Biological Invasions Affect Resource Processing in Aquatic Ecosystems: The Invasive Amphipod Dikerogammarus villosus Impacts Detritus Processing through High Abundance Rather than Differential Response to Temperature

SIMPLE SUMMARY: Ecosystems are affected by multiple stressors, which interact in ways that can be difficult to predict. Stressors such as climate warming and introductions of non-native species and parasites can impact processes vital to the functioning of ecosystems. In temperate freshwater ecosystems most nutrients originate from leaf litter, which is processed by invertebrate shredder species. Focusing on the invasive killer shrimp, which is replacing native species, we investigated how the stressors interact to impact rates of shredding and survival of the shredder species to make predictions of how temperature and invasive species may alter the function of temperate freshwater ecosystems. Increasing temperature was found to increase rates of shredding up to an optimum, after which shredding decreased. The native shredders had a higher rate of shredding than the invasive species at all temperatures. However, the invasive killer shrimp reached a much higher abundance than the native and we demonstrated that the total population of the introduced shredders processed far more leaf litter at invaded sites. While this may make the ecosystems more productive in the short term, it may lead to the exhaustion of the leaf litter resource, with negative consequences for the function of the ecosystem over time. ABSTRACT: Anthropogenic stressors such as climate warming and invasive species and natural stressors such as parasites exert pressures that can interact to impact the function of ecosystems. This study investigated how these stressors interact to impact the vital ecosystem process of shredding by keystone species in temperate freshwater ecosystems. We compared metabolic rates and rates of shredding at a range of temperatures up to extreme levels, from 5 °C to 30 °C, between invasive and native amphipods that were unparasitised or parasitised by a common acanthocephalan, Echinorhynchus truttae. Shredding results were compared using the relative impact potential (RIP) metric to investigate how they impacted the scale with a numerical response. Although per capita shredding was higher for the native amphipod at all temperatures, the higher abundance of the invader led to higher relative impact scores; hence, the replacement of the native by the invasive amphipod is predicted to drive an increase in shredding. This could be interpreted as a positive effect on the ecosystem function, leading to a faster accumulation of amphipod biomass and a greater rate of fine particulate organic matter (FPOM) provisioning for the ecosystem. However, the high density of invaders compared with natives may lead to the exhaustion of the resource in sites with relatively low leaf detritus levels.