Nearly a century ago, Lotka and Volterra generated a pair of differential equations that inspired the development of a rich theory on predator-prey dynamics. However, despite clear similarities between predators and parasites, predator-prey theory is only rarely applied to parasite-host interactions. Predators can kill their prey (consumptive effects), thereby triggering density-mediated indirect effects (DMIEs) on species with which prey interact. Alternatively, predators can cause their prey to adopt defensive strategies (non-consumptive effects), thereby triggering trait-mediated indirect effects (TMIEs) on species with which prey interact. Here I apply this framework to understand direct and indirect interactions between parasites and hosts. Parasites can kill their hosts, thereby triggering DMIEs on species with which hosts interact. Parasites can also cause their hosts to adopt defensive strategies that reduce the risk of infection, thereby triggering TMIEs on species with which hosts interact. However, unlike predators, parasites are not necessarily lethal to their victims, and even when they are, significant time lags typically occur between infection and death. Therefore, in addition to causing DMIEs through their consumptive effects and TMIEs through their non-consumptive effects, parasites can also trigger TMIEs through their consumptive effects.
Results/Conclusions
I will introduce five types of infectious consumers (parasitoids, parasitic castrators, pathogens, macroparasites, trophically transmitted parasites) and distinguish them from predators based on four dichotomies. Although parasites are appreciated mainly for their consumptive effects (i.e., infection), like predators, they can also cause non-consumptive effects. Furthermore, unlike predators, infectious agents can change victim traits through both their consumptive and non-consumptive effects. Next I will describe three food web modules in which infectious agents trigger indirect effects (consumptive DMIEs, consumptive TMIEs, non-consumptive TMIEs) on species (resources, consumers, others) with which hosts interact. I will illustrate each type of indirect effect with examples from the literature, and discuss their relative frequency and strength. Like the direct and indirect effects of predators, direct and indirect effects of parasites are likely to have important population-, community-, and ecosystem-level consequences. For instance, alteration of host traits creates diverse phenotypes within host populations, thereby expanding ecological niches available to species with which hosts interact. Because parasites are capable of altering host traits through both their consumptive and non-consumptive effects, they could be more likely than predators to maintain biodiversity in ecosystems.