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Crabs Killing Northeast Saltmarshes

Apr 29, 2014 12:24 PM EDT
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Sesarma crabs killing Northeast salt marshes
Field work conducted by Brown University provided evidence that crabs are killing the coastal saltmarshes of southern New England and Long Island. Pictured: Senior Sinead Crotty checks a cage that protects Sesarma crabs from predators.

(Photo : Bertness lab/Brown University)

Field work conducted by Brown University provided evidence that crabs are killing the coastal saltmarshes of southern New England and Long Island.

Sesarma crabs are eating grasses on saltmarshes, called chord grass, to their hearts' content because a lack of predators in the area is allowing them to do so. Mark Bertness, professor of ecology and evolutionary biology, and students published one paper in PLOS ONE and another in Ecology Letters supporting this explanation.

Overfishing, and not physical forces, are what is behind the dwindling saltmarsh grasses. The number of crab predators, including blue crabs, striped bass and cod, according to The New York Times, has been reduced and they are no longer keeping the crustaceans in check.

Sesarma crabs eat marsh grasses, feeding both above ground nocturnally and below ground through networks of burrows. Bertness was astounded by the destruction they caused Cape Cod's National Seashore back in 2007.

"These plants were chewed down to the nubbin and eaten up like corn on the cob," he told The Times.

Chord grass is vital to marsh land survival, allowing other species and plants to take root.

Bertness' team divided their time between from 14 sites around Rhode Island's Narragansett Bay and Cape Cod looking for the origin of the marsh death. In Narragansett Bay they ran several tests during the summer at sites where die-off ranged from less than 5 percent to 98 percent, as well as analyzed aerial shots of the sites from 1997, 2003, 2008, and 2012.

The team also considered erosion of the saltmarshes due to tides, local growing conditions and an overabundance of nitrogen as possible culprits.

But, the report concluded that "excluding predators for a single growing season rapidly led to a more than 100-percent increase in Sesarma herbivory, a more than 60-percent decrease in aboveground cordgrass biomass, a more than 95-percent increase in Sesarma substrate disturbance, and a more than 150-percent increase in unvegetated bare space in comparison to control plots."

The team plans on continuing their experiments in order to further understand the ecosystem dynamics that lead to the marsh die-off.

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