The banks of the Potomac River upstream from Washington, D.C., are often mounded with drifts of tiny shells bleached white by the sun. That invertebrate abundance startles me every time I walk the riverbank, a clue to an invisible city of bivalves under the water.

In this issue, freelance writer Stephen Ornes takes us to Appalachia, where he chronicles scientists’ dogged work to restore an endangered species of mussel, the golden riffleshell. Freshwater mussels used to be incredibly common in the United States, but dams and pollution have taken a toll. It’s a clear loss, because the humble creatures play an outsize role in riparian ecosystems, and in public health: More than two-thirds of U.S. homes get their drinking water from rivers, and mussels excel at cleaning water.

Ornes, who lives in Nashville, was looking forward to wading streams with the researchers doing the work, but the pandemic scotched those plans. Instead, multiple phone interviews helped him re-create the painstaking work of finding the rare mussels. “Tell me what it’s like being in the river,” he asked. “What does it feel like when you’ve been in the river for three hours and haven’t found one?” A video tour of Kentucky’s Center for Mollusk Conservation in Frankfort, led by director Monte McGregor, also helped. “Between his descriptions and the real-time tour, I felt like I could get a sense of what these people are doing every day,” Ornes says.

This summer Ornes, like many parents, found himself with children at loose ends. The family ended up spending a lot of time kayaking the Tennessee River, blending work and play in a particularly satisfying way. “We’ve learned so much about the biodiversity,” Ornes says. “It’s like the Amazon of America right now in our backyard.”

In this issue, we also examine the global challenges of protecting that biodiversity. Staff writer Jonathan Lambert digs into goals being drafted by the U.N. Convention on Biological Diversity to protect 30 percent of the planet’s land and sea by 2030 — and 50 percent by 2050. But even those ambitious targets may not be enough to slow extinction rates, with up to a million species at risk of disappearing in the next few decades.

“A lot of what’s been written about these targets gets less into the science behind them,” Lambert told me. “I thought it was useful to touch, in more detail, on how these numbers are arrived at, and the limitations of them, too.” Those limitations include the difficulty of agreeing on what protection actually means, given that countries vary greatly in their needs, resources and priorities for preserving biodiversity. And once again, the pandemic intrudes. A conference to hammer out the details was supposed to convene in China this October; it’s been delayed until 2021.

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