Foodweb structure in a tropical Asian forest stream

Abstract

A food web based on the gut contents of consumers (invertebrates and fishes) in pools of Tai Po Kau Forest Stream (TPKFS), Hong Kong (southern China), indicated the importance of periphyton and fine organic particles; coarse particulate organic matter was a less important food source despite its higher relative abundance in this shaded hillstream. Stable isotope analysis of consumer tissues was undertaken to confirm this result. IsoSource software was used to model n-isotope and >n + 1-sources, so that the relative contribution of the potential food sources could be determined. Results of an IsoSource mixed model of δ15N and δ13C stable isotope signatures of primary consumers generally supported evidence from gut content analyses about the importance of autochthonous resources. Inconsistencies between the results of gut content analysis and isotope signatures of consumer tissues occurred in a few cases but could be explained either by small sample size or the wide range of feasible solutions provided by the isotopic mixed model. Both techniques were needed to resolve the trophic position of omnivores. For instance, the gut contents of balitorid loaches indicated that they were primarily herbivorous but their stable isotope signatures revealed a significant dependence on animal food. Considerable overlap in the diets of predatory fishes and invertebrates (odonates, perlid stoneflies, palaemonid shrimps) was confirmed by both gut contents and stable isotope analyses. This finding, along with a lack of intraguild predation, resulted in a short mean and maximum foodchain length, high links per species, and high connectance for the TPKFS food web when compared with literature reports of other stream food webs. Periods of spate-induced disturbance during the wet season and limited algal productivity in TPKFS might also have contributed to the short food chains. Inconsistent levels of resolution for different taxonomic groups within the food web may have generated artefacts of low linkage complexity, high predator-prey ratio, and a small number of basal and intermediate species, a pattern that has been confirmed for stream food webs elsewhere. Our study is the first example of a food web based on complementary analyses of gut contents and stable isotope signatures for any tropical stream. This combined approach is recommended for future studies of food webs, especially in habitats where omnivores are an important component of the community.