In a tidal salt marsh, the cordgrass appears to dominate by sheer numbers, its stems crowding the mudflats from channel to upland. Yet ecologists who pulled the grass from test plots found the marsh did not flourish in its absence; instead the exposed sediment eroded, the burrowing crabs that aerate the soil declined, and the remaining plants weakened. The cordgrass, it turned out, was not merely the most abundant species but a kind of engineer, binding the mud, buffering the tides, and quietly maintaining the conditions on which its apparent rivals depended. To call it dominant, the researchers concluded, was to mistake the foundation of a structure for one tenant among many.