Evolution of induced plant responses to herbivores

Commensurate with the increased frequency of cost-of-defense studies, the conventional wisdom that inducibility is a cost saving strategy is enjoying growing empirical support. However, there may be other advantages to inducibility that have yet to be confirmed. To identify them, I consider the disadvantages of high levels of constitutive defense in the context of herbivores differing in response to the defense and in the context of potential predators and parasitoids of herbivores. Among the potential benefits of inducibility are that it allows herbivore-specific targeting, reliable signaling to predators/parasitoids, avoidance of kairomonal use by specialist herbivores, and avoidance of adverse effects of direct defense on third trophic level organisms. Inducibility also enables local and systemic variation in defense responses. These factors may figure prominently in certain systems, but I predict that their quantification will prove to be a more daunting challenge than the quantifying of costs. Moreover, there may be constraints on the evolution of inducibility that limit the fine-tuning of responses necessary to realize many of these hypothetical benefits. Nevertheless, the growing body of detailed knowledge of the physiology of multiple induction pathways will provide opportunities for testing some of these hypotheses in different systems.