Megafauna species living in freshwater habitats are some of the most vulnerable to extinction, new research shows.

Across the globe, large freshwater species are in rapid decline. Now, conservationists know why. In a newly published study, scientists in Germany detailed the threats facing large aquatic vertebrates, or freshwater megafauna.

Each species faces unique circumstances, but there are commonalities among their ecological vulnerabilities. For freshwater megafauna, habitat fragmentation appears to be a universal problem.

Large species, whether in the water or on land, need to move. They require more resources to survive, and as a result, require larger amounts of space to hunt and breed. Many large species migrate significant distances.

Increasingly, new research shows, the movements of large species are being restricted by human incursions. Dam construction, for example, has interrupted the migration routes and breeding patterns of the Russian sturgeon, the Ganges river dolphin and the Mekong giant catfish.

"The fragmentation of habitats is one of the central threats to freshwater megafauna, as well as overexploitation," Fengzhi He, a researcher at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, explained in a news release.

Of all the planet's freshwater megafauna species averaging more than 66 pounds, half are included on the International Union for Conservation of Nature Red List of Threatened Species.

These vulnerable species, scientists say, most of which sit at the top of the local food chain, are of extreme ecological value. Their presence provides balance.

"The importance of freshwater megafauna for biodiversity and humans cannot be overstated," said Fengzhi He.

In addition to habitat fragmentation, large freshwater species are also threatened by overexploitation, environmental pollution and climate change.

In their latest study, scientists argue conservationists should place an emphasis on the protection of freshwater megafauna and freshwater ecosystems.

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