Local and regional drivers of taxonomic homogenization in stream communities along a land use gradient

Aim The interaction of land use with local versus regional processes driving biological homogenization (beta-diversity loss) is poorly understood. We explored: (a) stream beta-diversity responses to land cover (forest versus agriculture) in terms of physicochemistry and physicochemical heterogeneity; (b) whether these responses were constrained by the regional species pool, i.e. gamma-diversity, or local assembly processes through local (alpha) diversity; (c) whether local assembly operated through the regional species abundance distribution (SAD) or intraspecific spatial aggregation; and (d) the dependence on body size, dispersal capacity and trophic level (producer versus consumer). Location USA, Canada and France. Time period 1993-2012. Major taxa studied Stream diatoms, insects and fish. Methods We analysed six datasets totalling 1,225 stream samples. We compared diversity responses to eutrophication and physicochemical heterogeneity in forested versus agricultural streams with regression methods. Null models quantified the contribution of local assembly to beta-diversity (beta-deviance, beta(DEV)) for both types of land covers and partitioned it into fractions explained by the regional SAD (beta(SAD)) versus aggregation (beta(AGG)). Results Eutrophication explained homogenization and more uneven regional SADs across groups, but local and regional biodiversity responses differed across taxa. The beta(DEV) was insensitive to land use. The beta(SAD) largely exceeded beta(AGG) and was higher in agriculture. Main conclusions Eutrophication but not physicochemical heterogeneity of agricultural streams underlay beta-diversity loss in diatoms, insects and fish. Agriculture did not constrain the magnitude of local versus regional effects on beta-diversity but controlled the local assembly mechanisms. Although the SAD fraction dominated in both land covers, it increased further in agriculture at the expense of aggregation. Notably, the regional SADs were more uneven in agriculture, exhibiting excess common species or stronger dominance. Diatoms and insects diverged from fish in terms of biodiversity, SAD shape and beta(DEV) patterns, suggesting an overriding role of body size and/or dispersal capacity compared with trophic position.