Crabs killing Northeast saltmarshes, study confirms

From ScienceDaily.com:

A marathon summer of field work by Mark Bertness, professor of ecology and evolutionary biology, and a squadron of students may finally help settle the heated debate about what’s killing the coastal saltmarshes of southern New England and Long Island. The group’s work has yielded two new papers that offer clear evidence of the cause.

In one paper, published March 20 in the journal PLoS ONE, they provide the results of numerous measurements at 14 sites around Rhode Island’s Narragansett Bay. They sought correlations between the exent of marsh death and evidence of any of several popular hypotheses. The winner by far was runaway herbivory of cordgrass by the Sesarma crab. In the other study, published in the journal Ecology Letters, they directly tested that hypothesis with experiments on Cape Cod. The results were that wherever they protected Sesarma from the pressure of predators, the crabs ruthlessly mowed the grasses down.

Both papers back up the explanation that Sesarma are ravaging the marshes because overfishing has reduced the predators that would naturally keep the crabs in check. Long-held beliefs that physical forces, rather than disrupted food webs, are killing the marshes just aren’t true, Bertness said.

Bertness’ team included lab manager Caitlin Brisson, seniors Matt Bevil and Sinead Crotty, and junior Elena Suglia. They divided their time between Rhode Island and Cape Cod from May to August, often working from 3 a.m. into the early evening. Brown alumnus Tyler Coverdale is a co-author on the Ecology Letters paper.

In Narragansett Bay they ran several tests during the summer at sites where die-off ranged from less than 5 percent to 98 percent. They determined the extent and progress of marsh death by examining aerial images of the sites from 1997, 2003, 2008, and 2012.

To gather data about whether tides were eroding the marshes away, the researchers made chalk blocks and placed them at each site so they could watch how quickly they dissolved. To determine whether local growing conditions were poor for the grasses, they planted healthy grass in each site and then protected it from all herbivores, including the crabs. To assess whether any site had too much nitrogen, they took leaves of the grasses at each site back to the lab, ground them up and analyzed their chemistry.

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