In 1540 Francisco Vasquez de Coronado introduced the first domestic livestock to the American Southwest. Over the subsequent four centuries, cattle, horses, and sheep have created a massive ecological experiment on these arid grasslands, changing them in ways we can never know with certainty. The Appleton-Whittell Research Ranch in the high desert of southeastern Arizona is an 8,000-acre sanctuary where grazing has been banned since 1968. In this...