In experiments along the Virginia and Georgia coasts, the researchers manipulated local populations of animals and found that when blue crabs disappeared from a salt marsh, one of their chief prey periwinkle snails flourished. Once free of predation from blue crabs, the periwinkles ate all of the cordgrass in a marsh. Cordgrass dominates the southern marsh, anchoring it and providing its animals with habitat. Without the plants to bind sediment and protect wildlife, the salt marsh ecosystem collapses, the scientists found.
In fact, the study shows that overgrazing by periwinkle snails will convert a southern salt marsh into a barren mudflat within eight months, said lead scientist Brian Silliman.
"Cut back the blue crab harvest, because even if we're half right, the results of over-harvesting could be disastrous," said Mark Bertness, the project's senior researcher. Their findings appear in the current Proceedings of the National Academy of Sciences.
Southern salt marshes stretch from Chesapeake Bay to the central-Florida coasts. They are some of the most productive grasslands in the world. The marshes also temper coastal flooding, filter mainland run-off and act as nurseries for commercially important fish and other species. The marshes also protect barrier islands from erosion, and barrier islands buffer the shorelines.
Hundreds of miles of southern salt marshes have died in recent years, particularly in Louisiana and Florida. Silliman and Bertness surveyed several of those dead and dying marshes and found very high densities of periwinkles. "Blue crab populations have been in rapid decline due to over-harv
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