Nature: Ecologist Thomas Lovejoy tucks his trousers into his socks with a casual warning about chiggers and then hikes off into the Amazon jungle. Shaded by a tall canopy and dense with ferns and underbrush, the old-growth forest looks healthy, but Lovejoy knows better. Three decades ago, the surrounding forest was mowed down and torched as part of a research project, and the effects have spread like a cancer deep into the uncut area. Large trees have perished. The spider monkeys have moved out, as have...