Disentangling Resource and Apparent Competition: Realistic Models for Plant-herbivore Communities

We analyse a model food web in which two plant species compete for a nutrient and are attacked by a herbivore species, with both resource and apparent competition occurring between plants. Simple rules based on equilibrium nutrient concentration (R*) and herbivore density (H*) have been proposed to govern competitive outcomes (e.g. Holtet al., 1994,Am. Nat.144,741–771), but these earlier analyses assumed linearity of the nutrient-dependent plant growth function and the herbivore functional response. We now relax these assumptions. When plant growth is nonlinear, there is a rich set of potential trade-offs among plant competitive ability, plant maximal growth rate, plant defenses against herbivores, and preferences of herbivore attack on plants. Given stability, asymptotic outcomes can include exclusion of one plant species, priority effects wherein exclusion depends on initial conditions, and herbivore-mediated coexistence. Nonlinear herbivore functional responses potentially frustrate the application of simple rules for competitive outcomes. Nevertheless, there are circumstances in which simple rules appear to perform well. In these cases, graphical theory relates properties of food chains (nutrient, herbivore, and one plant species only) to asymptotic outcomes in food webs (with both plant species). Competitive outcomes in the graphical theory are summarized byR* andH* quantities. Our approach works best when plant species differ greatly in nutrient-dependent growth rates, and herbivores preferentially attack plants with high ratios of nutritional benefit to cost (measured by handling and processing times). A parameterized model ofDaphniaand several algal species suggests that our approach could perform well in practice.