Vertical distribution pattern of ants in a Bornean tropical rainforest (Hymenoptera: Formicidae)

Abstract

To investigate vertical distribution pattern of ants in a tropical rainforest, we collected ants from three vertical strata - canopy, tree-trunk and ground - in a rainforest of Sabah, Malaysian Borneo. Ants were collected by hand collecting on ground, by hanging pit-fall traps on tree-trunks, and by fogging in the canopy. Further, a survey of ant nesting sites was carried out to confirm whether ground-nesting and above-ground-nesting ants also appeared in other strata from their nesting stratum, or not. Previous studies have demonstrated that ants in tropical rainforests have static stratification between canopy and ground strata, with clear faunal boundaries and existence of distinct ant assemblages in the two strata. In the present study, we found that 40% of all species collected appeared in more than one stratum, and half of the ground-nesting and above-ground-nesting ants extended their distribution from their nesting stratum to other strata through the tree-trunk. Further, we found that both stratum-specialists and stratum-generalists occurred within the same ant genera and subfamilies, with variations in the proportion of specialists and generalists between taxonomic groups. The results suggest that ant species have a mix of broad and narrow vertical distribution ranges in a tropical rainforest, rather than static vertical stratification. Our research is unusual among studies of ant vertical distribution in sampling ants from tree-trunks and comparing ant samples from canopy and ground strata, which has not been done in the previous studies. We consider that the tree trunk is an important middle stratum to providing nesting and foraging places for ants, and a pathway between canopy and ground for flightless insects, including ants. Hence, those studies, which have focused only on comparing ant samples from canopy and ground strata, might have underestimated the proportion of stratum-generalists in ant assemblages.