Consequences of resource environment for the interactions among ants, an ant-tended herbivore, and its host plant

In this project, I evaluated the effects of resource environment on the interactions among ants, a tropical myrmecophilous caterpillar, and its host plant. The larvae of Thisbe irenea (Riodinidae) produce nitrogen-rich secretions for ants that provide protection from predators in return. Ants also visit the extrafloral nectaries of Thisbe's host, the pioneer tree Croton billbergianus (Euphorbiaceae). Through experimental manipulations and surveys of plants in natural field sites on Barro Colorado Island, Panama, I showed that high light conditions resulted in both positive and negative effects on plant quality for herbivores: higher leaf production rates and leaf nitrogen concentrations, as well as higher phenolic contents in leaves. In addition, Croton in high light had higher ant visitation rates. Such variation in plant characteristics across light environments affects Thisbe's performance: larvae achieved higher growth rates on plants growing in high light. Also, caterpillars in high light were tended by more ants for a greater proportion of time. Higher ant tending rates likely contributed to the lower disappearance rates of larvae placed on plants in high light field sites. I also investigated the relationship between ant attendance and Thisbe behavior. More ants tended both high and low light caterpillars while they performed activities that might draw predator attention (feeding or moving) than while motionless. The resource-mediated effects on Thisbe and its interaction with ants ultimately impact Croton performance. Plants in high light field sites experienced greater herbivore damage, particularly from Thisbe. The identity of the ants visiting plants also had a significant effect on herbivory rates. In general, however, plants in high light sites and experimental treatments exhibited higher leaf production in spite of higher herbivory rates. These findings show that patterns in herbivory across environmental gradients can depend on how particular herbivores respond to variation in different plant traits and their vulnerability to predators or parasitoids that are also affected by plant quality. These results also demonstrate how resource availability can contribute to variation in the outcome and/or strength of species interactions and their impacts on other organisms.