Sensitivity of aquatic insect species richness to disturbance in the Adour–Garonne stream system (France)

Ephemeroptera, Plecoptera, Trichoptera and Coleoptera (EPTC) were sampled at 153 sites distributed among 47 rivers from the Adour–Garonne stream system (south-western France). Geographic differences in abiotic conditions (elevation, slope, distance from source, stream order, maximum water temperature) were assessed by statistical analysis of data from unstressed sites, and three clusters (or sub-regions) were thus derived from a Principal Component Analysis (PCA). In terms of number of species at unstressed sites, these three sub-regions (i.e. “brooks”, “streams” and “large rivers”) grouped sampling sites having significantly comparable richness, whereas between-regions differences in species richness distributions were significantly different. Therefore, species richness to disturbance relationships should be assessed differently according to the considered area. For each sub-region, we have determined five richness classes, using box-plots of species richness distribution among disturbed and undisturbed sampling sites. The results provided by the EPTC species richness and the Indice Biologique Global Normalisé (IBGN, a normalised index used for the biological surveillance of French streams, with a family level of taxonomic resolution) were significantly different at the whole-drainage-basin scale but had different responses for streams and large rivers. By using the species level of taxonomic resolution, the EPTC richness was more sensitive to slight disturbances in mountain rivers (streams), i.e. where invertebrate diversity and number of species per insect families were the highest. If the species richness of a restricted number of taxonomic groups is considered an integrative descriptor of the ecosystem health, then the method has the interest to involve little computation, and to be easily transposable to any large stream system.