At one time beavers numbered in the hundreds of millions, and like engineers they have expertly reshaped the contours of the continent. But their fur proved valuable enough to trappers that the North American beaver population dropped to near-extinction levels. Now, the plucky rodents have made a comeback, and our environmental health is benefitting. Correspondent Luke Burbank talks with Leila Philip, author of “Beaverland: How One Weird Rodent Made America,” and with scientist Emily Fairfax about why beavers are key to a more resilient ecosystem.