How Barrier Islands Work

Barrier-island Habitats
Barrier-island profile showing various habitats
Barrier-island profile showing various habitats
Photo courtesy U.S. Air Force

Even though barrier islands are narrow, they have several distinct habitats:

  • Beach
  • Dune
  • Barrier flat
  • Salt marsh

Each habitat has varying conditions and wildlife. We will examine some of them according to each habitat.

Beach Habitat

On the ocean side is the barrier island's beach habitat. The beach is much like a desert in that it lacks fresh water, but a large portion of the beach gets covered almost entirely with salt water twice daily (the entire beach gets covered to the dune base during storms). Animals and plants in this environment (known as the intertidal zone, between tides) must endure long periods of exposure to salt water and drying air. On the beach, the only plant life you'll see is some algae that get washed ashore. Bacteria live in the spaces between the sand grains where water from the surf percolates through. The animals on the beach itself include burrowing animals like mole crabs and clams that filter-feed during high tides, burrowing worms that feed on bacteria in the sand, scavenging crabs (ghost crabs) and various shorebirds (sandpipers, seagulls and pelicans) that eat the crabs, burrowing animals and offshore fish.

Brown pelicans often feed on both the ocean and sound sides of barrier islands
Photo courtesy USGS

Dune Habitat

The dunes receive moisture from rain and surf and are occasionally flooded during severe storms. The dunes are still a relatively hostile environment with high salt content, sandy soil and little fresh water. Plants such as sea oats and bitter pancum provide stability to the dunes. Their root systems hold the sand in place and their shoots slow the winds, thereby allowing sand to be deposited. Along the dunes, you will find many crabs, particularly ghost crabs. Again, you will find birds (gulls, terns) that feed on the animals that inhabit the dunes.

Scrub tree community of a maritime forest
Photo courtesy USGS

After the dunes, some islands may have maritime forests with shrubs and trees (Sand Live Oak, Myrtle Oak, Slash Pine and Magnolia). Animals in these forests include various snakes, opossums, skunk, raccoon and fox.

Barrier-flat Habitat

On other islands, the barrier flats come after the dunes. The primary vegetation includes cordgrass and sawgrass. These areas are often flooded daily during high tides.

The muds and sediments are full of anaerobic bacteria (there is little oxygen in the sediments). The bacteria decompose the rich organic material in the sediments and from dead plants and animals. Animals that live in the wet muds filter-feed bacteria and plankton from the tidewaters or feed on bacteria in the muds; these animals include clams, mussels, snails and worms. Various fish come and go with the tides. Fiddler crabs feed on the bacteria in the muds. Ghost crabs and blue crabs feed on the bacteria, small invertebrates and small fish. Various birds (seagulls, egrets, pelicans) feed on the fish, crabs and invertebrates.

Sawgrass community on a barrier flat (top) and salt marsh community on the sound side (bottom)
Photo courtesy USGS
Sawgrass community on a barrier flat (top) and salt marsh community on the sound side (bottom)
Photo courtesy USGS

Salt-marsh Habitat

The sound-side of an island is usually dominated by the salt marsh. The salt marshes that you find on the sound sides of barrier islands are similar to those found on the coastal mainland. Like the barrier flats, salt marshes are regularly flooded with seawater during high tide and the animals and plants that you find are similar to those in the barrier flats.