Dammed rivers: impoundments facilitate fish invasions

River damming and other anthropogenic disturbances of natural habitats are among the main drivers of species loss through a range of direct and indirect effects. While the effects of river damming on aquatic species are relatively well studied, particularly with regard to their impacts on diadromous species and stenotopic riverine specialists, there is a paucity of studies quantifying the effects of dam construction on whole communities. We conducted a global meta-analysis focussed on fish communities, comparing species richness, abundance and proportion of alien species between dammed and undammed rivers. Both longitudinal and cross-sectional studies were examined. We found that construction of dams did not have a noticeable effect on fish richness and abundance, but the increase in proportion of alien species was significant (mean effect size of 0.62). Our findings suggest that the conversion of lotic waterbodies into lentic habitats result in the extirpation of species unable to withstand a drastic change in environmental conditions, but the loss is compensated by colonising lacustrine or eurytopic species taking advantage of reduced competition and the availability of new niches specific to lentic habitats. However, when eurytopic natives are absent from waterbodies connected to the newly constructed reservoirs, vacant niches are instead exploited by alien species, resulting in impoverishment of native species richness although overall species richness may be maintained.