For the last few decades, salt marshes in the western Atlantic have been dying off. The alarming trend has been recorded in Canada, the Gulf of Mexico and New England, and it has scientists scrambling to figure out just what’s going on. Prevailing theories have focused on pollution, plant disease, boat wake or eutrophication (the result of an over abundance of nutrients which is caused by fertilizers, animal and human waste, and other nutrient-rich pollutants running off from the land.)
But recent studies in Rhode Island’s Narragansett Bay, and Cape Cod, Massachusetts, point to a different cause. An abundance of marsh crabs left to thrive when their predators were over fished.
Marshes are important because they serve as buffers, filters and nurseries. As buffers, they protect the land behind them from erosion and heavy storms. As filters they trap sediment and filter nutrients from rivers, streams and other waters flowing off the land. As nursery grounds, they provide shelter for crabs, shrimp and juvenile fish, and food for the adults that feed on them.
But when you remove those adults–the top predators in the ecosystem–their prey are left to multiply. In New England that means purple marsh crabs (Sesarma reticulatum) flourish. The crabs feed voraciously on cord grass. Ecologist Mark D. Bertness, from Brown University published two papers last month claiming that overfishing of predators, and the resulting abundance of marsh crabs, is behind the bulk of marsh die offs from Cape Cod to Long Island.
The first study published in PloS One looked at several sites in Narragansett Bay and compared the effects of cordgrass-munching crabs, excess nitrogen, pollution, disease and erosion from boat wake. He found that the marsh die-off was the worst where the crabs were most abundant.
The second study published in the journal Ecology Letters, described an experiment to test those findings. Bertness and his research team set one-meter square mesh cages into the marsh and observed what happened. The cages kept predator fish out, but allowed smaller critters, like marsh crabs in. Within one season, the amount of herbivory doubled in the caged enclosures. According to Carl Zimmer’s article in the New York Times:
In a matter of weeks, the cages were crowded with marsh crabs, and much of the cordgrass inside the cages was dying off. “We were planning on it being a two- or three-year experiment,” Dr. Bertness said. “But by the beginning of July, I thought, ‘My God, this is really going fast.’”
While the work hasn’t been replicated in other marshes yet, and other scientists are reluctant to blame overfishing (recreational or commercial) for marsh die off, Bertness and his colleagues are convinced that removing too many predators is what’s killing the marshes.
Press Release from Brown University: http://news.brown.edu/pressreleases/2014/04/sesarma