"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."