Global Climate Change May Spread Snails Into Coastal Salt Marshes
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16 December 2005
Snails mount attack on US wetlands
Coastal salt marshes might be overrun by snails if droughts
worsen with climate change.
by Charlotte Schubert
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.
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
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.
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
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.
1.Silliman B. R., et al. Science, 310. 1803 - 1806 (2005).
�2005 Nature Publishing Group
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