In certain traditional models, plants have been viewed as passive participants. There is now strong evidence, however, that plants are far from passive in their interactions with herbivores. In this paper, Rhoades reviews the evidence that plants are defended against herbivores, that plant nutritional quality can vary as a function of physical stress of plants and degree of herbivory experienced by plants, and that defensive communication between plants exists. Rhoades postulates that acquired immunity to herbivores may be an important component of plant defense. Finally, he suggests that there may be two alternative strategies of herbivores to counter plant defensive systems. These strategies may explain why some herbivore populations are variable whereas others are relatively invariant, why some species of herbivores exhibit phase polymorphism, and together with other considerations, why it has proved difficult to demonstrate competition between herbivores.
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