In the Amazon, the turn to swidden was unfortunate. Slash-and-burn cultivation has become one of the driving forces behind the loss of tropical forest. Although swidden does permit the forest to regrow, it is wildly inefficient and environmentally unsound. The burning sends up in smoke most of the nutrients in the vegetation--almost all of the nitrogen and half the phosphorus and potassium. At the same time, it pours huge amounts of carbon dioxide into the air, a factor in global warming. (Large cattle ranches are the major offenders in the Amazon, but small-scale farmers are responsible for up to a third of the clearing.) Fortunately, it is a relatively new practice, which means it has not yet had much time to cause damage. More important, the very existence of so much healthy forest after twelve thousand years of use by large populations suggests that whatever Indians did before swidden must have been ecologically more sustainable.