Food habits of invertebrate grazers in a forested stream: variations according to taxonomic affiliation, flow habitat, and body size

Macroinvertebrate grazers mainly feed on periphyton comprising of algae and detritus in streams, and sometimes feed on plant detritus or other invertebrates. We determined food habits of 21 grazer species (19 mayflies, 1 caddisfly, and 1 net-winged midge) from their gut contents, and systematically examined whether taxonomic affiliation, flow habitat, or body size are responsible for variations in food habits among or within the grazer species on a forested stream. We obtained some evidence that the food habits of grazer species varied according to family or genus; for example, species of Ephemerellidae ingested more animal materials. Species inhabiting slow-flow areas in riffles ingested fewer diatoms, while those inhabiting rapid-flow areas in riffles ingested less plant detritus. Larger individuals ingested more plant detritus or animal materials within a particular species (five grazer species), whereas we did not find the opposite patterns. Understanding the trophic roles of stream macroinvertebrates and the energy flow in stream food webs requires systematic exploration of the factors responsible for inter- and intraspecific variation.