BIOGENOSIS. — With the term b. or biological association, introduced by Moebius in 1877, is defined the entire complex of living beings that coexist within the same environment and thus present mutual relationships deriving both from simple spatial cohabitation and from the establishment of nutritional or other exchanges. In a b., plants provide nourishment for herbivorous animals, which in turn fall prey to carnivorous animals; autotrophic plants, capable of producing organic matter from mineral substances, thus act as producer organisms, and animals as consumers. For a complete circulation of matter to occur, however, the coexistence of the so-called reducer 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.
An autarchic b. is one in which the aforementioned components are all represented and functioning, and which therefore exists in full independence from the surrounding world. However, there are frequent b. that are less self-sufficient and dependent on other nearby biological associations: examples of these are found in the depths of the sea and in caves, where producer plants are lacking. In general, the totality of living beings that populate the earth can be defined as a gigantic b., more or less distinctly divided into smaller biological associations. Among the components of a b., in addition to the aforementioned relationship of mutual predation between prey and predators, there is always competition for the search for food and space; due to both of these factors, each animal or plant species is limited by the others in its multiplication and expansion, until a state of equilibrium, called biocenotic equilibrium, is reached. This also depends on environmental physical influences and is subject to frequent fluctuations due to changes in the factors that regulate II. A violent disruption 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. This is the basis of the so-called biological struggle, widely used against insects harmful to agriculture.
A mathematical theory for the study of biocenotic equilibria was developed by Lotka (1925) and especially by Volterra (1927).