Herbivory and allelopathy contribute jointly to the diversity-invasibility relationship

Although herbivory and allelopathy play important roles in plant invasions, their roles in mediating the effect of plant diversity on invasion resistance remain unknown. We performed a two-year field experiment in which we manipulated plant-species richness, herbivory and allelopathy, and then experimentally invaded the plots with the invasive Solidago canadensis. One year after the start of the experiment, there was no significant effect of species richness on biomass of the invader, as a negative indirect effect of species richness via increase light interception was compensated by a positive direct effect. However, in the second year, the relationship was negative, in line with the diversity-invasibility hypothesis. The negative relationship was strongest when herbivory and allelopathy were not manipulated. Therefore, we conclude that joint effects of herbivory and allelopathic interactions between the native plants and the invader play a major role in the observed species richness-invasibility relationship.