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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.

How environmental contexts shape the strength of species interactions, and their influence on community structure, remains a key focus for the field of community ecology. In particular, the extent to which local competitive interactions impact community structure, and whether this differs across contexts, persists as a general issue that is unresolved across a broad range of animal systems. Studies of arboreal ants have shown that competitive interactions over carbon‐rich exudates from extrafloral nectaries (EFNs) and homopteran aggregations can have positive and negative effects on the local abundances of individual species. Nevertheless, it is still unclear the extent to which these local effects scale to community‐level effects. Here we address the role of food from extrafloral nectaries on the structure of arboreal ant communities in a savanna of central Brazil. We did this with a combination of a diversity survey across tree species with and without EFNs, a repeated survey at times of peak EFN activity, and testing of our survey findings with two experimental manipulations of nectar availability that also provided supplementary nesting cavities. Species richness, but not composition, differed significantly between trees with and without EFNs. However, trees with EFNs had, on average, only 9% more species than those without EFNs. Furthermore, ant species richness did not differ significantly between periods of high and low EFN activity. Although nectar supplementation significantly affected nest occupation rates, this difference was seen solely in the experiment with a massive supply of nectar and there was no effect on total ant richness or identity of the focal assemblages. Our findings suggest that the effects of extrafloral nectar on the abundances of arboreal ants at local scales do not scale to a strong structuring force at the community level. We suggest that this is most likely due to a lack of specificity of community members for EFN tree species, and the diffuse temporal and spatial nature of the availability of active EFNs. These properties mean that observable short‐lived activity and competition over particular EFNs does not ultimately drive lasting changes in the associated assemblage of species, and therefore, the community as a whole.

Fifty years ago, Ehrlich and Raven proposed that insect herbivores have driven much of plant speciation, particularly at tropical latitudes. There have been no explicit tests of their hypotheses. Indeed there were no proposed mechanisms either at the time or since by which herbivores might generate new plant species. Here we outline two main classes of mechanisms, prezygotic and postzygotic, with a number of scenarios in each by which herbivoredriven changes in host plant secondary chemistry might lead to new plant lineage production. The former apply mainly to a sympatric model of speciation while the latter apply to a parapatric or allopatric model. Our review suggests that the steps of each mechanism are known to occur individually in many different systems, but no scenario has been thoroughly investigated in any one system. Nevertheless, studies of Dalechampia and its herbivores and pollinators, and patterns of defense tradeoffs in trees on different soil types in the Peruvian Amazon provide evidence consistent with the original hypotheses of Ehrlich and Raven. For herbivores to drive sympatric speciation, our findings suggest that interactions with both their herbivores and their pollinators should be considered. In contrast, herbivores may drive speciation allopatrically without any influence by pollinators. Finally, there is evidence that these mechanisms are more likely to occur at low latitudes and thus more likely to produce new species in the tropics. The mechanisms we outline provide a predictive framework for further study of the general role that herbivores play in diversification of their host plants.

A major goal of community ecology is to identify the patterns of species associations and the processes that shape them. Arboreal ants are extremely diverse and abundant, making them an interesting and valuable group for tackling this issue. Numerous studies have used observational data of species co‐occurrence patterns to infer underlying assembly processes, but the complexity of these communities has resulted in few solid conclusions. This study takes advantage of an observational dataset that is unusually well‐structured with respect to habitat attributes (tree species, tree sizes, and vegetation structure), to disentangle different factors influencing community organization. In particular, this study assesses the potential role of interspecific competition and habitat selection on the distribution patterns of an arboreal ant community by incorporating habitat attributes into the co‐occurrence analyses. These findings are then contrasted against species traits, to explore functional explanations for the identified community patterns. We ran a suite of null models, first accounting only for the species incidence in the community and later incorporating habitat attributes in the null models. We performed analyses with all the species in the community and then with only the most common species using both a matrix‐level approach and a pairwise‐level approach. The co‐occurrence patterns did not differ from randomness in the matrix‐level approach accounting for all ant species in the community. However, a segregated pattern was detected for the most common ant species. Moreover, with the pairwise approach, we found a significant number of negative and positive pairs of species associations. Most of the segregated associations appear to be explained by competitive interactions between species, not habitat affiliations. This was supported by comparisons of species traits for significantly associated pairs. These results suggest that competition is the most important influence on the distribution patterns of arboreal ants within the focal community. Habitat attributes, in contrast, showed no significant influence on the matrix‐wide results and affected only a few associations. In addition, the segregated pairs shared more biological characteristic in common than the aggregated and random ones. Co‐occurrence patterns of arboreal ants are defined more by competitive interactions than habitat selection. Moreover, it was shown that coexisting species typically have less key ecological characteristics in common.

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.