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Crabs may be killing Cape Cod’s marshes

Researchers see link to humans

By Carolyn Y. Johnson
Globe Staff / July 6, 2010

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HARWICH — For the past seven years, scientists have been alarmed by the mysterious death of marsh grasses on Cape Cod, which is transforming expanses of lush green wetlands into lumpy mudflats with the appearance of Swiss cheese.

Work over the past few years has provided strong evidence that the marshes are being eaten away by a particular crab, called Sesarma reticulatum, whose appetite for cordgrass is leaving marshes vulnerable to erosion. The work is also revealing the possibility that human disturbances may have set off the chain of events that caused the crabs’ hungry assault, in turn endangering some of the world’s most important ecosystems.

“One of the pretty scary things is the leading edge of these marshes is the front-line defense for sea-level rise,’’ said Mark Bertness, a biology professor at Brown University who has been working intensively to under stand so-called die-off on the Cape for three years. “They are hurricane buffers and nursery grounds to all kinds of shellfish and finfish, and buffers from run-off — an important filtering system.’’

This summer, Bertness and colleagues are monitoring 14 marsh sites — some dead or dying — to unravel the complicated chain of events that is unfolding.

Early findings suggest that predators of the Sesarma — blue crabs, striped bass, or fish called tautog — are less prevalent in marshes disturbed by human activity, including fishing. To Bertness, that suggests recreational fishing has reached a “tipping point,’’ altering nature’s balance by depleting the crab’s enemies and thus allowing them to thrive in greater number.

“This lumpy area — it would have been all grass a few years ago,’’ said Andrew Altieri, a postdoctoral researcher at Brown University, as he stood on the edge of Saquatucket Harbor one recent morning. He held up one of the purple-tinged, 2-inch-long nocturnal Sesarma crabs between his fingers.

“It’s obvious now, the crab is eating the marsh — essentially clear-cutting the vegetation,’’ Altieri said. “There are some marshes that are just wasted.’’

While many scientists agree the crabs are playing an important role in the die-off in the marsh, it is still an open question why the crab chowdown has suddenly become a problem, and how to combat it. Marsh health is complicated, affected by rainfall and run-off, the plant and animal life within it, and other factors that make simple explanations elusive.

Stephen Smith, a plant ecologist at the Cape Cod National Seashore, spent last summer engaged in the Sisyphean task of attempting to rid an area of the crabs. Now, he is experimenting with biodegradable netting that might be able to control erosion and keep the crabs out of an area to allow marsh to recover.

“It’s looking like a classic story of humans altering one link in a food chain and everything going nutty, having cascade effects,’’ Smith said.

Bob Prescott, director of Massachusetts Audubon Society’s Wellfleet Bay Wildlife Sanctuary, said that yellow-crowned night herons used to show up relatively rarely. But last year, about eight of the birds took up residence in the sanctuary, and in their roosting areas, scientists found remains of the Sesarma crabs. It’s difficult to know yet whether the herons will appear more regularly to feed on crabs, but the unusual number last year suggests one possible outcome of an exploding crab population: As the crabs become more bountiful, predators may find ways to take advantage.

To get a clearer picture of what is happening in die-off marshes, Bertness, Altieri, and colleagues are out in the field this summer, breaking down every step in the food chain. They are counting crabs, measuring the predator population, and quantifying the level of die-off.

They capture crabs by sticking empty tennis ball cans deep into mucky marshes. When the Sesarma crabs emerge at night, they fall into the cans and are taken by researchers in the morning.

In past experiments, researchers tethered those crabs to stakes and found that predators ate them at a faster rate in the healthy marshes than in the ailing marshes.

Now, researchers are growing stalks of cordgrass in protective nets to see if preventing the crabs from feasting on them saves the grass, which would provide evidence that the die-off isn’t caused by some other factor, like marsh conditions. They are also setting traps and dropping nets in tidal creeks to count the size and abundance of the blue crabs and fish that are normally predators of the Sesarma crabs.

On a recent afternoon, scientists took samples of cordgrass from healthy marshes and those experiencing die-off, to test whether one kind had more palatable grass.

Efforts to understand the cause will be important as scientists try to decide what can or should be done to help the marshes.

It’s not clear yet “whether this is an unmitigated disaster or whether it’s just an interesting phenomenon,’’ said Robert Buchsbaum, conservation scientist for Mass. Audubon. “There’s a concern about the viability of marshes anyway, with sea level rise, and now you’ve got this crab.’’

Carolyn Y. Johnson can be reached at cjohnson@globe.com.

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