BIOLOGICAL ENVIRONMENTS. — By biological environments (a. b.) are meant the various settings in which conditions are found sufficient to allow the life of organisms. In the manifold varieties of environmental conditions, which nonetheless share a common minimum allowing life, there are plant and animal organisms with perfectly adapted organic differentiation to each environment. The study of biological environments, both for the direct living conditions of man as an organism and for man’s relations with flora and fauna, is of extreme importance that transcends the purely scientific sphere to enter the realm of social and economic problems.
Soil, solid; water, liquid; air, gaseous: these are the three great environmental media in which organisms, both plant and animal, live, and these in turn determine in the three media new environmental conditions for other plants and animals. In all three of these media, temperature and light are conditions of the greatest direct importance, since for these physical conditions an optimum and extreme limit conditions exist that cannot be surpassed. The physical condition of the ground—rocky, gravelly, sandy, clayey, marly, etc.—has essential importance for the life of plants. Thus the property of the soil to absorb and retain water or to draw it from deeper layers is linked to a special type of vegetation; that of arid soils (xerophilous) is entirely characteristic. Similarly, the chemical conditions of the soil correspond to special plants. The condition of humus, soil rich in plant detritus, represents a new condition. Cultivated, prepared 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 particular. Finally, the movement of air is essential for anemophilous plants (graminaceous plants, conifers, many plants with catkin inflorescences), since it is the wind that, by transporting pollen, allows fertilization. Thus wind is necessary for the dissemination of many plants, whose seeds, equipped with wings or parachutes, are perfectly adapted. In considering air for plant life, the degree of humidity, i.e., the water content in the state of vapor, is of essential importance. In water and in relation to the type of underwater terrain there is an adapted vegetation, which creates a new environment for animal life. The chemical conditions of water, especially in relation to the quantity of salts, create different conditions, the most notable of which are those of fresh water and those of salt water.
For animals as well, 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 day and night, with the changing of the seasons, and for innumerable other factors (and human intervention is sometimes one of the lesser ones), shifts in fauna and flora are observed that may be either periodic or not exhibit conditions of periodicity. Naturally, periodic migrations are more evident in animals, which are capable of wider movement and free from the substrate, aside from free and unicellular plant forms. Thus plant and animal life itself gradually modifies a biological environment, with the result that some species end up no longer finding their optimum and become extinct or emigrate, while the optimum becomes suitable for other forms that immigrate. It is therefore a common phenomenon for animal and plant species to succeed one another in a given setting. The promiscuous life of multiple plant and animal forms reveals intimate relations among these living beings, 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 chlorophyllian synthesis (cf. PHOTOSYNTHESIS), eliminate molecular oxygen, the only kind usable by plants themselves and by animals in respiratory processes. The relations among living beings range from mutualistic conditions (such as that just cited between plants and animals, since animals in turn yield carbon dioxide necessary to plants for the synthesis of carbon and nitrogenous substances with which they manufacture proteins) to antagonistic relations.
When there is an association of organisms of different kinds (plants with plants, animals with animals, or plants and animals), one speaks, according to the concept of De Bary (1879), of symbiosis, although the modern concept attributes this name to associations favorable to both associates. There may be relations useful to the symbionts (inquilinism, commensalism, mutualism) and harmful to one for the benefit of the other, and then one speaks of parasitism. Especially in the condition of parasitism a new great biological environment (or rather numerous biological environments) is revealed: the organic environment. Parasitic forms present specific conditions of adaptation to the various organic environments (cf. MESOLOGY AND PARASITISM).