Interactive effects of rhizobia mutualism and herbivory on <i>Vicia faba</i>

Species constantly interact with many organisms simultaneously, and multiple species interactions can have unique effects when combined and potentially cause resource and fitness trade-offs to the host. For example, extrafloral nectar can be an effective defence against herbivory, yet this nectar production can be influenced by abiotic and biotic environmental factors, including other mutualisms. Here, we tested how traits associated with two different types of species interactions, herbivory and rhizobia-induced nodulation, may independently and interactively affect a host plant. We manipulated the presence or absence of rhizobia at two different levels of herbivory in the field. We measured multiple plant growth traits and fitness responses, such as plant height, above- and below-ground biomass, total biomass, the ratio between above- and below-ground biomass, rhizobia nodule number, and average nodule size, extrafloral nectar volume, extrafloral nectar sugar concentration, as well as ant and arthropod visitation to explore the potential ant-plant mutualism. Rhizobia inoculation did not affect ant or other arthropod visitation but did alter plant traits associated with growth as well as decreased allocation to sugar concentration within extrafloral nectar. Extrafloral nectar volume remained nearly constant in nearly all treatments, except for a decrease when plants did not have rhizobia and were not subjected to herbivory. Finally, increased herbivory led to reduced nodule size but did not alter nodule number, indicating potential shifts in resource allocation. Overall, multiple species interactions have compounding effects when combined, and plant traits alter differentially in response. In particular, we found a substantial benefit of rhizobia to the plants which did not result in either higher ant visitation or herbivory. However, greater herbivory did reduce plant growth, yet these effects varied depending on the presence or absence of rhizobia. This study demonstrates the importance of considering traits associated with multiple diverse species interactions simultaneously to better understand plant responses to environmental conditions.