Local and Regional‐Scale Food Web Structure in Nepenthes alata Pitchers1

Abstract

ABSTRACTTropicalNepenthespitcher plants provide small, isolated aquatic habitats. We examined inter‐pitcher variation in the community structure of the inhabitants ofNepenthes alataBlanco in West Sumatra, focusing on the conditions of the pitchers, bacterial density in the pitcher fluid, density and biomass of metazoan inhabitants, and the frequencies of interspecific encounters. Older pitchers contained more insect carcasses. The bacterial density increased with the age of the pitchers, but decreased in withered pitchers that contained finely decomposed detritus. In live pitchers, the bacterial density, the density, mass and species richness of metazoa, and the number of trophic levels per pitcher were positively correlated with detrital mass, which was correlated with volume of pitcher fluid. The metazoan fauna fromN. alataconsisted of 4 predators and 12 saprophages, among the richest known forNepenthesspecies. However, each individual pitcher harbored a limited numbers of species, owing to (1) the low incidence of many species, and (2) the aggregated distribution and different temporal colonization pattern of major species. Six dipteran taxa (one predator and five saprophages) accounted for the bulk of metazoan inhabitant biomass. Of 48 combinations of predator‐prey encountered, only four occurred frequently (in > 30% of pitchers), which included two predators and three saprophages. Thus, individual pitchers harbored relatively simple communities despite the regional species richness, and only limited kinds of predator‐prey encounters seemed to occur frequently in the regional food web. The local‐scale properties of the subdivided communities presented here provide the basic information for understanding the maintenance of regional species richness and food web complexity.