Understanding how top-down and bottom-up effects interact is integral to predicting how biological communities are structured. Because of the complexity of the processes regulating community structure, there may be interactions among factors that regulate top-down and bottom-up effects. However, few studies have explored the interactions of more than two factors. We examined how salinity stress (addition and control), nutrient levels (addition and control), the presence/absence of a top omnivore and the presence/absence of two herbivore species interacted to alter top-down and bottom-up control of a salt-marsh trophic system using a mesocosm experiment.  

Results/Conclusions

We found that bottom-up effects were strong and predictable at the plant level but that their effects at the herbivore level were variable. The omnivorous crab had weak direct top-down effects on plants but strong direct top-down effects on herbivores and strong indirect top-down effects on plants due to its flexible diet. While bottom-up forces did not alter trophic cascades, direct top-down forces on plants and herbivores were altered in complicated ways by bottom-up forces. Finally, side-to-side interactions (competition) affected the strength of top-down forces coming from the omnivore and from the herbivorous beetle. The results from our study show how bottom-up, top-down and side-to-side factors can interact in complicated ways to determine the structure of a community, and illustrate how the development of a predictive model of community structure can be complicated by the presence of interactions among regulating factors.