Assistant Professor Purdue University West Lafayette, Indiana, United States
Background/Question/Methods
How plants navigate intra- and interspecies competition while surviving attack from herbivores is key to understanding community structure, ecosystem function and economic production from plant products. Plants have plastic growth and allocation responses to both competition and herbivory that can enhance survival and reproduction. Niche theory predicts that diverse communities should be more productive than monocultures, and many experiments have confirmed this hypothesis. Here we ask: how does herbivory from a specialist interact with plant-plant competition to shape productivity in monocultures and mixtures? To address this question, we used a game theoretic model that included both plant-plant competition and herbivory. The specialist herbivore only attacks one plant species. The game theoretic model explores the effects of plant competition on growth and resource allocation of shoot, roots, and leaves. Further, the model explores how plants endure an herbivore specialist predation on leaf tissue. Plants have a compensatory growth response in their leaves, but this reduces production in stems and roots. For this model the evolutionarily stable strategy (ESS) maximizes plant and herbivore fitness by simultaneously considering herbivory, and root, stem, and leaf growth. Additionally, to examine the productivity – diversity relationship the model has monoculture and multi-species polyculture communities.
Results/Conclusions
We found that the ESS in all communities was to maintain somewhat similar root production regardless of the type of competition or herbivory. While communities with herbivory produced slightly less than communities without herbivory, there was little difference between monoculture and mixture communities. Interestingly, the ESS stem production in mixtures with herbivory was higher than mixtures without herbivory resulting in a compensatory response. In addition, the ESS stem production in mixtures was considerably more than monoculture. The ESS leaf production was highest in mixtures without herbivores, lower with herbivores, and lower still in monocultures. Using this model we gain an understanding of the interaction between plant competition along a diversity gradient and herbivory. Niche partitioning still resulted in a positive diversity productivity relationship, but specialist herbivory lowered the effect. These results are impactful considering the number of specialist invasive pests that have been introduced into forests around the United States. Indeed, the forest products industry is one of the top ten manufacturing employers in 48 states and generates over $200 billion a year in sales and $54 billion in annual payroll. With such an economically significant industry the importance of plant community diversity cannot be understated.
