Indirect Effects Explain the Role of Parasites in Ecosystems.

Parasites are increasingly recognized as integral members of ecological communities, but their ecological effects remain less clear. Here, I propose that, to uncover the unique role of parasites, we must understand their indirect effects, which differ in important ways from those caused by predators. Similar to predators, parasites can cause density-mediated indirect effects (DMIEs) through their consumptive effects, and trait-mediated indirect effects (TMIEs) through their nonconsumptive effects; however, because they can consume a host without killing it, parasites can also trigger TMIEs through their consumptive effects. I consider the relative importance of each parasite-induced indirect interaction type and demonstrate their population-, community-, and ecosystem-level consequences. This paper contributes to recent efforts to unite predator–prey and parasite–host theory under a general consumer–resource framework. Similar to predators, parasites can trigger DMIEs through their consumptive effects, and TMIEs through their nonconsumptive effects. Unlike predators, parasites can also trigger TMIEs through their consumptive effects. Parasite-induced indirect effects can reverberate at the population, community, and ecosystem levels. If the receiver of one indirect effect initiates another indirect effect, an indirect interaction web can result. By altering host traits, parasites create diverse phenotypes within host populations, thereby expanding ecological niches available to species that interact with their hosts. Thus, parasites might facilitate coexistence and contribute to the maintenance of biodiversity. Applying predator–prey theory to parasite–host dynamics can yield novel insights. [ABSTRACT FROM AUTHOR]