A simple plant mutation triggers a predator-diversity trophic cascade

Resource-use efficiency often increases with greater consumer biodiversity. This could result either from niche partitioning among consumers or the attributes of particular key species, and it is often difficult to differentiate between these two mechanisms. Differences in foraging behavior among species provide one obvious route to differing resource use, and we exploited a simple plant mutation (reduced production of surface waxes) to attempt to alter foraging by a community of aphid-predator consumers. We found that greater predator species richness dramatically increased prey suppression and plant biomass, but only on mutant, reduced-wax plants. On the waxier normal plants predator diversity never impacted prey densities or plant biomass. Thus, a change in plant surface structure acted to turn on, or off, the cascading effects of predator diversity. High predator diversity encouraged higher densities of true predators, but was not beneficial for the aphid parasitoid Aphidius ervi; surface wax did not impact densities of any natural enemy species. Behavioral observations indicated that although A. ervi was less likely to forage within diverse-predator communities, low-wax plants mitigated this interference by encouraging generally greater parasitoid foraging. Thus, only predator-diverse, low-wax plants encouraged simultaneous strong impacts of both true predators and parasitoid. Only the parasitoid initiated escape behaviors by aphid prey, likely an important non-trophic effect, providing further evidence of a unique functional role for A. ervi. In summary, in our study species-richness and species-identity effects were complexly interwoven, suggesting that it may sometimes prove impossible to fully separate the two.