At present, the invasive round and bighead gobies are the most abundant benthic fish species in the near shore zone of the Middle Danube. We compared their diet seasonally in natural and artificial habitats and contrasted it with the food supply. The composition of the macroinvertebrate community was determined mainly by seasonal changes, whereas habitat type had smaller effect. Round gobies followed these changes flexibly. They consumed mainly chironomids in the spring, whilst amphipods and molluscs in the summer and autumn. Bighead gobies relied on amphipods in each season and in both habitats, and consumed fish, too, including round goby (intraguild predation). Diet overlap was determined by the morphological differences of the species allowing a varying degree of differentiation according to the seasonally changing food supply. IntroductionOne of the most spectacular changes in the fauna of Central and Eastern European large river systems in the last decades has been the ongoing spread and proliferation of PontoCaspian fauna elements, primarily macroinvertebrates (crustaceans, molluscs) and gobiid fishes (BIJ DE VAATE et al., 2002;COPP et al., 2005). These aquatic invaders have been reported to cause serious changes in the structure and functioning of their newly invaded habitat including the displacement of their native counterparts or the reconstruction of the food web (RICCIARDI, 2001;MINCHIN et al., 2002;VAN RIEL et al., 2006). Round goby (Apollonia melanostoma (PALLAS, 1814), formerly Neogobius melanostomus) for example, the most successful invasive goby species caused substantial changes in the composition and abundance of benthic macroinvertebrates (BARTON et al., 2005), declines of native fish species (FRENCH and JUDE, 2001;CORKUM et al., 2004) and increased energy transfer to predators in the well studied Laurentian Great Lakes. Gobiid fishes * Corresponding author 610 P. BORZA et al.

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30The River Danube plays a central role in the spread of Ponto-Caspian species as a part of the

Summary Facilitative interactions among co‐evolved representatives of the endemic Ponto‐Caspian fauna are regarded as a major factor of their invasion success. Nevertheless, the most renowned examples represent interactions between different trophic levels or functional groups, while ecologically similar species can be expected to show competition‐based niche partitioning. Here, we test for differences in the realized niche of three invasive Dikerogammarus species (Crustacea: Gammaridae) in their co‐occurring range. We sampled multiple habitats within sites distributed along the River Danube to test whether some environmental variables could reveal spatial niche differentiation among the three species of Dikerogammarus, and if so, to test a predictive model outside the zone of co‐occurrence. Spatial niche differentiation was present among the species, primarily determined by current velocity (and associated substrate preference), likely reflecting a stress tolerance–competitive ability trade‐off. Suspended matter concentration was also relevant, suggesting food resources (through filter feeding) might represent another important niche axis, somewhat loosening the terms of co‐existence between D. haemobaphes and the other two species. Environmental variables could effectively explain the absence of D. bispinosus in the Lower Danube, implying that the co‐existence of the three species is possible only along a sufficiently wide current velocity gradient, and the observed turnovers are the result of niche expansion in the absence of the stronger competitor. Hence, differences in invasion success may be attributed to a stress tolerance–competitive ability trade‐off. Our results suggest the advantage of D. villosus is attributable to its competitive dominance, allowing it to monopolize lentic and/or structured habitats, which represents a fortunate pre‐adaptation to anthropogenic alterations of aquatic ecosystems. The presence of D. villosus does not greatly affect the expansion of D. haemobaphes; however, the exclusion of D. bispinosus from lentic habitats by D. villosus probably strongly limits its potential to spread by active dispersal.