Earth’s biggest creatures on the brink of extinction as freshwater animals decline by 88%
Overexploitation and loss of habitat could soon drive many big freshwater animals - fish, amphibians, reptiles, and mammals - to extinction.
Scientists, for the first time, have quantified the decline of freshwater megafauna - which are big freshwater animals that weigh 30 kilograms or more - and found that their global populations have declined by 88% from 1970 to 2012. This is twice the loss of vertebrate populations on land or in the ocean, says the team from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin; Institute of Biology, Freie Universität Berlin; University of Oxford, UK; and University of Nevada, Reno, among other institutes.
“The results are alarming and confirm the fears of scientists involved in studying and protecting freshwater biodiversity,” says Sonja Jähnig, senior author of the study and expert for global change effects on river ecosystems at IGB.
The big freshwater animals include species of river dolphins, beavers, crocodiles, giant turtles, and sturgeons, among others. The analysis shows that among these freshwater animals, large fish species such as sturgeons, salmonids, and giant catfishes are particularly threatened, with a 94% decline, followed by reptiles with 72% decline in their population.
“The results demonstrate that freshwater megafauna populations exhibit even larger declines (88%) than those in overall freshwater vertebrates (81%), which is twice of the decline reported in terrestrial (38%) or marine (36%) vertebrate populations. In addition to population declines, major range contractions of freshwater megafauna have been observed, which is more pronounced in Europe than in the US. The most notable declines have been in the Indomalaya (by 99%) and Palearctic (by 97%) realms - the former covering South and Southeast Asia and southern China, and the latter covering Europe, North Africa and most of Asia. The sharp decline of Indomalayan populations began in the late 1980s, while Palearctic populations show a continuous decline since 1970,” says the study published in Global Change Biology.
The researchers say that unlike big animals who live on the land - such as tigers, elephants or giraffes - the big freshwater animals have received much less public attention, research, as well as conservation efforts. “Considering the human fascination with megafauna species (big animals), freshwater megafauna could and should be leveraged to inform the public of the crisis in freshwaters and promote conservation for overall freshwater biodiversity,” says the team.
The researchers compiled global population data for 126 freshwater megafauna species - 81 fishes, 22 mammals, 21 reptiles, and two amphibians - totaling 639 individual time series and covering 72 countries or regions.
The researchers also examined the historical and contemporary geographic distribution data of 44 species in Europe and the US and found massive changes in distribution.
For example, the once very common European sturgeon (Acipenser sturio) has been eliminated from all major European rivers, except the Garonne River (France). In the Danube and Volga rivers, sturgeon species, including the Russian sturgeon (Acipenser gueldenstaedtii), Persian sturgeon (Acipenser persicus), ship sturgeon (Acipenser nudiventris) and Stellate sturgeon (Acipenser stellatus), are now restricted to downstream sections, primarily due to the construction of large dams.
In Europe, eight species (42% of all species) lost more than 40% of their historical distribution range, compared to a single species (the Colorado pikeminnow, Ptychocheilus lucius) in the US.
“Though range contractions were less pronounced in the US, species such as the lake sturgeon (Acipenser fulvescens), alligator gar (Lepisosteus spatula), and paddlefish (Polyodon spathula) have been eliminated in some parts of Missouri and middle Mississippi river basins, as well as in the Great Lakes region,” analysis reveals.
Overexploitation and loss of free-flowing rivers are major threats
Overexploitation by humans is a key threat, since meat, eggs, and skin from sturgeons, crocodiles, and turtles, for example, are used as luxury food and medicines. “In addition, conflicts between freshwater megafauna and humans have escalated due to their large habitat requirements and the rapidly increasing human population and expanding anthropogenic activities. This has led to increased mortality rates caused by direct killing, or through accidents, such as vessel collisions,” says the study.
Habitat loss and degradation associated with large dams and pollution also contribute to population declines. The researchers say that large‐bodied animals are particularly vulnerable to extinction owing to their complex habitat requirements, low reproductive output, and late maturity, making them less flexible toward environmental changes.
