The role of herbivory in determining plant population size is an unresolved question with important ramifications for applied problems involving weedy and invasive species. Although some recent studies have characterized the effects of herbivory on plant population growth rate, these studies have typically used population models that assume density independent growth, and thus cannot estimate herbivore effects on the equilibrium density distribution (or carrying capacity) of the plant. To test whether herbivores affect both the equilibrium density distribution and the population dynamics of a plant, we are incorporating density into matrix population models. We are parameterizing these models with data from experimental populations of the weedy perennial Solanum carolinense grown at different densities, with and without herbivores. Here we report effects of plant density on plant-herbivore interactions in this system, and preliminary demographic analyses based on the first two years of data (2007 and 2008).

Results/Conclusions

We find that at higher densities plants receive more damage from herbivores in both natural and experimental populations. Density also influences the quality of the plant as food for insect larvae, and both plant density and herbivore damage affect plant fitness. Using estimates from each experimental population, we have identified demographic transitions influenced by density and herbivory treatments. For example, for some transitions related to plant growth, the effects of density differed between plots with and without herbivores. We used these results to construct two matrices of average transitions across densities, one for plots with ambient herbivory and one for plots with reduced herbivory. We used fixed averages for the transitions that were not affected by density and density functions for the transitions that were influenced by density. These results are the first step toward evaluating whether herbivores affect the strength of density dependence in the plant and its carrying capacity.