Combined effects of habitat modification on trait composition and species nestedness in river invertebrates

Changes in catchment land-use and sedimentation have large ecological effects on rivers, but there is limited transferable understanding of the consequences for river conservation. In the Usk river system (Wales, UK), we assessed whether catchment-scale change in land-use and patch-scale sedimentation (i) affected organisms with specific life-history traits and (ii) resulted in nested assemblages with species-poor sites occupied mostly by sub-sets of organisms from richer sites. Reaches in catchments converted to agriculture had nested species assemblages characterised by greater representation of organisms with small body size, shorter life cycle and effective dispersal capacity. In contrast, richer sites in semi-natural catchments supported taxa with longer life cycles. Patch-scale sedimentation was also accompanied by nested patterns in which depauperate patches supported taxa mostly with shorter life cycles, small size and detrital feeding habits. Sediment-free patches were richer and characterised by larger taxa, poor dispersers and predators. Trait diversity was reduced by habitat modification at both scales. We conclude that habitat modification in this river catchment has led to the systematic drop-out at two different scales of specific groups of organisms with particular trait character. Large-scale agricultural intensification appears to have removed larger, longer-lived invertebrates that probably require stable conditions, and we advocate further studies to appraise whether such organisms are at risk more globally from land-use conversion. This river case study is one of the first to combine nestedness analysis with biological trait assessment and it might help in developing transferable methods to predict the conservation impacts of land-use change