In protecting land for wildlife, size matters—here's what it takes to conserve very large areas

Driving north on state Highway 66 through the Fort Belknap Indian Reservation in central Montana, it's easy to miss a small herd of bison lounging just off the road behind an 8-foot fence. Each winter, heavy snows drive bison out of Wyoming's Yellowstone National Park—the only place in the U.S. where they have lived continuously since prehistoric times—and into Montana, where they are either killed or shipped off to tribal lands to avoid conflict with cattle ranchers.