Assistant Professor University of Nevada, Reno Reno, United States
Abstract: Generalist predation pressure is theorized to increase local community diversity by enabling the coexistence of competing prey species. In comparison to predation, the effects of mutualism on community structure are less understood theoretically and empirically. However, increasing evidence suggests that mutualistic interactions can enhance community diversity. In ant-plant protection mutualisms, ant mutualists also act as predators. In these mutualisms, plants provide shelter to aggressive ant colonies in exchange for defense against leaf-eating herbivores. By decreasing herbivore abundances, ant defense may increase herbivore species diversity on ant-plants via amelioration of competition and through promoting the survival of herbivores with specialized counter-defenses. However, at the regional scale, ant-plants often span environmental gradients where plant investment in ant defense can vary with resource availability. Furthermore, the size of herbivore species pools can also vary along resource gradients, suggesting that the effects of ant defense on herbivore community structure may also vary with environmental context.
To test the effects of ant defense on caterpillar community diversity, we used a reciprocal common garden experiment using the tropical ant-tree species Cecropia peltata and C. obtusifolia. We established garden plots in two forests with different levels of annual rainfall and used a randomized block design to cross tree species with Azteca ant introduction or exclusion. We then performed biweekly caterpillar surveys for eight months. Although ant defense resulted in a ten-fold reduction in caterpillar abundance, across both plots, caterpillar community diversity on ant-defended trees was significantly higher compared to ant-excluded trees. This increase diversity was driven by community evenness along with higher species dissimilarity among ant-defended trees. However, at the plot level, trees defended by ants hosted higher caterpillar diversity only in the drier, deciduous plot. Ant defense had no effect on diversity in the wetter, rainforest plot. The effects of ants on evenness were driven by the adverse effects of ant defense on the abundance of the dominant caterpillar species in this system, Patania solis. Although ant defense reduced the abundance of P. solis in both plots, P. solis was less dominant in the community at the rainforest due to higher overall species richness.
