Predicting the ecological impacts of an alien invader: experimental approaches reveal the trophic consequences of competition

1. Ecological theory on the trophic impacts of invasive fauna on native competitors is equivocal. While increased inter-specific competition can result in coexisting species having constricted and diverged trophic niches, the competing species might instead increase their nichesizes to maintain energy intakes. Empirical experiments can test invasion theory on competitive interactions and nichesizes across different spatial scales and complexity. 2. The consequences of increased inter-specific competition from a model alien fish Leuciscus iduswere tested on two taxonomically and trophically similar native fishes, Squaliuscephalus and Barbus barbus. Competitive interactions were tested in tank aquaria using comparative functional responses(CFRs) and cohabitationtrials. The consequences of these competitive interactions for the trophic nichesizes and positions of the fishes were tested in pond mesocosms. 3. CFRs revealed that compared to B. barbus, L. idus had significantly higher attack and consumption rates; cohabitationtrials revealed B. barbusgrowth rates were depressed in sympatrywith L. idus. For L. idus and S. cephalus, differences in their functional responseparameters and growth rates were not significant. 4. Pond mesocosmsused stable isotope metrics to quantify shifts in the trophic nichesizes of the fishes between allopatry and sympatryusing a substitutive experimental design. Isotopic nicheswere smaller and more divergent in sympatric paired species than predicted by their allopatric treatments, suggesting trophic impacts from inter-specific competition. However, an all-species sympatric treatment revealed similar nichesizes with allopatry. This maintenance of nichesizes in the presence of all species potentially resulted from the buffering of direct competitive effects of the species-pairs by indirect effects. 5. Experimental predictions from tank aquaria assisted the interpretation of the constricted and diverged trophic nichesdetected in the paired-species sympatric treatments of the pond mesocosms. However, the all-species sympatric treatment of this experiment revealed greater complexity in the outcomes of the competitive interactions within and between the species. These results have important implications for understanding how alien species integrate into food webs and influence the trophic relationships between native species.