Differences in behavioural traits between two potentially invasive amphipods, Dikerogammarus villosus and Gammarus pulex

The behavioural trait axes of activity, exploration, boldness and sociability can help to understand the tendency of an invasive species to disperse, as may be expected at the fringe of an invasive population, or to socialise, as may be expected in well-established populations where densities of invaders are typically high. We compared behavioural traits between the gammarids Dikerogammarus villosus and Gammarus pulex from centre and fringe populations at Barton Broad, Norfolk, UK. Dikerogammarus villosus is invasive in Western Europe and has displaced many macroinvertebrate species, including the native G. pulex. Gammarus pulex is itself invasive in Northern Ireland and the Isle of Man, thus its displacement increases interest into what unique characteristics of D. villosus make it a dominantly successful invader. Dikerogammarus villosus was significantly less active, less explorative, and more social than G. pulex. We found no significant differences in the behaviours of D. villosus individuals from the central population and the invasive fringe. These patterns indicate active dispersal is likely important to the invasion success of G. pulex, while D. villosus might depend on passive movement. Our data suggest that behavioural factors determining invasive success within closely related taxa can differ considerably, and may lead to different patterns of invasion.