Background/Question/Methods
Diverse and ecologically important taxa are likely to be particularly useful in ongoing empirical efforts to understand the processes that regulate species diversity and coexistence. Arboreal ants are both diverse and ecologically dominant in the tropics. Nevertheless, empirical work on the processes structuring these assemblages at local scales is limited. Our study addresses how access to resources and the diversity of nesting resources affect arboreal ant assemblages at the local scales of habitat type and resource patch. Work was conducted in the low canopy of the Brazilian savanna, or “cerrado”. We focused on three cerrado habitats that differ in canopy connectivity, and thus access to tree-based resources, and our focal resource patches were individual trees. Ant nesting resources in this system are preexisting arboreal cavities, predominantly made by wood-boring beetles. We characterized arboreal ant diversity across habitats and tree size. We also conducted whole-tree experimental manipulations of canopy connectivity and the diversity of available nesting resources.

Results/Conclusions
The survey suggested that species richness was equivalent among habitats, with high species sharing. It also showed that the number of species per tree increased with tree size in all habitats, but the most open habitat had significantly fewer species per tree than the other two. The experiment demonstrated directly that connected canopy results in significantly higher species per tree. Cavity diversity, defined by entrance size, did not significantly increase overall species per tree. However, it did significantly increase the number of species using new cavities on each tree, and the number of new species colonizing each tree. Cavity diversity also significantly increased the total species using new cavities, and total cavity use across trees. These results represent the first experimental evidence that access to resources and cavity diversity have significant effects on arboreal ant assemblages. Moreover, they provide assemblage-level evidence of interspecific differences in cavity specialization. Overall, our evidence suggests that differences in cavity access and diversity do not significantly affect arboreal ant diversity at the scale of habitat type. However, greater cavity access and diversity does appear to interact with interspecific differences in cavity use to increase the number of species that coexist at the scale of individual trees. Future work will address the relationships between the diversity of wood-boring beetles, which appear to be ecosystem engineers in the system, and the diversity and ecological impacts of the arboreal ant assemblages.