Litter Decomposition can be Reduced by Pesticide Effects on Detritivores and Decomposers: Implications for Tropical Stream Functioning

Understanding which factors affect the process of leaf litter decomposition is crucial if we are to predict changes in the functioning of stream ecosystems as a result of human activities. One major activity with known consequences on streams is agriculture, which is of particular concern in tropical regions, where forests are being rapidly replaced by crops. While pesticides are potential drivers of reduced decomposition rates observed in agricultural tropical streams, their specific effects on the performance of decomposers and detritivores are mostly unknown. We used a microcosm experiment to examine the individual and joint effects of an insecticide (chlorpyrifos) and a fungicide (chlorothalonil) on survival and growth of detritivores (Anchytarsus, Hyalella and Lepidostoma), aquatic hyphomycetes (AH) sporulation rate, taxon richness, assemblage structure, and leaf litter decomposition rates. Our results revealed detrimental effects on detritivore survival (which were mostly due to the insecticide and strongest for Hyalella), changes in AH assemblage structure, and reduced sporulation rate, taxon richness and microbial decomposition (mostly in response to the fungicide). Total decomposition was reduced especially when the pesticides were combined, suggesting that they operated differently and their effects were additive. Importantly, effects on decomposition were greater for single-species detritivore treatments than for the 3-species mixture, indicating that detritivore species loss may exacerbate the consequences of pesticides of stream ecosystem functioning.