Imagine the North American wilderness as the explorers Lewis and Clark saw it: forests thick with chestnut trees in the East, prairies teeming with bison and rivers overflowing with salmon in the West. Now picture the continent today: superhighways link colossal cities, suburbs stretch farther and farther into the countryside, industrial farmland goes on for miles, and a few patches of greenery and a national park or two break up the monotony.