Most of what you see in a salt marsh is grass of the genus Spartina—cordgrass (Spartina alterniflora) near the water, and salt-marsh hay (Spartina patens) above the level of the average tides. Going inland, there are zones dominated by spike grass, black grass, and switch grass, with reeds and cattails where freshwater creeks enter the marsh. Finally, above the highest tides, is a belt of shrubs, including bayberry. At the base of the grasses growing in the flooded part of the marsh are found algae in the form of filaments or tiny diatoms.

The salt-marsh ecosystem is actually created, in part, by the plants that make up the bulk of the living things found there. The cordgrass is known as a pioneer species, which colonizes bare mud flats. Blades of dead cordgrass accumulate among the live plants, held by stems and roots, trapping sediments to form a layer of peat. Layers of peat eventually accumulate to raise the landward part of the salt marsh to the high-tide level. Salt-marsh hay can then begin to grow on the higher ground, protected from constant flooding by the tides. Peat forms from the salt-marsh hay as well, further raising the level of the marsh so that the landward edges are protected from flooding by all but the highest storm-driven tides.

The plant community supports an animal community by providing both food and shelter. Parts of the grasses are eaten by insects, while diatoms and filamentous algae are consumed by tiny worm-like and shrimp-like creatures, as well as fish like mummichog and sheepshead minnow, and mollusks, such as snails and mussels. These, in turn, are eaten by crabs such as the fiddler crab, cancer crab, blue crab, lady crab, and horseshoe crab—which really is more closely related to spiders than to crabs. Birds, such as herons, ducks, terns, and plovers, feed on fish, crabs, and worms, as well as on the seeds of the grasses. Mammals are represented by mice, shrews, raccoons, skunks, minks, and weasels.

This is the salt-marsh ecosystem, a community of plants and animals controlled by the tide.

—By Jim Donaldson, former URI oceanography graduate student. This article first appeared in A Guide to Rhode Island's Natural Places.