Functional feeding groups are a classification approach that is based on behavioral mechanisms of food acquisition rather than taxonomic group. The same general behavioral mechanisms in different species can result in the ingestion of a wide range of food items. The benefit of this method is that instead of hundreds of different taxa to be studied, a small number of groups of organisms can be studied collectively based on the way they function and process energy in the stream ecosystem. Individuals are categorized based on their mechanisms for obtaining food and the particle size of the food, and not specifically on what they are eating.
This method of analysis avoids the relatively non-informative necessity to classify the majority of aquatic insect taxa as omnivores and it establishes linkages to basic aquatic food resource categories, coarse particulate organic matter (CPOM), and fine particulate organic matter (FPOM), which require different adaptations for their exploitation. The major functional feeding groups are: scrapers (grazers), which consume algae and associated material; shredders, which consume leaf litter or other CPOM, including wood; collectors (gatherers), which collect FPOM from the stream bottom; filterers, which collect FPOM from the water column using a variety of filters; and predators, which feed on other consumers. A sixth category includes species that do not fit neatly into the other categories such as parasites. It is important to keep in mind, however, that many kinds of invertebrates use a variety of food acquisition methods.