Native parasite acquisition provides introduced species with the potential to modify native host-parasite dynamics by acting as parasite reservoirs (with the ‘spillback’ of infection increasing the parasite burdens of native hosts) or sinks (with the ‘dilution’ of infection decreasing the parasite burdens of native hosts) of infection. Exotic salmonids are frequently shown to acquire native parasites; however, as research into the threats posed by exotic salmonids has largely focused on predation and competition, threats posed by shared native parasites are poorly understood.
I used a multiple-pronged approach combining field observations, experimental infections and dynamic population modelling to investigate whether native parasite acquisition by exotic salmonids alters host-parasite dynamics in native fish populations from streams and lakes in New Zealand and Argentina. I also used a meta-analysis approach to investigate which trait(s) influence native parasite acquisition by exotic freshwater fish.
My research demonstrated that two key factors strongly influence whether the dynamics of native parasites will be affected by exotic fish. On one hand, the competency of exotic fish for native parasites is an important determinant of whether native parasite populations are likely to increase or decrease. On the other hand, the relative abundance of the exotic species determines whether its competency for a native parasite will actually translate into altered native host-parasite dynamics, with highly abundant exotic species more likely to induce changes in native parasite dynamics. I also demonstrated how exotic species may be able to override the influence of low host abundance, or competency by altering native host behaviour.
The meta-analysis suggested that traits known to influence parasite richness in native fish or invasion success of exotic species are not reliable predictors of native parasite acquisition by exotic fish. Instead, it is more likely that complex interactions between a variety of biological, geographical and historical factors govern parasite acquisition by exotic species, making it difficult to predict whether native parasites will be acquired.
