In competitive interactions, the presence of a shared enemy such as a parasite should favor the species that is least impacted (e.g., the less susceptible host species). It is also predicted that more resources in a community will reduce the strength of competition by allowing for niche partitioning, making co-existence between competitors more likely. However, more resources can also lead to greater population densities, increasing rates of parasite transmission and strengthening the effects of a shared parasite. We sought to understand the interactive effects of parasitism and resource levels on competition between a native and invasive species of zooplankton (Daphnia dentifera and D. lumholtzi, respectively). Previous work has shown that the invasive species is a superior competitor, but is more susceptible than the native species to a shared fungal parasite. In a series of laboratory microcosm experiments, we measured competition between these two host species while varying resource availability (algal food) and parasite presence.
Results/Conclusions
We found that more resources led to larger population sizes of both species, leading to higher rates of infection. However, the effects of competition and parasitism varied by species. The presence of the invasive species reduced native population densities and diminished the impact of resources on infected host density in the native species. In contrast, competition had little effect on the invasive host, but parasitism led to reduced invasive host densities. Finally, the invasive species was a stronger competitor in high resource environments, suggesting that high resource environments (e.g, those associated with eutrophication) and the absence of parasitism may increase the success of this invader. Our results suggest that parasitism and resource availability can have interactive, non-additive effects on the outcome of competition between an invasive and native species.