| Human activities, particularly shipping and transport, have introduced many exotic species worldwide. Although the effects of most introductions are undetectable, certain species have dramatic ecological and economic impacts. After habitat destruction, invasive species are the largest key threats to biodiversity. The only certainty about invasive species is that they will continue to emerge as an environmental problem. Despite heightened awareness about the threat of invasions to natural ecosystems, we still have a poor understanding of the mechanisms responsible for invasion success. A prominent, but untested, hypothesis explaining the success of introduced species is that they are relatively free of the effects of natural enemies. Most notably, they may be subject to fewer parasites in their introduced range compared to their native range. For my dissertation research I developed a theoretical framework to model invasion success. I also evaluated the effects of parasites on the invasion success and impacts of three exotic marine and estuarine species, a crab, a snail and a fish. Finally, to determine the generality of this hypothesis, in a meta-analysis, I examined parasite presence and prevalence in host species from a broad range of taxa, including molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles over a wide range of ecologies from terrestrial, freshwater and marine systems.; Some parasites of exotic hosts were introduced from the native range to the introduced range, and some native parasites colonized introduced hosts. Still, introduced populations of invasive species were substantially and significantly less parasitized compared to native populations, both in terms of parasite species richness and prevalence. Evidence from native green crab populations suggests that some parasites can significantly decrease both size of individuals and biomass of populations. Taken together, the loss of native parasites and the limitation of local parasites to become established in exotic hosts may explain why some invasive species are such successful invaders and cause significant ecological impacts. |
