Summary1. Arboreal ants are both diverse and ecologically dominant in the tropics. Such ecologically important groups are likely to be particularly useful in ongoing empirical efforts to understand the processes that regulate species diversity and coexistence. 2. Our study addresses how access to tree-based resources and the diversity of pre-existing nesting cavities affect species diversity and coexistence in tropical arboreal ant assemblages. We focus on assemblage-level responses to these variables at local scales. We first surveyed arboreal ant diversity across three naturally occurring levels of canopy connectivity and a gradient of tree size. We then conducted whole-tree experimental manipulations of canopy connectivity and the diversity of cavity entrance sizes. All work was conducted in the Brazilian savanna or 'cerrado'. 3. Our survey suggested that species richness was equivalent among levels of connectivity. However, there was a consistent trend of lower species density with low canopy connectivity. This was confirmed at the scale of individual trees, with low-connectivity trees having significantly fewer species across all tree sizes. Our experiment demonstrated directly that low canopy connectivity results in significantly fewer species coexisting per tree. 4. A diverse array of cavity entrance sizes did not significantly increase overall species per tree. Nevertheless, cavity diversity did significantly increase the species using new cavities on each tree, the species per tree unique to new cavities, total species using new cavities, and total cavity use. The populations of occupied cavities were consistent with newly founded colonies and new nests of established colonies from other trees. Cavity diversity thus appears to greatly affect new colony founding and colony growth. 5. These results contribute strong evidence that greater resource access and greater cavity diversity have positive effects on species coexistence in local arboreal ant assemblages. More generally, these positive effects are broadly consistent with niche differentiation promoting local species coexistence in diverse arboreal ant assemblages. The contributions of this study to the understanding of the processes of species coexistence are discussed, along with the potential of the focal system for future work on this issue.

Question: Plant communities in Paleotropical savannas are regulated by a combination of bottom‐up and top‐down effects. However, the paucity of ungulates and other large herbivores in Neotropical savannas has led to speculation that these communities are primarily structured by physical factors such as fire, precipitation and soil chemistry. We addressed the following question: How much plant biomass is consumed by leaf‐cutter ants in Neotropical savannas, and is it comparable to the amount of biomass consumed by herbivores in Paleotropical savanna sites? Location: Our study was conducted at the Estação Ecológica do Panga, located 30 km south of Uberlândia, Minas Gerais, Brazil. All field work was conducted in the vegetation type known as cerrado sensu stricto. Methods and Results Using direct measurements of herbivory, coupled with estimates of plant productivity and ant colony density, we found that leaf‐cutter ants (Atta spp.) consume 13–17% of the foliar biomass produced annually by woody plants in a Neotropical savanna (Brazilian cerrado). Although comparisons with other savanna systems are complicated by methodological differences among studies, the proportion of biomass consumed by Atta species is about 25% of that consumed by the entire ungulate community in some African savannas and greater than or comparable to the total herbivory observed in some terrestrial ecosystems. Conclusions: We hypothesize that this intense biomass consumption by Atta will have important ecological consequences for the cerrado ecosystem, because leaf‐cutter abundance increases in fragmented or degraded habitats. These effects are likely to be exacerbated as anthropogenic pressure in this biodiversity hotspot increases.

