A quick glance at the marsh next to Saquatucket Harbor in Harwich Port, Massachusetts, reveals right away that some of the grass is missing. The cordgrass there, and all around Cape Cod, has been slowly disappearing for decades. Marine ecologist Mark Bertness of Brown University studies this critical ecosystem, which protects our coastal environment by nurturing a complex web of plants and animals, filtering nutrients, and serving as a critical storm barrier. Bertness says the marshes are being overrun by purple marsh crabs because their main predators, blue crab and finfish, are being overfished. So, the purple marsh crabs are free to gorge on healthy fields of cordgrass and once done feeding, they leave behind nothing but lumpy fields of mud.