Pernilla Borgström

Insect herbivores can shift the composition of a plant community, but the mechanismunderlying such shifts remains largely unexplored. A possibility is that insects alter thecompetitive symmetry between plant species. The effect of herbivory on competitionlikely depends on whether the plants are subjected to aboveground or belowgroundherbivory or both, and also depends on soil nitrogen levels. It is unclear how these bioticand abiotic factors interactively affect competition. In a greenhouse experiment, wemeasured competition between two coexisting grass species that respond differently tonitrogen deposition:Dactylis glomerataL., which is competitively favoured by nitrogenaddition, andFestuca rubraL., which is competitively favoured on nitrogen-poor soils.We predicted: (1) that aboveground herbivory would reduce competitive asymmetryat high soil nitrogen by reducing the competitive advantage ofD. glomerata; and (2),that belowground herbivory would relax competition at low soil nitrogen, by reducingthe competitive advantage ofF. rubra. Aboveground herbivory caused a 46% decreasein the competitive ability ofF. rubra, and a 23% increase in that ofD. glomerata, thusincreasing competitive asymmetry, independently of soil nitrogen level. Belowgroundherbivory did not affect competitive symmetry, but the combined influence of above-and belowground herbivory was weaker than predicted from their individual effects.Belowground herbivory thus mitigated the increased competitive asymmetry causedby aboveground herbivory.D. glomerataremained competitively dominant after thecessation of aboveground herbivory, showing that the influence of herbivory continuedbeyond the feeding period. We showed that insect herbivory can strongly influenceplant competitive interactions. In our experimental plant community, abovegroundinsect herbivory increased the risk of competitive exclusion ofF. rubra. Belowgroundherbivory appeared to mitigate the influence of aboveground herbivory, and thismechanism may play a role for plant species coexistence.