The effect of herbivores in structuring plant communities is a contentious issue in community ecology. Results from large-scale herbivore removal studies show effects of herbivores on plant species richness and abundance but the mechanisms by which herbivores influence plant community structure are not well understood. One mechanism is through changes in plant competition where herbivores selectively feed on one plant species, reduce its abundance, and allow its competitors to increase in abundance.  Results from small-scale factorial experiments are consistent with this mechanism; however results from such studies cannot predict long-term consequences for plant community structure as they simply determine whether main or interaction effects exist.  In order to effectively link results from small-scale to large-scale experiments, interaction strengths can be quantified from small-scale experiments and incorporated into models to examine long-term patterns of coexistence.  The goal of this project was to examine the effects of insect herbivores on competition between two common perennial plants. In 2007-2008, I estimated competition coefficients between Solanum carolinense and Solidago altissima in the presence and absence of insect herbivores using a response surface experimental design.  Response surface designs allow for more realistic estimates of competition coefficients by incorporating density dependence into parameter estimates.    
Results/Conclusions

Results showed that the presence of insect herbivores affected competition between these two old-field species, such that herbivores exacerbated the negative effects of competition on the inferior competitor, S. carolinense.  In the absence of insect herbivores, S. carolinense grew much larger, produced more fruit, and became more competitive (shown by a larger competition coefficient).  These coefficients can be incorporated into mathematical models to determine differences in long-term patterns of coexistence in the presence and absence of insect herbivores.  These results show that herbivory can affect long-term patterns of coexistence by accelerating rates of competitive exclusion.