Herbivory can increase plant fitness via reduced interspecific competition. Evidence from models and mesocosms

Herbivores are generally considered to reduce plant fitness. However, as in natural communities herbivores often feed on several competing plant species, herbivores can also increase plant fitness by reducing interspecific competition among plants. In this study, we developed a testable model to predict plant fitness in the presence of an interspecific competitor and an herbivore that feeds on both plant species. Our model allows to predict at which herbivore and competitor densities the focal species will benefit from herbivory. This can be estimated by quantifying the effects of the herbivore on the fitness of the focal plant and on its competitor, and by estimating the levels of intra- and interspecific competition in a pair-wise fashion, respectively. We subsequently validated the model in indoor microcosms using three interacting species: an aquatic macrophyte (the giant duckweed Spirodela polyrhiza), its native competitors (green algae), and its native herbivore (the pond snail Lymnaea stagnalis). Additional outdoor mesocosm experiments supported our model under natural conditions. Together, this study provides a conceptual framework to understand how herbivores shape plant fitness in a community context.