Fire is an important agent of disturbance in tropical savannas, but relatively few studies have analyzed how soil-and-litter dwelling arthropods respond to fire disturbance despite the critical role these organisms play in nutrient cycling and other biogeochemical processes. Following the incursion of a fire into a woodland savanna ecological reserve in Central Brazil, we monitored the dynamics of litter-arthropod populations for nearly two years in one burned and one unburned area of the reserve. We also performed a reciprocal transplant experiment to determine the effects of fire and litter type on the dynamics of litter colonization by arthropods. Overall arthropod abundance, the abundance of individual taxa, the richness of taxonomic groups, and the species richness of individual taxa (Formiciade) were lower in the burned site. However, both the ordinal-level composition of the litter arthropod fauna and the species-level composition of the litter ant fauna were not dramatically different in the burned and unburned sites. There is evidence that seasonality of rainfall interacts with fire, as differences in arthropod abundance and diversity were more pronounced in the dry than in the wet season. For many taxa the differences in abundance between burned and unburned sites were maintained even when controlling for litter availability and quality. In contrast, differences in abundance for Collembola, Formicidae, and Thysanoptera were only detected in the unmanipulated samples, which had a lower amount of litter in the burned than in the unburned site throughout most of our study period. Together these results suggest that arthropod density declines in fire-disturbed areas as a result of direct mortality, diminished resources (i.e., reduced litter cover) and less favorable microclimate (i.e., increased litter desiccation due to reduction in tree cover). Although these effects were transitory, there is evidence that the increasingly prevalent fire return interval of only 1–2 years may jeopardize the long-term conservation of litter arthropod communities.

The selection of nutrient-rich leaves by leaf-cutter ants ( Atta spp.) is thought to be of indirect benefit to these ants by promoting the growth of their symbiotic fungus. However, relatively few studies have analyzed the influence of leaf nutrient content on host plant selection by leaf-cutter ants, and conflicting results have been found. We compared the content of eight nutritional elements plus the non-nutrient aluminum between leaves harvested by colonies of Atta laevigata (Smith) (Hymenoptera: Formicidae: Attini) and leaves collected randomly within their foraging areas. In addition, we evaluated whether leaf nutrient content explained the frequency with which these ants attacked and defoliated some of the tree species found in the study area. For 2 years, we monitored 17-26 trees from 15 species and determined the number of times each plant was attacked and the amount and type of foliage removed. Leaves harvested by A. laevigata presented significantly higher concentrations of N, P, K, Zn, and Cu than those collected randomly. This result is likely to reflect the foraging pattern presented by these ants, which were selective both in terms of the plant species and age of leaves most commonly attacked. Young leaves were the only or the main leaf type exploited in many species, and in comparison to mature leaves these presented significantly higher concentrations of P and K. Large differences in the mean number of ant attacks on the tree species studied were also observed, and those presenting more leaf N tended to be the most frequently attacked.

Summary 1.Herbivory has been shown to have prominent top-down effects on vegetation in Paleotropical savannas, where consumers of early stages of life history act as demographic bottlenecks. Such impact has been largely ignored in Neotropical savannas, however, despite insect consumption being linked to reduced recruitment of woody species. 2. We hypothesize that Atta leaf-cutter ants -the prevalent herbivores in the Neotropics -alter the establishment of woody plant seedlings in the Brazilian Cerrado by reducing seed availability and seedling survival. We also hypothesize that the demographic effects will be species-specific. 3. To test these hypotheses, we experimentally (i) compared seed removal in areas with and without Atta foraging and (ii) compared the survivorship of seedlings exposed to or protected from Atta foraging. Both experiments were performed with the same common Cerrado species which allowed us to evaluate the potential net effects of consumers at the population and community levels. 4. Overall seed removal rates in sites with Atta foraging were greater than those where ants were absent (59.2% and 39.2% of seeds removed, respectively). There were differences in removal for 10 of the 12 tested species, with per species removal 1.1-to 3.8-fold greater in areas with Atta foraging. On transplanted plants, 45% of seedlings exposed to Atta were attacked, resulting in a survival 7.6% lower than that of protected seedlings. Seedling survival was 11.8-31.5% lower in five species, with the largest differences in survival between treatments during the dry season. 5. Taken together, these results support the hypothesis that consumers can lead to demographic bottlenecks in Neotropical savanna plant species, with effects varying among life-history stages and between species. 6. Synthesis. Granivory by ants has been linked to reduced seedling recruitment of woody species in the Brazilian Cerrado. Our results show that leaf-cutter ants may largely limit early seedling establishment of woody species by reducing seed availability and seedling survival with differential species-specific effects. Atta ants may therefore be acting as an ecological filter, which coupled with potential selectivity in foraging on reproductive life-history stages, may ultimately influence the relative abundance of different species and hence the structure and composition of Cerrado vegetation.