The Vertical Distribution of Ants on Canopy Trees in a Bornean Lowland Rain Forest.

Abstract

ABSTRUCT The species composition of ants attracted to sugar baits was compared between four vertical levels ofeleven canopy frees in a mixed dipterocarp forest in Sarawak, Malaysia. Fifteen, six and four species were collected at the subcanopy, understory, and forest floor levels, respectively, while no ants werc attracted to the baits at the top layer (upper canopy) in the daytime. The ground ant fauna includes the predatory ponerine ants and carnivorous or omnivorous Pheidole ants. In contrast, many species of Canponotus, Crematogaster, ar:d Polyrlachis were found in the subcanopy. The latter ants may be thought of as omnivorous or herbivorous ants which feed on plant nectar and/or arthropod exudates. The overlap in species composition between ftees was relatively small. A large part of the biodivenity found on this planet exists in the canopy of ropical rain forests. Although the ant fauna is not particularly diverse compared to such other arthropod groups as beetles (e.g., Stork, 1987), ants have been found to be the most abundant insects and to constitule the largest proportion of the animal biomass in the canopy of tropical rain forests (Adis er al., 1984: Tobin, 1991) . It is still unclear, however, in what part of the canopy and at what time of the day ants are active, on what they forage in the canopy, and where they nest. Tobin (1991 , 1994) proposed that ants function as primary consumers in the canopy of tropical rain forests because their biomass is too large for them to function as secondary consumers (predators or scavengers). Although numerous studies in the temperate region have shown that in some species fluids are quite important (articles cited by Tobin, 1994; Yoshimoto & Yamane, 1990; Itino el a/., unpubl.), there has been no direct evidence showing that ants are primary consumers in tropical rain forest canopies. Our canopy-observation syst€m in Borneo is suitable for testing Tobin's (1994) hypothesis. As a first step towards this goal, we conducted a short-term survey of the species composition of ants in the upper canopy, subcanopy, and understory (on the tree trunk) as well as on the forest floor.