Window on Nature
By Lowell & Kaye Christie, F47246
Barrier islands are found on coastlines all over the world. In the United States, you won’t see them along the rocky coast of the Pacific Ocean, but they’re very common along the Atlantic coastline from New England down to the southern tip of Florida, and along the shoreline of the Gulf of Mexico.
Why barrier islands “” also known as barrier spits “” exist is still debatable, but the general agreement seems to be that they were formed approximately 18,000 years ago at the end of the last Ice Age. Now let’s take a look at what they are.
By definition, barrier islands are long and narrow; they lie just offshore, parallel to the mainland; and they’re made of sand and sediment tossed up by the ocean and by severe storms. The islands are separated from the mainland by a shallow sound, bay, or lagoon, and from each other by narrow tidal inlets. And they are constantly changing.
Coastal residents have good reason to appreciate barrier islands, because their presence lessens the severity of storm damage to the mainland, especially from hurricanes. Add to that the fact that the salt marsh ecosystems of the islands and coast help to purify the runoff from mainland streams and rivers. And humans aren’t the only ones to benefit. The islands provide a variety of habitats needed by wildlife. Each of these habitats has distinct animal and plant life, but first we’ll consider the structure of a typical barrier island.
On the ocean side is a sandy beach, thanks to waves scooping sand from the ocean floor and dumping it on the shoreline. Behind the beach is a series of dunes formed when the wind pushes the sand farther inland. Over time the dunes are stabilized by plants, as well as by sand fences strategically placed by man.
The area behind the dunes goes by several names “” barrier flat, backdune, overwash, or mud flat. It’s formed by sediment that is pushed across the dune system by major storms. Over time, grasses begin growing to stabilize these areas.
Finally we come to the salt marsh. It’s the low-lying area on the mainland side of a barrier island and generally is divided into high and low marshy areas. The low marsh areas are flooded twice daily by high tides, whereas the high marsh areas are flooded twice a month by spring tides. Cord grasses stabilize these salt marsh areas, producing one of the most ecologically productive habitats on Earth.
Even on a narrow island, a variety of habitats exists. On the ocean side is the beach habitat. It’s similar to a desert in that it lacks fresh water, but a large portion of the beach gets covered with salt water twice a day, and the entire beach is drenched during storms. Thus, the plants and animals living there must be able to handle almost constant exposure to salt water and drying air. On the beach itself, the only plant life visible is that which is washed ashore.
Many of the beach animals live underground. Mole crabs and clams filter-feed during high tides, burrowing worms feed on bacteria in the sand, and scavenging crabs (ghost crabs) grab anything edible washed up by the tide. Shorebirds are constant visitors to the beach to feed on the crabs, worms, and offshore fish.
In the dunes the pickings are slim, inhabited by pretty much the same animals as on the beach, only in far smaller numbers.
The larger islands may have stands of maritime trees such as sand live oak, myrtle oak, and slash pine. Animals that live in the forests might include snakes, opossums, skunks, raccoons, and foxes.
On the inland side of the island, you’ll probably find salt marshes similar to those on the coastal mainland. They, too, are typically flooded at high tide, and the wildlife is much the same.
Barrier islands are constantly changing. Ocean waves both deposit and remove sediment from the ocean side. At high tide, the water reaches into the salt marshes, so eventually they will be filled.
Longshore currents play a different role. They’re caused when waves hit the island at an angle, giving them the ability to redesign the island’s shape by moving sand from one end of the island to the other. Offshore currents on the Atlantic coast tend to remove sand from the northern end of a barrier island and deposit it at the southern end.
The wind gets into the act as well, blowing sand from the beaches to form dunes farther back. Savage storms can make dramatic alterations on barrier islands, eroding the beaches and creating new overwash areas.
If global warming continues as predicted, gradual changes in barrier islands are expected as the rising sea level gradually pushes barrier islands toward the mainland. But the greatest change in these islands comes from human activities.
We were first attracted to the islands by the terrific birding opportunities they provide. To our astonishment, some islands now have more people than birds. We saw resort communities, high-rise hotels, and golf courses. To make way for them, the sand dunes and the salt marshes were destroyed. It’s a shame, even though we realize that the people living there also love the coastline. We hope that enough barrier islands are protected as parks, seashores, and refuges so that the coming generations can continue to enjoy nature on the edge of the continent.