Exotic species and native parasites: Biotic resistance versus parasite spillback

Background/Question/Methods

Exotic species and native parasites can interact in a number of ways. When native parasites infect exotic species, one of two scenarios is predicted: 1) native parasites may act as a form of biotic resistance, reducing the ability of exotic species to invade, or 2) exotic species may amplify the parasite, increasing disease in native species (known as parasite spillback). We tested the relationship between native parasites and exotic species using an aquatic host-parasite system involving a fungal parasite (Metschnikowia bicuspidata) and two Daphnia host species (the native D. dentifera and the exotic D. lumholtzi). In experimental mesocosms, we manipulated the presence of the parasite in varying species combinations; native species only, exotic species only, and both species together. We monitored species abundance and infection rates for 55 days following parasite exposure. 

Results/Conclusions

Our results show initial signs of biotic resistance, but the parasite fails to harm the exotic species. Abundance of both species was reduced in the presence of the other, indicating that the species competed for resources. Thus, this exotic species has the potential to negatively impact the native species. The parasite also reduced population sizes of the native host, and the combination of parasite and exotic species had a less-than-additive negative effect on native species populations. Moreover, the exotic host species did not affect infection levels in the native hosts. In contrast, addition of the native species increased disease in the exotic species. Despite this increase in disease, there was no effect of the parasite on population sizes of the exotic species, suggesting that it is tolerant of the parasite. Thus, while the increase in exotic host infection mediated by the native host would initially suggest biotic resistance, the lack of population-level effects of the parasite suggests that it will not reduce the ability of the exotic species to invade. Therefore, the relationship between exotic species and native parasites is more complex than predicted by the biotic resistance versus parasite spillback dichotomy.