[LUM#16] The biodiversity of rivers in danger

[LUM Magazine, Podcast] - Published on May 2, 2022 in Science-Society

Between extinctions caused by habitat changes or pollution, and the introduction of species, human activities have profoundly disrupted fish biodiversity in waterways around the world.

Although rivers and streams cover only 3% of the Earth's surface, they are home to nearly 17,000 species of fish. How is this fauna affected by human activities? To find out, Sébastien Villéger, a researcher at the Marine Biodiversity, Exploitation and Conservation Laboratory (Marbec)* and his colleagues at the University of Toulouse have studied more than 2,400 waterways, covering nearly 80% of the world's rivers. The aim was to map the number of species in rivers to determine the changes in biodiversity that have occurred over the last two centuries. " This is the most comprehensive study ever conducted on this subject, " says the researcher.

Rivers preserved far from human activity

And according to their findings, which were published in the journal Science, these changes are widespread. "More than 50% of the waterways considered in the study have had their fish fauna significantly altered by human activities, " explains Sébastien Villéger. Are some rivers spared? "Our study shows that only 14% of the world's waterways are home to fish biodiversity that has been little affected by human activities." These waterways, which show no local changes in their fauna, are precisely those rivers located furthest from humans, in areas where there is little economic activity: "tropical areas such as the Amazon, Central Africa, part of Southeast Asia, northern Australia, and a few waterways in the far north of the Northern Hemisphere, " the researcher explains.

Elsewhere, the fauna that inhabits rivers is nothing like it was a few decades ago. Some species are on the verge of extinction, such as sturgeon, which has been decimated by overfishing. Others are struggling, such as trout, a particularly sensitive species that is as intolerant of warming waters as it is of river pollution. Still other species are affected by changes to their habitats, such as those that see their turbulent waters transformed into stagnant waters due to the construction of a dam, giving way to other fish more adapted to this new environment.

Introduced species

Because while some species are now absent, others are just waiting to take their place. "A third of the fish found in French rivers today were not historically present," says Sébastien Villéger. Carp, zander, gambusia, catfish, sunfish, and wels catfish are all fish that are now an integral part of our river landscapes, but they were all introduced by humans, mostly intentionally. This introduction is detrimental to native species, which often suffer from the arrival of newcomers.

So why move fish? "Introductions began in ancient times, notably with carp, which was introduced to Europe by the Romans for farming, followed by other species of fishing interest, especially after the 18th century, " explains the biologist. Some of these species also allow fishing enthusiasts to show off their catches, such as catfish and black bass. Many species have also been introduced through aquarium keeping, such as goldfish, which some aquarium owners have released into rivers. "The gambusia, meanwhile, has been introduced in many countries to combat the proliferation of mosquitoes, whose larvae it eats, " adds Sébastien Villéger.

Homogenization

But with so many new arrivals, hasn't the biodiversity of our rivers increased? "Yes, it has increased locally in some cases, but that's not necessarily good news," replies Sébastien Villéger. "This increase in biodiversity has been accompanied by a standardization of our rivers, which are all starting to look the same. This is a global problem because most of the world's waterways are subject to similar human pressures, so the faunal, functional, and phylogenetic differences between waterways subject to these same disturbances tend to diminish." This homogenization is all the more problematic because it reduces the ability of ecosystems to respond to global changes. "This will likely lead to local species losses in the future," warns the specialist.

Can this phenomenon be limited? "We can take measures to protect and restore habitats, as we did for salmon, which helped save populations. We can also consider taking more drastic measures to prevent the introduction of new species and limit the spread of those already established, " suggests Sébastien Villéger. In the United States, for example, electric barriers have been deployed in canals to prevent silver carp from invading the Great Lakes from the Mississippi basin. In France, only sunfish and catfish are currently officially considered pests.

*Marbec (UM – CNRS – IRD – Ifremer)

Keywords: water, Environment, LUM#16, LumLu, Research, Company