Agriculture is actually the rearrangement of the ecosystem in favour of human needs; it is often forgotten that cultivated land is an ecosystem. Peasant agroecosystems are a continuum of integrated farming units and natural ecosystems where plant gathering and crop production are actively practised.
Agroecology engages (rather than removes) and enhances as many natural ecological processes and interactions as possible. Agricultural ecosystems (or agro-ecosystems) are those "ecosystems that are used for agriculture" in similar ways, with similar components and similar interactions and functions. Agro-ecosystems are determined by three sets of factors: the genetic resources, the physical environment and the human management practices, which exhibit genetic, spatial and temporal variation, as well as by their interactions.
Agro-ecosystems may be identified at different levels or scales, for instance, a field/crop/herd/ pond, a farming system, a land use system or watershed. These can be aggregated to form a hierarchy of agro-ecosystems. Agro-ecosystems comprise polycultures, monocultures, and mixed systems, including crop-livestock systems, agroforestry, agro-silvo-pastoral systems, aquaculture as well as rangelands, pastures and fallow lands. Their interactions with human activities, including socio-economic activity and sociocultural diversity are determinant.
Tlaxcalan farmers (Mexico) grow corn in an array of polyculture and agroforestry designs that result in a series of ecological processes important for insect pest and soil fertility management. Often beans and squash are intermixed with the corn. Trees integrated into cropping systems modify the aerial and soil environment of associated understorey corn plants, influencing their growth and yields. With decreasing distance from trees, surface concentrations of most soil nutrients increase. Certain tree species affect corn yields more than others. Densities of predators and the corn pest Macrodactylus sp. depend greatly on the presence and phenology of adjacent alfalfa strips. Although the data were derived from non-replicated fields, they nevertheless point out some important trends, information that can be used to design new crop associations that will achieve sustained soil fertility and low pest potentials.
It is difficult to separate the study of agricultural systems from the study of the cultures that nurture them. The complexity of the production systems is matched by the sophistication of the knowledge of the people that manage them. There is a need to continue studying traditional agroecosystems; indeed rescue of this knowledge must occur rapidly, not only because it is being irreversibly lost, but also because it is crucial for the advancement of agricultural ecology.