Keystone Herbivores and the Evolution of Plant Defenses

Plants need to defend themselves against a diverse and dynamic herbivore community. Such communities may be shaped by keystone herbivores that through their feeding alter the plant phenotype as well as the likelihood of attack by other herbivores. Here, we discuss such herbivores that have a large effect on the interaction network structure with associated fitness consequences for the plant, as dominant agents of selection on plant defense traits. Merging the keystone herbivore concept with plant fitness and trait selection frameworks will provide an approach to identify which herbivores drive selection in complex multispecies interactions in natural and agricultural systems. Keystone herbivores affect the composition of plant-associated communities through plant-mediated species interactions and thus affect the integrative effect of the herbivore community on plant fitness.Through their role in mediating the outcomes of plant-herbivore community interactions, keystone herbivores are expected to be major agents of natural selection on constitutive and induced direct and indirect resistance although they may individually not significantly affect plant fitness. Thus, they are key to understanding evolution of plant defenses.Identifying keystone herbivores and measuring their plant fitness effects would be equivalent to identifying the fitness effect of plant-associated interaction networks and provide a shortcut to understanding diffuse (co)-evolution.