The Ecology of Nest Cavity Use by Arboreal Ants in the Brazilian Cerrado: Resource Availability, Nest Modification, and Trophic Interactions

Arboreal ants in the Brazilian Cerrado rely on cavities in living trees as nest sites. These cavities are created by a community of, wood-boring beetles, which act as ecosystem engineers. Despite the importance of these cavities as a resource, little is known about their natural abundance and heterogeneity, how ants use and modify them as nest sites, and how this interaction between cavities and their ant occupants influences trophic interactions on cerrado trees. Here I use natural history observations and manipulative experiments to address these questions. In the first chapter I quantified the occurrence, heterogeneity, and use of beetle-created cavities by ants in six cerrado tree species. I found that cavity abundance differs significantly among tree species and within different branch sizes. Furthermore, patterns of cavity use suggest that competition for large cavities is far greater than that for abundant smaller cavities. Finally, a strong correlation between ant body size and cavity entrance size suggests an important axis of variation upon which arboreal ant species partition cavity resources, allowing for high ant diversity on individual trees. In the second chapter, I describe how ants modify the entrance size of cavities to better correspond to their body size. I found that entrance modification reduced entrance area of otherwise unsuitable cavities. In doing so the ants expand availability of a limiting resource without sacrificing nest defensibility. In the third chapter, I report a year-long experiment to test the effects of ant exclusion and increased cavity resources on levels of herbivory for two species of cerrado trees, Caryocar brasiliense and Sclerolobium aureum. I found that while excluding ants significantly increased the amount of leaf tissue consumed by herbivores, adding cavities had no measurable effect on herbivory. These results point to the important role of specific ant species that use large nest cavities in reducing herbivory on trees. Overall this work has further developed our understanding of relationships between host trees, cavities, and arboreal ants by demonstrating that cavity availability and use by arboreal ants has significant ramifications for the ecology and evolution of ants, trees, and arthropod herbivores in the Cerrado ecosystem.