Island Biogeography

Island Types

Islands can be divided into two broad types: (1) true islands, land wholly surrounded by water, and (2) habitat islands, other forms of insular habitat, that is, discrete patches of habitat surrounded by strongly contrasting (hostile) habitats.

In terms of geology, geography, and biology, true islands can be subdivided into two types:

Oceanic Islands. These are islands formed over oceanic plates; they are of volcanic origin (although sometimes their subaerial parts are in part or wholly composed of sedimentary, and especially coral, formation as the volcanic core has sunk) and have never been connected to continental landmasses, from which they are separated by deep sea (e.g., Azores, Canaries, Hawaii, Iceland, Tristan da Cunha). They generally lack indigenous land mammals and amphibians but typically have a fair number of bird species and insects and usually some reptiles.

Continental Islands. These islands are located on the continental shelf. Typically, many of these islands have been connected to the mainland during the Quaternary ice ages (e.g., Britain, New Guinea, Sri Lanka, Sumatra, Trinidad). They are rarely remote and always contain some land mammals and amphibians, as well as representatives of the other classes and orders in considerable variety.

Continental Fragments. These are islands that formed on ancient fragments of continental rock stranded out in the oceans by plate tectonic processes, although by their location they might otherwise pass for oceanic islands (e.g., Crete, Cuba, Madagascar, New Zealand, Seychelles). Typically, they exhibit high endemism and relictualism.

The Significance of Islands

The biotas of islands, especially of oceanic islands, characteristically differ from continental biotas in four main ways: They are relatively impoverished, unsaturated, and disharmonic, and they harbor a disproportionally high number of endemic species. Considering New Guinea as the largest island, islands constitute only 3% of the total landmass of the Earth, while more than 15% of plant, land snail, and bird species are restricted to islands. In the case of higher plants, more than 13% of the world's species are endemic to just 13 islands/archipelagoes. The high endemism of islands means that island species are crucially important to global biodiversity. The first three traits mentioned above are often considered as being largely responsible for island species and communities being particularly fragile when exposed to external pressures.

The ecosystems of islands (coastal, marine, and inland) provide valuable services to more than 500 million people—namely, food, tools, industry, medicine, transport, and waste disposal. With increasing human population pressures through high migration and reproductive rates, island systems face several serious issues both in the immediate and in the near future. The natural land cover of island systems has changed drastically under the pressure of growing human populations and the consequent exploitation of the landmass (e.g., the Azores have lost more than 90% of their original native forest during the five centuries of human occupation). On some islands, the impact has exceeded critical thresholds, particularly along the coastal periphery. Anthropogenic changes range from deforestation for cropland to urbanization and the abandonment of degraded land. All these have immediate repercussions in destruction of the remaining habitat and loss of biodiversity.

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