European green crab, an invasive species on U.S. coasts, may be reversing decades-long damage to the cordgrass population in Cape Cod, Mass., caused by another pest. Dubbed "hell on wheels" by one scientist, the aggressive crab lives on most coastlines and has driven away or killed most native species. But in Massachusetts, scientists found that the green crab has been chasing away purple marsh crabs, which were eating up the cordgrass in the salt marshes. "It's important to acknowledge that introduced species will in some cases provide an unintended benefit, and this is a cool one," said Edwin Grosholz, an ecologist not involved in the study. "That means we have to pay attention as we eradicate [invasive] species."