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Background Info

An island is a land area completely surrounded by water. Familiar islands include Madagascar and the Hawaiian Islands. But Greenland is also an island, as is Cuba, Ireland, the United Kingdom, and Manhattan. The Bahamas, Japan, Indonesia, Philippines, and New Zealand are nations made up of islands. A group of related islands, such as the Philippines, is called an archipelago.

There are two types of islands: continental and oceanic. Continental islands are areas of the continental shelf that have become isolated because of a rise in sea level. For example, Tasmania is part of the Australian continent that has become cut off by ocean waters forming the Bass Strait. Oceanic islands form from volcanoes erupting on the ocean floor. Iceland and the Hawaiian Islands are examples. Sometimes coral reefs form where ocean waters are shallow, eventually building up exposed island land called an atoll.

Because of their isolation, many islands are home to unique species. Unfortunately, the isolation of islands also makes them and the species living there susceptible to environmental damage. People have brought nonnative plants and animals to islands that became invasive. These invaders often don't have natural predators and quickly crowd out native species. Cats, rats, goats, pigs, and numerous introduced plants have damaged fragile island ecosystems.

Climate change is also a growing threat to islands. Because they are so susceptible to rising sea level, islands are in danger of flooding should sea levels rise. Encroaching seawater may also contaminate fresh water sources on islands. The numerous islands of the Pacific, including those of Tuvalu, are threatened by rising sea levels. Warming seas also damage coral reefs and the diverse ecosystems they support.

Fast Facts

  • Surtsey is a volcanic island south of Iceland that formed during a four-year eruption beginning in 1963. The island has been protected as a natural, uninhabited area since its emergence, and scientists have studied it closely. They have observed how the ocean carried seeds to establish plants there, as well as the arrival of birds and other animals that colonized the new land. What was barren rock in 1963 now has hundred of species of plants and animals.
  • The Galapagos Islands are famous for animals found nowhere else. Giant tortoises, marine iguanas, Galapagos penguins, and flightless cormorants call these islands home. The isolated animal populations of the Galapagos helped Charles Darwin refine his theory of natural selection when he visited the islands in 1835.
  • Coral reefs, which are the basis of island atolls, are habitats teeming with life. While they only cover 1 percent of the ocean floor, they are home to one-quarter of marine species. Pollution and warming seas threaten these precious ecosystems.
  • Rats introduced on 17 of Alaska's Aleutian Islands quickly spread and began preying on ground-nesting seabirds. Scientists studying the rats' effect found that islands with rats have far fewer birds than those where rats have not spread. Without the birds to keep their numbers in check, snails and other invertebrates multiply and clear the tidal zones of seaweed and algae. On islands where rats have not invaded, birds are plentiful and the ecosystem balanced.
  • Half of all animals known to have gone extinct in the past 400 years have been island dwellers; 90 percent of those were birds. Madagascar and Hawaii have large numbers of species native only to those islands; they also have very high numbers of endangered and threatened species.


Term Part of Speech Definition Encyclopedic Entry



a group of closely scattered islands in a large body of water.

Encyclopedic Entry: archipelago



a coral reef or string of coral islands that surrounds a lagoon.

Encyclopedic Entry: atoll



all weather conditions for a given location over a period of time.

Encyclopedic Entry: climate

climate change


gradual changes in all the interconnected weather elements on our planet.

Encyclopedic Entry: climate change

continental island


land once connected to a continent but broken off by shifting tectonic plates.

continental shelf


part of a continent that extends underwater to the deep-ocean floor.

Encyclopedic Entry: continental shelf



tiny ocean animal, some of which secrete calcium carbonate to form reefs.

coral reef


rocky ocean features made up of millions of coral skeletons.



community and interactions of living and nonliving things in an area.

Encyclopedic Entry: ecosystem

introduced species


a species that does not naturally occur in an area. Also called alien, exotic, or non-native species.

invasive species


type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.

Encyclopedic Entry: invasive species



body of land surrounded by water.

Encyclopedic Entry: island



shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

Encyclopedic Entry: lagoon

non-native species


a type of plant or animal that is not indigenous to a particular area. Non-native species can sometimes cause economic or environmental harm as an invasive species.

oceanic island


land formed from the eruption of a volcano on the ocean floor.



an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.


Media Credits

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Rhonda Lucas Donald


Caryl-Sue, National Geographic Society


Sean P. O'Connor, National Geographic Society

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