On the importance of ecosystem engineers in the Neotropical canopy: How beetle-produced cavities influence community composition in cavity-nesting arboreal ants

Background/Question/Methods

Understanding the extent to which species interactions mechanistically shape ecological communities remains a central challenge to the field of community ecology. Much of the work on this issue has focused on competitive interactions, while the role of other types of ecological interactions have been studied far less. “Ecosystem engineers” produce persistent physical state changes in the environment that are used secondarily as resources for other organisms. While ecosystem engineering is acknowledged as a widespread ecological interaction, how it might influence community assembly is poorly understood. Here, we address how cavity-producing wood-boring beetles influence the diverse communities of cavity-nesting arboreal ants of the Brazilian savanna. To do this, we characterized the beetle cavities, occupancy patterns by ants, and overall ant diversity on six common species of savanna trees. Ant diversity was sampled on 40 trees of each species (240 total), covering a broad range of tree sizes for each species. Stems were subsequently collected from eight trees of each species for dissection in the lab. A suite of quantitative measures were recorded for all beetle cavities in the stems, and any ant occupants were identified. 

Results/Conclusions

Tree species differed significantly in the abundance of beetle cavities, as well as the mean and variance of cavity size and entrance size. For dissected stems, the percentage of occupied cavities and ant species composition differed significantly among tree species. Moreover, the percentage of occupied cavities was positively correlated with both the mean and variance of the entrance sizes across tree species. Overall ant species richness did not differ strongly among tree species. However, tree species differences in cavity abundance and entrance size had significant effects on ant species turnover among trees. In particular, low abundance of diverse cavity entrances was associated with particularly high richness and turnover of a diverse lineage of obligate cavity-nesting ants. This suggests a potentially important interaction between the availability of diverse beetle-produced cavities and cavity competition in these ants. Overall, these findings suggest that the nature of ecosystem engineering by wood-boring beetles differs among common tress species. The differences in the resulting cavity resources subsequently have significant effects on the community composition of cavity-nesting ants. Experiments testing the mechanistic effects of cavity properties on ant community assembly, and the subsequent trophic interactions of these communities, are currently underway.