The ALIEN IMPACT project aimed to provide a first integrated study of patterns and mechanisms of impact by alien invasive species in Belgium. It considered multiple, highly invasive plant species (HIPS), and combined large-scale screening of invader impact (to characterize patterns) with highly mechanistic studies at fixed sites to characterize impact pathways. Both terrestrial and freshwater ecosystems were studied. The main objectives were: (1) To identify the patterns of HIPS impact on the diversity of native plant communities, by characterizing communities that experience greatest impact and characterizing target native species, both in aquatic and terrestrial ecosystems. (2) To identify mechanisms of HIPS impact on native plants, both direct and indirect via pollinators or soil modification. (3) To estimate the impacts at other trophic levels by investigating whether HIPS impact on native plant diversity is associated with diversity loss or changes in community structure in other trophic groups, notably soil fauna and macroinvertebrates in water and sediment. (4) To analyse factors that may modify HIPS impacts on native plant species in the future.