Predictable variation in food web structure of three guilds of natural enemies: Predators, parasitoids, and pathogens of aphids

Most communities of insect herbivores are unlikely to be structured currently by resource competition as they feed on non-overlapping resources, but they may be structured by apparent competition mediated by shared natural enemies. To assess the potential of three major guilds of natural enemies to influence aphid community structure through indirect interactions, separate fully quantitative food webs were constructed for parasitoids, pathogens and predators.  Based on the biology of the three guilds we predicted that the scope for apparent competition would be greatest for the predator and least for the parasitoid guilds. The webs were analysed using standard food web statistics designed for binary data, and using information-theory based metrics that make use of the full quantitative data. The parasitoid webs showed the lowest connectance and the predator webs the highest while the pathogen webs are intermediate.  These results are clearest when measures based on quantitative data are used. There is evidence that a single aphid species has a particularly large effect on the structure of the pathogen food web. The predator and pathogen webs are not compartmentalised, and the vast majority of parasitoids are connected in a single large compartment. It was concluded that indirect effects are most likely to be mediated by predators, a prediction supported by the available experimental evidence.