Invasive Species Aids Salt Marshes

Ailing Cape Cod marshes are recovering with the arrival of European green crabs.

By | April 8, 2013

A European green crab.CATHERINE MATASSA/NORTHEASTERN UNIVERSITYThe green crab, an invasive species from Europe, threatens many native species in the United States, consuming shellfish, algae, and crustaceans. But its aggression has an unexpected silver lining. The hated species is indirectly preventing erosion in Cape Cod salt marshes by driving down the population of a native crab species that has multiplied out of control, according to a paper published in the journal Ecology last month (March 29).

The native species, the purple marsh crab, feeds on cordgrass. In recent years, overfishing has depleted the crab’s predators. As purple marsh crab populations have soared, they’ve eaten increasing amounts of marsh grasses, which hold the marsh soil together, preventing erosion.

Green crabs do not ordinarily live in marshes, preferring rocky shorelines. But invasive green crabs have begun to colonize unhealthy marshes, where they can take advantage of the extensive network of burrows dug by purple marsh crabs.

The researchers showed that green crabs evict purple marsh crabs from burrows, either killing them or scaring them away. When they put the two species in cages, only 15 percent of the purple marsh crabs survived, while all of the green crabs did. When the scientists put the crabs together in a larger enclosure, the purple marsh crabs hid and ate less grass than usual.

A survey of 10 Cape Cod marshes and found that the density of green crabs was associated with recovery in damaged marshes.

Mark Bertness, an ecologist at Brown University and one of the study’s authors, told ScienceNOW that he hoped the study would make people think harder before making blanket statements about needing to eradicate nonnative species. But Edwin Grosholz, an ecologist at University of California, Davis, who did not participate in the study, said that the green crab’s rehabilitation is far from complete. “It may have a positive effect in New England,” he told ScienceNOW. “It’s track record elsewhere is quite different.”

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Avatar of: WayneT


Posts: 1

April 16, 2013

Isn't the lesson here that we've overfished the native predators of purple marsh crabs, creating a better environment for an invasive? If native predators were present, purple marsh crabs would be less abundant and their burrows would be less plentiful for green crabs to inhabit. This story is nothing more than another example of an invasive outcompeting a native.

Avatar of: penbaywatch


Posts: 1

July 5, 2013

Maine coast scientists and clammers report that the green crab bloom is absolutely devastating softshell clams there and may be a key reason for a near complete disappearance of Casco Bay's once ubiquitous blue mussel beds from its shores - as well as elsewhere on the Maine coast.   Listen to Chad Coffin,  head of Maine Clammers Association testify before the Maine legislature's Marine Resources Committee earlier this year,  on the devastation wrought by green crabs. And to the questions the legislators responded with.   One wonders if one of the marsh crab's virtues is creating or enriching clam habitat, by stripping vegetation from low marsh and increasing sediment flow  from there into adjacent flats.    Might the green crabs thus be reducing softshell clams both by direct predation and by reducing sediment enrichment of intertidal flats through pushing out the purple marsh crabs?



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