Spatio-Temporal Variation in an Ant-Plant Interaction

The role of mutualisms in structuring communities is poorly understood, in large part because potential mutualistic interactions are often identified, but rarely quantified. I tested the hypothesis, proposed in 1979, that the interaction between the ant Myrmelachista flavocotea (Formicidae: Formicinae) and its obligate host plants Ocotea atirrensis and O. dendrodaphne (Lauraceae) is a mutualistic interaction. Despite the high abundance of Myrmelachista ants in tropical forests, relatively little is known about them, because of their timid nature and their habit of living inside plants. I used a combination of observations and experiments to analyze the interaction between M. flavocotea and Ocotea and to explore the potential higher-order community effects of the interaction. Myrmelachista flavocotea and Ocotea plants form an association early in the life history of both participants. Ocotea seedlings were colonized by multiple M. flavocotea queens. Mature colonies had only a single queen, apparently as a result of secondary monogyny. Presence of multiple foundresses may be critical in ensuring the successful founding by at least one queen, thus allowing the perpetuation of the Myrmelachista-Ocotea interaction. I found the outcome of the interaction is highly variable. Some ant colonies readily defend their host plant, while other colonies were never observed to defend their host plant. Ocotea inhabited by the most aggressive ant colonies suffer the least herbivore damage. The density of ants inside Ocotea stems, not colony size or body size of workers, was the best predictor of colony aggression and host plant defense. Myrmelachista flavocotea clearly acts as an inducible agent of biotic defense that responds to chemical cues from damaged leaves. Myrmelachista also influences the density of Ocotea seedlings by killing plants that grow in the vicinity of their host plant. Vegetative killing by M. flavocotea appears to be a mechanism to reduce competition with other M. flavocotea colonies and likely benefits host plants through decreased intraspecific competition. M. flavocotea and Ocotea receive reciprocal benefits from their partnership, which supports the hypothesis that the Myrmelachista-Ocotea interaction is a mutualism. The host plant defense behavior of the small, enigmatic, and relatively timid ant M. flavocotea has demonstrated community level effects