Evolutionary ecology of a multi-species mutualism at a regional scale

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Mutualist interactions between species, in which each species benefits the other, are vitally important in many ecosystems. Interactions involving more than two mutualist partners are common, but rarely studied. The exchange of services between mutualist partners varies in response to changing ecological conditions. This work will examine a three-way mutualism among trees, ants, and scale insects found in the tropical Americas. Cordia alliodora trees provide living space for Azteca ants in their branches; the ants defend the tree's leaves from herbivory by caterpillars and beetles; and scale insects live with the ants, consume fluids from the tree and feed the ants their sugar-rich excretion. The project will investigate three kinds of changes in the interactions among these mutualist partners: as the tree grows, in response to ecological differences in the quality of the habitat, and on the long-term evolutionary timescale. First, the project will examine how the interaction of scales and ants with the tree depends on the tree growth. New leaves produce more nutrients for the tree, so additional protection by ants would be beneficial. Are scale insects better fed near new leaves, and does this induce more ant defense near new leaves? Second, the project will examine how the mutualism depends on the amount of rainfall. Trees grow larger at sites with higher rainfall, and trees at these sites may be less dependent on ants to defend their leaves. Do trees in lower quality environments, with less rainfall, benefit more from the ants, despite the scale insects that the ants tend? Third, the project will examine differences among populations in the effectiveness of ant defense. By comparing the genetic structure of ant populations from Mexico to Argentina, it will be possible to distinguish how much the mutualism arises from interbreeding or from local evolutionary change. The results of the project will show how mutualistic interactions can respond to ecological change and how this influences their evolution.This project will train US undergraduates, and graduate students, including individuals from under-represented groups, and involve them in international collaboration. The project strengthens international scientific infrastructure and collaboration and includes outreach to K-12 teachers and students, as well as the general public, in Mexico and Costa Rica. The project uses local residents from Mexico and Costa Rica to help with field work and students from local Mexican and Costa Rican universities participate in the research. The focal tree species, Cordia alliodora, is an important forest tree of significant economic importance and the work has important implications for management.