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 maybe interactions among factors mediating top-down and bottom-up effects. We examined how salinity stress, nutrient levels and the presence of a top omnivore interacted to alter top-down and bottom-up effects in a salt marsh trophic system using a mesocosm experiment. We hypothesized that salinity stress would reduce the positive effects of nutrient addition on top-down forces. We found that the top-down control of Prokelisia sp. planthoppers by the omnivore grasshopper and the top-down damage of Prokelisia sp. planthoppers on Spartina alterniflora were both strong, independent of the bottom-up conditions. However, herbivory by the omnivore grasshopper was increased by nutrient addition if Prokelisia sp. was present. This was likely due to the fact that nutrient addition and Prokelisia sp. presence both reduced the leaf toughness of the plant. We also found that either salinity stress or nutrient addition alone increased Prokelisia abundance. However, when both factors were present, Prokelisia abundance did not increase further. This result is inconsistent with the environmental stress hypothesis but is consistent with the plant vigor and plant stress hypotheses. We conclude that generalizations about the regulation of the relative strength of top-down and bottom-up forces are difficult due to the presence of interactions and more studies examining such interactions will be needed to produce general paradigms.
