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.
Follow our blog to keep up to date with the latest island news:
Image credit
© Rick Tomlinson / naturepl.com
Nature Picture Library
5a Great George Street
Bristol
BS1 5RR
United Kingdom
Tel: +44 (0) 117 911 4675
Fax: +44 (0) 117 911 4699
info@naturepl.com
http://www.naturepl.com
Terms and Conditions of Use of Materials
Copyright in this website and materials contained on this website (Material) belongs to Wildscreen or its licensors.
Visitors to this website (End Users) are entitled to:
End Users shall not copy or otherwise extract, alter or manipulate Material other than as permitted in these Terms and Conditions of Use of Materials.
Additional use of flagged Material
Green-flagged material 
Certain Materials on this website (contributed under Wildscreen Licence 4) display green flags next to the Material and are therefore available for not-for-profit conservation or not-for-profit educational use. These green-flagged Materials may be used by End Users, who are individuals or not-for-profit organisations, for their not-for-profit conservation or not-for-profit educational purposes. Low resolution, watermarked versions of the Material may be copied from this website by such End Users for such purposes. If you require high resolution or non-watermarked versions of the Material, please contact Wildscreen with details of your proposed use.
Search Arkive for green-flagged Material.
Creative commons Material
Materials licensed to Wildscreen under a Creative Commons licence are clearly marked with Creative Commons buttons and may be used by End Users only in the way allowed by the specific Creative Commons licence under which they have been submitted. Please see https://creativecommons.org for details.
Any other use
Please contact the copyright owners directly (copyright and contact details are shown for each media item) to negotiate terms and conditions for any use of Material other than those expressly permitted above. Please note that many of the contributors to Arkive are commercial operators and may request a fee for such use.
Save as permitted above, no person or organisation is permitted to incorporate any copyright Material from this website into any other work or publication in any format (this includes but is not limited to: websites, Apps, CDs, DVDs, intranets, extranets, signage and interpretation, digital communications or on printed materials for external or other distribution). Use of the Material for promotional, administrative or for-profit purposes is not permitted.
More »Island species
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.
Sign up to our newsletter
Get the latest wild news direct to your inbox.
Get involved
Arkive relies on its media donors to donate photos and videos. Can you help? There are plenty of other ways you can get involved too!
Thursday 15 February
The World's Favourite Unloved Species 2018 - the Galapagos Racer
Thursday 01 February
Thursday 01 February
