A Science study mapping 45 threats across 21,490 species finds crops, dams and farm runoff each affect close to a quarter of freshwater species.
Farming is the most widespread of the threats to freshwater species worldwide, according to a study published in the journal Science. The paper maps 45 threats across 21,490 freshwater species of amphibians, fish, mammals, reptiles and odonates, the group that includes dragonflies and damselflies.
Three threats each affect close to a quarter of freshwater species in any sub-watershed on Earth, the authors estimate. Annual and perennial non-timber crops lead, with an average impact probability of 0.281. Dams and water management follow at 0.240, and agricultural and forestry effluents at 0.235.
The authors tie the farm finding to earlier research. Agriculture has become the single most important driver of land-use change, they write, and exponential increases in fertiliser and synthetic chemical use have caused widespread habitat loss and degradation.
The study, “The global geography of threats to freshwater biodiversity”, was led by Christophe Brabant, a PhD candidate in biology at McGill University in Montreal. Its senior author is Lars Lønsmann Iversen, a professor of biology at McGill. The ten authors include researchers from the University of Copenhagen, Lúrio University in Mozambique and the International Union for Conservation of Nature (IUCN).
How the maps were built
The team drew on the IUCN Red List of Threatened Species, which records the pressures known to affect each assessed species, and combined those records with species range maps. For every location, the method estimates the share of species present that are reported as affected by a given threat. The authors call this the impact probability.
The approach maps where pressures harm species, rather than only where the pressures occur, the paper says. Wastewater, farm effluents and dams can damage species far from their source, and the Red List records that damage. The result is 225 global maps, one for each of the 45 threats in each of the five animal groups, combined into a cumulative threat index.
Brabant said in a McGill University statement that “these maps give us a new, freshwater-specific baseline for conservation”.
Farms and pollution cover the most ground
Agriculture and aquaculture is the dominant threat category across 29.4% of Earth’s land area, much of it in the tropics, the authors found. Pollution from urban, agricultural and industrial sources comes second at 21.8%, and dominates across Europe, India and eastern China. Biological resource use, which includes fishing and hunting, ranks third at 15.3%, mainly in northern latitudes and parts of South-East Asia and sub-Saharan Africa.
Europe and the north-western Maghreb, the freshwater ecoregion the paper calls the Permanent Maghreb, show the highest cumulative threat. Pollution drives Europe’s score, according to the study. In the Maghreb, pollution combines with damming and intensive water use. The Nile, the Niger Delta and the Rift Valley Lakes also rank high. The lowest threat levels are in thinly populated Siberia and northern Canada, and in deserts including the Sahara and the Arabian Peninsula.
Each animal group faces a different main threat, the paper shows. Fishing and dams are the leading threats across about two-thirds of the global range of fish, at 37.0% and 30.5% respectively. Agriculture dominates for amphibians (45.8%) and odonates (31.45%), which depend on both land and water at key stages of life, the authors write. Hunting and other resource use lead for reptiles (37.8%) and mammals (43.4%).
Linked to extinction risk and declines
The authors tested the maps against two independent measures. In the 5% of sub-watersheds with the highest modelled extinction risk, 71% of the threats showed higher impact probabilities than their global average.
The team also compared the threat index with 2,568 populations of 600 freshwater vertebrate species in the Living Planet Index database. Populations are declining faster where the cumulative threat is higher, the study reports.
Freshwater mammals face higher impact probabilities than land mammals for all seven threats previously mapped for terrestrial vertebrates, the authors found. The gap is smaller for reptiles. The hotspots for freshwater species also differ from those for land animals, and the paper warns that conservation plans built on land data alone can overlook where freshwater species are most at risk.
The paper’s introduction sets out the stakes, citing earlier studies. Freshwater ecosystems hold almost a third of all described vertebrate species on 3.5% of Earth’s area. Wetlands are shrinking three times faster than forests, and only 37% of the world’s large rivers remain free flowing.
Limits of the method
The authors list the method’s limits. The Red List treats threats as acting independently, though many interact. Impact probabilities show whether a species is affected, not how severely. Some threats may be under-reported for certain animal groups or regions. Because the work relies on current range maps, it may miss areas where species have already disappeared.
Iversen said in the McGill statement that the work “can help indicate what actions should be taken”. The threat maps and the code behind them are freely available on the Zenodo and Dryad research repositories.
