Energy Flow and the Trophic Basis of Macroinvertebrate and Amphibian Production in a Neotropical Stream Food Web

Despite the typically high taxonomic and functional diversity of tropical habitats, little is known about the roles of individual consumers in their ecosystem structure and function. We studied the trophic basis of production in a tropical headwater stream by identifying major sources of energy, measuring energy flow through consumers and characterising interactions among trophic levels and functional groups. We examined gut contents of 18 dominant macroinvertebrate and two tadpole taxa and used these data, along with previously published estimates of secondary production, to quantify food-web structure and energy flow pathways. We also examined the prevalence of omnivory and patterns of resource consumption across seasons and habitats. Non-algal biofilm, a heterogeneous polysaccharidic matrix, was the most utilised food resource in the stream. Contrary to some studies of Old World tropical stream food webs, detrital energy sources were consumed at relatively high rates and contributed significantly to overall energy flow, although much of this was attributable to a single shredder taxon. Algal consumption rates were similar to values reported for temperate streams and were highest during the dry season. Omnivory was prevalent across all functional groups, particularly predators, suggesting traditional functional and trophic assignments based on temperate regions may not be appropriate for tropical systems. Seasonal patterns of resource consumption appeared linked to hydrological disturbance. This is the first study to provide quantitative estimates of energy flow through a neotropical stream food web. Extirpation and extinction rates in tropical freshwater habitats are high; our study provides baseline information for conservation and management of remaining systems, and for quantifying the consequences of further losses of biodiversity such as ongoing amphibian declines.