Beta Diversity in Riverine Ecosystems

Abstract

Beta diversity -- the variation in species composition between sites, quantifying species turnover and nestedness across spatial or environmental gradients -- is a fundamental measure of biodiversity organisation that determines how much of regional species richness is distributed across local communities versus concentrated in single locations. Riverine ecosystems present a distinctive spatial structure for beta diversity analysis: their linear, dendritic network geometry connects sites hierarchically through the drainage network, creating a tension between upstream-downstream connectivity (facilitating downstream dispersal and longitudinal species turnover) and drainage-level isolation (restricting cross-drainage dispersal and maintaining drainage-level compositional distinctiveness). This study presents the most comprehensive multi-taxon analysis of riverine beta diversity yet conducted, quantifying Sorensen dissimilarity and its turnover/nestedness components for macroinvertebrates, fish, macrophytes, and diatoms across 247 river reaches in 84 drainage basins spanning 18 European countries. Total beta diversity (mean Sorensen = 0.74 +- 0.08) was primarily driven by species turnover (turnover component = 0.61 +- 0.09) rather than nestedness (0.13 +- 0.06), indicating that beta diversity reflects genuine spatial species replacement rather than subset relationships. Hydrological connectivity - measured as network distance through the river network -- was the strongest predictor of pairwise compositional dissimilarity (Mantel r = 0.74; p < 0.001) across all taxa, outperforming Euclidean geographic distance (r = 0.54) and environmental distance (r = 0.47). Dams and weirs reduced upstream-downstream beta diversity by 28.4% relative to unimpeded reaches by blocking longitudinal species exchange