The impact of invasive gammaridean species on native biodiversity is well documented, but the potential for them to disrupt ecosystem functioning is less well understood. Native freshwater amphipods are considered to be archetypal leaf shredders and are considered key to leaf litter processing within rivers. It is possible that invasions may interfere with key ecosystem processes such as leaf litter breakdown, due to behavioural traits displayed by the invasive species. In two laboratory experiments, we compared the leaf shredding efficiency of the native Gammarus pulex and Dikerogammarus haemobaphes, a recently established Ponto–Caspian invader in the UK. We hypothesised that in isolation G. pulex would have a greater shredding efficiency than D. haemobaphes and that, in the presence of the invasive, leaf shredding and survival of G. pulex would be reduced. The results supported our hypothesis that, in isolated conditions, G. pulex consumed significantly more leaves than D. haemobaphes. Under mixed treatments, G. pulex leaf consumption and survival, although not statistically significant, appeared to be reduced. The implications of our findings suggest that the potential displacement of G. pulex from its native range, by D. haemobaphes, could lead to a decline in leaf litter processing and recycling in rivers within the UK.