Assessing the Complex Effects of the Invasive Amphipod <scp><i>Dikerogammarus villosus</i></scp> on Leaf Litter Breakdown in Rivers

ABSTRACT Allochthonous organic matter is an essential resource of aquatic food webs, and freshwater amphipods, particularly species of the genus Gammarus , play a significant role in leaf litter decomposition. However, this important ecological function may be affected by the invasion of alien amphipods such as Dikerogammarus villosus , which has been reported to show lower leaf litter decay rates than native gammarids. The aim of this study was to determine whether the invasion of D. villosus would disrupt the ecological function of leaf litter breakdown under field conditions by comparing leaf litter decay rates at invaded and uninvaded river sites. The experiment was conducted at six locations, four in Croatia and two in Germany, with one site upstream and one site downstream of an invasion front at each location. At each site, we placed preconditioned willow leaves in the river and sampled them three times to estimate leaf litter breakdown rates and invertebrate colonisation. We hypothesized that leaf litter breakdown rates would be higher at the sites without D. villosus (upstream of the invasion front) compared to the invaded sites. Contrary to this hypothesis, the results indicated contrasting and obviously context dependent outcomes. This indicates a change but no general disruption of leaf litter processing by the invasion of the omnivorous D. villosus , even when it displaces native shredders. The outcome of the experiments most probably depended on the biomass of native shredders in the leaf bags. At three locations with low biomass of native shredders in the upstream leaf bags, the downstream sites with D. villosus presence showed higher breakdown rates than the upstream sites. At the three locations with high biomass of native shredders, the downstream sites with D. villosus had lower breakdown rates than the upstream sites. These findings underscore the complex interactions between invasive and native species in freshwater ecosystems and their potential impact on ecosystem services.