Aggregation and the Dynamics of Host-Parasitoid Systems: A Discrete-Generation Model with Within-Generation Redistribution

Abstract

The hosts of parasitoids are often distributed in patches across the environment, and parasitoids often preferentially enter certain patches, either because they contain many hosts (density-dependent aggregation) or for reasons unconnected with host abundance (density-independent aggregation). There is a controversy about the importance of aggregation and patchiness in stabilizing the dynamics of host-parasitoid systems. Studies of discrete-generation models with no within-generation redistribution suggest both forms of aggregation are stabilizing, while studies of models with overlapping generations and frequent movement between patches suggest density-dependent aggregation is normally destabilizing while density-independent aggregation has no effect on stability. A mechanistic model is developed of a host-parasitoid system with discrete generations that allows within-generation redistribution. Exploration of this model suggests that, while density-dependent aggregation can contribute to stability, levels of within-generation movement that are realistic for most parasitoids can severely diminish its stabilizing influence. On the other hand, the stabilizing influence of density-independent aggregation is unaffected by within-generation redistribution.