The 4,900 bison that roam free in Yellowstone National Park are the largest congregation of genetically pure American bison—bison not interbred with cows—left in the world. The trouble with bison, though, is that they like to roam. Yellowstone bisons' historic wanderings took them across some 7,720 square miles of territory. The park offers just 3,500 square miles of space.
Sometimes, the bison roam where they're not wanted, like into farmers' fields in Montana, says Motherboard. As a result, the National Park Sevice has a plan to kill roughly 900 members of the herd—specifically, says Reuters, "those animals that stray from the park."
At the same time as the Park Service is planning its bison cull, its wildlife managers are also working on plans to reintroduce the massive mammals to an expanded range. Once, bison's range spanned the continent, from Canada's Northwest Territory in the north to Mexico in the south, bounded on the east and west by the Rockies and the Appalachians (and herds were so large they were measured in miles).
Yet some of Yellowstone's bison are a reservoir for a disease called brucellosis, or undulant fever, which can be transferred to cows, says Motherboard. These particular bison may not be prime candidates for relocation.