An explanation of island biodiversity should start with a definition of islands. Yet this definition is elusive. Although we can all agree that an island, strictly speaking, is a piece of land surrounded by water, beyond this stiupulation, there is no single accepted definition. The Millennium Ecosystem Assessment, for example, defines islands as “lands isolated by surrounding water and with a high proportion of coast to hinterland”; stipulates that they must be populated, separated from the mainland by a distance of at least two kilometres, and measure between 0.15 square kilometres and the size of Greenland (2.2 million square kilometres). Islands located within seas can be categorized in many ways, including by their area; by their altitude into high and low-lying islands; by a combination of the size of the land area, and political and demographic criteria to identify small island developing States; by their distance from the nearest continent; whether there are inhabited or not; the number of inhabitants; or whether they are continental (land areas that used to be connected to the mainland) or oceanic (those that rose from the sea as a result of coral deposits, volcanic activity or tectonic forces) islands. At the SCBD, work on island biodiversity emphasizes oceanic islands and particularly small island developing States (SIDS) because these systems are often perceived to be the most at risk.

In terms of biodiversity, the issue is clearer: islands boast a truly unique assemblage of life. Species become island dwellers either by drifting on islands, like castaways, as they break off from larger landmasses (in the case of continental islands) or by dispersing across the ocean to islands newly emerged from the ocean floor (oceanic islands). Henceforth they are confined to small, isolated areas located some distance from other large landmasses. Over time, this isolation exerts unique evolutionary forces that result in the development of a distinct genetic reservoir and the emergence of highly specialized species with entirely new characteristics and the occurrence of unusual adaptations, such as gigantism, dwarfism, flightlessness, and loss of dispersability and defence mechanisms. Genetic diversity and population sizes tend to be limited, and species often become concentrated in small confined areas.

The legacy of a unique evolutionary history, many island species are endemic—found nowhere else on Earth. Islands harbour higher concentrations of endemic species than do continents, and the number and proportion of endemics rises with increasing isolation, island size and topographic variety. For example, over 90% of Hawaiian island species are endemic. In Mauritius, some 50% of all higher plants, mammals, birds, reptiles and amphibians are endemic, and the Seychelles has the highest level of amphibian endemism in the world. The island of Cuba is home to 18 endemic mammals, while mainland Guatemala and Honduras, both nearby, have only three each. Madagascar is home to more than 8000 endemic species, making it the nation with the highest number of endemic species in sub-Saharan Africa.

It has often been remarked that islands make a contribution to global biodiversity that is out of proportion to their land area. In this sense, they can be thought of collectively as biodiversity “hot spots”, containing some of the richest reservoirs of plants and animals on Earth.