The search for sustainable agroecosystems

AGRICULTURE has changed the structure and function of natural ecosystems dramatically over much of the United States. This transformation, involving about 495 million acres of land (16), is historic and ongoing, affecting ecological processes at population, community, and ecosystem levels. It is also one of the most powerful anthropogenic forces shaping landscapes (31). While human intervention greatly modifies the ecology of agricultural systems, these systems still possess many of the components and processes characteristic of natural ecosystems (65). However, the size of agroecosystem components and the influence or importance of various ecological processes differ from natural terrestrial systems. These differences include patterns of energy flow, abundance and distribution of organisms, nutrient cycling, genetic selection, and internal feedback control or regulatory mechanisms (44). Inputs to agricultural ecosystems-fertilizers, pesticides, fuel, labor, and so forth-are largely of human origin. Inputs to natural terrestrial ecosystems are mainly meteorological. Material inputs to natural systems originating from rain, mineral weathering, and other processes are small compared with inputs to agricultural ecosystems. Consequently, anthropogenic inputs are a major influence on ecological processes in agroecosystems. Still, biologically mediated ecosystem processes, such as primary production, decomposition, and consumption, exist to varying degrees in all agroecosystems, whether managed …