Herbivory is among the most well-studied biotic interactions, yet most studies do not incorporate both sexual and clonal reproduction nor do they differentiate the consequences of different amounts of tissue lost, i.e., herbivory severity. We present a new approach that uses both size and herbivory severity as continuous predictors of ramet population growth rate within an Integral Projection Model of Asclepias syriaca. We followed > 5000 marked stems across five years and six sites in Virginia and Pennsylvania to create an IPM. We applied perturbation analysis to determine the relative impact of parameters related to clonal versus sexual reproduction, the probability of being eaten, and herbivory severity on ramet population growth.
Results/Conclusions
Although herbivory had no effect on survival to reproduction, herbivory significantly decreased sexual and clonal reproduction. Population growth rate decreased as herbivory severity increased largely due to negative effects of herbivory on clonal reproduction. The elasticities for all aspects of herbivory were negative: increasing the probability of being eaten, the average severity of herbivory experienced by an individual and the variance in herbivory severity among stems decreased population growth rate. Most studies of the effects of herbivory consider only the presence or absence of herbivory. Our approach to IPMs offers a powerful way to incorporate the individual-level effects of herbivores as a continuous predictor of vital rates and plant population growth. By using herbivory severity as a continuous state variable within the IPM, we make the novel conclusion that the variance among individuals in the severity of herbivory may be an important component of the population-level consequences of herbivory.