Between 1970 and 2012, Earth's largest freshwater animal populations declined by some 88 percent, according to a new survey.

There's a lot of water on Earth, but only 2.5 percent of it is freshwater. Of that 2.5 percent, just over 1 percent is surface water, taking the form of rivers, lakes, ponds and wetlands. Despite its relative scarcity -- accounting for just 2 percent of Earth's surface area -- the planet's freshwater is home to tremendous levels of biodiversity. One third of vertebrates make their home in freshwater.

Unfortunately, many freshwater ecosystems are severely imperiled. Freshwater species are struggling as a result. Over the course of just 42 years, according to analysis by researchers in Germany, populations of freshwater megafauna -- river dolphins, beavers, crocodiles, giant turtles, sturgeons and more -- declined 88 percent, twice the rate of decline among vertebrates on lands and in the ocean.

Scientists at Leibniz-Institute of Freshwater Ecology and Inland Fisheries, IGB, and the Forschungsverbund Berlin e.V., or FVB, Berlin's largest non-university research institution, surveyed studies on the abundance and distribution of 126 freshwater megafauna species worldwide, including 44 species in Europe and the United States.

"The results are alarming and confirm the fears of scientists involved in studying and protecting freshwater biodiversity," study author and IGB researcher Sonja Jähnig said in a news release.

The most severe population declines have been experienced by animals in the Indomalayan realm, a biogeographical realm that extends across south and southeast Asia, as well as southern China, and the Palearctic realm, a massive region covering Europe, North Africa and most of Asia.

According to the new study, published this month in the journal Global Change Biology, most of the losses can be blamed on overexploitation and river blockages. Numerous studies have documented the environmental damage caused by a global excess of dams.

With fewer and fewer free-flowing rivers, species become fragmented and unable to migrate and access necessary resources. Many dams disrupt the mating and reproductive patterns of large freshwater animals.

The problem is expected to get worse.

"Although the world's large rivers have already been highly fragmented, another 3,700 large dams are planned or under construction -- this will exacerbate the river fragmentation even further," said study author Fengzhi He. "More than 800 of these planned dams are located in diversity hotspots of freshwater megafauna, including Amazon, Congo, Mekong and Ganges river basins."

Like most surveys of Earth's biodiversity, the news is worrisome, but here are signs that conservation efforts are working. In the United States, numbers of green sturgeon and American beavers are rising. In Asia, the population of the Irrawaddy river dolphin is making a comeback. And in Europe, reintroduction efforts have helped boost Eurasian beaver numbers. European sturgeon and Atlantic sturgeon numbers are also increasing.

Still, more must be done to protect the world's largest freshwater animals.

"According to the International Union for Conservation of Nature's Red List of Threatened Species, over half of all assessed freshwater megafauna species are considered as threatened with extinction," Jähnig said. "Nonetheless, they receive less research and conservation attention than megafauna in terrestrial or marine ecosystems."

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