Warming alters cascading effects of a dominant arthropod predator on microbial community composition in the Arctic

Warming is expected to increase abundances of wolf spider, the top predator in soil communities in the Arctic, but we have little understanding on how increased wolf spider density under warmer conditions affects soil microbial structure through trophic cascades. We tested the effects of wolf spider density and warming on bacterial and fungal community structure in litter through a fully factorial mesocosm experiment in Arctic tundra over two summers. Replicated litter bags were deployed at the soil surface and underground in the organic soil profile and collected at 2- and 14-month incubation. The litter samples were analyzed for community structure of bacteria and fungi and mass weight loss. After 2-month incubation, bacterial and fungal community compositions were already structured interactively by the spider density and warming treatments. Such interaction effect was also found in litter microbial community structure as well as litter mass loss rates after 14-month incubation. Our results show that wolf spiders have indirect, cascading effects on microbial community structure but that warming can alter these effects. The non-linear responses of microbial communities and litter decomposition to warming and increased spider density cast uncertainty in predicting structure and function of Arctic terrestrial ecosystem under warmer conditions in the future. IMPORTANCEThis is one of the first studies demonstrating that predator abundances and increased temperature interactively structure litter microbial communities in the Arctic. The Arctic is one of the fastest warming regions due to climate change and contains disproportionately large amounts of soil organic matter, including thick litter which accumulated over the long time because of slow decomposition. The accelerated soil organic matter decomposition due to the rapid warming can cause positive feedback where resulting greenhouse gas emission contribute to further global warming. Since microbial structure can affect decomposition rates of litter, the observed non-linear responses of soil microbial community compositions and litter decomposition rates indicate challenges in predicting Arctic ecosystem responses in the future.