The Effect of Predation on Ant-aphid Mutualism in Ligusticum porteri

Predation and mutualism take up a large part of interactions within communities. These multi-trophic level interactions affect the makeup of the community. We explored how predation impacts the strength of mutualism between ants and aphids on the plant Ligusticum porteri. It has been a base assumption that ants mediate the effect of predation on aphids. We manipulated half of the plants by adding lygus bugs and measured ant density of each site. Through this study, it was discovered the presence of predators broke down the mutualism and disrupted it from forming (P < 0.0056). Ant density had no correlation to the efficiency of the mutualism forming or the strength of it (P = 0.360). Larger aphid colonies did attract more ants, and the larger the number of ants resulted in greater services for the aphids (P < 0.0056). Higher ant abundance resulted in a greater growth rate for aphid colonies. The predation impacts the mutualism potentially affecting aphid, ant and lygus abundance, as well as revealing how ants mediate the effect of predation. Introduction Top-down processes affect herbivore survival and their performance (Wimp and Whithman, 2001). Top-down studies have shown the effect of predators can influence multitrophic levels, such as herbivore abundance, plant growth, and plant reproduction (Moran et al., 1996). It is important to understand the top-down and bottom-up processes in trophic level interactions in order to grasp how the community is affected directly and indirectly (Wimp and Whitman, 2001). Predators have the potential to extirpate aphid population, therefore mutualism between ants and aphids can be crucial for survival. Ants perform a service by protection aphids from predators in return for honeydew (Ness et al., 2010). Mutualism allows both species in the interaction to have positive benefits and potentially expanding their distribution. In this case, mutualism is important for the survival, protection and reproduction of aphids. For ant’s honeydew is a valuable nutrient resource that contributes to their survival (Ness et al., 2010). Predators have the potential to alter this mutualism by changing ant and aphid behaviors (Mooney, 2006). An example of predation affecting the mutualism revealed birds reduced the abundance of ants as well as the quality of services they performed (Mooney, 2006). The birds had more of an effect on the aphids with a mutualism than the aphids without a mutualism. Other studies have also shown aphid populations can be affected positively and negatively by organisms of higher trophic levels (Wimp and Whitham, 2001). The performance of aphids would vary with the absence of predators and ants. Therefore, aphid’s realized abundance is altered by their mutualism with ants as well as the intensity of predation. The strength of the mutualism is influenced by the abundance and behaviors of other species (Boucher et al. 1982; Stanton, 2003). These species can alter the mutualism until it is no longer a mutualism, but a different relationship such as commensalism or a complete breakdown (Bronstein and Barbosa, 2002). In this system, there are a variety of trophic levels interacting and impacting abundance and behavior. These interactions take place on the host plant Ligusticum porteri, which has multiple flowering stocks that will go through a variety of phenological stages. As the flower progresses through these stages it will attract the aphids, as well as lygus bugs (Robinson et al.,