What is the term for a species found nowhere else on Earth?

Endemic, and the concept is why islands matter out of all proportion to their area. Isolation lets populations diverge, so islands accumulate species found nowhere else - Madagascar's lemurs, New Zealand's flightless birds, Hawaii's honeycreepers - and about a fifth of the world's plant species live on islands making up perhaps five per cent of the land.

The same isolation makes them fragile. An endemic species has nowhere to retreat to, usually a small population, and frequently no defences against introduced predators or diseases it never evolved with. The great majority of recorded bird and mammal extinctions since 1500 have been island species, and the usual agent is a rat, a cat or a goat that arrived on a ship.

Islands need not be surrounded by water. A mountaintop, a lake, a cave system or an isolated patch of forest works the same way - the tepuis of Venezuela, flat-topped plateaus rising sheer from the jungle, each carry their own endemics because their summits have been separated from the ground for millions of years. That is also why habitat fragmentation is so damaging: cutting a forest into pieces manufactures islands, with all the vulnerability and none of the time to adapt.