Summary 1. The amphipod Gammarus pulex, introduced to Irish rivers with the aim of enhancing trout feeding, is displacing the native Gammarus duebeni celticus. These two species are generally associated with different environmental conditions and macroinvertebrate communities, confounding assessment of effects of the invader as compared with the native on fish populations. Here, we uncouple effects of the two Gammarus species from environmental gradients.
2. A weir dissects a lowland stretch of the River Lissan, slowing the upstream invasion by G. pulex and resulting in contiguous G. pulex, mixed species and G. d. celticus reaches. Total invertebrate abundance and biomass in the benthos were significantly higher in the G. pulex reach, driven by high invader abundance, with low abundance of other taxa. Gammarus pulex was particularly prominent in night-time drift.
3. Correspondingly, densities and biomass of 0+ trout were significantly higher in the G. pulex reach, while instantaneous loss rates were lower. Fish growth rates were similar among the three reaches.
4. In the G. pulex reach, this invader dominated the diet of 0+ trout, leading to ingestion of significantly higher invertebrate biomass than fish in the other reaches. Fish generally preyed on Gammarus in proportion to its abundance, but exhibited some positive selection for G. pulex in the invaded reach.
5. The negative effects of the invader on native invertebrates are contrasted with positive effects on juvenile trout. This indicates changes in energy flux after invasion, with differential resource use or assimilation by G. pulex probable underlying mechanisms. As the frequency of amphipod invasions increases globally, investigations of their role as strong interactors at multiple levels of ecological organisation is required if the consequences of deliberate and unintentional introductions are to be predicted, and ultimately, prevented.