Debris-flow deposits on a major river influence aquatic habitats and benthic macroinvertebrate assemblages

Natural barriers formed by avalanche deposits, landslides, and debris flows are commonly distributed along deep incised rivers where they block river channels and create diverse river habitats. However, little is known about the effects these naturally-occurring features have on the diversity of macroinvertebrate assemblages. We, therefore, conducted a field study to assess how alpha- and beta-diversity of stream macroinvertebrates were influenced by these deposits. The Nujiang River (upper Salween River) basin in China has more than 200 debris-flow deposits in the 300-km long Grand Canyon reach. We assessed differences in environmental variables and macroinvertebrate assemblages among 5 habitat types, including portions of the main channel and tributaries without these deposits and 3 other habitat types associated with debris deposits. We calculated alpha-diversity for each habitat type as individual-based rarefied richness and beta-diversity among sampled patches within each habitat type with the Whittaker Index. Generally, patches within the debris-flow deposits had lower alpha-diversity, density, and biomass compared with patches in the non-deposit reaches. These results indicate that the different habitat types in the debris-flow reaches had markedly different abiotic and biotic conditions than the non-deposit main channel reaches and tributaries. However, patches in the debris-flow deposits had higher beta-diversity than non-deposit reaches. By creating diverse habitat types, the debris-flow deposits enlarged the environmental gradients for macroinvertebrate assemblages, which increased regional aquatic biodiversity.