Invasive Snails Take a Toll on Native Ducks

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The number of lesser scaup ducks is dwindling, and it could be an invasive species that does them in. Invasive snails and parasites are attacking these and other ducks on the Upper Mississippi.

U.S. Fish and Wildlife Service officials say with no natural predators, the snails and the parasites are thriving, and killing off a duck population that is already in trouble.

The agency has made a huge effort to create more and better habitat for the shrinking duck population, but it may be for naught.

As with a lot of other invasive species, once the creatures take hold, it's a challenge to get rid of them.

Nearly 150 invasive species live in the Mississippi River Basin, and while not all of them are destructive to native habitat, this snail has become a duck killer: The snail has helped kill nearly 50,000 ducks in the last few years in the Upper Mississippi Wildlife Refuge, which borders Wisconsin, Minnesota and Iowa.

The area is a rest stop of sorts for more than 450,000 migrating ducks every year, which snack on wild rice and snails. Pools on the river provide safe havens for birds from hunters.

Three types of invasive intestinal parasites are killing the birds.

"All three use an invasive snail, called the mud bithynia, or faucet snail, as an intermediate host," said Jim Nissen, who works for the Fish and Wildlife Service.

The snail slowly made its way from New England. Waterways along the Great Lakes played a role. In another circumstance, the faucet snail might be a new dish on the menu for ducks.

But Nissen says when the ducks eat these snails, the parasites they carry bore into the ducks' intestinal walls.

"They gorge on blood and then lay eggs," Nissen said. "The eggs are passed through the birds' feces, and that's how they reach the snails. That's how the cycle is perpetuated."

The lesser scaup duck is particularly susceptible to this parasite, killing them only in a few days. Their bodies litter the water like decoys. There were eight million lesser scaup in North America in the late 1970s now their numbers have dropped by half. Nissen worries that this snail-parasite duo could spell the end of the lesser scaup.

The snail and the parasites have been found as far south as Dubuque, Iowa, near another bird hot spot. In Texas, the invasive apple snail is thriving on wild rice beds. That's also duck food — and those snails, too, carry an intestinal parasite that is fatal to ducks.

Biologists have tried 15 poisons on the snails. Nothing has worked.

Scientists are currently redesigning islands on the river in an attempt to make them less hospitable to the snails. They're also hoping the ducks can develop a resistance to the parasites that live in the snails that they find irresistible.

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