Biological invasions continue to alter the structure and functioning of ecosystems worldwide. While the vectors and pathways of establishment of Invasive species are understood, predicting the ecological changes, or impacts, these species can exert on recipient ecosystems is lacking. Using invasive freshwater amphipods as models, this thesis aimed to investigate the role of intraspecific variation and interspecific interactions in understanding and predicting invasive species' impacts. This work first looks at the inherent variation in feeding behaviour within and between populations of an invasive amphipod, and discusses the implications of such variation for the use of functional responses, or the relationship between feeding rate and prey density. in invasion ecology. Secondly, behaviours associated to dispersal and impacts, as well as parasitic profiles, are compared between two UK populations of Dikerogammarus villosus. Differences between the populations show that monitoring population level differences, including behaviour and parasitism, can inform on invasive species' spread and impacts. Thirdly, interspecific interactions are used in order to elucidate the potential impac s of Dikerogammarus haemobaphes. Two aspects in particular are highlighted: predator-prey interactions, using the comparative functional response framework, and interference competition with D. villosus in terms of habitat use. Finally. cross-ecosystem interspecilic interactions are considered and their effects on a critical ecological process: leaf litter degradation. Using two invasive/native plant pairings, the individual leaf shredding abilities of an invasive and a native amphipod are compared in a laboratory setting. While both native plant species were consumed more readily, no difference was detected between the two amphipods. However, an in situ leaf litter bag experiment showed that the indirect effect of the invasive amphipod on macroinvertebrate community diversity may modify the degradation of certain plant species. This thesis therefore provides empirical evidence of the importance of intraspecific variation and interspecific interactions for the furthering predictive ecology