Effects of agricultural land use on stream assemblages: Taxon-specific responses of alpha and beta diversity

Although biodiversity loss and homogenization of communities with increasing stress is an established axiom of pollution ecology, few studies have compared the concurrent response of multiple taxonomic groups to determine if environmental perturbation results in coherent patterns in assemblage composition. Here, data from 35 lowland European streams was used to explore the responses of multiple taxonomic groups to disturbance from agricultural land use, mainly gradients in total phosphorus (TP). We studied responses of fish, invertebrate, macrophyte and benthic diatom assemblages to TP, based on univariate metrics that characterize α-diversity (taxon richness, Simpson diversity, evenness, taxonomic distinctness) and abundance, and multivariate community structure. Multivariate dispersion was used to infer β-diversity characteristics in two groups of streams with low and high levels of TP. Three of the five metrics of α-diversity and abundance of primary producers were significantly related to resources, however, the direction of change was not always consistent with our predictions; macrophyte diversity decreased, whereas benthic diatom diversity increased with elevated nutrients. For consumers, taxon richness increased (fish) or decreased (invertebrates), while fish assemblages had lower phylogenetic diversity (taxonomic distinctness) associated with disturbance, implying a loss of functional diversity. In line with our predictions, fish and invertebrate assemblages showed homogenization of β-diversity with disturbance. Our finding that β-diversity of the two primary producers remained relatively high, while α-diversity either decreased (macrophytes) or increased (benthic diatoms), suggests that local disturbance is to some extent compensated for by among-site diversity. Our finding that secondary producers showed homogenization of β-diversity with disturbance has strong management perspectives, suggesting that these groups will need to be more carefully managed to maintain diversity at the landscape level, potentially sustaining resilient communities.