Direct and ecological costs of resistance to herbivory

Herbivores can consume significant amounts of plant biomass in many environments. Yet plants are not defenseless against such attack. Although defenses might benefit plants in the presence of herbivores, herbivore attack varies both spatially and temporally, and the expression of plant resistance to herbivores can be costly in the absence of plant enemies. Costs can be described as allocation costs, resource-based tradeoffs between resistance and fitness, or as ecological costs, decreases in fitness resulting from interactions with other species. Here, we update the seminal 1996 Bergelson and Purrington review of resistance costs and find that many more studies have documented costs of resistance (sensu lato) than found during the 1996 survey. Eighty-two percent of studies in which genetic background is controlled, demonstrate significant fitness reductions associated with herbivore resistance. We categorize studies by type of resistance, induced or constitutive, by type of cost, and also by the degree to which investigators controlled for genetic background. Recent work has commonly detected both direct resistance costs, such as resource-based tradeoffs, and ecological costs, which depend on interactions with other species.