Yusah, K.M. & Foster, W.A.

Tree size and habitat complexity affect ant communities (Hymenoptera: Formicidae) in the high canopy of Bornean rain forest

Canopies of tropical rain forest host a considerable proportion of the world's biodiversity. However, the factors driving variation in this diversity are poorly known, particularly for the upper-most canopy strata, and for species-rich groups such as arthropods. We surveyed high canopy ants from emergent trees (height 35 - 60 m) in rain forest in Malaysian Borneo and measured a range of aspects of canopy structure that might affect the structure of these communities, including variables relating to tree dimensions and within-tree habitat complexity. We used two methods: a baitingbased method (purse-string trapping), which samples more behaviourally dominant ants foraging on the trunk and on major branches, and canopy insecticide fogging, which samples ants from the entire canopy. As expected, there were positive correlations between measures of tree size and ant abundance and species richness (baiting: abundance-crown width, richness-crown width; fogging: richness-tree height). However, fewer ant individuals were found in fogging samples from trees with larger trunk diameter, and fewer ant species were found at baits in taller trees, suggesting complicated relationships between tree size and ant community size. Habitat complexity also affected ant communities: More ant species were found in trees with more hollows (based on baiting) and with more dead branches (based on fogging). Furthermore, greater numbers of ant individuals were found in fogging samples from tree crowns with greater epiphyte / climber cover. These results suggest that higher levels of habitat complexity increase both the total number of ant workers that a tree canopy can support and the number of species that can co-exist there. There were also more ant individuals in crowns with higher connectivity, based on baiting, potentially as a result of increased foraging rates from nearby trees. Trees of the species Parashorea malaanonan supported more species of ants, but lower abundances of ants, than trees of the species P. tomentella, indicating the presence of larger numbers of colonies with smaller colony size. Finally, species composition for both fogging and purse-string traps samples were affected by trunk diameter and the presence of epiphytes and climbers, indicating that forests in which these factors are more variable are likely to present a wider range of niches for ants and hence support higher diversity. Taken together, these results demonstrate how variation in canopy structure and complexity can contribute to the high diversity of ants in the canopy of tropical rain forest.