A global survey has found that 24% of freshwater fauna is under threat of extinction.

“Freshwater ecosystems are highly biodiverse and important for livelihoods and economic development, but are under substantial stress,” write the authors of the study which appears in Nature.

More than 10% of all known species live in freshwater habitats. These ecosystems are critical for nutrient cycling, flood control and climate change mitigation.

The authors of the new study say that global assessments of extinction risk in freshwater ecosystems have been lacking.

“Consequently, data from predominantly terrestrial tetrapods are used to guide environmental policy and conservation prioritization, whereas recent proposals for target setting in freshwaters use abiotic factors,” they write.

To plug this gap, the researchers assessed representation of freshwater fauna in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species.

They analysed 23,496 species of freshwater crustacean, fish and odonates (insects like dragonflies and damselflies). Nearly a quarter of these species are currently listed as “vulnerable” or worse.

Decapods (crustaceans such as crabs, crayfishes and shrimp) have the highest number of threatened species, with 30% at risk of extinction. About 26% of fish species and 16% of odonates are at risk.

“Prevalent threats include pollution, dams and water extraction, agriculture and invasive species, with overharvesting also driving extinctions,” the authors explain.

Pollution is the biggest threat, affecting 54% of the species at risk. Dams and water extraction threaten 39%, land usage change and agriculture affect 37%, and 28% are threatened by invasive species and disease.

The authors say that the broad trends of extinction threat in freshwater species parallels research based on land-dwelling animals. However, they warn that research on land animals alone can’t be assumed to be enough to conserve local freshwater ecosystems.

While the authors are optimistic that there is a global shift to conservation of freshwater biodiversity, they say more work is required.

For example, they say that further research is needed to assess the extinction threat facing freshwater molluscs, other insects, plants and fungi.