More than half of the planet’s freshwater river basins have been heavily impacted by human activities, according to a new study. Although freshwater bodies—such as rivers and lakes—cover less than 1% of the planet’s surface, they support a considerable amount of the planet’s biodiversity.  This includes a quarter of Earth’s vertebrates, comprising various species of fish, reptiles, and amphibians. They also support the functioning of a variety of ecosystems and are largely responsible for human civilisations. 

However, this human activity over centuries has led to climate change, pollution, overfishing, over-irrigation, and many other practices that destroy riverine health. But little was known about the scale and magnitude of human impact. Now, scientists have found only 14% of the world’s river basins have fish populations unaffected by human activities. 

The study, published in the journal Science, evaluated the extent to which freshwater biodiversity has changed due to human activity over the past 200 years, by studying around 2,500 rivers worldwide, excluding polar regions and deserts. It developed an index to measure the change in a specific area, based on six key biodiversity indicators, called the Cumulative Change in Biodiversity Facets (CCBF). The index provides a score ranging from 0 – 12, where higher scores represent a greater change in ecology. Previous work focused simply on species numbers, but this study included the ecological roles of the species, as well as how closely related the different species are. 

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The study found that the worst-hit regions were western Europe and North America, where exploitation of river resources has historically been the highest—such as Thames in the U.K. and Mississippi in the U.S. According to researchers, rivers in these nations were practically unrecognisable compared with how they were before the Industrial Revolution. “Then we had sturgeon that were more than 2 metres in size, we had thousands of salmon, and many other fishes that have almost disappeared today,” Sébastien Brosse, of  Université Paul Sabatier in Toulouse, France, who was a part of the research team, told The Guardian. The least-affected rivers, on the other hand, were in the remote and sparsely populated regions of tropical Africa and Australia. 

A major change was in the alien species that were now living in the rivers. Researchers found big fish such as sturgeon have been largely replaced by invasive species such as catfish and carp. In temperate rivers, where human impact is greatest, biodiversity changes were primarily due to the interruption of the river’s natural flow by human activity such as building dams, as well as existence of new, invasive species. This homogenisation, where rivers contained similar species and fewer unique flora and fauna, show large fish populations had gone extinct. 

The researchers, however, add the study’s assessments are probably an underestimate; more fish extinctions than officially recorded are probable. “We are increasingly aware of human impacts on biodiversity across our planet, especially in terrestrial and marine systems,” write the authors in the study. “We know less about freshwaters, including large rivers.” Protection must also focus on areas where biodiversity has already been eroded by human activity, say the researchers, and add that the CCBF framework that they have developed provides a method to quantify the impacts of humans on global biodiversity.