Density-dependent distributions of hosts and parasitoids resulting from density-independent dispersal rules: implications for host-parasitoid interactions and population dynamics

Background: The distribution of hosts and parasitoids across patches is a key factor determining the dynamics of host-parasitoid populations. To connect behavioral rules with population dynamics, it is essential to comprehend how individual-level dispersal behavior influences the distribution of individuals. Typically, a simple deterministic model has been used to describe this connection. This study explicitly derived the relationship between individual-level dispersal behavior and the distribution of individuals across patches, contrasting it with the conventional deterministic model.

Methods: A stochastic individual-based model was developed from a widely used deterministic host-parasitoid population model. Individual-level dispersal rules were simulated in the stochastic model without assuming the resulting distributions. The models assume that the dispersal of hosts and parasitoids is independent of conspecific density. The deterministic model can be seen as an approximation of the stochastic model, describing the outcomes of stochastic processes as their expected patterns. In addition to describing the relationship between dispersal behavior and distribution across patches, its consequences for population dynamics were also examined.

Results: The stochastic model revealed that the distribution of individuals among patches varies with the number of dispersing conspecifics, whereas the deterministic model assumes independence from conspecific density, indicating that the deterministic model fails to capture the outcomes of stochastic dispersal. The resulting density-dependent distributions of hosts and parasitoids lead to other density-dependent interactions between them, such as density-dependent parasitism risk for hosts and density-dependent searching efficiency for parasitoids, ultimately affecting population dynamics. For instance, while aggregation of parasitoids is stabilizing in the deterministic model, it can be both stabilizing and destabilizing in the stochastic model.

Conclusions: The stochastic model revealed that density-dependent distributions of hosts and parasitoids emerge when individuals disperse in a density-independent manner, significantly impacting existing host-parasitoid theory, which assumes density-independent distributions. To address this, the implications of emerging density dependencies for well-known results, such as the pseudointerference of parasitoids and the CV² > 1 rule, were discussed. Explicitly considering individual-level dispersal behavior is essential for understanding host-parasitoid interactions and population dynamics.