Herbivory reduces the positive effects of elevated CO2 on plant growth and reproduction

Background/Question/Methods: Global environmental changes, such as rising atmospheric CO₂ concentrations, can impact plant populations directly. Indirect effects also are likely to occur, however, if the environmental change alters the intensity or likelihood of interactions with other species in the community. Rising CO₂ concentrations have been hypothesized to alter plant-herbivore interactions because of the changes in plant quantity and quality that occur when plants are grown under elevated CO₂ (eCO₂). Several empirical studies support this hypothesis, with herbivores frequently consuming more plant material under eCO₂ conditions. Because of the increased damage from herbivore antagonists, one might expect a strong, negative herbivore-mediated indirect effect of eCO₂ on plant fitness. The magnitude of these indirect fitness effects could either be reduced or magnified, however, if global changes such as eCO₂ not only change the intensity of species interactions, but also impact the fitness consequences of these interactions. Here we investigated how rising CO₂ concentrations impact both the intensity of plant-herbivore interactions and the fitness consequences of these interactions to obtain a more complete understanding of how eCO₂ affects plant fitness in complex communities. We used insecticide to manipulate the amount of insect herbivory and simulated browsing by clipping apical meristems on Arabidopsis thaliana plants grown under ambient CO₂ or eCO₂ conditions.

Results/Conclusions: We found no evidence that eCO₂ altered the amount of damage plants received, but plants grown in eCO₂ were less tolerant of herbivory – simulated browsing reduced biomass and fruit production by 13% and 22% respectively when plants were reared under eCO₂, but plants fully compensated for browsing in ambient CO₂ environments. These results illustrate how environmental perturbations can indirectly impact species not only by altering the intensity of species interactions, but also by altering the fitness consequences of those interactions.