BIOLOGICAL ENVIRONMENTS. — By biological environments are meant the various settings in which conditions sufficient to permit the life of organisms are found. Within the multiple varieties of environmental conditions, which nevertheless possess a common minimum sufficient to permit life, there are plant and animal organisms with organic differentiation perfectly adapted to each environment. The study of biological environments, both with regard to the direct conditions of human life as an organism and to human relations with flora and fauna, is of the utmost importance, extending beyond the purely scientific field into the sphere of social and economic problems.
Soil, solid, water, liquid, and air, gaseous, are the three great environmental media in which organisms, both plant and animal, live; and within these three media they determine further new environmental conditions for other plants and 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 limiting conditions that cannot be exceeded. The physical condition of the soil—rocky, gravelly, sandy, clayey, marly, etc.—is essential for plant life. Thus, the property of the soil to absorb and retain water, or to draw it from deeper levels, is associated with a special type of vegetation; that of arid soils (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, as are its movements (winds). In windy places the flora is distinctive. Finally, the movement of the air is essential for anemophilous plants (grasses, conifers, and many plants with catkin-like inflorescences), since it is the wind that, by carrying pollen, permits fertilization. Thus, wind is necessary for the dissemination of many plants, whose seeds, equipped with wings or parachutes, are perfectly adapted. In considering air in relation to plant life, the degree of humidity, that is, its water content in the state of vapor, is of essential importance. In water, and in relation to the type of underwater soil, there is a suitable vegetation that creates a new environment for animal life. The chemical conditions of the water, especially in relation to the quantities of salts, create different conditions, the most notable of which are those of the freshwater environment and those of saltwater.
For animals, too, soil, air, and water represent the three great environmental media; moreover, the plants that populate these three biological environments represent further environmental conditions for animals.
Since environmental conditions are not fixed but vary with night and day, with the changing seasons, and through innumerable other factors (and human intervention is sometimes by no means the least among them), movements of fauna and flora are observed that may either be periodic or display no periodicity. Naturally, periodic migrations are more evident in animals, which possess greater freedom of movement and are not attached to a substrate, apart from free-living and unicellular plant forms. Thus, plant and animal life itself gradually modifies a biological environment, so that some species eventually cease to find
their optimum and become extinct or emigrate, while it becomes optimum for other forms that immigrate. The succession of animal and plant species in a given location is therefore a common phenomenon. The coexistence of numerous plant and animal forms reveals intimate relationships among these living beings, often particularly evident. It is well known that the life of animals on earth is made possible by the existence of green plants, which, through the process of chlorophyll synthesis (v. FOTOSINTESI), release molecular oxygen, the only oxygen usable by plants themselves and by animals in respiratory processes. Relations among living beings range from mutualistic conditions (such as the one just mentioned between plants and animals, since animals in return release the carbon dioxide necessary for plants to synthesize carbon and the nitrogenous substances with which they manufacture proteins) to antagonistic relationships.
When there is an association of organisms of different kinds (plants with one another, animals with one another, or plants and animals), according to De Bary’s concept (1879) one speaks of symbiosis, although the modern concept restricted this name to associations beneficial to both associates. There may be relationships useful to the symbionts (inquilinism, commensalism, mutualism), or harmful to one and advantageous to the other, in which case one speaks of parasitism. It is especially under the condition of parasitism that a new broad biological environment (or rather, numerous biological environments) is revealed: parasitic forms display specific adaptive conditions for the various organic environments (v. PARASSITISMO).