Crop communities can be described in simple terms. Species and cultivar define genetic content while density, spacing pattern, plant size, and stage of development define structure. The type of community is termed a monoculture when only one crop species is grown in a field at a time; the terms polyculture and mixed cropping apply to communities with two or more cohabiting crop species. Other definitions and terms exist but these are the traditional ones employed by agronomists. Most arable farming involves rotations of monocultures over time whereas pastures are mostly polycultures.

Critical issues in crop ecology, which we will examine in detail in this chapter, include the impact of community structure on resource capture and yield of crop production systems and interactions between component plants.

Community change

Concepts of community structure evolved from complementary work by agronomists who study managed communities and by botanists concerned with natural communities. In agriculture, small differences in production are important and agronomists study how production rate, competition, limiting factors, and genetic expression influence behavior of simple communities. Botanists, faced with highly diverse, natural systems give greater attention to community species composition in relation to adaptive traits and evolution. A background in plant ecology is useful for agriculturalists, and vice versa. Natural communities are subject to continuing change as different species of plants invade a site and displace earlier occupants. This process is termed succession and the sequence followed, the sere.