For some conservationists, preserving the wilderness no longer means just setting land aside and leaving animals alone.

Although the entire county of West Sussex could fit nearly 100 times in just the eastern half of Montana, Beth Saboe and her co-workers at the American Prairie Reserve want more space. The reserve already owns land or grazing rights to more than 400,000 acres. But the bison being brought back to America’s Great Plains by the organization need plenty of room.

At the same time as the human population in the region is decreasing, private, state, and federal lands host small but steadily growing herds of bison. The American Prairie Reserve, along with the Fort Peck, Blackfeet, and Fort Belknap tribal reservations, is home to genetically pure wild bison—the gold standard in bison conservation. These animals, which make up only a tiny fraction of the nearly half million bison now in the United States, have been carefully sourced from Canada and, in a new arrangement between the federal government and the tribes, Yellowstone National Park.