Decoupled structure and function of leaf-associated microorganisms under anthropogenic pressure: Potential hurdles for environmental monitoring

Legal frameworks that assess stream ecological integrity rely mostly on structural metrics and implicitly assume that good structure will result in good function. This assumption may, however, be challenged. The results of our literature screening show that the structure and function of leaf-associated microbial assemblages can be decoupled, meaning that structure and function are not similarly affected by anthropogenic pressures. This decoupling can be characterized by microbially-mediated leaf litter decomposition remaining stable, increasing, or exhibiting a U-shaped response as structural metrics (e.g., taxonomic diversity) change gradually. When these types of decoupling occur, stream ecological integrity may be misinterpreted when solely based on structural metrics. This misinterpretation may be driven by a structurally-simplified microbial assemblage (e.g., fewer taxa) effectively decomposing leaf litter, which may limit the contribution of structurally-simplified assemblages to further ecosystem level processes (e.g., nutrition of food webs). At the same time, structurally-simplified microbial assemblages may exhibit reduced ecological resilience against pressures. Therefore, we suggest that incorporating both structural and functional metrics will improve stream biomonitoring by increasing our ability to determine the consequences of anthropogenic pressures. Additional use of molecular biological methods and information on microbial traits should improve the assessment of stream ecological integrity.