KIAWAH ISLAND — A tiny snail could turn the sweeping marshes along Capt. Sam's Spit into mud flats. That's a little-talked-about, worst-case possibility in the controversy over building an embankment along the river bank here.

When the marsh periwinkle snail takes over, cordgrass dies. The snail has ribbons of teeth that scrape like tongues to eat algae from the grass. That can shred it enough to let in a fungus that kills it. When the snails turn up in hordes, the marsh becomes a mud flat.

One of the few predators of the marsh periwinkle is the diamondback terrapin, the marsh turtle, which is in decline along Capt. Sam's Spit. And on the Kiawah River, the marsh turtles nest primarily along the bank where the embankment would be built. It might be one of the terrapins' last strongholds along the river.

When Davidson College biologist Michael Dorcas first studied the turtles in nearby Fiddler Creek, in the late 1990s, he would see 20 to 30 per day.

“Now I see three or four,” he said. Meanwhile, he sees evidence that marsh die-off is occurring in spots. “The collapse of the (turtle) species is probably going to have an effect on the entire ecosystem.”

The embankment

Capt. Sam's is an exposed, narrow-necked sandscape at Kiawah Island's western edge. The development company that planned to build 50 homes in the sand dunes there has been sold; the new owners “are still committed to a low-density development on no more than 17 of the 185 acres,” said Mike Touhill of Kiawah Partners.

The embankment would be a porous, sloping wall needed to support an access road along the eroding bank of the Kiawah River where the spit's neck is at its narrowest point.

Conservationists and community groups have fought the development for more than two years, because of the natural importance of the spit.

The access road and embankment would be built along a stretch that is part of a rare, out-of-water “strand-feeding” ground for dolphins, where they drive fish up on the bank to eat.

The homes would be built across a dune field near the tip of a changeable inlet where the tidal flats are a relatively undisturbed feeding ground for flocks of shorebirds.

And the embankment is a breeding ground for the terrapins.

The development is being held up by legal challenges that still have not been decided.

The balance

Marshes die off periodically for any number of reasons, and biologists suspect that cordgrass has to be weakened before the snails can kill it. But Dorcas isn't just speculating here.

He's in charge of the longest-running study of diamondback terrapins in the world, dating back to 1983. He was one of the experts called by conservationists to testify in a court hearing over the wall.

Dorcas suspects the turtles' decline is due to a combination of factors, such as crab-trap snarls and a degradation of the waterway as Kiawah and Seabrook islands develop around it. As a control, researchers also surveyed undeveloped Little Townsend Creek off Botany Island nearby. Its turtle population remains robust.

If the Kiawah waterway is degrading, the cordgrass is also. When it comes to ecosystem health, the key is the balance of species, said biologist Shelley Dearhart, of the South Carolina Aquarium. “Every piece of that ecosystem is equally important. If you eliminate one of those parts, it messes up the dynamics of the balance.”

The proposed half-mile-long embankment would still leave 3,000 feet of bank open, Touhill said. It has been designed to be porous and sloping to cause the least habitat disruption while stabilizing the bank.

“That's why we went this route. It's an environmentally sensible and reasonable approach,” he said. Dorcas' court testimony supported that, Touhill said. Dorcas doesn't agree.

“I testified that it would be detrimental and potentially (could) completely block the ability of terrapins to nest there,” he said.

The turtles

Strand-feeding dolphins and the hordes of shorebirds on Capt. Sam's Spit are the natural phenomena that draw onlookers and have raised concerns over the proposed development. The diamondback terrapin isn't usually seen and has nowhere near as high a profile — except to its devotees.

Dorcas' research team will be back out there this month, surveying Fiddler Creek and Little Townsend Creek among others. The terrapin's lifespan is thought to be 25-40 years. For years, Dorcas would recapture turtles that had been captured at the study's start.

“Very few nowadays,” he said. “The thing is, we catch far fewer recaptures (overall) than we used to.”

The terrapin is the only salt marsh turtle. Very few of its eggs survive. If it's going to survive along the Kiawah River, it needs the bank on Capt. Sam's Spit, Dorcas said.

“They need to be left alone to persist.”

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