Consequences of body size variation among herbivores on the strength of plant–herbivore interactions in a seasonal environment

Classical theory in community ecology assumes that smaller-scale details such as individual traits can be abstracted safely and that community dynamics can be simply characterized in terms of net changes in population densities. Here we use a mechanistic simulation model of a three-level food web to explore the effect of initial body size variation among herbivores on final plant abundance resulting from a non-linear relationship between population demography and body size. We show that initial herbivore body size variation has a negative effect on their survival and consequently a positive effect on the final plant biomass. We then use trait distribution, in combination with body size-survival and body size-fitness curves estimated through simulations, to generate predictions for comparison with observed food web effects. We show that, owing to frequency-dependence, our ability to predict herbivore population dynamics is limited. However, at the community-level, this frequency-dependence, as well as changes in herbivore population size, can be abstracted safely and the strength of plant–herbivore interactions can be simply predicted from initial body size distribution in combination with the survival curve. Our findings suggest a need to revisit classical theory in community ecology. Doing so will require the mechanistic study of population demography and experimental testing of the effect of trait variation on community dynamics.