Highly variable, unpredictable activity patterns in invasive, but not native amphipod species

Behavioral differences between native and introduced species may contribute to the invasiveness of certain species. This includes differences at the species level, consistent variation among individuals (personality) and within-individual variation (e.g., behavioral plasticity). Here, we investigated swimming activity of individuals from four different amphipod species occurring in the river Rhine system, three of which were native or naturalized (> 100 years present) while one is a recent invader (Dikerogammarus villosus, < 25 years present). At the species level, D. villosus did not show higher average swimming activity than the three non-invasive species. However, the non-invasive species, on average, changed their behavior predictably over the course of the experiment (average behavioral plasticity), while D. villosus did not exhibit any consistent change in activity. At the individual level, D. villosus exhibited greater among- and within-individual variation in activity levels than all non-invasive species. The non-invasive species further showed significant individual differences in plasticity, that is, individuals of these species differed consistently in how they changed their activity over time. The high within-individual variation in D. villosus translated into a lack of consistent individual differences in plasticity in this species. We hypothesize that the highly variable and unpredictable patterns of individual activity variation in D. villosus might help this successful invader to cope with new environmental conditions encountered in the river Rhine system.