Introductions of non-native species (NNS) are major drivers of biodiversity loss. Gammarids (Crustacea, Gammaroidea) have been particularly successful in establishing and spreading in their non-native range, especially in Europe. While their impacts are wide-ranging, interference competition with native species has received limited study to date. Here, we assessed the competitive abilities of the successful North American NNS Gammarus tigrinus relative to the European native Gammarus duebeni, over a chironomid larva as a single food resource. We staged four types of dyadic contest encounters, with individuals of the native or NNS added to the experimental arena containing the food resource, and inter- or intraspecific competitor individuals added upon the first individual taking possession of the resource, or after 20 minutes. Gammarus tigrinus were more likely to take hold of the bloodworm in the opening 20 minutes, and did so more quickly than G. duebeni. During this period, they were also less thigmotactic than the native, being more explorative and spending a smaller proportion of time in the outer zone of the arena. They exhibited more aggressive interactions and activity with increasing size and mass, whereas larger G. duebeni were shown to be less aggressive and less active. Gammarus tigrinus were found to be significantly less likely to lose possession to G. duebeni than they were to conspecifics, whereas G. duebeni were similarly likely to lose possession to G. tigrinus as to conspecifics. Overall, our findings indicate that the behaviour and competitive ability of G. tigrinus demonstrated here add to a list of traits that facilitate its invasion success. In addition, our method offers potential as an effective, standardisable means of assessing the competitive abilities of gammarid NNS. We encourage future studies to develop it further, incorporating alternative resources, such as habitat, and to assess the role of ecologically relevant abiotic stressors in determining contest outcomes.
