Elevated carbon dioxide concentrations indirectly affect plant fitness by altering plant tolerance to herbivory.

Global environmental changes, such as rising atmospheric CO(2) concentrations, have a wide range of direct effects on plant physiology, growth, and fecundity. These environmental changes also can affect plants indirectly by altering interactions with other species. Therefore, the effects of global changes on a particular species may depend on the presence and abundance of other community members. We experimentally manipulated atmospheric CO(2) concentration and amounts of herbivore damage (natural insect folivory and clipping to simulate browsing) to examine: (1) how herbivores mediate the effects of elevated CO(2) (eCO(2)) on the growth and fitness of Arabidopsis thaliana; and (2) how predicted changes in CO(2) concentration affect plant resistance to herbivores, which influences the amount of damage plants receive, and plant tolerance of herbivory, or the fitness consequences of damage. We found no evidence that CO(2) altered resistance, but plants grown in eCO(2) were less tolerant of herbivory-clipping reduced aboveground biomass and fruit production by 13 and 22%, respectively, when plants were reared under eCO(2), but plants fully compensated for clipping in ambient CO(2) (aCO(2)) environments. Costs of tolerance in the form of reduced fitness of undamaged plants were detected in eCO(2) but not aCO(2) environments. Increased costs could reduce selection on tolerance in eCO(2) environments, potentially resulting in even larger fitness effects of clipping in predicted future eCO(2) conditions. Thus, environmental perturbations can indirectly affect both the ecology and evolution of plant populations by altering both the intensity of species interactions as well as the fitness consequences of those interactions.