The importance of riverflies for freshwater ecosystems

A flat-bodied stone clinger (Ecdyonurus sp.) nymph. Image courtesy of The Riverfly Partnership.

Despite covering less than 1% of Earth’s surface, freshwater ecosystems are home to at least 10% of the species on the planet. Rivers, streams and lakes are home to an abundance of wildlife and the presence and numbers of some key species, such as riverflies, can give us valuable insights into river health and water quality.

The term riverflies is used to designate predominantly caddisflies, mayflies, and stoneflies. These are invertebrates that spend most of their life cycle in freshwater habitats – rivers, streams, ponds, or lakes and don’t roam far from their place of origin. They lay their eggs in water and when they hatch the larvae live and grow in the water until they reach maturity and become winged insects. There are more than 270 species of riverfly in the UK, eight of which have Biodiversity Action Plan status, meaning they’re recognised as of priority for conservation by the government.

Riverflies form the basis of many food chains as an important food source for fish, birds, and mammals, so any changes in their numbers directly impact the whole ecosystem. By feeding on organic matter, these invertebrates also help to keep the water clean. Due to their sensitivity to environmental change, which makes them great indicators of the health of a river, riverflies are known as the ‘canaries of our rivers’. The key factors that can affect their populations are water quality, habitat availability and water flow.

According to the 2021 Riverfly Census Report conducted by WildFish, there’s been an average 41% decrease in mayfly species in chalkstreams between 1998 and 2021. Habitat loss, pollution, siltation (high levels of silt in the water), and reduced flows are some of the main issues threatening riverflies.

Monitoring and collecting data on these invertebrates are extremely important if we’re to understand more about them and their distribution, as well as the health of our rivers, as we mentioned before.

Philippa Nicholls, our Evidence Officer, has recently delivered a Riverfly Monitoring Initiative (RMI) training for citizen scientists based in the Silk Stream, a sub-catchment of the River Brent. RMI is a UK-wide citizen science programme run by the Riverfly Partnership that contributes to the protection and improvement of rivers’ health through monthly aquatic invertebrate sampling.

If you are interested in becoming a trained riverfly citizen scientist in a catchment in the Thames Basin and helping us protect our freshwater ecosystems, please keep an eye on our What’s On page as dates for more training sessions will be confirmed soon.