Foodweb structure in small streams: do we need different models for the tropics?

Abstract

Prevailing notions of foodweb structure and trophic relations in fresh waters are based on research undertaken in a limited range of latitudes or habitat types. This limitation had led to the general view that stream food webs are detritus-based with short food chains and simple interactions that often are dominated by a few key species. We used gut-content analyses and stable-isotope signatures to characterize feeding relationships and foodweb attributes of benthic communities in riffles in 2 forest streams in Hong Kong. We compared them with existing data on foodweb structure from pools in 1 of the streams and data from the literature. The 2 approaches to dietary analyses yielded complementary results, providing confidence that trophic relations and foodweb structure were adequately characterized. Food webs in both streams were remarkably similar regardless of habitat (riffle vs pool). Consumers in both streams depended primarily on autochthonous resources, as has been reported from some other tropical streams, and food chains were short although connectance was higher than has been recorded previously for stream food webs. Very few omnivores were found, and omnivory was even rarer than is typical of temperate streams, although it is common in other tropical streams and rivers. No evidence was found for dominance by a few common macroconsumer species, as observed elsewhere in the tropics. The apparently high levels of autochthony in tropical running waters imply that models of ecosystem functioning for northern temperate streams are inadequate for describing tropical systems. However, marked differences in the degree of omnivory and dominance of tropical stream food webs by macroconsumers is evidence that characterization of trophic interactions and stream ecosystem functioning cannot be captured by a simple tropical vs temperate dichotomy. Successful management of these systems will depend upon development of conceptual models that reflect the diversity of food webs within and between regions.