Effects of invasive and indigenous amphipods on physico-chemical and microbial properties in freshwater substrates

Native benthic amphipods play a key role in freshwater ecosystem services such as leaf litter decomposition. The replacement of native by invasive species with different functional traits has the potential to alter hyporheic processes, communities, and food webs. Despite the increasing number of publications on invasive species, there is a lack of studies that compare the effects of native versus invasive amphipods on physico-chemical habitat properties and microbial communities in the substrate. We compared the effects of an indigenous (Gammarus roeseli, Gervais 1835) and an invasive (Dikerogammarus villosus, Sowinsky 1894) amphipod on leaf litter decomposition, as well as on bacterial communities and physico-chemical conditions in freshwater substrata. We hypothesized that the different amphipod species distinctly alter habitat conditions and microbial community composition, depending on functional differences in leaf litter decomposition. We detected strong differences between G. roeseli and D. villosus in the feeding rates on alder leaves, with 11-fold greater decomposition of alder leaves by the native species. Additionally, our study revealed differences in microbial community composition between treatments at the substrate surface, but almost no differences in physico-chemical parameters in the interstitial and open water. These results support the hypothesis that the replacement of indigenous amphipod species by functionally different invasive amphipods can lead to a decrease in leaf litter decomposition and an altered microbial community composition with possible effects on benthic food webs. These effects on ecosystem services should be taken into account when assessing the impacts of invasive species on freshwater habitats.