BioEd Online

Swarms of fungus-eating snails have laid waste to miles of US coastal wetlands that are already reeling from drought, say researchers in the southern United States. And global warming, they say, might exacerbate ecological breakdowns that give these plant-munching beasts the upper hand.

The marsh periwinkle (Littoraria irrorata) could be considered charming by some standards. It is about 2.5 centimetres long and has a clever feeding trick: it rakes the surface of marsh grass, creating a home for tasty and nutritional fungi. But when the snails get together, it can get ugly.

The snails graze on marsh cordgrass (Spartina alterniflora), which grows in vast fields in coastal areas down the eastern seaboard and into Louisiana. Usually the grass and snails live in harmony. But from 1999 to 2001, the region suffered from a drought. This made the grass more susceptible to fungal infection and snail infestation, says Brian Silliman, lead researcher of the study at the University of Florida in Gainesville.

Climate models predict that droughts will last longer in a warming world, putting more stress on coastal ecosystems.

Overkill

In one part of a Louisiana wetland, Silliman counted 2,000 snails per square metre. "There were layers and layers of snails pressing down on the grass. Normally the grass is four feet tall, but it had been compressed to just one foot," he says. "There were snails on every inch of it." When the grass has been worn away, the snails take off at the unsnail-like pace of 15 metres per hour to find new gardening grounds.

More than 250,000 acres of southern coastline has died off in the past six years. In places such as Louisiana, other factors related to human development clearly come into play. But the snails have certainly contributed to large patches of barren mud in many of the wetlands, says Silliman.

To show this, he and his colleagues created snail-free zones in marshes by caging out the creatures. Spartina flourished in these zones but died outside of them. The researchers also mimicked the effects of drought by increasing the salinity of patches of marsh. In these areas, the grass became sickly and the snails thrived. The findings appear in the current issue of Science¹.

Snail explosion

The idea that weakened wetlands can make way for explosions in herbivore populations is not new, says Bill Mitsch, a wetlands expert at Ohio State University in Columbus. But Silliman and his colleagues have gone a step further by proposing that the snails can cause so much havoc. Mitsch is not entirely convinced of the heavy impact of the snails, but notes that the work disturbingly suggests that other coastal areas, such as mangrove forests, might be similarly vulnerable.

Silliman is now looking at a coastal environment in Argentina that is dominated by another herbivore, plant-eating crabs, to see whether they too take over vegetation weakened by drought.

Since 2001, the US wetlands have begun to recover from the drought, and the grass is growing back, although many areas were again devastated by flooding after Hurricane Katrina.