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Effect of rhizobia and nectar composition on ant interaction in Vicia faba
Ecological interactions can vary in strength depending on environmental conditions. The outcomes of these interactions are often dependent on the abiotic environment, but can also depend on the presence of other biotic species, including mutualists, which can further alter these effects due to trade-offs in nutrient allocation for multiple interactions. For example, some plants produce extrafloral nectar, a specialized nectar used to attract ants that will defend the plant against herbivory. If plants with extrafloral nectaries also have a second mutualistic relationship with the soil bacteria rhizobia, this second mutualism has been shown to alter the attraction of ants to the extrafloral nectar of the plant. This change in ant attraction could be caused by changes in nectar production or nectar composition, although the exact reason is unknown. Plants do alter extrafloral nectar production and composition for other reasons, for example if a plant is experiencing herbivory the plant will increase the quantity of the extrafloral nectar which will attract more ants. What is not well understood is what specific changes to the nectar, if any, cause it to become more attractive or if the attraction is simply due to a higher quantity of the extrafloral nectar. In my thesis research I explored how the chemical composition of extrafloral nectar, specifically nectar quantity and sugar, is affected by rhizobia mutualists, soil nutrients, and herbivory. Moreover, I explored how these changes influence the ant community and herbivorous arthropod community, as well as plant traits and overall fitness of the Vicia faba plant and its associated mutualistic rhizobia. I tested the effects of these factors through a series of greenhouse and field experiments, running the extrafloral nectar through a total carbohydrate assay to quantify carbohydrate (i.e. sugar) content. I found that under most conditions, the composition of the extrafloral nectar remained constant. However, when plants associated with the soil bacteria rhizobia, the rhizobia mutualism caused a decrease in sugar content in extrafloral nectar but increased the overall quantity of extrafloral nectar produced when outside of herbivory pressure. The reason for this could be due to limited resources, as the plant has a finite amount of carbon and has to allocate this carbon to both the extrafloral nectar and to rhizobia. This need for resource allocation and the altered quantity and composition of extrafloral nectar had no effect on plant fitness traits nor on ant or arthropod visitation in my study, although so few ants were present that conclusions on ant visitation effects cannot be made. These results allow for other new and exciting questions in plant and insect ecology, such as what effect these resource allocations have on plant competition, growth, fitness, and how it impacts the broader ecological community.
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