Agriculture is the human enterprise by which natural ecosystems are transformed into ones devoted to the production of food, fiber, and, increasingly, fuel. Given the current size of the human population, agriculture is essential. Without the enhanced production of edible biomass that characterizes agricultural systems, there would simply not be enough to eat. The land, water, and energy resources required to support this level of food production, however, are vast. Thus agriculture represents a major way in which humans impact terrestrial ecosystems.