Parasitoid Competition and the Dynamics of Host-Parasitoid Models

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Abstract
The standard host-parasitoid models assume that each parasitized host yields the same number of parasitoid progeny; but in many parasitoid species, the number of progeny per host depends on the number of times that host was encountered. I present a family of new host-parasitoid models incorporating this encounter-density-dependent variability in parasitoid reproduction per host. Such within-host competition (which could be between ovipositing females, between larvae, or both) interacts with the distribution of encounters (i.e., aggregation) to determine the net parasitoid density dependence. Greater density dependence increases host equilibrium abundances while generally increasing the stability of the interaction. Overcompensating scramble-like parasitoid competition can be more stabilizing than contest-like competition, especially when the host has a moderate to high intrinsic rate of increase and encounters are moderately aggregated; overcompensation is destabilizing only at very high host reproductive rates. These results suggest that we must study parasitoid density dependence directly, both within a host and between generations within a population, in order to understand the dynamics of any actual host-parasitoid system. The most practical method appears to be to (1) describe the mean yield (parasitoid recruitment) per parasitized host as a function of the density of either parasitoids or parasitized hosts; and (2) relate the number (not the percentage) of parasitized hosts to parasitoid (and perhaps host) densities.

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