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Wood-boring beetle larvae act as ecosystem engineers by creating stem cavities that are used secondarily as nests by many arboreal ant species. Understanding the heterogeneity and distribution of available cavities and their use by ants is therefore key to understanding arboreal ant community assembly and diversity. Our goals were to quantify the abundance and diversity of beetle-produced cavity resources in a tropical canopy, reveal how ants use these resources, and determine which characteristics of the cavity resource contribute to ant use. We dissected branches from six common tree species in the Brazilian Cerrado savanna, measuring cavity characteristics and identifying the occupants. We sampled 2310 individual cavities, 576 of which were used as nests by 25 arboreal ant species. We found significant differences among tree species in the proportion of stem length bored by beetles, the number of cavities per stem length, average entrance-hole size, and the distribution of cavity volumes. The likelihood that a cavity was occupied was greater for cavities with larger entrance-hole sizes and larger volumes. In particular, there was a strong positive correlation between mean head diameters of ant species and the mean entrance-hole diameter of the cavities occupied by those ant species. Wood-boring beetles contribute to the structuring of the Cerrado ant community by differentially attacking the available tree species. In so doing, the beetles provide a wide range of entrance-hole sizes which ant species partition based on their body size, and large volume cavities that ants appear to prefer.

The maleo (Macrocephalon maleo) of Sulawesi, Indonesia, is culturally iconic and Critically Endangered, but the causes of its decline have never been systematically analyzed nor its nesting grounds comprehensively surveyed. We visited 122 previously known and 58 previously unrecorded sites, collecting data and interviewing local people at each site. We used ordinal logistic regression to fit models with combinations of 18 different predation, habitat, and nesting ground variables to determine the strongest predictors of nesting ground success, as represented by maleo numbers. At least 56% of known nesting grounds are now inactive (abandoned), and 63% of remaining active sites host ≤ 2 pairs/day at peak season. Egg-taking by humans is the single biggest driver of maleo decline. Protecting eggs in situ predicts higher maleo numbers than protecting eggs through hatchery methods. After egg-taking, quality (not length) of the travel corridor connecting nesting ground to primary forest best predicts nesting ground success. Being inside a federally protected area is not a primary driver of success, and does not ensure persistence: 28% of federally protected nesting grounds have become inactive. Local conservation efforts protected nesting grounds 2‒3 times better than federal protection. We update the methodology for assessing nesting ground status, and recommend five measures for maleo conservation, the foremost being to protect nesting grounds from egg-taking by humans at all remaining active sites.

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.