[Parasites as a natural element of any ecosystem].

The article presents some ideas upon the functioning of the parasite populations and communities in relation to the whole ecosystem. The author underlines that the parasites are one of the natural elements of every biocenosis and their existence submittes to similar ecological rules as the populations and communities of free-living organisms. Colonization of their particular environment (living organism) demands some adaptations allowing to resist immunological defense of the host, and to coexist with other parasite species harboured by the same host, not exploiting the supplies over the host "carying capacity", what could provoke the strong pathogenic effects, including host death. The mechanims maintaning the state of relative stability in host-parasite system are similar as in the case of free-living organisms, and are of both parasite- and host origin. They can confine the number of parasites (elimination of some specimens) or their biomass (growth restraint), limit their fecundity (maturation restraint), or make particular species to use different supply (niches separation). On the other hand parasite can regulate the demography of host population: restrict their fecundity, or kill their host directly or indirectly (making them an easier prey for predators). The author presents and discusses some examples illustrating the activity of the regulating mechanisms in host-parasite relationship, as well as the role of parasites in energetic ballance of ecosystem and in "engineering of ecosystem".