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. / A Dissertation submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2014. / October 29, 2014. / ants, bears, ecological web, mutualism, nitrogen deposition, trophic cascade / Includes bibliographical references. / Nora Underwood, Professor Co-Directing Dissertation; Brian Inouye, Professor Co-Directing Dissertation; Thomas Miller, Committee Member; Walter Tschinkel,
Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_252835 |
| Contributors | Grinath, Joshua B. (Joshua Bradley) (authoraut), Underwood, Nora (professor co-directing dissertation), Inouye, Brian D. (professor co-directing dissertation), Parker, William C., 1952- (university representative), Miller, Thomas E. (committee member), Tschinkel, Walter R. (Walter Reinhart), 1940- (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Biological Science (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (111 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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