41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

Parasites can affect the outcome of biological invasions in different ways. Outbreaks of parasites may lead to host population crashes and resultant community change. But parasites do not only act on host population density. We present studies of short-term, behavioural effects of parasites and their effect on invasions. We focus on parasite regulation of crustacean invasions. Using empirical studies and mathematical modelling, we show that two parasites play keystone roles in UK amphipod invasions. Firstly, the microsporidian Pleistophora mulleri may facilitate invasion by two smaller species of amphipod; it has no direct effect on the survival of the native G. d. celticus, but infected animals are less likely to prey on the two smaller invaders. Secondly, the acanthocephalan Echinorynchus truttae may promote coexistence, as infection of the invading species Gammarus pulex reduces its predation on native G. d. celticus. Microsporidia may also drive crayfish invasions. We provide evidence from sequence data that the invading signal crayfish has acquired Thelohania contejeani (porcelain disease) from the native. However, whilst the invader may suffer little from the infection, transmission to the native can cause reduced activity and mortality and so increase the rate of extinction of this species.