American Prairie. The end of the ranching era and the onset of the natural regulation era set into motion a chain of events that have led to the bison of Yellowstone Park migrating to lower elevations outside the park in search of winter forage. The presence of wild bison in Montana is perceived as a threat to many cattle ranchers, who fear that the small percentage of bison that carry brucellosis will infect livestock and cause cows to abort their first calves. However, there has never been a documented case of brucellosis being transmitted to cattle from wild bison. The management controversy that began in the early 1980s continues with advocacy groups arguing that the herd should be protected as a distinct population segment under the Endangered Species Act. Native American bison conservation efforts Many conservation measures have been taken by Native American tribes in order to preserve and grow the bison population as well. Of these Native conservation efforts, the Inter Tribal Bison Council was formed in 1990, composed of 56 tribes in 19 states. These tribes represent a collective herd of more than 15,000 bison and focus on reestablishing herds on tribal lands in order to promote culture, revitalize spiritual solidarity, and restore the ecosystem. Some Inter Tribal Bison Council members argue that the bison's economic value is one of the main factors driving its resurgence. Bison serve as a low cost substitute for cattle, and can withstand the winters in the Plains region far easier than cattle. A Native American conservation effort that has been gaining ground is the Buffalo Field Campaign. Founded in 1996 by Mike Mease, Sicango Lakota, and Rosalie Little Thunder, the Buffalo Field Campaign hopes to get bison migrating freely in Montana and beyond. The Buffalo Field Campaign challenges Montana's DOL officials, who slaughtered 1631 bison in the winter of 2007-2008 in a search for food away from Yellowstone National Park. Founder Mike Mease commented in regards to DOL officials: "It's disheartening what they're doing to buffalo. It's marked with prejudice that exists from way back. I think the whole problem with white society is there's this fear of anything wild. They're so scared of anything they can't control, whereas the First Nations take pride in being part of it and protecting the wild because of its importance. Our culture is so far removed from that, and afraid of it." Additionally, many smaller