Keystone mutualism strengthens top–down effects by recruiting large-bodied ants
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Determining the impacts of mutualistic interactions and predator diversity on food webs are two important goals in community ecology. In this study, we examined how predator community variation mediates the strength of top–down effects in the presence and absence of mutualistic interactions. We examined the impacts of predatory ant species that simultaneously prey on leaf-chewing herbivores (Lepidoptera) and engage in food-for-protection mutualisms with sap-feeding herbivores (Hemiptera) in the lower canopy of Connecticut forests. In this 2-year study, we examined three hypothetical mechanisms by which mutualisms can alter the top–down effects of ants: (1) sap feeders increase ant abundance, thus strengthening predatory effects; (2) sap feeders increase the relative abundance of a species that has stronger predatory effects; and (3) changes to predator diversity (species richness) are caused by sap feeders mediating top–down effects of the ant community. Experiments revealed that host plants occupied by sap feeders favored large-bodied ant species in the genus Camponotus, but there were no changes to community-wide ant abundance or ant species richness. Fitting predictions of predation strength based on the functional trait of body size, large-bodied Camponotus suppressed caterpillars and reduced leaf herbivory. This work shows that the ant–hemipteran mutualism, which has been characterized as a keystone interaction, can generate strong top–down effects on leaf-chewing herbivores and herbivory via increasing the relative abundance of species with functional traits relevant to predation, such as body size. Therefore, the emergence of specific ants as keystone predators in a community can be contingent upon their mutualism with sap-feeding Hemiptera.
KeywordsPredator–prey interactions Mutualism Food webs Ants Caterpillars
This work was supported by an NSF Doctoral Dissertation Improvement Grant DEB-1404177. We thank Jacob Feder, Erin Smith, Henok Alemu, Mattheau Comerford, Taiga Araki, Max Atkinson, Pierre Gerard, and Delaine Winn for assistance in fieldwork, curation of specimens, and collection of data in spring and summers of 2013–2015. Sonia E. Sultan, Frederick M. Cohan, and Manuel A. Morales provided helpful feedback on design and analysis of experiments. Matthew S. Wallace and Mark J. Rothschild aided in identification of Membracidae and Coccidae specimens. We thank Carmen Blubaugh and three anonymous reviewers for helpful feedback on the previous versions of this manuscript.
Author contribution statement
REC ran field experiments and performed statistical analyses. Both REC and MSS designed experiments, participated in fieldwork, evaluated experimental results, and wrote and revised manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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