Two recent assessments of the world’s freshwater ecosystems catalogue the scope and severity of human impact on these once-bountiful, biodiverse habitats that contain a quarter of the world’s known vertebrate species.
Humanity’s ever-expanding footprint has slashed biodiversity in more than half of Earth’s freshwater river basins, with only 14 percent remaining pristine, according to new research published last week in the journal Science. This week, 16 conservation organizations released a global assessment of the world’s freshwater fish species, finding nearly a third are at risk of extinction. This most recent assessment, titled The World’s Forgotten Fishes, also finds that the biggest fishes—species weighing more than 60 pounds—have undergone a particularly calamitous decline, with their numbers plummeting by 94 percent over the past half century.
The World’s Forgotten Fishes frames this lost biodiversity—the 80 species declared extinct, 16 disappearing in 2020 alone—as not just a tragic draining of our planet’s natural beauty and evolutionary grandeur, but levies a heavy human cost. Some 200 million people are fed by protein from freshwater fishes and 60 million people depend on hauling in that essential catch to support themselves and their families.
Perhaps unsurprisingly, the researchers behind the paper in Science observed that the river basins surrounded by heavy human presences were the most severely degraded, reports Karina Shah for New Scientist.
“Rivers which have the most economic development around them, like the Mississippi river, are the most strongly impacted,” Sébastien Brosse, an evolutionary biologist at Paul Sabatier University in France, tells New Scientist.
In contrast, the rivers that were mostly spared this fate were primarily in Africa and Australia, he adds, speculating that this is probably due to slower industrialization in Africa and sparse human populations around rivers in Australia.
Brosse and his co-authors created an index to quantify changes in fish biodiversity in almost 2,500 rivers across the globe—excluding the polar and desert regions. Prior attempts to study the extent of human impact on the world’s river ecosystems have focused only on changes in the number of species, reports Damian Carrington for the Guardian. But the new effort also incorporates the ecological roles and evolutionary relationships of the freshwater species.
Per New Scientist, overfishing and climate change are the most significant and pervasive drivers of the global decline in freshwater biodiversity, but the blockages created by dams and the introduction of non-native species have also played significant roles.
Dams and other modifications of waterways can kill off native species and even hasten their replacement by invasive fish. By turning rivers that once flowed swiftly into still or slow-moving waters, dams give invasive fish such as carp, largemouth bass and tilapia, which are adapted to such conditions, a competitive advantage. In this way, the prevalence of dams homogenizes rivers—a 2019 study found just around a third of the world’s longest rivers remain free-flowing. These chopped up habitats also undoubtedly contributed to the 76 percent decline of migratory freshwater fishes recorded by another report published in 2020.
Brosse tells the Guardian that the 14 percent of river basins that have remained relatively untouched are not enough to “maintain global biodiversity of fish” because they only contain 22 percent of the world’s nearly 18,000 freshwater fish species. “We also need to conserve the biodiversity in basins highly impacted by humans,” he says.
Speaking with Helen Briggs of BBC News about The World’s Forgotten Fishes, Jeremy Biggs, director of the Freshwater Habitats Trust, a successful conservation plan will need to consider waters large and small, from rivers and streams to lakes and ponds.
Carmen Revenga, a senior fisheries scientist at the Nature Conservancy, tells BBC News, "it's now more urgent than ever that we find the collective political will and effective collaboration with private sector, governments, NGOs and communities, to implement nature-based solutions that protect freshwater species, while also ensuring human needs are met."