The gophers were grumpy, but they understood the assignment. Brought by helicopter to a barren landscape with pumice stones the size of marbles and golf balls, the animals did what they’ve always done: They started digging. Just two years earlier, a cataclysm erased life in the landscape. The explosive eruption of Mount St. Helens in 1980 blew out the mountain’s northern flank and destroyed some 135 square miles of forest. …When scientists returned to the fenced plots six years later, they were stunned to find some 40,000 plants there, while nearby patches of land remained desolate. In the decades since, the effects have kept compounding. …So, why did it happen? Part of the credit goes to the gopher’s diligent digging, which cycled fertile materials back toward the surface. But they also left things behind — from their droppings to spores and fungi.