PLANT DEFENSES AND OPTIMAL FORAGING BY MAMMALIAN HERBIVORES

We examined, using an optimal-foraging model, the conditions under which particular plant defenses (e.g., structural, digestive-inhibiting chemicals, toxic chemicals, and nutritional content) should be effective in reducing intake of plants by mammalian herbivores. We demonstrated that no type of plant defense can guarantee reduced consumption, because defense efficacy depends not only upon other characteristics of the defended plant (e.g., its nutrient content, its passage rate through the herbivore's digestive tract, and its ability to be harvested by the herbivore), but also the characteristics of other plants available to the herbivore in the environment. Consequently, some defenses will be effective in some environments, but not others, making generalizations about the efficacy of defenses conditional.