Oceanian realm

Hawaii Fiji French Polynesia
Map of Oceanian realm. It extends further east to include Rapa Nui and Sala y Gomez.

The Oceanian realm is one of the World Wildlife Fund (WWF) biogeographic realms, and is unique in not including any continental land mass. It is the smallest in land of an area of the WWF realms.

This realm includes the islands of the Pacific Ocean in: Micronesia, the Fijian Islands, the Hawaiian islands, and Polynesia (with the exception of New Zealand).[1] New Zealand, Australia, and most of Melanesia including New Guinea, Vanuatu, the Solomon Islands, and New Caledonia are included within the Australasian realm.

Conversely, New Guinea, New Caledonia, Solomon Islands and New Zealand are included in the Oceanian realm in the classification scheme developed by Miklos Udvardy in 1975.[2][3]


Oceania is geologically the youngest realm. While other realms include old continental land masses or fragments of continents, Oceania is composed mostly of volcanic high islands and coral atolls that arose from the sea in geologically recent times, many of them in the Pleistocene. They were created either by hotspot volcanism, or as island arcs pushed upward by the collision and subduction of tectonic plates. The islands range from tiny islets, sea stacks and coral atolls to large mountainous islands, like Hawaii and Fiji.


The climate of Oceania's islands is tropical or subtropical, and range from humid to seasonally dry. Wetter parts of the islands are covered by tropical and subtropical moist broadleaf forests, while the drier parts of the islands, including the leeward sides of the islands and many of the low coral islands, are covered by tropical and subtropical dry broadleaf forests and Tropical and subtropical grasslands, savannas, and shrublands. Hawaii's high volcanoes, Mauna Kea and Mauna Loa, are home to some rare tropical montane grasslands and shrublands.

Flora and fauna

Since the islands of Oceania were never connected by land to a continent, the flora and fauna of the islands originally reached them from across the ocean (though at the height of the last ice age sea levels were much lower than today and many current seamounts were islands, so some now isolated islands were once less isolated). Once they reached the islands, the ancestors of Oceania's present flora and fauna adapted to life on the islands.

Larger islands with diverse ecological niches encouraged floral and faunal adaptive radiation, whereby multiple species evolved from a common ancestor, each species adapted to a different ecological niche; the various species of Hawaiian honeycreepers (Family Drepanididae) are a classic example. Other adaptations to island ecologies include gigantism, dwarfism, and among birds, loss of flight. Oceania has a number of endemic species; Hawaii in particular is considered a global 'center of endemism', with its forest ecoregions having one of the highest percentages of endemic plants in the world.


Land plants disperse by several different means. Many plants, mostly ferns and mosses but also some flowering plants, disperse on the wind, relying on tiny spores or feathery seeds that can remain airborne over long distances notably Metrosideros trees from New Zealand spread on the wind across Oceania. Other plants, notably coconut palms and mangroves, produce seeds that can float in salt water over long distances, eventually washing up on distant beaches, and thus Cocos trees are ubiquitous across Oceania. Birds are also an important means of dispersal; some plants produce sticky seeds that are carried on the feet or feathers of birds, and many plants produce fruits with seeds that can pass through the digestive tracts of birds. Pandanus trees are fairly ubiquitous across Oceania.

Botanists generally agree that much of the flora of Oceania is derived from the Malesian Flora of the Malay Peninsula, Indonesia, the Philippines, and New Guinea, with some plants from Australasia and a few from the Americas, particularly in Hawaii. Easter Island has some plants from South America such as the totora reed.


Kagu (Rhynochetos jubatus), a flightless bird from New Caledonia

Dispersal across the ocean is difficult for most land animals, and Oceania has relatively few indigenous land animals compared to other realms. Certain types of animals that are ecologically important on the continental realms, like large land predators and grazing mammals, were entirely absent from the islands of Oceania until humans brought them. Birds are relatively common, including many seabirds and some species of land birds whose ancestors may have been blown out to sea by storms. Some birds evolved into flightless species after their ancestors arrived, including several species of rails. A number of islands have indigenous lizards, including geckoes and skinks, whose ancestors probably arrived on floating rafts of vegetation washed out to sea by storms. With the exception of bats, which live on most of the island groups, there are few if any indigenous mammal species in Oceania.

Impact of settlement

Many animal and plant species have been introduced by humans in two main waves.

