SMITH ISLAND, Md. -- For four centuries, islanders in this Eastern Shore community have scratched out a living from the crabs, oysters and fish that thrive in these waters. Yet natural and economic forces are chipping away at the island. Sea-level rise could put much of it underwater by the end of the century, even as the population dwindles and young people leave for opportunities elsewhere. Some of these problems are hardly new, but for the first time, residents are making a united stand to preserve their way of life.