Based on extensive field studies, the work challenges six decades of salt marsh science. Ecologists have long thought that stressed soil - too much salt, not enough oxygen - was the main killer of this critical marine habitat.
But Brian Silliman, a Brown University research fellow and a University of Florida assistant professor, said drought-stressed soils pave the way for predatory periwinkles that spread fungal disease as they graze on cordgrass.
"Snails can transform healthy marsh to mudflats in a matter of months," said Silliman, lead author of the Science paper. "This finding represents a huge shift in the way we see salt marsh ecology. For years, scientists thought marsh die-off was simply a 'bottom-up' problem related solely to soil conditions. We found that the trouble also comes from the top down. Drought makes the marsh vulnerable, then the snails move in."
Thousands of acres of salt marsh have disappeared from South Carolina to Texas since 2000, according to several scientific studies. In Louisiana alone, more than 100,000 acres of marsh were severely damaged between June 2000 and September 2001. This drastic decline poses a serious threat to the ecology and economy of the southeastern seaboard and the Gulf Coast. Salt marshes serve as nursery grounds that support commercial fisheries, protect coastline from storm-induced floods, and filter fresh water before it flows out to sea.
Mark Bertness, chair of the Department of Ecology and Evolutionary Biology at Brown and a co-author of the paper, said a better understanding of the causes of salt marsh loss will point to better ways to protect them.
"Loss of blue crabs and turtles, which prey on periwinkles, allows the snails to flourish," Bertness said. "Protect the crabs and turtles and you can help save the marshes."
Silliman came up with the periwinkle premise as a graduate student conducting field research in Virginia. Silliman found that removing snails from cordgrass, the dominant plant species in salt marshes, bumped up grass growth as much as 50 percent.
Silliman earned his Ph.D. at Brown and worked in the Bertness lab along with Johan van de Koppel, a former postdoctoral research associate now at the Netherlands Institute of Ecology. For more than two years, the trio tested Silliman's top-down theory of marsh ecology along the Georgia, South Carolina and Louisiana coasts in conjunction with Lousiana State University researchers Lee Stanton and Irving Mendelssohn.
In 12 randomly selected die-off sites, the team surveyed periwinkle populations. They found the largest concentration of snails - as many as 2,000 per square meter - along dead-zone borders. To test the idea that the snails contribute to cordgrass death, they created dozens of deterrents - wire mesh enclosures measuring about one meter square. Enclosures were placed ahead of fronts of grass-grazing snails and monitored for more than a year.
The results: Inside the enclosures, snail-free cordgrass thrived. In fact, plant biomass increased more than threefold. Outside the cages, in 11 of the 12 sites, snail overgrazing converted healthy marsh to exposed mudflats in as little as three months. When snail density was high, destruction was more extensive.
Researchers also wanted to test the notion that increased soil salinity, brought on by drought, acts in concert with snails to kill marshland. So in one healthy site in Georgia, the team elevated soil salt concentrations in areas with and without snails. Sites were monitored for eight months.
In the experimental plots, increased salinity reduced grass growth by 45 percent while high salt levels, in combination with the presence of snails, reduced grass growth by 84 percent.
How do periwinkles contribute to marsh destruction? Silliman has shown that they kill the grass by slicing the stems during grazing, leaving plants vulnerable to harmful fungi. In a process called "fungal farming," snails then consume this fungi living off injured grass.
"We've found a synergism between climate change and grazers," Silliman explained. "Severe drought triggers formation of traveling fronts of grazing snails. Then there is runaway consumption, which leads to waves of marsh destruction. Given predicted increases in climate change-induced drought, these results highlight the potential for marsh die-off to be even more intense and extensive in the future."
The findings, the authors argue, underscore the interplay of climate and consumers in the worldwide collapse of coastal systems. While an overabundance of snails may fuel southeastern salt marsh destruction, they point to other examples of habitat destruction that may be caused, in part, by a plethora of grazers: sea urchins wiping out California kelp beds, sea stars devastating Australian coral reefs, snow geese decimating marshes along the Artic Sea, bark beetles killing off Arizona pine forests.
Georgia Sea Grant, Louisiana Sea Grant, The Nature Conservancy, the National Science Foundation and the Schure-Beijerinck-Popping Fund supported the work.