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.