Stable coexistence in insect communities due to density- and trait-mediated indirect effects

Density-mediated and trait-mediated indirect interactions between species may have important roles in structuring ecological communities. Here we dissect their contributions to community stability in a model herbivore-natural enemy interaction consisting of two aphid species (Acyrthosiphon pisum and Megoura viciae) and a specialist parasitoid (Aphidius ervi) that attacks only one of the aphids (A. pisum). In replicated cage experiments, we found that the two aphid species alone were unable to coexist, with A. pisunt competitively excluding M. viciae. We also found that the simple host-parasitoid interaction between A. pisum and the parasitoid was unstable. However, the three-species community persisted for at least 50 weeks. We constructed a series of models to explain the stability of the full community and conclude that it is due to a combination of density-mediated and trait-mediated indirect interactions. Parasitoid attack on the susceptible host reduces the interspecific competition experienced by the non-host (a density-mediated effect), and the presence of the non-host reduces the searching efficiency of the parasitoid (a trait-mediated effect).