Resource partitioning among the fishes of rainforest streams in Sri Lanka

The fish assemblage typical of small rainforest streams of Sri Lanka was investigated to see if the fishes were characterized by a high degree of specialization, expected of equilibrium communities, or if they were relatively unspecialized, expected of more stochastic (non-equilibrium) communities. Morphological features, habitats, microhabitats and diets of the 20 most abundant fish species were measured. The fishes included 11 Cyprinidae (including seven Barbus spp. and two Rasbara spp.), two Gobiidae, two Channidae, and one species each in the Cobitidae, Cyprinodontidae, Belontidae, Belonidae, and Mastacembelidae. Microhabitats were determined by measuring (for 6760 fish) water column depth, distance of fish from bottom, mean water column velocity, water velocity at fish, and substrate. Morphologically, the species showed a high degree of specialization, especially in structures related to feeding. Although several species were habitat generalists, most species occurred in distinct habitats. Within habitats, microhabitat overlap among co-occuring species was low, particularly in relation to position in the water column. Principal component analysis of the microhabitat measurements produced three new variables. Most species not clearly segregated by the overlap analysis showed segregation on the new variables. Fishes not segregated by habitat or microhabitat tended to show low dietary overlaps. Specialization in feeding habits was more pronounced among the Sri Lankan fishes than noted for fishes in small rainforest streams elsewhere, in part because of the greater reliance of the Sri Lanka fishes on autochthonous foods. Overall, the fish assemblage had the characteristics expected of an equilibrium (deterministic) assemblage.