A PLAGUE of snails unleashed by drought has killed off huge areas of salt marsh in the south-eastern US. The event demonstrates that ecological interactions can turn a small climate change into a catastrophe.
Salt marshes along the Gulf of Mexico suffered severe diebacks of cordgrass, their dominant plant species, after a severe drought that lasted from 1999 to 2001. More than 100,000 hectares have disappeared over the last six years. Most ecologists put this down to the drought, but Brian Silliman, a marine ecologist at the University of Florida in Gainesville, noticed unusually high densities of periwinkle snails along the boundary between the denuded mudflat and living grass.
鈥淚t鈥檚 an army of snails,鈥� he says. 鈥淚t鈥檚 difficult at some points to see the grass for all the snails on top of it.鈥� In earlier studies, Silliman had found that the snails graze fungi off the grass blades, damaging the grasses and so allowing the fungi to infect them, sometimes lethally.
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鈥淚t鈥檚 difficult at some points to see the grass for all the snails on top of it鈥�
So Silliman and his colleagues built cages in 2002 to protect cordgrass from the snails. A year later, they found the grass flourishing within the cages, while adjacent areas exposed to the army of snails were bare (Science, vol 310, p 1803).
What happened, Silliman thinks, is that when the drought killed off small patches of cordgrass, the snails that had lived in them clustered at the edge of the remaining grass. Now at densities of up to 2000 snails per square metre, compared with 100 to 400 per square metre on healthy marsh, the snails formed a hungry 鈥渇ront鈥� of grazers. Coupled with the effects of the drought, this seems to have been enough to kill off the grass. 鈥淵ou go from a stable to an unstable system, and that can lead to greater collapse than we would predict by just focusing on drought stress alone,鈥� says Silliman.
Such runaway interactions in salt marshes, not to mention other ecosystems, could become more prevalent as global warming increases the risk of drought, says Robert Paine, a marine ecologist at the University of Washington in Seattle.
