Trophic Organization of Understory Birds in a Malaysian Dipterocarp Forest

-Single-sample studies suggested that understory flowers and fruits and their avian consumers are scarce in the Malaysian rain forest as compared with African and Central American rain forests. Results from my longer-term studies at Pasoh Forest Reserve (Negeri Sembilan, Peninsular Malaysia) established that flowers and fruits were consistently rare as food for birds. A comparison of two forest types at Pasoh revealed the effect of lower food availability on avian trophic organization. Food resources (e.g. flowers, fruits, arthropods) were less abundant in the regenerating than in the virgin forest, and bird species richness and individual abundance were also lower in the regenerating forest understory. However, the two forests did not differ significantly in the relative importance of the various foraging guilds, suggesting that similar types of resources were present in similar proportions. None of the birds sampled in the Malaysian rain-forest understory was a specialized consumer of understory flowers or fruit, whereas birds feeding mainly on foliage-dwelling arthropods were abundant and were represented by many species. This trophic organization is contrary to that reported for rain forests in other tropical regions but may simply reflect an allocation of harvestable productivity that is different rather than lower. Received 16 January 1985, accepted 23 July 1985. THE absence of avian frugivores in short-term mist-net samples from the Malaysian rain-forest understory (Karr 1980) hinted at a departure from the trophic organization observed in similar short-term studies in African and Central American rain forests, where frugivorous birds comprised 25-33% of mist-net captures (Karr 1980). Similar short-term work prompted Janzen (1977) to comment on the unusually low density of plants flowering or fruiting in the Malaysian rain-forest understory as compared with similar vegetation in Costa Rica. Janzen (1977) independently suggested that frugivores were not likely to be an important component of the Malaysian understory fauna because there would be little for them to eat. However, such observations of low frugivore density and low reproductive activity of plants based on single visits can arise from one of two rather different ecological situations. Reproductive activity of understory plants and frugivore density may indeed be consistently low. Alternatively, frugivores may have been absent temporarily from the area because the lo' Present address: Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Republic of Panama. cal availability of seasonally abundant flowers and fruits was low during the short sampling period. Certain attributes of the Malaysian rain forest promote the expectation that understory flowers and fruits, and their consumers, are consistently low in abundance. The tree family Dipterocarpaceae dominates the canopy of the Malaysian rain forest. A compilation of surveys covering 7,900 acres of lowland and hill forest in peninsular Malaysia revealed that dipterocarps comprise 30% of the trees ('30 cm in diameter) by number and 55.5% by volume (Symington 1974). In a given forest, there may be 10-40 dipterocarp species (Symington 1974) that reproduce synchronously at irregular intervals of 2-8 yr (Burgess 1972, Medway 1972, Janzen 1974, Ng 1977). Many tree species from other families (e.g. Bombacaceae, Guttiferae, Sapindaceae) also follow this reproductive cycle and either will only fruit or will fruit more heavily during dipterocarp mast years (Whitmore 1984). Although as yet undocumented, the amount of harvestable productivity (the portion of primary productivity that goes into the production of flowers, fruits, and new leaves and that is consumable by animals) in the canopy of dipterocarp forests must vary in accordance 100 The Auk 103: 100-116. January 1986 This content downloaded from 157.55.39.235 on Fri, 07 Oct 2016 06:28:28 UTC All use subject to http://about.jstor.org/terms January 1986] Birds in Malaysian Forest Understory 101 PASOH FOREST RESERVE