ABSTRACT
TYPES OF POSSIBLE RELATIONS A group of individuals belonging to the same species and close to each other in space and time, constitute a population. All the populations of a defined ecosystem form a community (that can also be called a settlement). No matter at what level the ecosystem is considered, the populations are not merely juxtaposed; each within a settlement interacts with the other populations. The more diversified a community, the more complex the multiple interactions, which simultaneously exercise pressures in varying directions. Overall, the effect of these divergent forces is to oppose all drastic changes and to cushion all the variations caused by modifications of environmental conditions: the functioning of a community is thus much more orderly than that of each of its components. The aptitude of communities to sustain stability is called homeostasis. This equilibrium, the result of dynamic processes, can be maintained only within certain limits, however. A very intense or very prolonged stress can destroy it and provoke either the disappearance of the settlement in its entirety, or uncontrolled proliferation of a particular population. Such phenomena can be observed, for example, following treatment with a pesticide.
