Disappearance of Southern Salt Marshes Explained

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A few years ago, for reasons nobody fully understood, salt marshes in the Southeast started turning into barren mudflats, where the few remaining patches of grass were covered with hordes of snails. A new paper in Science explains what happened and why it might happen again in the future if global temperatures rise.


And now some environmental news. Climate scientists have tallied the numbers and discovered that the Earth's average temperature reached a near-record high this year. On average, the global thermometer reached 58.1 degrees Fahrenheit; that's just 1/10th of a degree lower than the all-time record set in 1998. The new numbers mean the last decade is shaping up to be the hottest since people started keeping detailed records some two centuries ago.

Ecologists have noticed a worrisome phenomenon in the salt marshes of the Southeastern United States: the sudden disappearance a few years ago of marshes lining more than a thousand miles of coast land. Some see the cause as a record drought, but others point to another factor: hordes of voracious snails that turned the marshlands into mud flats. NPR's John Nielsen has more.

JOHN NIELSEN reporting:

Healthy salt marshes suck the power out of hurricanes headed for cities. They filter out industrial pollutants, and they protect a lot of fish we like to eat. When they're healthy, they're also a lot of fun to tromp around in, say biologists like Courtney Hackney. He's a salt marsh expert at the University of North Carolina at Wilmington.

Mr. COURTNEY HACKNEY (University of North Carolina at Wilmington): It's flooded about up to my knees, and I can feel the oysters underneath my feet, hopefully not cutting into my boots or my shoes--various kinds of fish splashing around on the surface, very clear water, very yellow-green plants with little heads of flowers on them. That's sort of my vision of a perfect salt marsh.

NIELSEN: But in recent years many of the region's salt marshes have been far from perfect. Several hundred thousand square miles of marshlands have all but vanished, leaving murky mud flats in their place. It happened first along the coast of Louisiana and then spread out, eventually reaching parts of the coast of North Carolina. Salt marsh experts like Brian Silliman of the University of Florida say they've never seen a regional die-off this extensive, nor has anybody found what Silliman found at the edges of the disappearing grasslands: millions and millions of inch-tall snails stretched out like an attacking army.

Mr. BRIAN SILLIMAN (University of Florida): The common name of this snail is the marsh periwinkle, and we're talking 2,000 to 2,500 snails per meter squared. And so when you look down at the marsh, you don't see green grass; you see grass bent over with a snail on every inch of every piece of grass. Instead of looking green and brown, the marsh just looked gray, the color of the snail shells.

NIELSEN: NPR reached Silliman in a small town in Argentina, where he's doing field work. He says periwinkle snails have long been common in Southeastern marshes, where they eat both grasses and a fungus that grows in their slime trails. Normally the grasses fight back by toughening their skins and releasing chemicals, but Silliman says those defenses fell when the record drought kicked in. The snails multiplied and converged on the edges of weakened marshlands, mowing them down after slathering them with a heavy coat of fungus. Silliman says the periwinkle hordes would then move on to their next meal.

Mr. SILLIMAN: The snails have this interesting behavior in that they always hone towards marsh grass. If you put a snail in the middle of a mud flat, it will move 30 meters in two hours to get to the closet marsh grass.

NIELSEN: Silliman reports on the periwinkle wars in the latest edition of Science. He notes that in some marshes the snail attacks were less than overwhelming for reasons he does not understand. He also notes that many of these marshlands have bounced back since the record Southeastern drought ended last year, but not all of them. And that's a source of real concern to Courtney Hackney, the wetlands expert at UNC-Wilmington. Marshlands are already stressed out enough by rising sea levels, he says, and by rising human populations near the coastlines.

Mr. HACKNEY: You know, at one time when sea level rose, the marsh just rose along with it and just sort of covered up what were previously uplands. And now what we have are all those marshes being essentially blocked off from doing that by bulkheads 'cause people live there.

NIELSEN: If all these problems worsen, there may come a day soon when the marshes stop bouncing back, he says. The fish will go away, the pollutants won't be filtered and the mud flats will be endless. John Nielsen, NPR News, Washington.

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