An ecosystem is the community of organisms in an area and their non-living environment (Figure 1). The concept of communities interacting with their physical environment such as dead matter, minerals, water; and energy is the basis of ecology. It can be defined in terms of energy and matter fluxes and can be described at various scales such as crop ecology, landscape ecology or global ecology.
2.4.1. Food chain and food web
Energy flow drives the ecosystem, determining limits of the food supply and the production of all biological resources. Light energy from the sun is captured by green plants and converted to chemical energy. Energy is stored in plants as carbohydrates and used by the plant to support all functions such as vegetative growth, fruit maturation and respiration. Other organisms use and convert this chemical energy to various forms through food chains (Figure 3). A food chain is a succession of organisms in a community that constitutes a feeding sequence in which food energy is transferred from one organism to the next as each consumes a lower number and in turn is preyed upon by a higher number (Figure 4). At the bottom of the chain is a photosynthesizing plant, usually followed by an herbivore, a successions of carnivores, and finally decomposers. At each step, some of the chemical energy is assimilated and used by the organism and the rest is released in respiration and waste products. The Food web is the complex interrelated food chains in a community (Figure 5). Trophic structure is the series of links in a food web that describe the transfer of energy from one nutritional level to the next. The goal of crop production is to maximize ecosystem energy into a harvestable product; utilization of plant energy by pests is undesirable as it takes away from crop production.
Figure 4. Links of food chain (source - Flint, M.L. and Gouveia P. 2001)
Agroecosystems or agricultural ecosystems are predominantly monocultures. In a monoculture, the age and genotype of crop plants are relatively uniform and species diversity is limited. Complex food webs are changed into simple, short food chains. The physical diversity of the system is much lower than natural ecosystems. The uniformity of monoculture systems encourages pest outbreaks.
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