BIOLOGICAL ENVIRONMENTS. - By biological environments are meant the various habitats in which conditions sufficient to permit the life of organisms are found. In the manifold varieties of environmental conditions, which nevertheless share a common minimum for permitting life, plant and animal organisms are found with an organic differentiation perfectly adapted to every environment. The study of biological environments, both for the direct conditions of human life as an organism and for man's relations with flora and fauna, is of extreme importance that transcends the purely scientific field to enter the sphere of social and economic problems.
Soil, solid, water, liquid, and air, gaseous, are the three great medium-environments in which organisms, both plant and animal, live, and these determine in the three media other environmental conditions new for other plants and other animals. In all three of these media, temperature and light are conditions of very great direct importance, since for these physical conditions there is an optimum and extreme limit-conditions, which cannot be exceeded. The physical condition of the terrain — rocky, gravelly, sandy, clayey, marly, etc. — is of essential importance for the life of plants. Thus the property of the terrain to take up and retain water or to draw it from deeper layers is connected with a special type of vegetation; that of arid terrains (xerophilous) is entirely characteristic. Likewise, special plants correspond to the chemical conditions of the soil. The condition of humus, soil rich in plant debris, represents a new condition. Prepared, cultivated soil represents another environment.
The conditions of the air are also important for plant life, and so are its movements (winds). In windy places the flora is particular. Finally, the movement of the air is essential for anemophilous plants (grasses, conifers, many plants with catkin inflorescences), since it is the wind that, by carrying the pollen, permits fertilization. Thus the wind is necessary for the dissemination of many plants, whose seeds, provided with little wings or parachutes, are perfectly adapted. In the consideration of the air for plant life, the degree of humidity, that is, of the content of water in the vapor state, is of essential importance. In water, and in relation to the type of underwater terrain, there is a suitable vegetation, which creates a new environment for animal life. The chemical conditions of water, especially in relation to the quantities of salts, create different conditions, among which the most notable are those of the freshwater environment and those of salt water.
For animals too, soil, air and water represent the three great medium-environments; moreover, the plants that populate these three biological environments represent other environmental conditions for animals.
Since environmental conditions are not fixed but vary with night and day, with the changing of the seasons and through countless other factors (and human intervention is at times not among the least of them), displacements of fauna and flora are observed that may be either periodic or show no conditions of periodicity. Naturally, periodic migrations are more evident in animals, endowed with broader movements and free from the substrate, apart from free and unicellular plant forms. Thus plant and animal life itself gradually modifies a biological environment, so that some species end up no longer finding their optimum and become extinct or emigrate, while it becomes optimum for other forms that immigrate. It is therefore a common phenomenon for species of animals and plants to succeed one another in a given habitat. The promiscuity of life of manifold plant and animal forms reveals intimate relations among these living beings that are often particularly evident. It is well known that the life of animals on land is made possible by the existence of green plants which, through the process of chlorophyll synthesis (v. FOTOSINTESI), release molecular oxygen, the only kind usable by the plants themselves and by animals in respiratory processes. The relations among living beings range from mutualistic conditions (such as the one just mentioned between plants and animals, since animals give in exchange the carbon dioxide necessary to plants for the synthesis of carbon and nitrogenous substances with which the latter manufacture proteins) to antagonistic relations.
When there is an association of organisms of different nature (plants among themselves, animals among themselves, or plants and animals), one speaks, according to De Bary's concept (1879), of symbiosis, although the modern concept applied that name to associations favorable to both associates. There may be relations useful to the symbionts (inquilinism, commensalism, mutualism) and harmful to one for the advantage of the other, and then one speaks of parasitism. Especially in the condition of parasitism a new great biological environment is revealed (or rather numerous biological environments), namely the organic environment: parasitic forms present conditions of adaptation specific to the various organic environments (v. MESOLOGIA E PARASSITISMO).