What is the relationship between biodiversity on an island and its size and distance from a mainland?

According to the Millennium Ecosystem Assessment, islands are defined as ‘lands isolated by surrounding water and with a high proportion of coast to hinterland’. It stipulates that they must be populated, separated from the mainland by a distance of at least two kilometres, and measure between 0.15 km2 and the size of Greenland (2.2 million km2).

In relation to biodiversity, islands are unique places that are home to a variety of species and habitats including endemic as well as threatened biodiversity. Endemism is a feature of many islands. From a global biodiversity perspective islands are therefore considered as biodiversity ‘hot spots’. They provide for the livelihood, economy, well-being and cultural identity of 600 million island dwelling people (one-tenth of the world population). Island species are also unique in their vulnerability: of the 724 recorded animal extinctions in the last 400 years, about half were island species.

The characteristics of the species assemblages found on islands are determined by many factors, including size, age, distance to other islands and the mainland, climatic history, current climate, relief and geology. Thus, although as a general rule islands are poorer in species than comparable mainland areas, their biodiversity often exhibits unique features and a high degree of endemism, from the genetic to the ecosystem level. Although islands make up only some 5 % of the global land area, their endemic biota are estimated to include about 20 % of the world’s vascular plant species and 15 % of all mammal, bird and amphibian species.

Islands harbour numerous discrete ecosystems, from mountain forests to wetlands and beyond, that provide services in the form of food, fresh water, wood, fibre, medicines, fuel, tools and other important raw materials. In addition they provide aesthetic, spiritual, educational and recreational values. This services support island livelihoods, economies and cultures. Island ecosystems also contribute to the maintenance of ecosystem functions: they provide defence against natural disasters, support nutrient cycling, and soil and sand formation; and they contribute to the regulation of climate and diseases.

Over the past century, island biodiversity has been subject to intense pressure from invasive alien species, habitat change and over-exploitation and, increasingly, from climate change and pollution. The impacts of climate change are particularly relevant in the island context and include sea level rise and the possibility of increasing incidence of (already problematic) invasive alien species.