BIOCENOSIS. - The term b., or biological association, introduced by Moebius in 1877, defines the entire complex of living beings that coexist within the same environment and therefore have mutual relationships, arising both from simple cohabitation of space and from the establishment of nutritional or other kinds of exchange. In a b., plants provide nourishment to herbivorous animals, which in turn fall prey to carnivorous animals; autotrophic plants, capable of producing organic matter from mineral substances, thus function as producer organisms, and animals as consumers. In order for a complete circulation of matter to take place, however, the coexistence of so-called reducing organisms is necessary, represented essentially by bacteria that degrade the organic substances of dead animals and plants, transforming them back into mineral substances usable again by plants.
A b. in which the aforementioned components are all represented and functioning, and which therefore subsists in full independence from the surrounding world, is called autarchic. Frequently, however, there are b.'s less sufficient for their own needs and therefore dependent on other nearby biological associations: we have examples of this in the deep sea and in caves, where producer plants are lacking. In general, the whole of living beings that populates the earth can be defined as a gigantic b., more or less sharply divided into smaller biological associations. Among the components of a b., besides the aforementioned relationship of mutual overpowering between prey and predators, there is always a competition for the search for nourishment and space; because of both these facts, each animal or plant species is limited by the others in its own multiplication and expansion, until a state of equilibrium is reached, called biocenotic equilibrium. This also depends on physical environmental influences, and is subject to frequent fluctuations due to the variation of the factors that regulate it. A violent rupture of a biocenotic equilibrium can be produced by the introduction of a new species, which multiplies excessively if it does not find effective competitors in the b.; the subsequent introduction of a natural enemy, however, restores a state of equilibrium. The so-called biological control, used widely against insects harmful to agriculture, is based on this.
A mathematical theory for the study of biocenotic equilibria was developed by Lotka (1925) and above all by Volterra (1927).
Bibliography2 works
Enrico Vannini