Herbivory as a continuous state variable in an IPM: Increasing herbivory decreases population growth of Asclepias syriaca through its effects on clonal reproduction

Herbivory is among the most well-studied biotic interactions, yet most studies do not incorporate effects on both sexual and clonal plant reproduction or the consequences of different amounts of tissue lost, i.e., herbivory severity. We address both of these gaps using a novel extension of an Integral Projection Model of Asclepias syriaca that uses both plant size and herbivory severity as continuous predictors of ramet population growth rate. Herbivory severity was a significant predictor of survival, growth, as well as sexual and clonal reproduction. We saw these effects using both observational data from across seven sites and five years as well as an experimental approach where we removed plant tissue. Increases in all three aspects of herbivory (probability of herbivory, and the mean and the variation among individuals in herbivory severity) led to decreases in population growth. Population growth rate decreased with herbivory largely due to negative effects of herbivory on clonal reproduction. Our approach to IPMs offers a powerful way to understand the individual-level effects of several aspects of herbivory on plant population growth.