Biotic factors of rivers include insects, fish, periphyton and benthic macroinvertebrates categorized into functional feeding groups such as shredders and grazers. These living organisms make up some of the most important biotic factors in any river ecosystem, according to Nature Education.
The river ecosystem provides a dynamic environment for biotic diversity. Aquatic ecologists tend to focus on benthic macroinvertebrates in their studies on river ecosystems due to their abundance and complexity. The four functional feeding groups of benthic macroinvertebrates include shredders, which each leaves; collectors, which consume finely particulated organic matter; grazers, which feed on periphyton; and predators. The physical habitat of the river itself determines which of these four organism categories is most pronounced. Small temperate streams, for example, provide the ideal conditions for shredders and collectors to thrive, while canopies opening in larger streams give grazers better access to their main food source, periphyton.
Fish are also important contributors to the river ecosystem, especially anadromous fish that spend most of their lives in a marine environment and travel upstream to reproduce. These fish bring marine nutrients to the environment, which at least one experiment has linked to an increased abundance of insects in the river ecosystem, as stated by Nature Education.