The overarching goal of my research program has been to understand the ecological and evolutionary dynamics of mutualism. My research addresses this goal using a variety of study systems, but focuses on the interaction between ants and the treehopper Publilia concava. In this mutualism, treehoppers feed on the sugary but nitrogen-poor phloem (sap) of the host-plant Tall Goldenrod (Solidago altissima). Treehoppers filter large quantities of sap to meet their nutritional needs, and the sugary excrement (honeydew) is collected by ants as a food resource. In return, ants protect treehoppers from predators, and the act of removing honeydew facilitates feeding by treehoppers. Below, I highlight four active research areas.
Tri-trophic population dynamics of mutualism
An NSF-funded project that my lab is currently involved in is to understand the consequences of mutualism in a community context. I have addressed this question using both modeling and empirical approaches. For example, a simple model of mutualism involving ants, treehoppers, treehopper predators, and host-plants shows that by reducing the impact of predators on treehoppers, protection by ants can allow treehoppers to overexploit their host plants. Thus, while ant protection can provide short term benefits, it can generate population cycles over the long term. Experiments testing these predictions showed that treehopper mothers avoid plants that were occupied by treehoppers in previous years, but with no apparent effect on survivorship. Future studies are planned to evaluate whether the disconnect between plant-quality and insect performance is a function of mothers following an ideal free distribution during oviposition (high-quality plants attract more herbivores thus canceling out heterogeneity in plant quality).
Population genetics of an ant-dependent herbivores
A second project that I have become interested in is the population genetics of the ant-protected herbivore, Publilia concava. In collaboration with Patrick Abbott from Vanderbilt University, we have developed 10 highly variable microsatellite loci for P. concava. Preliminary analyses, conducted in collaboration with Luana Maroja, have shown strong inbreeding in this species at the level of local patches, with moderate levels of population isolation between patches. Future studies are planned to further explore the underlying mechanisms driving these patterns.
The European Fire Ant
A third project that I am involved in is a collaboration with colleagues at Skidmore College and the University of Connecticut to assess the role of mutualism in the spread of invasive species. In the Spring of 2003, I discovered the invasive European Fire Ant (Myrmica rubra) in Williamstown MA, previously recorded outside of its native range almost exclusively along the coast of northern New England. Research in my lab found that this population of M. rubra appears to be concentrated along the Hoosic River watershed from North Adams, MA to Hoosic Falls, NY. Interestingly, the presence of this ant species is correlated with the abundance of a second invasive species, the plant Japanese knotweed. Japanese knotweed has extrafloral nectaries that attract ants who defend these plants against their natural enemies. While there are few herbivores of Japanese knotweed in its introduced range, a third invasive species, Japanese beetles, can inflict high levels of herbivory. In these cases, ants effectively defend plants from beetle herbivory. Ongoing research is aimed at identifying how mutualistic interactions can affect the population dynamics of participants in these invaded communities.
A final area of my research is in exploring the role of interspecific communication in regulating the mutualism between ants and treehoppers – both ants and treehoppers are capable of producing substrate-borne vibration signals which can be thought of as “sound” that travels through plants. Results in my lab and in field experiments have shown approximately a two-fold reduction in the time taken until predator discovery by ants associated with the playback of treehopper alarm signals. Overall, our results show that P. concava treehoppers produce alarm signals in response to predator threat and that this signaling increases the efficacy of predator-protection by ants. Future studies are planned to evaluate these results in a phylogenetic context, and to characterize additional aspects of vibrational communication in these species.