The Effects of Inducible Plant Defenses on Herbivore Populations. 1. Mobile Herbivores in Continuous Time

Abstract

Recent empirical evidence suggests that many plants respond to herbivore damage by producing inducible defenses. Although several workers have suggested that inducible defenses may be responsible for fluctuation in some herbivore populations, little formal justification for this suggestion exists. We describe here a quantitative framework that may be used for understanding the effects of inducible plant defenses on herbivore population. In particular, we use procedures for modeling continuous, structured populations to examine the dynamics of interactions between inducible defenses and mobile herbivores. Our models indicate that inducible defenses can by themselves regulate herbivore populations under a wide variety of conditions and that, in conjunction with other regulatory agents (e.g., predators, parasitoids), they can significantly depress herbivore populations. However, only under unusual conditions can inducible defenses cause persistent flucuations in herbivore populations. Finally, our model suggests that herbivores cannot maintain heterogeneity in the level of inducible defenses within a plant population. These conclusions may not apply to sedentary or selective herbivores or to herbivores with discrete, nonoverlapping generations.