DENSITY-DEPENDENT AND DENSITY-INDEPENDENT PARASITOID AGGREGATION IN MODEL HOST-PARASITOID SYSTEMS

This article analyzes a model for host-parasitoid interactions with discrete generations to explore the different effects of density-dependent and density-independent parasitoid aggregation on population densities and dynamics. I demonstrate six points. First, density-independent parasitoid aggregation always decreases average parasitism rates and leads to greater equilibrium host densities. Second, density-independent parasitoid aggregation always stabilizes population dynamics. Third, an intermediate level of density-dependent parasitoid aggregation produces the greatest average percentage parasitism among patches and the lowest equilibrium host densities. Fourth, the effect of density-dependent parasitoid aggregation on stability depends on the functional response of parasitoids to host density within patches, with greatest instability associated with the lowest variability in the percentage parasitism among patches. When parasitoids show a type 1 functional response, greatest instability occurs when there is no density-dependent parasitoid aggregation, while type 2 functional responses give greatest instability at intermediate levels of density-dependent aggregation. Fifth, there is a necessary trade-off between instability and the ability of the parasitoid to depress the host population density for density-independent parasitoid aggregation but not for density-dependent parasitoid aggregation. Finally, self-regulation of the host population density can act in conjunction with parasitoid aggregation to stabilize population dynamics.