Knowledge about what controls plant population size is fundamental to our understanding of natural systems, and is also at the root of applied problems involving weedy and invasive species. We are examining the role that herbivores play in the population dynamics of a weedy plant, Solanum carolinense. To fully characterize herbivore effects, we are using an approach that incorporates herbivore effects on population growth rates and density dependence (change in growth rate with plant density) across the life cycle of the plant.

Results/Conclusions

We have documented effects of plant density on plant-herbivore interactions in this system. At higher densities, for example, plants receive more damage from herbivore communities in both natural and experimental populations. This pattern occurs even though females of some species prefer to lay eggs on plants growing at low densities, perhaps because plants are poorer quality food (as measured by bioassays of larval growth in two specialist herbivores) at higher densities. Here we present preliminary matrix models exploring the implications of these density-dependent interactions on S. carolinense’s population dynamics. We parameterized matrix models using data from the first year of a long-term factorial experiment manipulating both plant and herbivore densities. We identified the demographic transitions with the highest elasticity, and thus the greatest influence on population growth, and determined whether these transitions change with plant density or the presence of herbivores. These results are the first step toward evaluating whether herbivores affect the strength of density dependence in the plant and its carrying capacity.