Surface freshwaters, including rivers and lakes, cover approximately 1% of Earth’s surface but are home to one-third of all vertebrates and nearly half of all fish species globally. At the same time, freshwaters are exposed to multiple persistent and emerging threats, and are, therefore, among the most threatened ecosystems globally.
Since current conservation actions fall short in safeguarding freshwater habitats and biodiversity as they are rarely targeted in conservation management strategies and actions, a third of all classified freshwater species are threatened with extinction, according to the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species.
Mega‐fish are in a particularly grim situation as they are prone to both overexploitation and dam construction.
Researchers say the decline of large fish species can be attributed to the loss of free-flowing rivers as access to spawning and feeding grounds are often blocked by dams. For example, approximately 3,700 additional large hydropower dams are planned or under construction, increasing the fragmentation of rivers worldwide. More than 800 of these planned dams are located in diversity hotspots of freshwater megafauna, including Amazon, Congo, Mekong, and Ganges river basins.
The Russian sturgeon, Amur sturgeon (Acipenser schrenckii), Chinese sturgeon, and ship sturgeon have experienced population declines of over 90% during the past three generations, says the study.
Further, in megafauna-rich basins, such as the Mekong and Amazon, the situation is continuously deteriorating.
“For example, populations of mega‐fish in the Mekong River basin have dropped close to zero. This includes the Mekong giant catfish (Pangasianodon gigas), giant Siamese carp (Catlocarpio siamensis), and giant pangasius (Pangasius sanitwongsei). Further, Arapaima (Arapaima spp.) have been locally extirpated (destroyed) from 19% of surveyed fish communities along the main stem of the Amazon River,” the findings state.
Tackle glaring gaps in monitoring and conservation efforts
According to the research team, currently, there are massive gaps in freshwater megafauna monitoring, assessment, and conservation actions, which is the first challenge that must be tackled.
“Our results show a clear decline of freshwater megafauna across the globe. However, there are significant sources of uncertainty in tracking changes in population inventory and distribution range of freshwater megafauna. Primarily, this is due to a general lack of long‐term monitoring data available for these species,” they say.
For example, says the study, while IUCN Red List assessments suggest that many freshwater megafauna species, including the Mekong giant salmon carp (Aaptosyax grypus), pangas catfish (Pangasius pangasius), and yellowcheek (Elopichthys bambusa), have experienced a severe population decline, these species were not included in the analysis due to a lack of available time-series data.
“Consequently, an even sharper population drop would likely have been demonstrated if more species had been monitored and data made available,” says the team.
The gaps in population data are particularly seen in the case of mega‐reptiles and mega‐fish other than sturgeons and salmonids. Sturgeons and salmonids account for just 18% of all mega‐fish species, yet contribute 60% of all-time series for mega‐fishes in this study.
“Further, for 73% of all mega‐mammals, one or more time-series were available. Conversely, data for 52% of mega‐reptiles were not available. This is consistent with the current monitoring prioritization - that is, a focus on mammals and economically valuable species,” says the study.
The researchers also observed gaps in monitoring of freshwater megafauna populations.
According to the team, when taken together, Africa, Asia, and South America have contributed a mere 35% of all-time series data, even though they are home to 77% of global freshwater megafauna species.
“This mirrors the current biodiversity and conservation research distribution. For mega‐reptiles, such as crocodilians and giant turtles, six species in the Australasia and Nearctic realms contributed 54% of all-time series, yet more than 80% of all mega‐reptiles inhabit the Afrotropical, Indomalaya, and Neotropical realms,” they state.
Accordingly, the team recommends future studies focusing on population monitoring, distributions (critical habitats, migratory routes), and life‐history traits of freshwater megafauna, to develop proactive conservation strategies.
They say this is particularly necessary for megafauna‐rich basins like the Amazon, Congo, Mekong, and Ganges, which must account for rapidly increasing and emerging threats.
“In addition, a comprehensive and regularly updated database of freshwater megafauna species is urgently needed, alongside a global initiative to combine and consolidate knowledge and data on freshwater biodiversity,” says the team.