ABSTRACT
Insularity is a topospheric feature that greatly influences geoecosystems. True islands form where submarine hills and mountains reach the sea or lake surface. They are typically considered to be smaller than continents, Greenland being the largest at present. Logically, continental land-masses are islands, albeit enormous ones. To be sure, Australia and Antarctica are island-continents, and South America was an island-continent from 65 to 3 million years ago. Whether designated islands or not, continents that have been isolated for a considerable time manifest insular features in their fauna and flora. At the other end of the spatial scale, a true island conventionally ceases to be an island when it cannot sustain a supply of fresh water; it is then simply a beach or sand bar. The critical area required to carry a stock of fresh water is about 10 ha. True islands are divided, on the basis of geology, into two broad groups: oceanic islands and land-bridge islands. Oceanic islands are all either of volcanic origin, or made of coral rock, or both. Land-bridge islands either lately formed part of a nearby mainland or else, even though recent separation cannot be proved, have a structure similar to continental lands. They were formerly called continental islands. Useful though this classification be, it fails to capture the infinite variety of details displayed by oceanic and land-bridge islands.
