A barrier island is a long, thin stretch of sand deposited parallel to the mainland. One side of a barrier island faces the ocean, while the other side faces an estuary, bay or lagoon that separates the island from the mainland. Barrier islands are popular tourist attractions and contain fragile ecosystems with distinct habitats and wildlife. Due to their location, barrier islands serve as protection from storms for the mainland.
Only 15 percent of the world's coastlines are rimmed by barrier islands. How these barrier islands formed is a matter of debate. Some scientists believe barrier islands formed due to waves pushing sand toward the shoreline. According to this theory, the sand formed a sand bar that eventually grew large enough to form a barrier island. Others propose that barrier islands were once long fingers of sand extending from the mainland. These fingers were cut off from the mainland by violent storm surges at key points, forming island structures. A final theory suggests that barrier islands were once dunes along coastlines that were flooded during a period of rising sea levels. According to this theory, the dunes rose above the flooded areas to become islands.
Barrier islands contain some of the world's most interesting habitats. Dunes, barrier flats and salt marshes are a few of the environments that shelter many organisms that live on barrier islands.