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The Yellowstone Area Bison Herd

Bison are native to the Greater Yellowstone Area and were observed there by early travelers both before and after the creation of Yellowstone National Park in 1872 and the Yellowstone Timber Land Reserve in 1891.

Hunting and poaching of bison in the late 1800s substantially reduced the number of bison in the Yellowstone herd, and by 1901, only 25 bison were counted. Fearful the small wild herd might vanish, park managers imported 21 bison from captive herds into the park in 1902. These bison were raised using livestock techniques on the “Buffalo Ranch” in Lamar Valley until the 1930s, when the National Park Service gradually began efforts to restore the bison to a more natural distribution (NPS, USDI, Meagher 1973). Although the native and captive herds were initially kept separate, they began to intermingle between 1915 and 1920. After the 1920s, little or no effort was made to keep the two populations separate.

By 1922, the park suggested that a law be passed authorizing the sale or disposition of some bison (Albright 1922, as cited in Skinner and Alcorn 1942–1951). Authority for this was granted in the Appropriation Act of 1923.

By the 1930s, the total number of bison wintering in the Lamar area had increased to over 1,000, and the park began reductions by shipping bison to public parks, zoos, and private estates. Bison were also used to begin herds in other areas of the park. Artificial feeding of the Lamar Valley herd, herd reductions to achieve range management goals, and other manipulation of the population continued from the 1920s until the late 1960s, and were often quite intensive. The highest reported bison count during this period was 1,477 in 1954.

In 1967, when herd reductions in the park ceased as part of a larger redirection of park policies, 397 bison were counted. Since that time bison, elk, and other animals have been allowed to reach population levels dictated by environmental conditions.

Brucellosis was first diagnosed in the Yellowstone herd in 1917 (Mohler 1917, as cited in Tunnicliff and Marsh 1935). In 1968, in response to livestock industry concerns over brucellosis, the National Park Service proposed a program to control bison at the boundary of the park. In addition, an early version of parkwide capture, test, and slaughter or vaccination efforts took place in the mid-1960s (Yellowstone National Park Bison Management Plan 1964–65). These efforts were reviewed by park management and determined to be ineffective and “never-ending” (Meagher 1972). Beginning in 1967, this type of bison management ceased in the park. More recently, a series of four interim bison management plans (the latest in 1996) put specific boundaries and lethal control measures in place. In 1996–97, a particularly harsh winter with deep snow and ice conditions sent hundreds of bison toward park boundaries, seeking accessible forage at lower elevations. Implementation of the interim plan, combined with the severe winter conditions, resulted in the removal of 1,123 bison in the five months between November 14, 1996, and April 15, 1997 (1,084 bison were shot or slaughtered, and 39 were used for research purposes). Others died of starvation or other natural causes inside the park, bringing the total population down from an estimated 3,500 in fall 1996 to an estimated 2,000 animals by early spring 1997. The federal agencies and the state of Montana discussed the situation and in 1977 began to implement adjustments to the interim plan that were aimed at reducing the number of bison shot or shipped to slaughter. These adjustments include increased emphasis on hazing bison back into the park, holding bison up to the capacity of the Stephens Creek capture facility until weather conditions moderate, and allowing low-risk bison that evade capture in the West Yellowstone area to remain on public lands for 30 to 60 days before cattle are released on federal grazing allotments.

Brucellosis in Cattle and Bison

Brucellosis is a contagious bacterial disease, caused by various species of the genus, Brucella, that infects domestic animals, wildlife, and humans worldwide. Brucella abortus is the species that infects both cattle and bison. There is no cure for brucellosis in these species. Vaccines developed so far are not 100% effective, and are to date less effective with bison than with cattle. The first known case of brucellosis in the bison herd was reported in 1917. It is generally agreed that the transmission of brucellosis to the Yellowstone bison herd was from cattle, and occurred either through contact with infected cattle or from infected cows’ milk fed to captive bison calves.

In cattle, the organism is shed primarily in aborted tissues, reproductive tissues, and discharges, especially just before, during, or soon after abortion or live birth. Ingestion by other cattle of contaminated material is the primary route of infection. Cows infected with brucellosis characteristically abort their first calf after the fifth month of gestation.

Less is known about the disease in bison, particularly free-ranging bison. Transmission from bison to cattle has occurred under experimental conditions in confined spaces, but has not been documented under free-ranging conditions. Since the release of the Draft Environmental Impact Statement, the National Academy of Sciences finalized a summary of pertinent literature on several aspects of brucellosis (NAS 1998). Relevant material from this summary is used throughout volumes 1 and 2 of the final environmental impact statement to clarify discussions on epidemiology and pathology of the disease in both cattle and bison.

Diagnosis. In cattle, diagnosis is based on the results of blood tests, herd history, clinical signs, and other information. The diagnosis can be confirmed by positive cultures. B. abortus may be isolated from tissues collected at slaughter, milk or udder secretions, biopsy of lymph nodes, reproductive tract exudates, discharges from live animals, or fetal or placental materials collected at the time of abortion or calving. In Yellowstone bison, agencies have used a blood test for the presence of Brucella antibodies. For a number of reasons, these blood tests tend to overestimate the number of bison actually harboring the bacteria. Difficulties in isolating the bacteria from tissues and other factors have also meant fewer positive culture tests than the number of infected bison.

