Islands vary greatly in size, from just 0.15 square kilometres up to the size of Greenland at 2.2 million square kilometres. There are two main types of island: continental islands such as Borneo and Barbados, which lie on a continental shelf, and oceanic islands such as Hawaii and Tonga, which are usually formed by volcanoes. Atolls are islands formed from coral reefs that have grown on submerged volcanic islands.
The isolated nature of islands results in them being home to a wide diversity of plant and animal species that can be found nowhere else on Earth, making islands attractive destinations for both biologists and tourists alike.
Islands and their surrounding waters account for around a sixth of the world’s total area, but for their size they are home to a surprisingly high proportion of the world’s animals and plants. Estimates suggest that 20 percent of all species of birds, reptiles and plants are found on islands. In addition to this, what makes islands special is that many of the animals and plants found there are endemic, meaning they are found nowhere else on Earth. For example, over 90 percent of the species found in Hawaii are endemic, and Madagascar alone has over 8,000 endemic species.
Some of these species are descended from mainland ancestors, after the land they inhabited drifted away from the main continent as the Earth’s tectonic plates moved, while others colonised newly formed land which had emerged from the sea bed. Over time these isolated species developed adaptations to suit their habitat, distinguishing them from their mainland counterparts, and they ultimately evolved into separate species.
Remote islands often provide important habitats for migratory species, some of which, such as the Chatham albatross, may only breed on a single island. Island biodiversity isn’t just restricted to the land either. Over half of the world’s marine biodiversity is found in the waters surrounding islands, including some of the world’s most spectacular coral reefs.
Over the past 400 years, around half of all animal extinctions have occurred on islands. Island species are often only present in relatively small numbers, putting them at greater risk, and the limited habitat available to them means they cannot easily disperse elsewhere. This also means that island species are particularly vulnerable to the effects of climate change, such as rising temperature and sea levels, extreme weather events, and fires.
Island species are also vulnerable to the introduction of invasive species, as they have typically evolved in isolation with limited competition. Many island species also lack the adaptations to cope with introduced predators. As humans have travelled the world they have introduced large numbers of non-native species to islands, sometimes with devastating consequences, and on some islands the alien species now outnumber the native ones.
The other main risks to island species are natural disasters, habitat destruction, tourism development, overexploitation and pollution. These pressures show their impacts on islands before they would be visible on larger land masses.
Although once widespread throughout Southeast Asia, today the Bornean orangutan is restricted to the island of Borneo. Currently, the main threat to this species is the loss of forest habitat. In the past 20 years, 80 percent of this species’ habitat has been lost to illegal logging, gold mining and conversion to permanent agriculture such as oil palm plantations.
The Juan Fernández petrel faces its greatest threats on its tiny breeding island, where numerous introduced species are causing extensive damage to the natural ecosystem.
The Lord Howe Island stick-insect was believed Extinct due to predation by introduced black rats until it was rediscovered surviving on a single island outcrop known as Ball’s Pyramid.
Although the small size of many islands can be a risk factor for island species, it can also prove beneficial to conservation projects that would not be feasible in larger areas.
For example, small, predator-free islands are providing a safe haven for the kakapo, a large flightless parrot from New Zealand. The kakapo was once widespread throughout New Zealand but its population was decimated by hunters, introduced predators and forest clearing. In 1989, the Kakapo Recovery Plan was developed and all the surviving individuals were relocated to suitable predator-free islands where they could breed. Prior to relocation, the islands underwent extensive re-vegetation, and introduced mammalian predators and competitors were eradicated. The project is intensive and ongoing, but kakapo numbers have increased from 51 individuals in 1995 to 126 individuals in 2013.
The Antiguan racer is a species of snake once believed to be Extinct, before being rediscovered in 1989 on Great Bird Island, Antigua. The Antiguan Racer Conservation Project eradicated rats and mongooses from Great Bird Island and 11 other offshore islands. After captive breeding and reintroductions to the now predator-free islands, the population increased to around 300 individuals by 2009.
The pink pigeon, from the island of Mauritius, has been rescued from the brink of extinction by an intensive management programme. A captive breeding and release programme, supplemented by efforts such as the establishment of Conservation Management Areas, habitat restoration, control of predators and supplementary feeding, has enabled this species to increase in abundance.
These conservation projects are ongoing and need to be sustained, but are great examples of what concerted conservation efforts can achieve on islands.
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Islands supported by...
Islands have been profiled with support from The Bromley Trust to help raise awareness about the importance of islands and their distinctive biodiversity, and the need for their protection and conservation.
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