Bison by the thousands have been slaughtered by the federal government this decade to appease Montana ranchers, prompting a lawsuit from conservationists. About 3,500 bison have been killed in and around Yellowstone National Park since 2000 because of concerns the animals will spread the bacteria brucellosis to cattle, causing them to lose weight and not reproduce offspring or milk. The Western Watersheds Project and other organizations have filed suit in federal court to stop the killings, arguing federal officials in the Department of the Interior have erred in believing brucellosis is the threat that ranchers claim. Environmentalists say that new scientific evidence shows the spread of the bacteria from bison is low, and that elk are more likely to infect cattle.