| abstract | Populations of water-mites (Hydrachnellae, Acari) have been studied at 108 stations (86 species) in the central Pyrenees. Canonical analysis and factorial analysis have revealed: - on the one hand, a correlation between well known ecological factors (temperature, current speed, etc,) and a certain number of topographical, hydrological, geological parameters which form a locality complex, that is stable in time at each station; - on the other hand, a correlation between the locality complexes and a certain number (8) of water-mite associations. Five parameters (altitude of the source, altitude of the station, slope, catchment area, and water regime) are sufficient to define the locality complex and to explain, together with the type of substratum (gravel or moss), the species distribution. The water regime, notably, plays a fundamental role in this distribution and justifies the distinction of German-speaking authors between the fauna of Mittel-and Hochgebirge. Different biological types of water-mites are identified as a function of substratum, rate of reproduction and biological cycle. But, to explain the structure of communities and especially their diversity, it is necessary to take account of other factors such as biotope stability (time), its area (space), the distance between equivalent biotopes, - that is to say MacArthur and Wilsonʼs theory of islands. Populations of unstable biotopes, of limited areas and isolated are at the uncertain limit. |