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Tough guy. Invasive green crabs will freak out herbivorous marsh crabs, allowing cordgrass to regrow.

Catherine Matassa/Northeastern University

Hated Invasive Species Helps Restore an Ecosystem

It's not quite redemption, but one of most loathed invasive species in the world—the European green crab (Carcinus maenas)—has had a surprisingly positive effect on an ecosystem. On Cape Cod, Massachusetts, researchers have found that the crab is reversing a decades-long trend of damage that another species has inflicted on salt marshes. It might be the first nice thing that the green crab has done for anyone.

Green crabs are aggressive and have colonized most of the world's coastlines, where they've chased away or killed native species. "It eats about everything," says marine ecologist Mark Bertness of Brown University. "In terms of biodiversity, it's hell on wheels." But in New England, green crabs are now fixing an ecological problem that humans caused.

Recreational fishing and crabbing on Cape Cod have removed most of the native predators that used to eat a common native species called the purple marsh crab (Sesarma reticulatum). With the predators off their backs, the marsh crabs are now up to four times more common than before. And they're hungry, feasting on the tender young shoots of cordgrass (Spartina alterniflora), which is the predominant plant in the salt marshes. In addition, the crab burrows make the soil more likely to erode. Over the past 3 decades, more and more salt marsh has been laid bare.

But about 3 years ago, Bertness noticed that cordgrass was starting to recover. "This was quite surprising," he says. Soon, he realized that salt marshes with lots of green crabs were doing the best. It was a bit odd to find the green crabs there, because these crabs, which invaded New England more than 100 years ago, tend to live near the shore, where they can hide under rocks.

Bertness thinks that the salt marshes became more appealing for green crabs once the marsh crabs had dug so many burrows. (Even though predators are half as common as before, both crabs need burrows to escape from birds.) The 5-centimeter-wide green crabs, Bertness figured, would have no trouble kicking the smaller marsh crabs out of their burrows and making them skitter away.

To test the idea that this was how green crabs are indirectly helping the cordgrass, Bertness and his lab manager, Tyler Coverdale, surveyed 10 marshes on Cape Cod in August 2012. They found that, in fact, recovering patches of salt marsh did have denser populations of green crabs. And they confirmed the brutal nature of green crabs: When confined in cages and pitted one against another, green crabs evicted marsh crabs from their burrows. More than 85% of the marsh crabs died in the cages, they report online this week in Ecology, which were littered with broken shells and severed limbs.

Just the fear factor alone may have a big effect in reducing herbivory. Another test in a larger enclosure showed that the presence of a single green crab caused marsh crabs to spend the entire month of the experiment in hiding. Even if the green crab was locked up in a cage, the marsh crabs rarely dared to venture out. By the end of the experiment, the marsh crabs had eaten an order of magnitude less cordgrass than they usually did. Once cordgrass gets a break, Bertness says, it can rapidly regrow.

What does it all mean? Bertness says that many ecologists have a "knee-jerk reaction to invasives" and that removing them just because they are invasive may not be the best use of limited conservation funds.

"It's important to acknowledge that introduced species will in some cases provide an unintended benefit, and this is a cool one," says Edwin Grosholz, an ecologist at the University of California, Davis, who was not involved in the study. "That means we have to pay attention as we eradicate species" that have invaded an ecosystem. But he's not ready to put the green crab on a pedestal. "It may have a positive effect in New England," he says. "Its track record elsewhere is quite different."