Parasites alter freshwater communities by modifying behavior of invasive crayfish hosts

Background/Question/Methods

While parasites can alter communities by reducing densities of keystone species, few studies have examined how parasite-induced behavioral changes to hosts can alter communities. We tested how trematode parasites (Microphallus spp.) that affect the behavior of an invasive crayfish host (Orconectes rusticus) alter the impacts of this host on lake littoral communities. O. rusticus are major drivers of community composition in north temperate lakes, and predatory fish can reduce the impacts of O. rusticus by reducing crayfish activity and feeding. Microphallus can also alter O. rusticus behavior.  In laboratory studies, infected O. rusticus consumed fewer macroinvertebrates and were bolder in the presence of predatory fish than uninfected individuals. Therefore, we predicted that when predators are absent, infected crayfish would have reduced impacts on macrophytes and macroinvertebrates compared to uninfected crayfish. However, when predators are present, we expected infected crayfish to have the greatest impact on lower trophic levels because of increased boldness. We tested the combined effects of predators and parasites on O. rusticus impacts in large mesocosms (1880 L) over four weeks. Mesocosms were stocked with macrophytes, macroinvertebrates, and either infected or uninfected O. rusticus. Half of the mesocosms also contained rock bass (Ambloplites rupestris).

Results/Conclusions

Trematode parasites significantly altered O. rusticus growth, behavior, and ecological impacts. Infected crayfish were more active and had greater growth than uninfected crayfish across treatments, suggesting that the reduction in feeding behavior observed in short term experiments does not occur over longer timescales. In addition, crayfish infection had a significant impact on the composition of the macroinvertebrate community. Further, as we predicted, when predatory fish were present, infected crayfish consumed a greater biomass of macrophytes and macroinvertebrates than uninfected crayfish, likely due to increased boldness. The behavioral effects of parasites that we observed were similar in magnitude to the behavioral effects of predators. Our results suggest that parasites can substantially alter aquatic communities merely by modifying host behavior.