When atolls like the Cocos Keeling Islands emerge from the ocean, they contain no terrestrial life at all. Any plants and animals must reach them by crossing the ocean and then establishing themselves in the hot, salty waterless conditions of the new island.

As atolls grow large enough to retain fresh water and the inside of the islands become further from the effects of salt spray, conditions become easier and more species are able to take root and survive. Plants help to stabilise and enrich the soil with organic matter as they die and decay.