Why is the Midwest covered with prairies instead of forests?

 THE MIDWESTERN PRAIRIES used to be forests. The shift likely started between 20 million and 28 million years ago, and it accelerated during the last 5 million years. Possible reasons include a drier climate with more natural fires, as well as s the appearance of larger animals. Herbivores such as elk and bison needed a complex digestive system —housed in a large body—to derive nutrition from grasses. These big animals also tended to

form herds, which made it easier to defend themselves against predators while migrating long distances to stay fueled. Their grazing played a role in preventing large trees from returning to the Midwest.

Jun Wen, Chair of Botany,

National Museum of Natural History