DAVID GREENE, HOST:
We take certain parts of the natural world for granted - right? - trees cleaning the air, worms enriching soil, bees pollinating crops. But increasingly, these natural systems that we all rely on are failing. They're under attack from climate change, human development and disease. NPR's Nathan Rott has the story of one critical species that's dying and the team racing to find out why.
NATHAN ROTT, BYLINE: Slip into your waders and slosh out into the knee-deep waters of the Clinch River in southwest Virginia. The Clinch flows at the feet of the age-rounded southern Appalachian Mountains, slowly descending towards Tennessee. Its water is sharp, cold and gin-clear, which is good because what we're looking for lives on the rock-crusted river bottom.
(SOUNDBITE OF WATER SLOSHING)
JORDAN RICHARD: So we start walking along, and it's just a matter of, like, how long's it take till we see something that died very recently?
ROTT: That is Jordan Richard, a biologist with the U.S. Fish and Wildlife Service. And it doesn't take him long for him to find what he does not want to see.
RICHARD: See that one? So that's a pheasantshell that's just laying there. You can see it's not buried.
RICHARD: And that's - it should be buried in by that foot.
ROTT: He pulls a palm-sized object from the water.
RICHARD: And it's not. That's dead.
ROTT: Richard is holding a freshwater mussel, a less-edible version of its saltwater cousin that lines river bottoms across the country, cleaning water and providing habitat to other species. Its shell is gold-brown and glistening. But the milky white mussel inside is turning a gray-brown at its edges - the color of decay. Biologist Rose Agbalog brings over another mussel in similar condition.
ROSE AGBALOG: That's that smell.
ROTT: Oh, yeah.
AGBALOG: It's real, real bad.
AGBALOG: So that's been dead maybe a couple days - day or two.
ROTT: They find another dead mussel - and another...
RICHARD: By the time I stopped right there, I found, like, five.
ROTT: ...And then more after that. Standing mid-river a bit later, Richard lifts his gaze from the water and looks upstream at seemingly nothing in particular.
RICHARD: This is not what I was expecting.
ROTT: Not in a good way.
RICHARD: No, which I'm pretty used to. I'm pretty used to, like, coming out here, thinking I know what I'm going to see and then just getting completely, like, bombed with dead mussels. But it sucks, man.
ROTT: The U.S. Fish and Wildlife Service estimates that hundreds of thousands of freshwater mussels have perished since the die-off was first noticed in 2016. Biologists and fishermen were finding fresh dead mussels week after week after week, and not just here. Unexplained freshwater mussel die-offs have since been documented in Oregon, Washington, Wisconsin and Michigan. There's even been one in Spain.
EMILIE BLEVINS: It's a major concern for the future and for the future health of our freshwater.
ROTT: Emilie Blevins is a conservation biologist with the Xerces Society for Invertebrate Conservation, a nonprofit that focuses on some of the world's more, let's say, under-loved critters.
BLEVINS: Yeah, people don't tend to get quite as excited about things that lack backbones, unfortunately.
ROTT: But many of those species are critically important to the natural world. Take freshwater mussels. They're filter feeders that remove algae, sediment and heavy metals from passing water.
BLEVINS: There's research that's shown they can remove pharmaceuticals from the water and pesticides and flame retardants. And they remove E. coli from the water.
ROTT: So think of them as nature's equivalent to a Brita filter, cleaning the water we drink while also providing suitable habitat to countless other species. The problem is freshwater mussels are also one of the most imperiled species on the planet. Dozens of mussel types have already gone extinct in North America, wiped out by water pollution, human development and habitat loss.
The current die-off is just one more threat, widespread and fast-moving. And its cause - Richard, the biologist, says that's the challenge. It could be a million things. But because of limited time and resources...
RICHARD: We've had to just pick and choose what we think is the most likely things and then start running them down. And that takes years' worth of effort.
ROTT: A team at the University of Wisconsin that's working with Richard and Blevins has detected a virus and a bacteria that are statistically associated with the die-off, but they're not willing to call either the culprit just yet. Climate change is stressing ecosystems and threatening species around the world, but it does not seem to be the driver here.
So with the clock ticking and a cause unknown, the team in Virginia has something of a contingency plan - a hatchery - or nursery, more or less - for freshwater mussels.
TIM LANE: This is one of our living streams. So pheasantshell is in here. That's the one that's been going through the die-off.
ROTT: Tim Lane is a mussel recovery coordinator with the Virginia Department of Game and Inland Fisheries. He's walking us through what's essentially a long shed lined with PVC pipes, troughs and dozens of shallow black buckets half-filled with sand. These buckets are home to a who's who of endangered mussel species.
LANE: And here's an endangered Cumberland combshell.
ROTT: Not to be confused with the endangered Cumberland monkeyface nearby.
LANE: Over here, we have a golden riffleshell over here. I think we have 50. That's more than probably live in the wild that we haven't put out there.
ROTT: This facility is a last line of defense for some freshwater mussel species. Lane and other biologists are reproducing them here and then keeping them safe until they're mature enough to be brought back into the wild. When the recent die-off started on the Clinch, they brought a bunch of pheasantshells here from an unaffected part of the river. The mussels could be used as a baseline, a healthy sample to use as a search for the die-off's cause. But in a worst-case scenario where the die-off continues unabated, they could also be used as stock.
LANE: And we're not going to stand idly by and just watch them lose their way. We're going to do the best we can to help them produce progeny so the species isn't gone forever.
ROTT: Inside the hatchery's office, away from the troughs and pumps, Lane and Richard say they know mussels aren't as photogenic as a rhinoceros or as easy to communicate as the plight of the polar bear. But Richard says freshwater mussels and many other lesser-known species are like the foundation of a house that everything lives in.
RICHARD: It's not sexy to care about the foundation of your house when you could renovate your kitchen.
ROTT: But if that foundation is crumbling and you ignore it...
RICHARD: By the time you notice a problem because you fall through the floor, it's too late to do anything about it.
ROTT: And everything else falls through, too.
Nathan Rott, NPR News, Abingdon, Va.
(SOUNDBITE OF FOALS SONG, "2 TREES") Transcript provided by NPR, Copyright NPR.