Antagonistic interactions between competition and insect herbivory on plant growth

1 Although substantial theoretical work suggests that competition and herbivory should exhibit a wide variety of interactions in their effects on plant growth, empirical studies have shown that the predominant interactions are simply multiplicative. 2 To determine both the relative strengths of, and interactions between, competition and herbivory, we conducted a field experimental study in a native grassland using four focal species: Koeleria macrantha, Coreopsis tinctoria, Linum lewisii and Helianthus petiolaris. 3 The effects of competition and herbivory on plant growth, biomass allocation and survival varied among species. When effects were present, neighbouring vegetation reduced plant growth and survival, and insect herbivores decreased relative biomass allocation to roots and increased plant mortality. 4 Although reduced herbivory caused by insecticide application had little direct effects on plant biomass, it did interact with competition to affect growth for three of the four species. Herbivory reduced the strength of competition experienced by Coreopsis, but increased it in Linum. For both Coreopsis and Helianthus, the combined effects of herbivory and competition on plant growth were less than expected from a simple multiplicative response. 5 We suggest that interactions were found because we used an experimental design that modified insect densities on both neighbours and the focal plant, allowing for determination of both direct and indirect effects. 6 We suggest that the antagonistic interactions between competition and herbivory may have occurred because herbivores altered the competitive environment by harming the neighbouring plants, or because the presence of neighbours facilitated focal plant growth by distributing herbivore loads over the greater amount of plant biomass available. 7 Although as an isolated factor competition is more intense in this system than insect herbivory, herbivory can alter the strength of competitive interactions and thus its importance cannot be determined by measuring intensity alone. This result emphasizes that these two ecological processes are not truly discrete, and further suggests that studies that simultaneously manipulate multiple ecological processes are needed.