The invasion success varies among populations of a species. This project investigates intra-specific variation in invasion success by comprising experimental and field studies centred on a reciprocal relationship between bitterling fish (Rhodeus amarus) and unionid mussels in Europe. This relationship is affected by recent invasions of an Asian unionid (Anodonta woodiana), zebra mussel (Dreissena polymorpha) and potential invasion of Chinese rose bitterling (Rhodeus ocellatus). We look at individual, population, and community effects of these invasions, including the effect of adult mussels on R. amarus demography, ability of parasitic mussel larvae to exploit local fish hosts, impact of A. woodiana on unionid communities, demographic and evolutionary processes during A. woodiana invasion, potential of R. ocellatus to establish in Europe, and impact of D. polymorpha on native and non-native unionids and bitterling species. This should illuminate how non-native species interact with native and other non-native species and how population-specific attributes affect such interactions.