Everyone knows ethnic hatred between Hutus and Tutsis was the main reason for Rwanda's 1994 genocide. Everyone but Jared Diamond, that is. In his new book, "Collapse: How Societies Choose to Fail or Succeed," the UCLA geography professor and Pulitzer-winning author of "Guns, Germs, and Steel" acknowledges the ethnic strife but insists that a more elementary factor was ecological. A vast population explosion had left Rwandan farmers unable to feed their families, making them desperate and primed for violence. To prove his point, Diamond cites a study from the Kanama district, where farms had shrunk to postage-stamp size and rampaging Hutus murdered more than 5 percent of the local populace. The number of Tutsis among the victims: one.

"Guns, Germs, and Steel" charted the environmental factors that allowed European and Asian societies to conquer the world. "Collapse" explores the reverse: how some complex, successful societies have courted ecological suicide by abusing their natural resources. Most Easter Islanders starved to death after a competitive frenzy between chiefs to erect the famed giant stone statues led them to cut down every tree on the island. Descendants of the Vikings who settled Greenland died out after they squandered valuable fields for sod houses and hay to feed impractical cows rather than copy their Inuit neighbors and eat fish. When the climate turned colder, the Vikings perished. The Inuit survived.

Diamond looks to the past and present to sound a warning for the future--which he distilled into a chiding New Year's piece in The New York Times. "These cases raise the question: if they collapsed, what about us?" he tells NEWSWEEK. (The book cites instances where foresight prevented disasters: the 17th-century Japanese shoguns stopped rampant deforestation caused, in part, by the build-ing of ostentatious wooden palaces.) Diamond also warns against thinking that advanced technology leaves us immune. Several years ago, he says, Bill Gates told him he believed technology would prevent environmental disaster. Diamond disagreed, but at the time he hadn't formed a solid rebuttal. Now he has.