Different Effects of Induced Plant Defense on Population Densities of Specialist and Generalist Insects: An Explanation by Mathematical Modeling.

In the wild, plant-insect interactions are highly influenced by plant defense chemicals. These chemicals (or toxins) are usually harmful to attacking insect herbivores, especially generalists. Plant defense is categorized into two types: Constitutive and induced defense. Constitutive defense is present in plants from germination on, while induced defense is activated by biotic stress, such as herbivory, or abiotic stress. This work focuses on modeling the multifaceted ecological roles of defense chemicals and investigates how the community structures of specialists and generalists in an ecosystem are determined by induced defense. The ecological roles of defense are very diverse. Specialist insects are well adapted to plant toxins and thus become less affected than generalists. Specialists are even attracted by the defense chemicals, although both insect groups could be killed at high concentrations. Moreover, natural enemies of insects are recruited by plants through toxins. In this study, these fundamental ecological functions of plant defense are expressed by a deterministic model based on nonautonomous ordinary differential equations. By analyzing this model, we obtain a stable non-zero equilibrium for specialists and a zero equilibrium for generalists at high concentrations of induced defense. Thus, it is proved theoretically that specialists are endemic and generalists are non-endemic in the wild, a common real-world situation, due to the defense induction in plants.