AGGREGATION AND THE DYNAMICS OF HOST-PARASITOID SYSTEMS - A DISCRETE-GENERATION MODEL WITH WITHIN-GENERATION REDISTRIBUTION

The hosts of parasitoids are often distributed in patches across the e nvironment, and parasitoids often preferentially enter certain patches , either because they contain many hosts (density-dependent aggregatio n) or for reasons unconnected with host abundance (density-independent aggregation). There is a controversy about the importance of aggregat ion and patchiness in stabilizing the dynamics of host-parasitoid syst ems. Studies of discrete-generation models with no within-generation r edistribution 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 dest abilizing while density-independent aggregation has no effect on stabi lity. A mechanistic model is developed of a host-parasitoid system wit h discrete generations that allows within-generation redistribution. E xploration of this model suggests that, while density-dependent aggreg ation can contribute to stability, levels of within-generation movemen t that are realistic for most parasitoids can severely diminish its st abilizing influence. On the other hand, the stabilizing influence of d ensity-independent aggregation is unaffected by within-generation redi stribution.