Grazing insects reduce algal biomass in a neotropical stream

Herbivorous insects have received relatively little attention in studies of consumer resource interactions in tropical compared to temperate streams. In this study I examined the effects of small, highly mobile, herbivorous insects on epilithic algae in riffle habitats of a stream located on the Pacific coast of Costa Rica. By elevating artificial substrates 20-30 cm off the streambed, while holding other abiotic factors constant, I successfully reduced the densities of grazing insects (primarily mayflies and dipterans) throughout an 8 week experimental period. This manipulation resulted in approximately 30% more algal biomass (measured as AFDM and chlorophyll a) on raised (grazer exclusion) than on control substrates placed directly on the streambed. Observations of the one common epibenthic grazing fish species, Sicydium salvini (Gobiidae), indicated that there were no consistent differences in their visitations to raised and lowered substrates, suggesting that fish grazing pressure did not contribute to observed differences in algal biomass between treatments. These results show that insects can be effective in reducing algal biomass in a tropical stream. While previously neglected, the role of grazing insect in contributing to the regulation of algal resources in tropical benthic communities warrants closer examination.