Above, a bison and its calf graze on the grasslands of the El Uno Ecological Reserve in Janos, Mexico, about 140 miles from Ciudad Juarez. Environmental authorities in the United States and Mexico are working with the Nature Conservancy to reintroduce the American bison, which was on the verge of extinction in the 19th century, in the grasslands of northern Chihuahua state. The conservancy sees the bison as an essential part of the reserve’s ecosystem, supporting populations of other species. The animal’s heavy tread breaks the soil, allowing seeds to grow and promoting water filtration. By grazing, they keep the grass at a height perfect for other species, including the prairie dog, to thrive.