Community-wide trophic cascades and other indirect interactions in an agricultural community

A current goal in ecology is to elucidate the relative roles of primary and secondary consumers versus plant resources in determining community structure and dynamics. The complexity and diversity of terrestrial communities has been hypothesized to strongly influence the strength of these top-down and bottom-up forces, and in turn, trophic interactions are predicted to have significant consequences for species diversity within trophic levels. To examine the relative strengths of top-down and bottom-up trophic interactions and their relationship to arthropod diversity in an agricultural ecosystem, we conducted experiments in two alfalfa fields with different management regimes in which we manipulated light availability, nutrients, and arthropod abundance. We employed path analysis to examine how variation generated by these manipulations influenced top-down and bottom-up trophic pathways, focusing on alfalfa productivity and the abundance and species richness of arthropods. These analyses revealed a number of complex interactions between treatments and each of the three trophic levels. Shade structures had a strong effect on enemy, herbivore, and plant trophic levels, but much of the effect on plant biomass appeared to be mediated indirectly through changes in enemy and herbivore abundance. Potential positive effects of nutrient addition on alfalfa biomass were negated due to increases in abundance of soil microbes in the intensively managed field and herbivores in the weedy field. In the intensively managed, low arthropod abundance field, alfalfa biomass increased with herbivore diversity. However, in the lightly managed fields, herbivore diversity, which increased significantly with nutrient addition, reduced alfalfa biomass. The indirect top-down and bottom-up effects uncovered in this experiment were strong, but were not limited to the classic trophic cascades that have been the subject of intense recent investigation.