Dilution effects enhance variation in parasitism risk among hosts and stabilize host–parasitoid population dynamics

In host–parasitoid interactions, parasitism risk experienced by each host may decrease as the host density increases (referred to as the dilution effect) when parasitoids can parasitize a limited number of hosts at a given time no matter how abundant hosts are. In the relationship between parasitism risk and host–parasitoid dynamics, the dilution effect is generally considered as a destabilizing factor, whereas variability in parasitism risk among hosts is regarded a stabilizing factor. However, although the dilution effect and variability in parasitism risk have been studied in isolation, the simultaneous expression of these two factors in host–parasitoid systems has not been investigated. This study examines the dilution effect in a patchy environment using a simulation model, with a specific focus on the CV2>1 rule (which states that CV2>1 is a condition for stability where CV is the coefficient of variation in parasitism risk among hosts). The simulation model shows that the CV2>1 rule is a good predictor of stability of the model regardless of the presence or absence of the dilution effect. It was also found that the dilution effect increases CV2 and stabilizes the host–parasitoid dynamics. ? 2020 Elsevier B.V.