American prairies were once home to as many as 60 million bison. But when ranching and agriculture displaced elk and bison from the prairies, America’s grasslands all but disappeared. Now, the Nature Conservancy manages the largest remaining protected tract of tallgrass prairie in the entire world, the Joseph H. Williams Tallgrass Prairie Preserve in northeastern Oklahoma. This 40,000-acre expanse is much more than grass. It is a biologically rich habitat that harbors a diverse collection of plant and animal species—more than 750 species of flora and 80 different mammals. This prairie is also a vital component in nature’s fight against climate change. The key to keeping the prairie healthy? Bison.