ECOLOGICAL EFFECTS OF SPECIES INTRODUCTIO NS IN HYDROSYSTEMS - AN ATTEMPT OF EVALUATION

The present attempt at evaluating the ecological impacts of species in troductions is based upon the communications given at this meeting and the literature. The inescapable character of species dispersion over time and the low efficiency of within-continent barriers are stressed. The mechanisms implied - mobility and fecundity of the introduced spe cies, efficiency of the dispersion-limiting barriers, structure of the recipient trophic networks and degree of saturation of their ecologic al niches - are detailed in the context of hydrosystem functioning. Th ey reveal the complexity of the problem. Introductions have produced q uite diverse ecological effects, many of them unwanted and/or unexpect ed. An exam pie from the literature shows that the same introduction h as had different consequences according to the environment. Before any introduction, it is therefore necessary to study both the biology of the species to be introduced and the functioning of the receiving ecos ystem. Great wariness is recommended before any wilful introduction. S evere and efficient regulation must be applied against unintentional i ntroductions. The need for regular field monitoring by permanent ecolo gical personnel is stressed. Considering the lack of proper informativ e data, scientific investigations on the ecological effects of past or possible introductions must be undertaken or continued, at least on t he ecosystems exposed to introductions.