Malayo-Polynesian settlers brought pigs, dogs, chickens and polynesian rats to many islands; and had spread across the whole of Oceania by 1200 CE. From the seventeenth century onwards European settlers brought other animals, including cats, cattle, horses, small Asian mongoose (Herpestes javanicus), sheep, goats, and the brown rat (Rattus norvegicus). These and other introduced species, in addition to overhunting and deforestation, have dramatically altered the ecology of many of Oceania's islands, pushing many species to extinction or near-extinction, or confining them to small islets uninhabited by humans.

The absence of predator species caused many bird species to become 'naive', losing the instinct to flee from predators, and to lay their eggs on the ground, which makes them vulnerable to introduced predators like cats, dogs, mongooses, and rats. The arrival of humans on these island groups often resulted in disruption of the indigenous ecosystems and waves of species extinctions (see Holocene extinction event). Easter Island, the easternmost island in Polynesia, shows evidence of a human-caused ecosystem collapse several hundred years ago, which contributed (along with slave raiding and European diseases) to a 99% decline in the human population of the island. The island, once lushly forested, is now mostly windswept grasslands. More recently, Guam's native bird and lizard species were decimated by the introduction of the brown tree snake (Boiga irregularis) in the 1940s.


Ecoregions of the Oceanian realm. OC0101: Carolines tropical moist forests; OC0102: Central Polynesian tropical moist forests; OC0103: Cook Islands tropical moist forests; OC0104: Eastern Micronesia tropical moist forests; OC0105: Fiji tropical moist forests; OC0106: Hawaiian tropical rainforests; OC0107: Kermadec Islands subtropical moist forests; OC0108: Marquesas tropical moist forests; OC0109: Ogasawara subtropical moist forests; OC0110: Palau tropical moist forests; OC011: Rapa Nui and Sala-y-Gomez subtropical broadleaf forests; OC0112: Samoan tropical moist forests; OC0113: Society Islands tropical moist forests; OC0114: Tongan tropical moist forests; OC0115: Tuamotu tropical moist forests; OC0116: Tubuai tropical moist forests; OC0117: Western Polynesian tropical moist forests; OC0201: Fiji tropical dry forests; OC0202: Hawaiian tropical dry forests; OC0203: Marianas tropical dry forests; OC0203: Yap tropical dry forests; OC0301: Hawaiian tropical high shrublands; OC0302: Hawaiian tropical low shrublands; OC0303: Northwestern Hawaii scrub.
Carolines tropical moist forests Federated States of Micronesia
Central Polynesian tropical moist forests Cook Islands, Johnston Atoll, Kiribati, Palmyra Atoll
Cook Islands tropical moist forests Cook Islands
Eastern Micronesia tropical moist forests Marshall Islands, Nauru, Kiribati, Wake Island
Fiji tropical moist forests Fiji, Wallis and Futuna
Hawaiian tropical rainforests Hawaii
Kermadec Islands subtropical moist forests New Zealand
Marquesas tropical moist forests French Polynesia
Ogasawara subtropical moist forests Bonin Islands (Japan)
Palau tropical moist forests Palau
Rapa Nui and Sala-y-Gomez subtropical broadleaf forests Easter Island (Chile)
Samoan tropical moist forests American Samoa, Western Samoa
Society Islands tropical moist forests French Polynesia
Tongan tropical moist forests Tonga
Tuamotu tropical moist forests French Polynesia
Tubuai tropical moist forests French Polynesia
Western Polynesian tropical moist forests Kiribati, Tokelau, Tuvalu
Fiji tropical dry forests Fiji
Hawaiian tropical dry forests Hawaii
Marianas tropical dry forests Guam, Northern Marianas
Yap tropical dry forests Federated States of Micronesia
Hawaiian tropical high shrublands Hawaiʻi
Hawaiian tropical low shrublands Hawaiʻi
Northwestern Hawaii scrub Hawaiʻi


  1. ^ Olson, D. M., Dinerstein, E., Wikramanayake, E. D., Burgess, N. D., Powell, G. V. N., Underwood, E. C., D'Amico, J. A., Itoua, I., Strand, H. E., Morrison, J. C., Loucks, C. J., Allnutt, T. F., Ricketts, T. H., Kura, Y., Lamoreux, J. F., Wettengel, W. W., Hedao, P., Kassem, K. R. (2001). Terrestrial ecoregions of the world: a new map of life on Earth. Bioscience 51(11):933-938.
  2. ^ Udvardy, M. D. F. (1975). A classification of the biogeographical provinces of the world. IUCN Occasional Paper no. 18. Morges, Switzerland: IUCN.
  3. ^ Udvardy, Miklos D. F. (1975) World Biogeographical Provinces (Map). The CoEvolution Quarterly, Sausalito, California. link.