How One City May Put an Alien Species to Good Work

Cameron Walker
for National Geographic News
April 12, 2005
The city of Olympia, Washington, has proposed using a pesky invasive
species of water snail to help prevent storm-water flooding.

The idea: Use millions of shells from the non-native aquatic snail that infests Puget Sound. Packing them beneath city sidewalks should slow down storm-water runoff.

In December the city applied for a grant from the U.S. Environmental Protection Agency to test the plan. The results are expected soon.

The project would be a unique way of creating an economically valuable use for an invasive species, said Scott Smith, aquatic invasive species coordinator for the Washington State's Department of Fish and Wildlife. "That's what really intrigues me," he said.

Native Oysters Knocked Out

During the gold rush days, big spenders along the U.S. West Coast developed a taste for a new delicacy—the Olympia oyster. Since then the tiny Puget Sound oyster, about the size of a half-dollar coin, has remained a favorite of connoisseurs. But during the last century oyster populations began to plummet. Pollution in Puget Sound, combined with intensive oyster harvesting, made the oysters decline to near extinction.

Improved water quality in Puget Sound has allowed the oysters to recover somewhat. But now a new menace lurks below the water. An alien aquatic snail species, the arched slipper limpet (Crepidula fornicata), has made its way to the sound from its native habitat on the East Coast.

Invasive species like these snails often travel as stowaways, hitching rides in the ballast water of ships or on boat hulls. Species like this often flourish in new areas because they have no natural predators.

Slipper limpets have also made their way across the Atlantic, taking over areas off the coast of the British Isles and Northern Europe.

Researchers aren't exactly sure when this particular snail arrived in Puget Sound, which hosts dozens of other alien invaders, including some species of sea grasses and crabs. But it's certainly made itself at home, forming thick carpets of shells on the seafloor, crowding out indigenous species.

"Native species must either adapt to the new habitat created by invasive species, or die," Smith said. But management plans like this one could help mitigate the effects of the invasive, giving native oysters a fighting chance, he said.

Storm-Water Problems

On the shores of the Puget Sound, Olympia, Washington's capital city, gets an average of 51 inches (130 centimeters) of rain each year, causing the city's streams to swell during a storm.

Rain that hits city streets and sidewalks runs off in a torrent known as storm water. The same roads that cars and people use become conduits for rainwater during a storm. Storm water engulfs local creeks and streams.

Olympia is growing rapidly, and with more development comes more pavement. Water running off the paved areas of the city is funneled directly into streams, often causing floods. Waterways are often contaminated by pollutants washing off the roads.

Storm-water drainage may be the biggest environmental problem the city faces, said Emmett Dobey, a program manager at the city's Department of Public Works.

This is where the slipper limpet can come to the rescue.

The scoop-shaped shell of the snail could help slow storm water. Currently the city uses rock and gravel beneath sidewalks to stabilize the concrete. City officials want to try using snail shells instead.

Water Traps

Millions of snail shells could act as tiny traps for storm water, releasing it slowly to retard the flow to the city's streams and storm-water holding ponds, Dobey said.

Dobey and others have tested the shells under sample sidewalk. They found that the shells held the water back for much longer than sidewalks' traditional underlay—enough time, Dobey said, to significantly slow the flow from street to stream.

The process also seemed to reduce the amount of pollutants that typically come along for the ride, he said.

Streets and sidewalks aren't the only paces that may benefit from shell underlays.

Olympia is trying to encourage its growing population to get out of their cars. It is using bike paths and sidewalks to make the city more friendly for walkers and cyclists.

One project would connect a series of parks via new paths, and this is where Dobey wants to put in shell-lined surfaces. An EPA grant would be used to make the new sidewalks. But even if the city doesn't get EPA funding for the project this year, it will begin construction on a few sidewalks this spring and summer, Dobey said. "We're starting to design them right now."

The project could be a bonanza for oyster harvesters. Right now the snail shells they scoop up while harvesting oysters are a nuisance. But what if they could get some economic benefit from the bycatch?

The city is "thinking outside the box, and that's great," said Tim McMillin, the president of Olympia Oyster, a Shelton, Washington, company that has harvested Puget Sound's oysters since 1878. If the project goes ahead, McMillin said, "I could sell these pest snails, make a little money, and clear out the ground for more oysters."

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