Parasite-mediated predation determines the infection in a complex predator-prey system

The interplay of host-parasite and predator-prey interactions is critical in ecological dynamics because both predators and parasites play an important role in regulating populations and communities. But what is the prevalence of infected prey and predators when a parasite is transmitted through trophic interactions, particularly when stochastic fluctuations of demographical changes are allowed arising from individual-level dynamics? Here, we analysed the system stability and the frequency of infected and uninfected host subpopulations in a complex predator-prey-parasite system, where infection happens through trophic interactions transmitting parasites from prey to predators. We varied the parasite virulence implemented as reproductive costs imposed on infected hosts and the probabilities of parasites infecting the hosts per encounter, to investigate how those important evolutionary factors will determine the species coexistence and population composition. We further explored the role of stochasticity in our system by comparing our deterministic analysis with stochastic simulations. Our results show that parasites go extinct when the infection probabilities of either host are small. The success in infecting the final host (the predator) is more critical for the survival of the parasite species, as the threshold for infection probability of the predator is higher than that of the prey for three-species coexistence. While our stochastic simulations agree with deterministic predictions well in most parameter regions. However, in the border parameter regions between coexistence and extinction typically with high infection probabilities, while only one possible outcome in deterministic dynamics, both coexistence and extinction can happen in stochastic repeats under the same parameter values. This illustrates the importance of stochasticity and demographic fluctuations in species coexistence. In addition, the proportion of infected individuals increases with the infection probabilities in our deterministic analysis and stochastic simulations as expected. Interestingly, we found that in some parameter space, the relative frequencies of infected and uninfected individuals are different between the intermediate host (prey) and the final host (predator) populations. This counterintuitive observation shows that the interplay of host-parasite and predator-prey interactions lead to more complex dynamics than a simple resource-consumer relationship.