Positive species interactions (e.g , mutualisms, symbioses) play a principal role in determining the distribution and abundance of organisms. Mutualistic interactions are widespread in all ecosystems, involve diverse organisms, and can contribute to complex interaction networks in which interactions among closely associated species have cascading effects on associations between other species in the community. These mutualistic networks can have profound positive and/or negative effects on community composition, dynamics, and structure, though the dynamics of interaction networks remain poorly understood in most ecosystems. Furthermore, the outcomes of many positive species interactions are dependant on environmental conditions such that an interaction can be mutually beneficial under certain conditions, but negative in different environments or communities. It follows then that changes in environmental conditions may predictably alter the outcomes of positive interactions, with consequences for species distributions, abundance, and community structure. This research explored the dynamics of positive species interactions in order to better understand the factors that affect their strength, frequency, and stability. This project measured the effects of a plant-microbe mutualism on ant-aphid mutualism and arthropod abundance, providing a novel study of the effects of a "foundational mutualism" on higher order processes across multiple levels of the food-web. A well-described experimental system of soybean (Glycine max) strains was used to examine the direct and indirect impacts of belowground interactions (plant-microbe) on aboveground interactions (plant-insect, insect-insect). Two soybean strains were used: R+, which contained nutrient-fixing bacteria (rhizobia) in the plant roots; and R-, which did not harbor any nutrient-fixing microbes. The project addressed three main questions: 1) How do rhizobia affect ant-aphid mutualisms? 2) How do the indirect effects of plant-rhizobia interactions change in the presence of ant-aphid interactions? 3) How are arthropod community properties (richness, abundance, composition) affected by these above- and below-ground mutualisms? Preliminary data analysis has yielded surprising results in the context of previous studies of rhizobia and arthropod communities. Whereas rhizobia have been shown to affect plant traits and arthropod community traits, all effects (positive or negative) of rhizobia appear to be lost when aphids are included in the experimental setup. Preliminary results suggest that the presence of the ant-aphid mutualism had a buffering effect on the indirect effects of the plant-rhizobia mutualism. These results highlight the importance and complexities of species interactions and their effects on community properties.