It began decades ago, with a few hardy pioneers slogging north across the tundra. It’s said that one individual walked so far to get there that he rubbed the skin off the underside of his long, flat tail. Today, his kind have homes and colonies scattered throughout the tundra in Alaska and Canada — and their numbers are increasing.... ...Scientists have come to view their landscape engineering as beneficial, and even critical in some vulnerable ecosystems. In many places south of the tundra, conservationists have moved to protect and reintroduce beavers to restore stream and wetland habitats. But in the...