Predation on infected host promotes evolutionary branching of virulence and pathogens' biodiversity

Traditionally, theoretical works on the evolution of virulence of wildlife infections have focused on interactions between just the host and its parasite. In a large number of study cases, however, infected host individuals also incur severe mortality due to predation of higher trophic levels. Such mortality should be virulence-dependent since the population size of predators is determined by the available amount prey they consume, which, in turn, is a function of pathogen virulence. The potential role of trophic pressure by predators in the evolution of virulence of their prey remains largely unaddressed in the literature. Here we investigate the possible role of predators in promoting biodiversity and disruptive evolution (evolutionary branching) of pathogen strains infecting the prey that those predators consume. Our theoretical study is based on principles of adaptive dynamics and evolutionary game theory. With the help of a fairly simple model we demonstrate that predation on infected prey can result in evolutionary branching of pathogen virulence, which would be impossible in the same system without predators. We show that predator-mediated evolutionary branching can occur within a large range of species life-history traits and for various types of transmission-virulence trade-off relation. We argue that predation can play an important role in explaining the existing polymorphism and biodiversity of pathogen strains in wildlife.