Two Brown researchers, graduate student Brian Silliman and Professor of Ecology and Evolutionary Biology Mark Bertness, have just issued a similarly radical new understanding of coastal wetlands. Their paper in the August 16 Proceedings of the National Academy of Sciences could well have been titled "Thinking Like a Marsh."
The vast tidal marshes that fringe the East and Gulf coasts of the United States are among the earth's most productive ecosystems, sources of nourishment and refuge to a wealth of seafood and wildlife. Traditional scientific thinking holds that the source of this productivity is nutrients in seawater, swished twice daily throughout the marshes by tides. Ecologists
call this "bottom-up control" of the ecosystem. But what if the vitality of the marsh, like that of Leopold's mountain, was also controlled from the top down - by the periwinkle snails that cluster on marsh grass and the blue crabs that prey on snails? The implications for management of crab harvesting would loom large.
In the late 1990s Silliman began studying the gray, half-inch-long periwinkles in the marshes of Hog Island, Virginia. Ecologists thought the snails ate only dead or dying plants, but Silliman found them "farming" the marsh by making tiny wounds in the living grass blades and then grazing the fungi that grew there. At high enough population densities, they could inflict sufficient damage to kill the marsh, and could even switch to eating the marsh outright. Could the ubiquitous snails be playing a role in the mysterious die-offs now affecting marshes throughout the Southeast?
After Bertness suggested to Silliman that they test this hypothesis, the researchers placed 400 wire-mesh cages containing varying densities of snails throughout the cordgrass marshes of Sapelo Island, Georgia. "Everywhere we put snails at moderate to high densities, we ended up with a mudflat," Bertness says. Unchecked by predators, the snails could turn the world's most productive grassland to bare mud in as few as eight months. Even outside the cages Silliman found snails mowing down grasses at a rate of up to ten meters a year along a front some 300 meters across.
Factors such as rising global sea levels are also clearly influencing marsh die-offs. But the two scientists say their results show the potential for marshes to be altered from the top down, and with growing evidence that blue crabs are overfished, this could be contributing to marsh die-offs. "We're not saying stop all crabbing," Silliman explains. "I love to eat blue crabs myself. But this shows there can be consequences for the marshes."
Bertness says their study is one of several documenting the overlooked importance of top-down control of ecosystems. A classic West Coast study linked the overharvest of sea otters to an explosion of sea urchins and the subsequent devastation of kelp beds when urchins overran them."If we're right, and I'm confident we are," Bertness says, "these [salt marsh] systems are more like the kelp-urchin-otter system - a real shaking of the foundations of how people have seen salt marshes."
This top-down take on salt marshes will no doubt generate debate. "I'll need a lot of convincing to think the snails are driving marsh losses," says J. Court Stevenson, a leading salt marsh ecologist on Chesapeake Bay. He believes sea level rise might better explain the marsh losses occurring at Sapelo Island.
The next step, Bertness and Silliman say, is to scale up their experiments from small cages to the real world. One question, Silliman says, is to what extent other snail predators, like terrapins, compensate for the decline in blue crabs.
