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Dendronsicyos Soqotrana: © Royal Botanic Garden Edinburgh Biologists find islands such as Soqotra irresistible because they are "living laboratories" for the study of evolution and ecology. Islands represent a small part of the Earth's land area but a large part of its biodiversity, including about one-sixth of the total flora, so they are critical to global conservation. But these floras are particularly susceptible to extinction. For a start, they often cannot compete with weedy plants brought in from outside. And because they usually evolve in the absence of large grazing animals most of these plants lack anti-grazing defences such as thorns or poisons and are killed by livestock.

Soqotra's rare species have escaped this fate. Relics of ancient species are so abundant that the island looks like most people's idea of a prehistoric world. Until at least 10 million years ago Soqotra was part of the African mainland and, before that, part of the African-Arabian tectonic plate. Today the ancestors of plants from these ancient landmasses can still be found growing on the island.

Island Lives

The number of endemic plant species on an island depends on its age, size, topography, climate, degree of isolation and geological history. The Canary Islands, for example, are home to over 500 endemic species40 per cent of the flora. But Britain, which is over 30 times bigger, has only about 16 endemic plants-less than I per cent of the flora-according to figures from the World Conservation Union (IUCN). The reasons for this difference include the Canaries' more diverse topography and warmer climate, and the fact that the islands have not experienced glaciations and have been isolated from the mainland for many more millions of years than Britain.

Broadly there are three types of island endemics-relict species, newly adapted species and adapted-relict species. Relict species are found on islands which, like Soqotra, were once part of larger landmasses but have become isolated through continental drift and changes in sea level. The species may die out on the mainland, leaving remnants of once widespread ancient floras on the island. Most of the plants of Madagascar and New Caledonia are relicts.

Newly adapted species are the product of accidental colonisation of an island by individual plants, which then adapt to the new environment. This type of endemic is commonly found on islands which have never been part of larger landmasses and are volcanic in origin, such as the Galdpagos and Hawaiian Islands. The third type of island endemic, adapted-relict species is the result of a second burst of evolution in the relict species.

Today, island endemics make up a third of the world's threatened plants, and many are, already extinct. On Saint Helena, alone, 96 per cent of the, endemic flora is rare or threatened with extinction, says the, IUCN. Seven endemic species are definitely extinct and another 50 or more are believed to have been wiped out since the introduction of goats in the 16th century.