High diversity and community permeability: the riparian Collembola (Insecta) of a Pyrenean massif

This work deals with the structure of Collembolan communities in riparian habitats. Sixty samples collected along running waters in a Pyrenean massif have been analysed. Diversity was much higher than for any comparable habitat studied in the literature. The proportion of rare species was particularly high, and explained a large part of the observed diversity. The contribution of specialized hydrophilous species to overall diversity was low: they were 5 times less numerous than non-hydrophilous species for a similar global abundance. Correspondence analysis showed that the riparian community was loosely structured, without any strong determining factor. Epigeomorphic species of hydrophilous Collembola were, however, clustered in a well-defined group. The forest type, of major importance for soil fauna diversity in the area, was a poor predictor of diversity in the riparian habitats. Conversely, richness was significantly related to distance from water, in spite of the samples having been all collected from permanently water-saturated substrates. The importance of non-hydrophilous species for the diversity of riparian habitats along running water is interpreted as reflecting the ecological permeability of the hydrophilous community, resulting from the spatial organisation of the habitat in narrow strips, and its frequent disturbance by flooding. Finally, the riparian habitat may act as a refuge for a significant proportion of the soil species affected by reafforestation, presently the most severe disturbance of Pyrenean ecosystems.