December 19, 2005
Periwinkle Snails Harming Louisiana Marshes
NEW ORLEANS -- There are plenty of culprits from which to choose in assigning blame for the destruction of Louisiana's marshes: oil companies that carved wetlands to tatters with their exploratory canals, levees that hemmed in the nourishing waters of the Mississippi River, burrowing nutria that left marshes exposed to grass-killing saltwater.
Now, the state's dwindling coastline must deal with another nemesis: the marble-sized periwinkle snail.Researchers from Brown and Louisiana State universities added it to the list after witnessing millions of the snails chomping their way through drought-weakened marshlands across the Gulf of Mexico and up the eastern seaboard. Over the past six years, the snails have left in their wake thousands of acres of bare mud flats where there were once thriving wetlands.
Marsh destruction removes a natural buffer against hurricanes and robs fish, crabs and waterfowl of crucial habitat.
"These marshes have one of the most potent grazers (the snails) in the animal kingdom, especially in a stressed environment," said Brian Silliman, a former Brown researcher and lead author of a study on the snails published in this week's issue of the journal Science.
"As the grass becomes more stressed (by drought), the snails stay on those grass stems until they're completely dead. Then they fan out."
The article describes the snails as a "major contributing factor" to marshland erosion. Silliman said they killed as much as 11 percent, or 11,000 acres, of the estimated 100,000 acres of marsh lost in Louisiana during the 2000-01 drought.
But Mark Schexnayder, a coastal specialist at the Louisiana Sea Grant program, has his doubts. He buys the premise that the periwinkle snail eats marsh grasses, but he said ranking it anywhere near levees or oil company canals diverts attention from the root of the coastal erosion problem.
"Obviously they exist and are part of the web," Schexnayder said. "But I'm not dropping the fight against the main causes of the erosion to take on a snail."
In a healthy marsh ecosystem, one periodically refreshed by flooding rivers and uninterrupted by man-made canals, the snails would be irrelevant, he said.
The periwinkle snail, he said, has been around for thousands of years - long before Louisiana's coastal marshes began dying off at a rate of 25 to 30 square miles per year.
"I just don't see how something that has been with us all these millennia could all of a sudden turn into an agent of destruction," Schexnayder said. "A vigorous system could always respond to attacks by native species."
Silliman agreed that the periwinkle snail ranks well behind other marsh killers. But he said his team's study represents a "huge shift" from earlier assumptions that the health of marsh soils was the determining factor in erosion.
And if global climate change leads to more frequent droughts, Silliman said, the snails could play an even greater role in coastal erosion in the future.
In his work along the coasts of Louisiana, Georgia and South Carolina, Silliman said, he has seen lines of the snails up to 200 meters long and 2 meters wide, steadily destroying everything in their path.
They strike first where marshes already are dying, devouring struggling stalks of cordgrass at a rapid rate. When those are gone, the snails turn their attention to healthier swaths of marsh. The snails leave wounds that attract a fungus fatal to the host plant.
"The drought is the trigger in a domino event that leads to the concentration of snails," Silliman said.
Irving Mendelssohn, an LSU wetlands researcher and collaborator of Silliman's, said he, too, was initially skeptical of the snail grazing theory. But after conducting experiments in which some areas of marsh were protected from the snails while others left exposed died off at a more rapid rate. Mendelssohn realized their contribution to marsh loss was significant.
"We know the snails did not cause the initial die-back," he said. "After the plants died, the snails expanded the area. They form these large density fronts and move like herbivores through the marsh."
Information from: The Times-Picayune, http://www.timespicayune.com