM and Forest Service lands. Although the Forest Service manages wildlife habitat on its lands, the states manage the wildlife. For example, in Gallatin National Forest, the Forest Service manages wildlife habitat, while the Montana Department of Fish, Wildlife, and Parks manages wildlife within the forest’s borders. The Fish and Wildlife Service manages wildlife refuges, such as the National Elk Refuge south of Yellowstone, and the Bureau of Land Management manages land used by both wildlife and cattle in the Greater Yellowstone Area. This past winter, park officials estimated the size of the northern elk herd at about 17,000 in Yellowstone and the total number of elk in the Greater Yellowstone Area at about 120,000. The population of Yellowstone’s northern range elk herd has ranged between 16,000 and 20,000 since 1991. At the beginning of this past winter, about 3,500 bison lived within the park, 900 of which occupied the northern range. Subsequently, about 1,100 bison left the park and were shot or shipped to slaughter because of concerns about brucellosis. About 700 other bison were killed by the severe winter, leaving approximately 1,700 bison in the park this spring, including about 300 in the northern range. For thousands of years, various animal species have routinely migrated in and out of what is now Yellowstone National Park. Bison and elk herds seasonally migrate out of the park to seek forage, especially in severe winters like that of 1996-97. While elk have traditionally migrated widely in the Greater Yellowstone Area, bison have more recently left the park, primarily through its northern and western borders, to seek available winter range. Appendix I illustrates the Greater Yellowstone Area elk herds’ winter ranges and migration routes. Appendix II illustrates the Greater Yellowstone Area bison herds’ winter ranges and migration routes . Because bison that migrate outside Yellowstone may be infected with brucellosis and may interact or share rangeland with domestic cattle, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) and its state counterparts also have a strong interest in the management of Yellowstone’s wildlife. APHIS is responsible for eradicating brucellosis from cattle in the United States. According to APHIS, it also has statutory authority to eradicate brucellosis in all animals—including bison. Since a national brucellosis control program was first instituted in 1934, more than $3.5 billion in federal, state, and industry funds have been spent trying to eradicate the disease. According to APHIS, nationwide, only 22 herds of domestic cattle and bison are now known to be infected. The states also play a major role in the effort to eradicate brucellosis. Because federal statutes on controlling disease in livestock pre-empt the states’ authority only when cattle and bison are moving in interstate commerce, most states have enacted their own statutes to supplement federal regulatory efforts. The Brucella abortus organism, a bacterium, is transmitted among animals primarily through exposure to infected reproductive material, such as aborted fetuses. APHIS tests cattle and bison for antibodies to the Brucella abortus organism. Antibodies in blood samples may indicate either past exposure to the disease or current infection. Positive tissue cultures for Brucella abortus confirm the presence of live bacteria and the potential for animals to be infectious. However, according to APHIS, negative tissue cultures do not prove the absence of bacteria because the organism cannot always be isolated even when it is present. After surveillance tests and procedures are conducted to ensure that cattle and bison herds are free of the disease, APHIS may certify states as brucellosis-free. This certification allows the states to ship their cattle and bison in interstate commerce without having to perform expensive testing to assure importing states that the cattle or bison do not pose a threat of the disease to their livestock industry. As of June 1997, Idaho, Montana, Wyoming, and 34 other states were certified as brucellosis-free. The economic consequences of infection with brucellosis could be significant. Under the requirements of APHIS’ eradication program, if a single herd of cattle or bison in a state that is designated brucellosis-free becomes infected, the infected animals must be slaughtered, the herd quarantined, and the herds in the surrounding area tested to ensure that the disease has not spread. If the herd is slaughtered and no additional infection is found, the state can remain classified as brucellosis-free. If the herd is not slaughtered or additional infection is found, the state’s classification will be lowered and additional interstate testing requirements implemented. Montana estimates that it saves between $1 million and $2 million annually because it does not have to test cattle for brucellosis. A state with infected cattle or bison may also be subject to restrictions imposed by other states. For example, because of the increased movement of brucellosis-infected and -exposed bison out of the Greater Yellowstone Area, the state of Oregon decided in March 1997 to protect the interests of its cattle industry by immediately requiring the testing of any cattle entering Oregon from Montana or Wyoming. Other states have imposed, or threatened to impose, similar restrictions. The management of Yellowstone’s wildlife, especially of bison and elk, has gone through many phases as wildlife managers have gained experience and scientific knowledge has grown. When the park was founded in 1872, there were numerous elk, estimated at 25,000 in 1891, and, according to park officials, bison were also very common. However, no estimates of the bison population exist for that period. After almost two decades of slaughter by market hunters, the bison population in Yellowstone dwindled to about 44 in 1901-02. Yellowstone officials saved the bison from extinction by aggressively protecting the remnant population and supplementing it with bison imported from Montana and Texas. For several decades, Yellowstone also aggressively reduced the populations of wolves and other predators. As a result, the park’s bison population gradually increased, growing to more than 1,000 in 1930. However, from about 1935 to 1968, park rangers controlled the elk and bison populations by shooting or by trapping and removing animals. This “culling program” reflected the then-prevailing view that wildlife populations had to be controlled to meet an area’s carrying capacity—a determination of how many animals can live in an area without degrading the range. In the early 1960s, however, elk kills initiated by park officials to reduce the size of a herd that was considered too large, led to a public outcry, studies, and U. S. Senate hearings on Yellowstone’s wildlife management policy. As a result, in the late 1960s, Yellowstone’s wildlife management policy changed significantly. According to park staff, although little information was available on how functioning elk and bison populations might respond in a natural environment, park managers thought that Yellowstone might be a place to develop this knowledge and resolve the controversy over the size of the herds by letting natural forces regulate the populations. Therefore, in Yellowstone, natural regulation replaced the capture and culling of elk and bison herds. The park’s master plan, written in 1974, reflects the shift to natural regulation, stating that “Yellowstone should be a place where all the resources in a wild land environment are subject to minimal management”. For wildlife, the plan proposes to reduce or eliminate disruptive human influences, relying, whenever possible, upon natural controls to regulate animal numbers. For the past 30 years, the Park Service has been implementing natural regulation in Yellowstone, in essence, following the park’s master plan. However, the Park Service recognizes that because of the pervasiveness of human influences in today’s world, true natural process management is seldom feasible. In the lower 48 states, the Park Service believes that Yellowstone is the only park large enough to test the effects of natural regulation. At Yellowstone today, the Park Service relies on natural forces within the park—mainly animal behavior, climate, food supply, and predation—to regulate bison and elk populations. In addition, elk have always been hunted in the surrounding states. More recently, bison have been killed when they have migrated out of the park, and some public hunting of bison has occurred in both Wyoming and Montana. However, in 1991, Montana discontinued public hunting. According to park officials, once humans stopped controlling the size of the herds and Yellowstone adopted the natural regulation policy, the bison and elk populations increased considerably. For example, from 1967 to 1988, the bison population rose from 397 to more than 2,500 and then peaked at about 4,200 in the summer of 1994.