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Agricultural Ecology: Food Web
Organisms in a biotic community are ecologically related through their dependence on one another for food. These relationships make up either a food chain or a food web.
In simple feeding relationships, organisms are linked in an unbranched chain. For example, a plant is eaten by a herbivore, which is itself eaten by a carnivore. A second carnivore may then feed on the first, and so on. Such simple food chains are characteristic of certain feeding relationships in polar oceans; there, tiny plants, known as diatoms, are eaten by small crustaceans, which are later eaten by fish. The fish are in turn eaten by seals and porpoises, which are then preyed on by killer whales—the terminal link in the food chain.
In the simple diatom-killer whale food chain, each organism is represented as having no more than one kind of prey and one kind of predator. Usually, however, organisms have several kinds of prey and several kinds of predators. As a result the simple food chains are interlinked into food webs.
Pyramid of Numbers and Biomass
The relative abundance of organisms at different levels in food webs may be expressed as numbers of individuals or as weight ("biomass," or "standing crop"). The fact that prey organisms are often more abundant than are their predators may be represented by a pyramid with the lower levels (the primary producers) forming the broad base. Pyramids of numbers or biomass may also be inverted, however, and numerous large predators may be supported by a smaller population of individually tiny, but rapidly reproducing, prey. In many aquatic communities, a relatively large biomass of fish may feed on small, highly productive populations of microscopic organisms.
Energy Transfer and Length of the Food Chain
Food chains and food webs are essentially energy webs, because the relationships represented involve the flow of energy (in the form of food) from one population to another. Although one can imagine food chains with an indefinite number of links, the actual number of links in a simple chain rarely exceeds five since only a small part of the food energy entering a given link is transmitted to the next link, and after about five links the remaining energy is inadequate to support a population of organisms other than decay organisms.
Probably the main reason for the great loss of energy from link to link is that most of the food energy taken in by the members of one link is expended in the course of the organisms' normal activities rather than used for growth. Thus, in a wide variety of food chains, each group of predators obtains from its prey only about 10% of the energy that was contained in the prey's food. Hence, in the simple diatom-killer whale chain, only about 0.01% of the energy available to the diatom is available to the killer whale. The quantitative relationships in food webs are far more complex than those in food chains.
Stability of Food Webs
Since the late 1950s, much attention has been given to the idea that ecological communities in which feeding relationships are relatively complex are more stable than communities characterized by simpler chain-like relationships. The idea is attractive because it suggests a basis for the diverse ecological relationships found in many communities.