Buoyed by the effects of an intense drought, otherwise harmless snails likely killed off thousands of acres of salt marsh in the Southeast in recent years.
Periwinkle snails, known to science as Littoraria irrorata, normally coexist happily with salt marsh. But the drought, which lasted from 1999 to 2001, weakened and killed marsh grasses such as cordgrass, or Spartina alterniflora, so extensively that the snails moved from finishing off stressed patches to decimating large pockets of otherwise healthy marsh in concentrated waves. The result: the loss of an estimated 250,000 acres of marsh stretching over 900 miles on the Gulf and southeastern coasts between 1999 and 2003.
So says a paper set to appear Friday in the journal Science.
"It's important to note that drought was the trigger that initiated these events - and because drought stress is becoming more extreme with global warming, events like this could become both more frequent and intense," said Brian Silliman, the paper's lead author and an assistant professor in zoology at the University of Florida.
Salt marshes are key to healthy shorelines and oceans. They provide nurseries for juvenile fish and shellfish, filter water-borne pollutants and calm storm-driven waves, reducing the threat of hurricane-induced flood and erosion.
So scientists and citizens alike watched with alarm as the marshes started dying off from Louisiana to South Carolina beginning in 1999. Most earlier research pointed to the effects of a severe drought as the cause. The drought dried up soils, raised their acidity and boosted estuarine water and soil salinity levels -- all of which were blamed for stressing cordgrass and other marsh grasses beyond their limits.
Silliman and four other authors of the Science paper don't dispute the drought's impact on what scientists call "bottom-up" factors such as increased salinity. But, they say, decades of scientific tradition emphasizing only these types of influences resulted in overlooking "top down" ones - in this case, top-down controls potentially spurred by climate change.
"For ecology in general, the take-home lesson here is that increasing climatic extremes, such as drought stress, can trigger formation of grazer fronts and subsequent waves of habitat die-off in an otherwise stable ecosystem," Silliman said. "For marsh ecology, the message is even clearer: The long-standing paradigm that bottom-up forces rule is officially dead."
Climate change aside, the study also falls in line with other recent studies highlighting the role of predators or other top-down animals. "This study adds to a growing body of evidence showing strong top-down regulation of ecosystems processes, including sharks in the Gulf of Mexico and wolves in Yellowstone Park," Silliman said.
Native and abundant, dime- to quarter-sized periwinkle snails can often be seen hanging on cordgrass above the water line. Contrary to appearances, they don't actually eat the grass, or at least not much of it. Instead, they crunch up the surface to make it easier for colonization by fungi. In a process described as fungal farming, the snails then eat the fungi.
Periwinkle snails normally coexist happily with marsh grasses. But the drought so weakened cordgrass that it began dying off. Snails then moved in, finishing off weakened and dying patches. When these disappeared and exposed mudflats emerged, large numbers of snails moved off the flats and concentrated on the edge of the die off, where healthy grass remained. This migration resulted in the formation of grazer "fronts," which attacked and destroyed more marsh in "waves" of runaway consumption, the authors write.
A team of scientists from UF, the Netherlands Institute of Ecology, Louisiana State University and Brown University reached that conclusion after observations and experiments at 12 randomly selected die-off sites in Louisiana, Georgia and South Carolina. The research was funded by Georgia Sea Grant and The Nature Conservancy.
For part of the work, the scientists simply counted snails. They discovered "extreme densities" of 400 to 2,000 snails per square meter (about 10 square feet) on the borders between healthy and dying marsh. Those numbers compared to almost no snails on exposed grassless mud flats, and far fewer snails in healthy marsh set back from the die-off border.
The researchers also removed snails from plots of healthy marsh, then enclosed the plots with wire mesh, preventing snails from reaching the grass. They put enclosures in the path of expanding die offs, as well as in remnants of healthy marsh. After 14 months, the patches were "robust and green" compared with denuded areas where snails moved through.