food chain(redirected from Why we have a food Chain)
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food chain:see ecologyecology,
study of the relationships of organisms to their physical environment and to one another. The study of an individual organism or a single species is termed autecology; the study of groups of organisms is called synecology.
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the interrelation of various species of plants, animals, fungi, and microorganisms that are connected to each other as food and food consumers. Each of the successive feeding links consists of organisms that consume those of the preceding link in the chain; the resulting transfer of energy and matter lies at the foundation of the cycle of matter in nature. As much as 80 or 90 percent of the potential energy is lost in each such transfer, being dispersed in the form of heat. Hence the number of links, or sets of species, in a food chain is usually no more than four or five.
At the base of every food chain are the producer species—the autotrophic organisms that synthesize organic matter. These are primarily green plants, consisting of water, inorganic salts, and carbon dioxide, that synthesize organic matter by assimilating the energy of sunlight; also included in this category is a group of bacteria, such as sulfur and hydrogen bacteria, that use the energy obtained from the oxidation of chemical compounds to synthesize organic matter.
The next link in a food chain consists of the consumer species—the heterotrophic organisms that consume organic matter. The primary consumers are herbivorous animals that feed on grass, seeds, fruit, the underground portion of plants (roots, tubers, and bulbs), and even—in the case of some insects—wood. Carnivorous animals are classified as secondary consumers and are subdivided into (1) those that subsist on great quantities of small prey and (2) active predators, which frequently attack prey larger than themselves. The great majority of these consumers subsist on a variety of foods, including a certain amount of plant food. Thus, for example, the size of the marten and sable populations depends not only on the plentifulness of small mammals and birds but also on fruit and seed crops, and particularly the available yield of pine nuts. At the same time, herbivorous animals also consume a certain amount of animal food, thereby obtaining essential and irreplaceable amino acids of animal origin.
Finally, the saprophytic organisms—chiefly fungi and bacteria —obtain their essential energy by decomposing dead organic matter.
In the case of animals that develop by metamorphosis, the larvae and the adult individuals require different types of food and occupy different positions in the food chain. The ecological niche of a given species (or of a given phase in its development) is determined by its position in the food chain and its relation to its partners—the organisms at higher and lower levels of the food chain. The various populations or age groups of a single species may be part of several different food chains, thus forming a more complex set of relationships.
Two major types of food chains are found in biocenoses, or biotic communities—the “pasture” and the “detritus” type. The pasture type of food chain begins with photosynthesizing green plants and usually forms the basis of the biocenosis. The detritus type begins with saprophytic organisms, which utilize the energy liberated by the decomposition of dead organic matter (fungi and many microorganisms). Together, these two types of food chains ensure the functioning of the three principal stages in the cycle of matter, as reflected in three trophic levels: the producers— plants—are at the first level; the consumers—primary consumers (herbivorous animals) and secondary consumers (carnivores)—at the second; and the reducers—the saprophages that break down organic matter—at the third.
The designation of trophic levels groups together types of activity rather than species; the population of one species may occupy one or more trophic levels, depending on its energy source. The flow of energy through a trophic level equals the total amount of energy assimilated at this level; in its turn, the total energy assimilated equals the total biomass produced plus respiration.
Biotic communities usually contain a number of parallel food chains—for example, the chain of herbaceous vegetation-rodents-small predators together with the chain of herbaceous vegetation-ungulates-large predators. While the inhabitants of different strata (soil, grass, and trees) often belong to parallel food chains, relationships may also be found to exist between them. The complex structure of food chains ensures both the integrity and the dynamic nature of the biotic community. Any reduction in the number of individuals in a species constituting a link in a food chain—whether due to human activity or other factors—inevitably brings injury to the integrity of the biotic community.
REFERENCESNaumov, N. P. Ekologiia zhivotnykh, 2nd ed. Moscow, 1963.
Odum, E. Osnovy ekologii. Moscow, 1975. (Translated from English.)
Williamson, M. Analiz biologicheskikh populiatsii. Moscow, 1975. (Translated from English.)
N. P. NAUMOV