Optimal defense in a model incorporating ecological costs in a tri-trophic context
Plants exhibit great diversity both in the traits that confer defense against herbivory and the degree to which they appear to invest in these traits. Although most theoretical explanations for variation in plant defenses rely upon allocation costs, there is a growing realization the ecological context in which a plant is growing may also influence the costs of plant defense. In this study, we hypothesize that plant defenses may be particularly costly when specialist herbivores are able to utilize plant traits, such as toxins or trichomes, to protect themselves from predation. We modify the resource availability model (Coley et al. 1985) to incorporate a term for predation, and we investigate the implications of variation in predation on optimal defense when plant defenses not only reduce plant growth and herbivory, but also reduce predation.
We find that when plant defense reduces predation such that the effect of defense is proportional to the degree of predation, variation in top down forces can generate variation in optimal defense. In these cases, environments with high maximal predation favor low levels of defense. In our revised model, the magnitude and shape of the defense-herbivory and defense-predation curves also affect optimal defense. Our model demonstrates the potential for the tri-trophic context to influence optimal defense in plants and highlights the importance of considering both top-down and bottom-up factors that may influence the evolution of plant defenses.