Predicting Trophic Cascades in an Antagonistic/Mutualistic Web

In food webs, which are composed of antagonistic species interactions (with negative component effects), predators frequently suppress herbivores and indirectly benefit plants via "trophic cascades". Yet, ecological webs commonly contain multiple types of species interactions. Mutualistic species interactions (with only positive component effects) are ubiquitous across ecosystems and often co-occur with antagonistic relationships. A mechanistic understanding of cascades in ecological webs with both mutualistic and antagonistic species interactions is largely lacking, especially for cascades across four trophic levels. This dissertation focuses on how mutualistic interactions between predators and herbivores affect trophic cascades from fourth trophic level predators to plants. I used field experiments to evaluate the mechanisms leading to plant responses to predators in an ecological web that includes a mutualism between predacious ants and herbivorous treehopper insects. In this ecological web, ants have both mutualistic and antagonistic interactions with herbivores, resulting in a net effect of ants on plants that depends on the magnitude of positive and negative indirect pathways. These ant effects provide opportunities for testing the relative importance of antagonistic and mutualistic interspecific interactions for cascades. I tested the following predictions for cascades of effects on plants: 1) antagonistic interactions are more important than mutualistic interactions, 2) predators in the fourth trophic level will indirectly benefit plants, and 3) elevated nitrogen deposition will cause stronger effects from predators to plants. Among studies that have evaluated the effects of mutualisms on tri-trophic cascades, paths of antagonistic effects were generally considered stronger than indirect effects including mutualism. To test this expectation, I first demonstrated that ants simultaneously had a positive effect on treehoppers, and a negative effect on beetles, resulting in less beetle damage to plants hosting the mutualism. Then I compared interaction strengths through the entire insect herbivore community on plants with and without ants. The ant's mutualism with treehoppers was the sole strong interaction contributing to the net indirect effect of ants on plants. Predation, herbivory, and facilitation were weak, and the net effect of ants was reduced plant reproduction. This net indirect effect was also partially due to behavioral changes of herbivores in the presence of ants. An additional treehopper manipulation showed that the treehopper's effect on ant foraging was largely responsible for the ant's net effect on plants; ant workers were nearly ten times as abundant on plants with mutualists and effects on other herbivores were similar to those in the ant-manipulation experiment. This study shows that mutualistic interactions can be strong relative to negative direct interspecific interactions, and can drive indirect ecological dynamics. I next asked whether a predator in the fourth trophic level indirectly benefits plants in this ecological web. Fourth trophic level predators consume lower-level predators, which can indirectly harm plants when herbivores are released from predator suppression. However, I found that black bears, which are predators of ants, were beneficial for plants. Plants near bear-damaged ant nests had greater reproduction than those near undamaged nests, due to weaker ant protection for herbivores, which allowed herbivore suppression by arthropod predators. These results suggest that bears and other predators should be managed with the understanding that they can influence primary producers through many paths. Lastly, I asked whether low-level nitrogen (N) deposition alters the cascade from bears to plants. Widespread, low-level N deposition often enhances plant quality for herbivores and increases herbivory, but it is not known if this small anthropogenic nutrient enrichment affects trophic cascades. A previous study found that high levels of N enrichment caused this ant species to more strongly decrease beetle damage to plants; therefore I expected the indirect effect of bears on plants to become stronger under low-level N enrichment. I found that low-level N enrichment and bear damage to nearby ant nests had interacting effects on plants. The cascade was weak in N-enriched conditions because plants were more resistant to herbivores and predator effects were not stronger. This research indicates that cascades are currently being disrupted by N deposition. Overall, my studies show that mutualistic species interactions can be influential for trophic cascades. These results highlight the need to integrate mutualisms into trophic cascade theory, which is based primarily on antagonistic relationships. Positive and negative species interactions frequently co-occur and may often have interdependent effects on ecological dynamics. Progress in community ecology may be greatly promoted by adopting a perspective that integrates multiple types of species interactions in the same ecological network.