Contribution of beta diversity in shaping stream macroinvertebrate communities among hydro-ecoregions

Rivers are heterogeneous and patchy-structured systems in which regional biodiversity of aquatic communities typically varies as a function of local habitat conditions and spatial gradients. Understanding which environmental and spatial constraints shape the diversity and composition of benthic communities is therefore a pivotal challenge for basic and applied research in river ecology. In this study, benthic invertebrates were collected from 27 sites across three hydro-ecoregions with the aim of investigating patterns in alpha- and beta diversity. We first assessed the contribution to regional biodiversity of different and nested spatial scales, ranging from micro-habitat to hydro-ecoregion. Then, we tested differences in alpha diversity, taxonomic composition and ecological uniqueness among hydro-ecoregions. Variance partitioning analysis was used to evaluate the mechanistic effects of environmental and spatial variables on the composition of macroinvertebrate communities. Macroinvertebrate diversity was significantly affected by all the spatial scales, with a differential contribution according to the type of metric. Sampling site was the spatial scale that mostly contributed to the total richness, while the micro-habitat level explained the largest proportion of variance in Shannon-Wiener index. We found significant differences in the taxonomic composition, with 39 invertebrate families significantly associated with one or two hydro-ecoregions. However, effects of environmental and spatial controls were context dependent, indicating that the mechanisms that promote beta diversity probably differ among hydro-ecoregions. Evidence for species sorting, due to natural areas and stream order, was observed for macroinvertebrate communities in alpine streams, while spatial and land-use variables played a weak role in other geographical contexts.