LAUSR

Dikerogammarus villosus (Sowinsky, 1894) is a Ponto–Caspian amphipod expanding in European freshwaters and posing a threat to biological diversity through several biological and behavioural traits, including high carnivory and voracity. The species spreads in Europe through two major corridors from two geographically and genetically different sources: the Danube Delta (the Western Group) and the Dnieper Delta (the Eastern Group). These two intraspecific groups differ also phenotypically. The goal of our study was to compare the food consumption and food preference of these groups depending on the season and amphipod size using two separate experiments. In the food consumption experiment, amphipods were provided with one of the three food types: willow leaves, dead fish tissue or live chironomid larvae. In the food preference experiment, they were provided with all three food types together. Both experiments were conducted five times (every second month). We analysed the consumption rate and food preference after 24 h of exposure. Amphipods of both groups preferentially consumed chironomid larvae in all months (maximum in May). However, amphipods from the Western Group selected chironomid larvae significantly more often than amphipods from the Eastern Group. Also, amphipods in the Western Group consumed a higher weight of chironomid larvae per mg body weight, but this relationship was not observed in the Eastern Group. In both groups, the consumption rate and selection of chironomid larvae were higher in warmer months (May–September). Our results show that Dikerogammarus villosus from the Western Group is a voracious predator of animal tissue. Although amphipods from the Eastern Group also prefer chironomid larvae, their consumption of all kinds of food is more uniform, making them more omnivorous. As a consequence, Western populations of D. villosus may pose a greater threat to macroinvertebrate communities, but their Eastern counterparts may be more successful invaders due to their higher diet flexibility. Our study shows differing behaviours underpinning the invasion dynamics of different groups of Dikerogammarus villosus. Our results highlight the importance of examining traits that underpin species' invasiveness at the population level.