Decomposer communities in human-impacted streams: species dominance rather than richness affects leaf decomposition.
Abstract
There is compelling evidence that anthropogenic disturbance can decrease biodiversity and impair ecological functioning. A major challenge to biodiversity-ecosystem function research is to disentangle the effects of biodiversity loss on ecosystem functions from the direct effects of human disturbance. We studied the influence of human disturbance (acidification and eutrophication) and a natural stressor (low pH due to bedrock geology) on leaf-shredding macroinvertebrates, fungal decomposers and leaf decomposition rates in boreal streams. We used pyrosequencing techniques to determine fungal richness and assemblage structure. Decomposition rates were higher in anthropogenically disturbed than in circumneutral reference or naturally acidic sites, but did not differ between the latter two groups. Macroinvertebrate richness was higher in circumneutral than in human-impacted or naturally acidic sites, and shredder evenness was highest in circumneutral sites. Fungal evenness was also lower in human-disturbed than in reference sites, whereas fungal richness did not vary among site groups. Decomposition rate in fine-mesh bags was related positively to current velocity and fungal dominance, while in coarse-mesh bags, it was related positively to total phosphorus. In anthropogenically disturbed streams, the effects of low pH were overridden by eutrophication, and increased decomposition rates resulted from disturbance-induced increase in species dominance rather than richness. Furthermore, decomposition rates were positively correlated with abundances of dominant taxa, suggesting that ecosystem processes may be driven by a few key species. Synthesis and applications. Our results suggest that leaf decomposition rates are insensitive to natural background variation, supporting the use of decomposition assays, preferably accompanied by molecular analysis of fungal assemblages, to assess stream ecosystem health. Instead of focusing solely on diversity, however, more emphasis should be placed at changes in dominance patterns, particularly if management aims are to improve stream ecosystem functioning. Our results suggest that leaf decomposition rates are insensitive to natural background variation, supporting the use of decomposition assays, preferably accompanied by molecular analysis of fungal assemblages, to assess stream ecosystem health. Instead of focusing solely on diversity, however, more emphasis should be placed at changes in dominance patterns, particularly if management aims are to improve stream ecosystem functioning.