River biodiversity at risk

Between extinctions caused by habitat modification or pollution, and species introductions, human activities have profoundly affected fish biodiversity in rivers around the world.

Although rivers account for just 3% of the world's surface area, they are home to some 17,000 species of fish. How is this fauna impacted by human activities? To find out, Sébastien Villéger, a researcher at the Marine Biodiversity, Exploitation and Conservation (Marbec) laboratory*, and his colleagues at the University of Toulouse surveyed more than 2,400 watercourses, covering almost 80% of the world's rivers. The aim: to map the number of species in rivers to determine changes in biodiversity over the last two centuries. "This is the most exhaustive study ever carried out on this subject," says the researcher.

Rivers preserved far from mankind

And according to their findings, published in the journal Science, these changes are legion. " More than 50% of the rivers considered in the study have had their fish fauna strongly modified by human activities," explains Sébastien Villéger. Are some rivers spared? "Our study shows that only 14% of the world's rivers are home to fish biodiversity that has been little affected by human activity. These rivers, which show no local change in their fauna, are precisely those located farthest from man, in areas where there is little economic activity: "tropical areas such as the Amazon, central Africa, part of south-east Asia, northern Australia and a few rivers in the far north of the northern hemisphere", explains the researcher.

Everywhere else, the fauna that populates our rivers is nothing like it was just a few decades ago. Some species are on the brink of extinction, like the sturgeon, which has been decimated by overfishing. Others are in poor health, such as the trout, a particularly sensitive species that is just as intolerant of warming waters as it is of river pollution. Still other species are affected by changes to their habitats, such as those which see their turbulent waters transformed into stagnant waters by the construction of a dam, giving way to other fish better adapted to this new environment.

Introduced species

While some species are now absent, others are eager to take their place. " A third of the fish now found in French rivers were not there in the past," explains Sébastien Villéger. Carp, pike-perch, gambusia, catfish, sun perch, sheatfish - these fish are now an integral part of our river landscapes, but they have all been introduced by humans, most of them deliberately. This introduction is detrimental to native species, which often suffer from the arrival of newcomers.

So why move fish? " Introductions began in ancient times, with the Romans bringing carp to Europe for farming. Other species of interest to the fish industry were introduced after the 18th century," explains the biologist. Some of these species, such as catfish and blackbass, also provide good catches for anglers. Many species have also been introduced through the aquarium trade, such as the goldfish, which some aquarium owners have released into rivers. " As for the gambusia, it has been introduced in many countries to combat the proliferation of mosquitoes, whose larvae it eats", adds Sébastien Villéger.

Homogenization

But then, with so many newcomers, hasn't the biodiversity of our rivers increased? " Indeed, it has sometimes increased locally, but that's not necessarily good news... answers Sébastien Villéger. This increase in biodiversity is accompanied by a standardization of our rivers, which end up all looking the same. This is a problem on a planetary scale , as most of the world's rivers are subject to similar human pressures, and the faunistic, functional and phylogenetic differences between rivers subject to the same disturbances tend to diminish". This homogenization is all the more problematic as it reduces the capacity of ecosystems to respond to global change. " This willprobably lead to local losses of species in the future," warns the specialist.

Can we limit this phenomenon? "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 expansion of those already established", envisages Sébastien Villéger. In the USA, for example, electric barriers have been deployed in canals to prevent silver carp from invading large lakes from the Mississippi basin. In France, however, only the sun perch and catfish are officially considered pests.

*Marbec (UM - CNRS - IRD - Ifremer)