WEST DENNIS BEACH - From the road, the salt marsh looks just fine. A broad expanse of long-stemmed cordgrasses, their tops tinged golden with the arrival of fall, wave lazily in the breeze. On closer inspection, though, the marsh is not so healthy. Along the banks of the tidal channels are muddy areas devoid of the grasses that should reach the water's edge.
In recent years, scientists have been marking the loss of cordgrass on Cape Cod but have been mystified as to the cause. They have identified drought, rising air and water temperatures, sea-level rise, currents, a fungus, and ice damage as possibilities. Now, a salt marsh expert at Brown University has added another one: A native crab named sesarma reticulatum.
Mark Bertness, chairman of the Department of Ecology and Evolutionary Biology at Brown, believes the crab's predators have been eliminated, allowing its population to explode and its feeding to go unchecked.
On a recent excursion, Bertness pointed at a patch of ground near a tidal channel where he said cordgrass had been replaced by crab burrows resembling honeycombs.
"These are plants that won't come back," he said. Looking at another bank he added: "Everything you see here has been totally eaten away."
In 2002, scientists began noticing that salt marshes on the outer Cape - roughly from Dennis to Wellfleet were retreating. Stephen Smith, a plant ecologist with the Na tional Park Service station at the Cape Cod National Seashore, has documented cordgrass losses since 1984 using historical and aerial photographs. Bertness and his researchers estimate that 80 percent of those marshes are receding, a figure that Smith agrees with.
In a healthy salt marsh, the crabs eat the cordgrass to the nub. As they feed, they dig burrows that incidentally help aerate the soil and create channels for nutrients that allow the plants to regrow.
But now, the runaway crabs are munching too much of the plant and creating too many burrows. That prevents the cordgrass from regrowing and exposes too much of the underlying peat, which is then "replaced by mud," Bertness said.
To determine that the crabs were eating the cordgrass, Bertness in 2006 placed cages around some stands of cordgrass and left other grassy areas unprotected. He found the cordgrass in the caged areas was left unscathed, while the grasses in the unprotected areas had been eaten away by the crabs.
This summer, the researchers went to two Cape Cod marshes to prove that the crabs' predators had disappeared. They tethered hundreds of crabs to stakes and left them overnight. In the morning, all but one of the crabs were still alive. By contrast, in identical experiments in marshes along Narragansett Bay in Rhode Island, just 5 percent of the tethered crabs made it through the night - the rest had been eaten by predators.
Bertness presented his findings for the first time a week and a half ago at the Estuarine Research Federation conference in Providence. However, some scientists remain skeptical.
Ivan Valiela, a senior scientist in the Ecosystems Center at the Marine Biological Laboratory in Woods Hole, said that the crabs may be contributing to the salt marshes' regression. But the crabs' role may be localized, and other factors may be in play too, he said.
"The whole story is not in yet, and certainly there could be multiple causes in different places," said Valiela, also a biology professor in the marine program at Boston University who has studied estuarine ecosystems for nearly four decades.
Smith acknowledged that he, too, initially doubted the crabs' role. But he changed his mind, he said, when he observed Bertness's cage tests.
If the grass were being killed by an environmental cause, Smith said, he would expect to see dead leaves and other plant remains. Instead, the cordgrasses "are there one year and they're gone the next, and that fits into the idea that they're being consumed by crabs at night."
Ron Rozsa, a coastal ecologist at the Connecticut Department of Environmental Protection, said he's not sure what's the cause and what's the effect.
"One of the questions I have is which came first, the crabs or the dieback?" he said.
Another hole in Bertness's theory is that it's not clear who the crabs' predators are.
One known predator, the black-crowned night heron, has been in decline, Smith said. But Valiela suspects there are other predators as well.
Perhaps, Bertness said, it's fish species whose populations have declined from overfishing.
He plans to begin testing that theory next year.