Pressures on Predators Lead to Marshland Die-Offs Swaths of Cape Cod's salt marshes are slowly disintegrating. For the human observer, the most notable sign of their decline might be the increase in night herons. They crouch like low, dark smudges on the salt marshes at dawn after feeding on the surfeit of Sesarma crabs through the night. Anyone looking will probably see pieces of crabs on the ground around night heron roosts. "You can find little balls of Sesarma legs and carapaces and eyeballs," said Mark Bertness, an ecologist at Brown University who has been studying New England's salt marshes for 20 years. These crabs, which measure a little over an inch at most, have many more predators than merely herons, especially when young. "When they're small, basically anything with a mouth will eat them," Dr. Bertness said. Blue crabs, striped bass and cod feast on Sesarma crabs, for example. But these fish and shellfish are being fished out by recreational fishermen, so over all, there aren't enough predators eating Sesarma crabs, according to a study by Dr. Bertness and his colleagues that will appear this week in the journal Ecology. As a result, the crab population on Cape Cod is exploding. And their unchecked proliferation isn't only drawing in hungry herons; it's destroying valuable Cape marshland. For their own meals, Sesarma crabs eat marsh grasses, feeding both above ground nocturnally and below ground through networks of burrows. With fewer predators curbing their population growth, the crabs have run rampant. Dr. Bertness was amazed by the destruction he first saw from the crabs on Cape Cod's National Seashore in 2007. "These plants were chewed down to the nubbin and eaten up like corn on the cob," he said. The grasses on salt marshes, especially a grass called cordgrass, are fundamental to a marsh's existence. Cordgrass takes root in a shallow bay and sediment builds up around it, forming clods of land that rise up out of the sea. "It's like coral in a coral reef," Dr. Bertness said. It forms the initial habitat that allows other plants and species to thrive. Without the grasses, the marshlands wash away. Marshlands are critical ecosystems, serving as nurseries for fish and filtering chemicals from runoff from the land. "Maybe most importantly, they prevent erosion and buffer entire coastlines from storm damage," Dr. Bertness said, noting how the marshy Outer Banks help protect mainland North Carolina from hurricanes. Yet the salt marshes of Cape Cod are also popular places for recreational fishing, which appears to be detrimental for the marshes. "The marshes that had severe die-off were marshes with a lot of fishing access," Dr. Bertness said. He said that recreational fishermen fish for "top predators" like striped bass and blue crab, leaving fewer to munch on the Sesarma. The new study explored other reasons why more marsh grasses might be dying off where recreational fishing is most intense - disturbances from boat wakes and the construction of docks and houses, for example, or varying nutrient levels in the marsh soils - but none of them explain the die-off except recreational fishing, the researchers said. "There are a lot of people, including scientists, that really like to recreationally fish," Dr. Bertness said. He said he and other scientists had suggested a couple of ways to mitigate the loss of marshland to Sesarma crab populations. Perhaps predators like night herons or green crabs (an invasive species introduced from Europe during colonial times) could be used to try and control them, Dr. Bertness said, or the authorities could encourage catch and release recreational fishing.