The processing of organic matter in freshwater ecosystems is crucial for integrating terrestrial plant material into aquatic food webs, with many systems depending on allochthonous inputs. In a study published in Oikos (https://doi.org/10.1111/oik.10774) we investigated how the population density of native and non-native amphipods (Gammarus fossarum and Gammarus roeselii) affects leaf litter processing, revealing a negative density-dependent relationship in both laboratory and field experiments. Interspecific competition confirmed functional redundancy between the species and showed that processing rates plateau at defined breakpoints, which reflect minimal metabolic needs. The findings emphasize the importance of accounting for nonlinear density-dependencies and breakpoints in aquatic ecosystem models, as population density significantly impacts ecosystem functions.