A quarter of all freshwater species are at risk of extinction

Thousands of fish, crab and dragonfly species could become extinct in the coming decades – and many more could follow.

A landmark assessment of the health of nearly 24,000 freshwater species found that just under a quarter are at risk of extinction. Of these, almost 1,000 species are considered Critically Endangered, with 200 having potentially been lost already.

These numbers may only represent the tip of the iceberg, with scientists lacking the information needed to properly understand the extinction risk of thousands of species. Catherine Sayer, the lead author of the study, says that urgent action is needed to understand and protect these animals.

“Lack of data on freshwater biodiversity can no longer be used as an excuse for inaction,” she says. “Freshwater landscapes are home to 10% of all known species on Earth and key for billions of people’s safe drinking water, livelihoods, flood control and climate change mitigation, and must be protected for nature and people alike.”

The findings of the study were published in the journal Nature.

What challenges do freshwater habitats face?

While fresh, clean water is vital for all life on land, freshwater ecosystems are some of the most threatened on Earth. Freshwater environments are being put under pressure as demand for food, water and resources increases.

Wetlands in particular, including bogs, mangroves and saltmarshes, are bearing the brunt of these losses. It’s estimated that an area the size of India – a staggering 3.4 million square kilometres – of wetland has been lost since 1700.

The loss of wetlands harms far more than just the animals and plants that live there, as it also limits our ability to fight climate change and stop flooding.

Freshwater sources are also suffering from a cocktail of different challenges. An increase in water abstraction and dams on rivers are reducing the available habitat for wildlife. Some rivers, like the Colorado River, no longer even reach the sea.

The animals that persist in these reduced habitats are then impacted by sewage, industrial and plastic pollution. Yet, despite the rising risk to these species, freshwater environments are significantly understudied compared to the oceans.

Dr Topiltzin Contreras MacBeath, a co-author of the study, says that this needs to change if we are to ensure that freshwater species have the best chance of survival.

“It is essential that freshwater species data are actively included in conservation strategies and water use management to support healthy freshwater ecosystems,” Topiltzin says. “Increased investment in measuring and monitoring freshwater species is needed to ensure conservation action and water use planning is based on the latest information.”

How threatened are different freshwater animals?

While around 10% of all species depend on freshwater, the study was particularly concerned with four groups that are intimately linked to it – the decapods, odonates, molluscs and fishes.

Decapods are a large group of crustaceans that include crabs, lobsters, crayfish, shrimp and prawns. While many species live in the sea, plenty of others live in rivers and streams across the world where around 30% are threatened with extinction.

The leading threat to decapods is pollution, especially from agricultural runoff. Many pesticides used to kill insects also harm crabs and shrimp in waterways, particularly when these animals are moulting. These chemicals are also a threat to fishes and can influence their development, fertility and behaviour.

The odonates are the group of insects that contains dragonflies and damselflies. While they might be most recognisable as airborne predators, these animals spend most of their lives in freshwater as growing nymphs.

This makes them vulnerable to habitat loss, with more than half of threatened species affected by the conversion of wetlands to farmland. A similar proportion are affected by logging, which removes the hunting grounds and shelter they rely on as adults.

While conservation data was readily available for freshwater decapods, fishes and odonates, a lack of information meant the study was unable to include molluscs in its analysis. This is the group that not only contains snails, but also other invertebrates such as freshwater mussels and oysters.

Previous research focusing on molluscs suggests that around a third are at risk of extinction, which would likely increase the overall proportion of threatened freshwater species.

The vast and often interconnected nature of freshwater environments means that reversing these declines is not easy. Co-operation between different stakeholders and nations will be vital to give threatened crabs, dragonflies and other freshwater wildlife a fighting chance.