Indirect effects and an invasive species: Complexity in a simple food-web

Indirect effects are known to play significant roles in structuring ecological food-webs. Despite this importance, few studies have highlighted the multiple outcomes of indirect effects in food-webs incorporating an invasive species. Here, I investigate these outcomes using a simple food-web incorporating two prey species, the invasive soybean aphid and the resident pea aphid, and a mobile, generalist predator that preys upon both, Harmonia axyridis (Harmonia). Chapter One demonstrates that via an indirect effect known as apparent competition, an invasive species can negatively impact species outside the ecological system it invades. Since soybean aphid arrival in soybean fields, the abundance of Harmonia greatly increased. Increase in Harmonia abundance has lead to significant declines in pea aphid density in alfalfa fields. In Chapter Two, I demonstrate that the outcome of a simple, three-species, indirect effect can vary by altering the density and ratio of preferred and alternative prey, over short time scales. Harmonia exhibited prey preference for soybean aphid over pea aphid. When the densities of the two prey were manipulated, both positive and negative indirect effects were observed, but the magnitudes of the effects were not symmetrical; soybean aphid consistently had a greater effect on pea aphid. In Chapter Three, I increase the complexity of the food-web to four species by including a specialist parasitoid that attacks pea aphid. To determine the effect of the invasive soybean aphid on the parasitoid, I manipulated the presence and absence of both soybean aphid and Harmonia. When alone, soybean aphid and Harmonia had independent, negative effects on the parasitoid. However, when both soybean aphid and Harmonia were together, the density of Harmonia was significantly greater than when soybean aphid was absent. Greater Harmonia abundance led to a reduction of pea aphid. This negative indirect effect on pea aphid negated any independent effects of Harmonia or soybean aphid that could disrupt pea aphid control by reducing the effect of the parasitoid. By highlighting the magnitude and complexity of indirect effects in a very simple food-web, these studies show the importance of indirect effects in ecological food-webs, particularly those incorporating an invasive species.