European green crabs help restore salt marshes on Cape Cod

Invasive species are usually the bad guys in conservation. But an invasive crab is helping restore salt marshes on Cape Cod by forcing out more destructive crabs, a new Ecology study suggests.

Along the New England coast, fishing has left many marshes bereft of predatory animals. As a result, marsh crabs that would otherwise have been eaten by the predators have multiplied. The marsh crabs have gobbled cordgrass along creek banks, making the land erode more easily.

Enter the European green crab, which invaded North America a couple of hundred years ago and has settled on Cape Cod. Researchers studied ten recovering marshes in the area last summer and discovered that the more green crabs they found at a site, the more the cordgrass had regrown.

The team wondered whether green crabs were stealing the marsh crabs’ homes. So they performed experiments in which they placed a green crab and a marsh crab into an enclosure with one burrow. In other experiments, they put a marsh crab alone into the enclosure.

Nearly all the lone marsh crabs staked out the burrow as their home. But when a green crab was present, none of the marsh crabs was able to settle into the burrow, and more than 85 percent of them were killed by the green crabs.

Finally, the team placed green crabs into enclosures with marsh crabs and cordgrass. After a month, the amount of cordgrass left was several times higher than in enclosures without green crabs. “These results suggest that invasive species can contribute to restoring degraded ecosystems,” the team concludes. ❧

—Roberta Kwok

Bertness, M.D. and T.C. Coverdale. 2013. An invasive species facilitates the recovery of salt marsh ecosystems on Cape Cod. Ecology doi:10.1890/12-2150.1.