A new study has found that half of all river systems in the world have been seriously affected by human activities, with significant impacts on their biodiversity, informs The Guardian. Globally, only less than 14% of the river basin area and river length have eluded damages and remain least impacted.
Even though river and lake ecosystems represent less than 1% of the earth’s surface, they are home to 17,000 fish species, about 25% of all vertebrates. They provide food for millions, and are a key source of freshwater.
The research, published in Science, looked at almost 2,500 rivers across the planet, taking into account biodiversity changes occurred over the past 200 years. The study found the impact has been worst in rivers found in temperate regions, with changes in biodiversity mainly due to river fragmentation and the introduction of non-native species. Other pressures on river ecosystems are pollution, overfishing, irrigation and global warming.
In western Europe and North America, prosperous and densely populated, human activities have had the largest impact. Examples of heavily affected rivers are the Thames in the UK and the Mississippi in the United States. Researcher Sébastien Brosee, from Paul Sabatier University in Toulouse, France, noted that rivers in wealthy nations have drastically changed compared to what they looked like before the industrial revolution.
Populations of migratory fish such as salmon, and large species have experienced the steepest declines. Introduced exotic species have flourished all over the globe. Some of the most common ones, like common carp, largemouth bass and tilapia, are adapted to live in still waters and do well in dammed rivers. Rivers have thus become more homogeneous, with similar species, something that will render them less able to adapt to environmental changes in the future.
South American rivers have the highest biodiversity, but only 6% of the least affected rivers are in this region. Predictably, the authors found most intact rivers are in remote areas, sparsely populated, mostly in Africa and Australia. But Brosee warns “They only host 22% of the global fauna, so we also need to conserve the biodiversity in basins highly impacted by humans.”