Scientists point to a sobering reality: Our planet appears to be entering a “sixth great extinction” as animal and plant species die out at an alarming rate. It’s a calamity, unlike the previous five, wrought mostly by man.

With the peril to plant life comes a threat to countless foods, medicines, building materials, and means of absorbing pollutants and regulating climate. Celebrated botanist Peter Raven examines the implications and the need to take measures he says could save most of the endangered species.

Hailed as one of TIME magazine’s “Heroes of the Planet,” Raven is president emeritus of the Missouri Botanical Garden, chairman of the National Geographic Society’s Committee for Research and Exploration, and a recipient of the U.S. National Medal of Science.