Risk of Transmission. Scientists and researchers disagree on even some of the most basic factors influencing the risk of transmission. These include whether studies on cattle are applicable to bison, whether controlled studies are applicable in the field, and the best ways to conduct additional research to determine the risk of transmission.

These disagreements and a paucity of information on brucellosis in bison make it impossible to quantify the risk of B. abortus transmission from bison (and elk, although the environmental impact statement does not analyze brucellosis in elk) in the Yellowstone area to domestic livestock. Instead, the agencies have identified factors that affect risk. They include the following:

1. The degree of association between potentially infectious and susceptible animals. Management actions emphasize separation to minimize risk.
2. The number and density of infectious animals in the host population.
3. The number of susceptible animals that may associate with infectious animals.
4. Environmental factors such as weather, sunlight, and other factors that determine the viability of the organism outside its host.
5. The class of the infectious animals. Because the disease is transmitted in cattle through ingestion of contaminated birth materials, pregnant bison are considered higher risk than other classes.
6. Vaccination and neutering reduce the transmission of the disease.
7. Some animals are naturally resistant to infection.

Since bison and cattle are prevented from interacting under each of the alternatives in the environmental impact statement, it is the presence and persistence of bacteria in birth materials that are at issue in determining the risk of transmission. Research completed since the release of the Draft Environmental Impact Statement has direct bearing on this discussion. In one study, 30 known bison birth or abortion sites in the park from 1996 to 1998 were sampled. The B. abortus bacterium was isolated at two of those sites immediately following the birth or abortion event and persisted for a maximum of 18 days (Coffin, pers. comm.). Cook (1999) studied B. abortus strain RB51 on samples taken from the exposed surface of bovine fetuses in

Alternative Interpretation of Risk. The above information represents areas where scientists generally agree on the interpretation of available data. However, considerable debate and need for additional research remain. The bulk of brucellosis research and disease management has focused on domestic livestock, yet limited published information suggests the disease may be transmitted differently and have different clinical, pathological, and population effects in bison (Williams et al. 1994; Meyer and Meagher 1995a).

Those who suggest the risk is negligible point out that there have been no documented cases of brucellosis transmission from wild, free-ranging bison to cattle.

It is possible that, although brucellosis may be endemic in the Yellowstone area bison herd, few of the animals are capable of transmitting the disease. This suggestion is supported by noting the discrepancy between the number of bison that test seropositive for brucellosis but culture tissue negative (Roffe et al. 1999). This discrepancy and the infrequency of observed abortions in the Yellowstone bison herd (usually required for transmission of the disease between cattle) has led to the theory that the primary route of transmission among cattle (abortions and birthing events) may be different from that among bison. In bison, the bacteria may be transmitted through milk (Meyer and Meagher 1995a).

Bison Distribution

The Yellowstone bison population uses three different wintering areas in the park: Pelican Valley (the smallest), Mary Mountain (the largest, in the Hayden Valley-Firehole River area), and the northern range. Yellowstone National Park grooms roads in the winter for snowmobile use, which allows bison to easily traverse the park. Bison seem to use the roads to exit in severe winters, such as the 1975–76 and 1996–97 winters, and retain the memory of the access routes (Meagher 1989a). While experts agree that bison traveling on groomed routes are traveling in a more energy-efficient manner than bison traveling through deep snow, there is disagreement about what bison would do if grooming ceased. What result this would have on bison numbers and distribution is not known. Bison migrate across the north and west ends of the park during the winter into Montana. In the north they exit primarily across the Reese Creek boundary of Yellowstone National Park, and move immediately onto adjacent private land where several hundred cattle are present year-round. Through the purchase of lands and conservation easements, a portion of this adjacent private land has been acquired by the U.S. Forest Service since the release of the Draft Environmental Impact Statement (see Royal Teton map). One of the designated uses is as a wildlife winter range. After an existing cattle lease on them expires in 2002, it is anticipated that bison would be allowed to use these acquired lands under alternatives where the lands are designated as a bison management area. These alternatives include 2, 3, 7, and the modified preferred alternative.

Bison may also enter national forest land in the Eagle Creek/Bear Creek area east of Reese Creek, where they occasionally enter private lands in the Gardiner area by traveling along the Maiden Basin hydrographic divide and Little Trail Creek drainage. These lands are collectively referred to as the Eagle Creek/Bear Creek “special management area” (areas outside the park where bison are allowed) in this document. To the east of these lands (and north of the park) lie Hellroaring and Slough Creek drainages and the Absaroka-Beartooth Wilderness, part of the national forest where cattle are not present. A few bison use these higher elevation, more rugged lands in winter and summer.

From the west side of the park, bison move along the Madison River, Duck Creek, and Cougar Creek in the vicinity of West Yellowstone. From here, bison infrequently move north (usually along Highway 191) onto public lands administered by the U.S. Forest Service in the Cabin Creek Recreation and Wildlife Management Area and the Monument Mountain Unit of the Lee Metcalf Wilderness. The western special management area (SMA) in this document includes these lands south to the West Yellowstone area. Up to a few hundred cattle may occupy select public and private lands in the West Yellowstone area in the summer months. No cattle are present in the winter.

Economic Impacts of Brucellosis in Cattle

Brucellosis (B. abortus) has the following direct impacts on the livestock industry:

• Abortion of calves
• Decreased weight gain by   calves
• Delays in calf production
• Increased rates of culling   and replacement
• Increased testing and   vaccinating costs

The presence of livestock disease may also affect each state’s classification by the Animal and Plant Health Inspection Service.