Why are invaders invasive? Development of tools to understand the success and impact of invasive species

Biological invasions are a major facet of anthropogenic global change, with severe negative environmental and socioecological impacts. Effective and efficient management of biological invasions requires a mechanistic understanding of the factors driving invasion success and impact. I investigate three factors likely to have broad relevance in explaining success and impact of alien invaders: resource use, behaviour and propagule pressure. Alien decapod and amphipod crustaceans may have different patterns of trophic resource use to native analogues. Through quantification of functional responses and food ‘choice’, I highlight an exceptionally large predatory impact of alien Eriocheir sinensis on invertebrate prey, relative to both native and alien crayfish. Through similar methods, I suggest the larger size of alien Dikerogammarus villosus relative to native Gammarus pulex could facilitate higher predatory impacts on fish eggs and larvae. I quantify personality traits (boldness, exploration, activity, sociability and voracity) of invasive and native decapod crustaceans in the laboratory. Invasive E. sinensis and Pacifastacus leniusculus were bolder than European Austropotamobius pallipes. Boldness may a common trait of successful, high-impact invaders. I provide the first evidence of personality (consistent within-individual behaviours) in these decapods, but find no evidence that it drives dispersal in signal crayfish. Comparisons of core and invasion-front populations of P. leniusculus suggest its spread is driven by density rather than behaviour. Using experimental invasions of ciliate protists into laboratory microcosms, I provide quantitative data to show how propagule pressure – the number of introduced organisms and introduction events – can increase invasion success (rate and population density) and invader impact. In general, resource use, behaviour and propagule pressure all have potential to predict the identity, impact and dynamics of successful invaders and thus inform management strategies. Having measured metabolism alongside these other factors, I propose that metabolic rate could provide another readily-measurable, general predictor of invasion success and impact.