Food Chains and Webs

Food chains and food webs show how energy is transferred in an ecosystem (an area in which living things interact with each other and their environment) from one organism to another in the form of food. Since all organisms in an ecosystem need nutrients in the form of food, the terms food chain and food web describe the feeding relationship between an ecosystem's different populations.

Among the many processes that occur in every ecosystem, none is more important than the transfer of energy from one living thing to another. Without this energy exchange, no organisms would be able to survive. Ecologists use the term food chain to describe the typical path or route that energy (food) takes as it moves from one group of living things to another. A simple food chain would be: green plant to mouse to snake to eagle. In this example, the green plant is the first link in the chain, producing chemical energy from sunlight through a process of photosynthesis. The plant is eaten by the mouse, which absorbs the plant's energy. In turn, the mouse is eaten by the snake, who absorbs the mouse's energy.

Finally, the eagle consumes the snake and obtains the snake's energy. This chain becomes circular when the eagle dies and its organic matter is reduced to nutrients, which are returned to the soil by organisms called decomposers.

Ecologists now prefer the term "food web" as a more realistic description of what really happens in this process. Since they feel that the word "chain" implies an orderly linking of equal parts, they use instead "food web" which can be described as a collection of food chains. In the natural world, food chains are extraordinarily complex, since there is no exact order stating which creature has to eat which. The notion of a web is more complicated than that of a chain and suggests a network of connections rather than a direct, one-to-one linking.

The first link in a food chain (or first stage of a food web) are the organisms known as primary producers. These are also called "autotrophs" since they can make their own food. Green plants and some forms of bacteria are primary producers since they begin the chain by performing photosynthesis. By capturing sunlight and using its energy to carry out a series of chemical reactions, green plants make glucose which is packed with energy. In the sea, primary producers are floating microorganisms known as plankton. On land, green plants are eaten by herbivores (planteating animals) who are considered to be the primary consumers. In turn, these primary consumers become food for other animals called secondary consumers. Each step up the ladder consists of fewer flesh-eating animals. Every food chain then begins with an autotroph and ends with a carnivore (flesh-eating animal) that itself is not eaten by a larger animal. The bear is a good example of a carnivore that is at the top of its food chain. When top carnivores die, their bodies are usually eaten by scavengers, like vultures. What remains is broken down by decomposers (bacteria and fungi) and returned to Earth as nutrients to be used by autotrophs (green plants).

Ecologists also use a model called an "energy pyramid" to depict the actual energy-transfer relationships in a food web. They have found that each species occupies a certain place or level on that pyramid that they call its "trophic level." Trophic refers to nourishment or nutrition, and a certain trophic level is the stage which a given organism occupies on the pyramid. In an energy pyramid, energy flows from one trophic level to the next, but only in one direction (bottom to top). At the base or bottom of the pyramid are the producers or plants, the most plentiful resource. Primary consumers are at the next level and are just below the secondary consumers. The pyramid is not just another way of showing a pattern, but demonstrates that the same amount of energy is never transferred up the pyramid. Studies have shown that the amount of energy passed on to the next higher pyramid level is only about 10 percent of the energy that the organism at the lower level received. This is because a great deal of energy is lost into the environment as heat, which cannot be reused. As energy passes up to the higher levels of the pyramid, less and less of it is left to reach the top. As a result, the total number of organisms that can participate in a food web is severely limited. It also shows how important the producers at the bottom of the pyramid are, since the less energy they produce the less energy is passed on up the pyramid.

Ecologists use models like energy pyramids to evaluate how much energy an ecosystem can produce, and therefore how much life it can sustain. They usually try to measure the total productivity of the producers and turn that into an amount of heat energy per unit of area.

Another way of measuring productivity is to measure the "biomass" (total amount of organic matter produced) of an ecosystem. Certain ecosystems are naturally more productive than others. For example, a forest is more productive than a desert. Also, human activity can have a disruptive effect on a food web if it causes a major change at one of its levels.

[See alsoEcosystem ]