Roberta Attanasio, IEAM Blog Editor
On 18 February, the United Nations Environment Programme (UNEP) released a report highlighting the three major emergencies that are currently challenging the entire planet— climate, biodiversity, and pollution. “Making peace with nature: A scientific blueprint to tackle the climate, biodiversity and pollution emergencies” is based on evidence from global environmental assessments. “Humanity is waging war on nature. This is senseless and suicidal. The consequences of our recklessness are already apparent in human suffering, towering economic losses, and the accelerating erosion of life on Earth,” said António Guterres, Secretary-General of the United Nations in the report foreword.
In timely fashion, “Human impacts on global freshwater fish biodiversity” published in Science on February 19 and points out the human impacts on the global biodiversity of freshwater fish. While there is clear awareness of the loss of biodiversity in terrestrial and marine systems, the extent of loss in freshwater has not received comparable attention—until now. In the Science article, Guohuan Su and his collaborators recently found that half of the global river systems have been heavily affected by human activities, resulting in the homogenization of rivers. In other words, rivers in temperate climates now contain mostly similar species. They note that very large tropical river basins were less affected. Changes in freshwater fish biodiversity were assessed over two centuries in almost 2,456 river basins, hosting more than 14,000 species, and covering almost the entire continental surface of the planet. However, while previous studies evaluated the number of species, the study by Su and collaborators also examined how closely related the species were, along with their ecological role.
The researchers found that the impact on biodiversity is greatest in temperate rivers. A major factor in biodiversity loss in these rivers is habitat fragmentation, a process—in this case caused mostly by dams—in which continuous habitats are divided into smaller, more isolated habitat fragments. Indeed, habitat fragmentation is well recognized as one of the most important causes of population decline, biodiversity loss, and alteration of community structure across the many ecosystems of the world. A global assessment published in 2015 found that dams had already altered 48% of all the river flow worldwide. Notably, only a quarter of long rivers that originally flowed freely to the sea were still following the same course—most had been instead rerouted to an inland lake or to another river.
The study by Su and collaborators also shows that another factor involved in the loss of temperate river biodiversity is the introduction of non-native species. Senior author Sébastien Brosse told The Guardian that rivers in many rich nations were unrecognizable compared with how they were before the industrial revolution: “Then we had sturgeon that were more than 2 meters in size, we had thousands of salmon, and many other fish that have almost disappeared today.” He added that a major change is the number of alien species introduced to rivers. “In western Europe, you will see North American salmon, black bullhead, which is a North American catfish, carp and goldfish that come Asia, and mosquitofish.” He also told Bloomberg Green that individual rivers face different challenges, from overfishing in the Mekong River to increased damming along the Amazon.
Interestingly, the authors point out that although present-day rivers are more similar to each other in terms of fish species they host, at the local level the introduction of alien species results in more diversity. Furthermore, the study found that, depending on the regions, other factors contribute to biodiversity changes. For example, in North America, a large role is played not only by fragmentation but also by the large agricultural and industrial water use.
In addition to their effects on river biodiversity, current agricultural practices are projected to cause the loss of large chunks of natural ecosystems—and therefore biodiversity—as result of the increasing demand for food, animal feed, and bioenergy crops. The United Nations Decade on Biodiversity 2011–2020 is at its end, and the International Union for Conservation of Nature (IUCN) actively supports the development of an ambitious new global biodiversity framework. Combined, the recent results on the loss of biodiversity and its causes provide information to develop the new framework, safeguard biodiversity, and manage global conservation targets.