The sympatric amphipods Gammarus fossarum, Gammarus pulex, and Echinogammarus berilloni show a typical distribution pattern in the intermittent karst streams of the Paderborn Plateau, Germany. While G. fossarum dominates the upstream reaches, the exotic E. berilloni invades the catchment of the main river Alme from downstream and apparently displaces G. pulex from the temporary mid-reaches. We conducted field and experimental studies to elucidate possible reasons for this distribution pattern and the invasion success of E. berilloni. We tested the response to abiotic factors related to occasional drought situations (temperature, flow, stream bed structure, dessiccation, oxygen concentration) and considered different species traits (e.g., mobiliy, life history strategies) as key components of niche separation of the species. Our phenomenological studies and experimental results suggest that, by comparison, E. berilloni has better adaptations to survive stream drying. We conclude that this advantage will contribute to the invasion success of this species and to the displacement of native species in the area. These findings have implications for climate change scenarios at the community level.