A healthy stream has a constant input of nutrients and energy needed for food chains to begin. This energy is in the form of algae that grows in the stream, leaves, pine needles, twigs, and other organic debris that drop into the stream from the surrounding forest. Sunlight is also an important energy source. Some material, however, remains trapped in pools, behind dams of debris, in back-eddies behind rocks, or in the nets some aquatic insects spin to catch small particles. Aquatic insects eat much of the debris from plants.
Some aquatic insects are collectors which trap bits of debris to eat.
Others are shredders. They cut up and eat leaves and other larger pieces of debris. Their leftovers become good pieces for bacteria or collectors to eat. Some insects are scrapers. They scrape algae and other material from the surfaces of rocks and logs in the stream.