By now, it should be clear that plants invest substantial resources in defense, resources that might otherwise be used for growth and reproduction. Given the considerable cost of plant defenses, their benefits must be correspondingly large. If a commonly encountered pathogen or herbivore is lethal, the resources invested in defense provide a clear reproductive advantage. But what if the pathogen or herbivore usually damages only half of a plant’s leaves? Or 10%? Or encounters the plant only infrequently?