North Keeling is an isolated oceanic island that has never been near a large land mass. Before it was first visited by people, colonisation of the island by plants and animals could have occurred only by wind or pelagic drift, flight or animal carriage.
The island flora could have originated from a variety of sources depending on their geographic location and suitability of their habitats for immigrant species. The origins and development of the flora and fauna of the island into a unique assemblage of 'travelling' species has long fascinated biologists. The Cocos (Keeling) Islands have similarities with some of the islands of the central and western Indian Ocean Groups, e.g., the Maldives and the Farquhar Group. All of these low islands have evolved in isolation from a continent, through the combined forces of vulcanism, subsidence and coral growth and presently rise less than 10 m above sea level.
The low habitat diversity of these islands leads to a flora characterised by very low endemicity with indigenous taxa of pantropical or Indo-Pacific distribution dominating (Renvoize 1979). Cocos (Keeling) is no exception to this general pattern with only one endemic sub species of plant, Pandanus tectorius cocosensis (Renvoize 1979).
Sixty-one indigenous plant species have been recorded on Cocos (Keeling) Islands. Of these, 32 native species are found in Pulu Keeling National Park. North Keeling Island has seven species not found on the southern atoll. Some species are more abundant on North Keeling Island than on the islands of the southern atoll, either because of greater areas of suitable habitat on North Keeling Island or due to clearing over the last 160 years on the southern atoll (Williams 1994). Other species on North Keeling Island have a restricted distribution and most of these are found on the northern peninsula at the entrance to the lagoon or on the north-west shore and adjacent habitats.
The vegetation of North Keeling Island was divided into four zones by Gibson-Hill (1948), pisonia (Pisonia grandis) and coconut (Cocos nucifera) forest, octopus bush (Argusia argentea) shrublands, tea shrub (Pemphis acidula) thickets and finally open grassy areas.
Much of the island is dominated by the pisonia forest, mixed in many areas with coconut, with a few stands of pure coconut. Octopus bush is common on the eastern shore, dominating the crest of the shingle or rubble ridges. In some cases it forms monospecific stands while north of the lagoon entrance it occurs with cabbage bush (Scaevola taccada).
Around the margins of the lagoon, tea shrub forms dense thickets, replaced in some places by ironwood (Cordia subcordata).
The open grassy areas often have a covering of sea purslane (Sesuvium portulacastrum), such as the clearing to the north-west of the lagoon.
The richest floristic units, apart from the herblands, are the forest types found near the lagoon entrance and on the north-west side of the island.