Salt marshes rank among the most productive ecosystems on earth. Live Spartina (a salt marsh plant)  is not a source of food but dead marsh plants are a source of nourishment for many species. Decaying Spartina breaks into small pieces called detritus that fuels the marsh and its animals. In spring and summer, marshes are lush green, highly productive and grow in height. In late fall, the green Spartina begins to turn brown as leaves die and decomposition begins. Water, waves, wind and storms dislodge and break up decaying leaves, and transport them to mud flats and other locations around the marsh. This dead plant matter, or detritus, forms an attachment site for microscopic organisms such as bacteria, fungi and small algae. These organisms colonize the broken bits of plant material and break down portions of the detritus that are not digestible by animals.  

For the most part, this decomposition occurs on or in the sediments where bottom-dwelling scavengers such as worms, fishes, shrimps and crabs live. These animals eat the decaying plant material, along with the bacteria, fungi and attached organisms. They then digest the material and excrete the undigested plant remains in feces that can be colonized again by microorganisms. As the microorganisms utilize detritus and reduce it to smaller and smaller pieces, the remaining detritus becomes fertilizer for the next Spartina crop. In this way, the whole food web cycle is repeated.

Microscopic animals associated with detritus also cover the surface of mud in the salt marsh. They help stabilize sediments, are food for larger organisms and contribute to an enrichment of the sediments. Large numbers of more sizable invertebrates (animals without backbones) inhabit salt marshes. Rapid changes in salinity, temperature and exposure create stressful conditions and thereby limit how many species occur in this habitat. Fiddler crabs, marsh snails and marsh mussels are typical invertebrate species which live in salt marshes. The popular and highly prized oyster generally borders salt marshes. Fiddler crabs and marsh snails shred dead plant material during feeding, aiding the decomposition process.  

Insects are also abundant in the salt marsh. Most of these salt marsh invertebrates consume living plants, or fluids secreted by the plants. Some insects also feed on detritus, though the importance of their role in the food web as grazers and detritus feeders is small compared to their importance to the abundant species of birds who depend on them for food. The undigested grass eaten by insects is deposited as feces on the marsh surface where it becomes part of the detrital food web. Many fish species living near the salt marsh rely on insects for food during part of the year.

Fishes, crabs, and shrimps live in salt marshes where stems, leaves, and roots provide food and shelter from predators. The young of many species, such as the blue crab, white shrimp and spot tail bass utilize the salt marsh as a nursery. Without benefit of an abundance of food and protection given by marsh plants, few younger animals would survive to adulthood.  Many fishes which inhabit marshes move on and off the marsh surface with the tide. Once they leave the protection of the marsh surface to enter the adjacent tidal creek, they become more susceptible to being eaten by large predators living in creeks. Some marsh-dwelling fishes and shrimps remain on the marsh surface after the tide recedes. They live in potholes and standing pools of water. These common marsh inhabitants include mummichogs and grass shrimp.

Few reptiles live in salt marsh habitats. Diamond back terrapins are probably the most common species in the marsh, where they lay eggs and forage during high tide. American alligators do occur in brackish salt marshes but are not often found in high salinity marshes. The regularly flooded salt marshes of South Carolina provide excellent habitat for birds, with many places for feeding, reproducing and roosting. Species such as the red-winged black bird alternately eat insects and seeds depending on the season. Other birds, such as herons and egrets, feed on fishes, shrimps and fiddler crabs. These graceful predators are year-round residents of our marshes and frequently perch on mud banks watching for movement of prey in tidal pools. The commonly heard but seldom seen clapper rail forms roosting areas on the marsh surface within the protective cover of marsh grass. Birds contribute important nutrients to the salt marshes through their feces, which accumulate in large quantities around nesting colonies. In turn, feces fertilize marsh grass, an important function in the marsh food web.