TheOceanian realm is one of the eightbiogeographic realms and is unique in not including anycontinental land mass. It has the smallest land area of any of theWWF realms.
This realm includes the islands of thePacific Ocean inMicronesia, theFijian Islands, theHawaiian Islands, andPolynesia (with the exception of New Zealand).[1]New Zealand,Australia, and most ofMelanesia includingNew Guinea,Vanuatu, theSolomon Islands, andNew Caledonia are included within theAustralasian realm.
Conversely, New Guinea, New Caledonia, the Solomon Islands and New Zealand are included in the Oceanian realm in theclassification scheme developed byMiklos Udvardy in 1975.[2][3]
TheJuan Fernández Islands have been included in both the Oceanian andTemperate South American realms.[4] Despite only being a few hundred miles removed from the South American coast, the islands have strong Hawaiian and southeastPolynesian biogeographic influences, and the presence of an endemic insect and plant family.[4] The uninhabitedFrench territory ofClipperton Island, 1,000 kilometers off the coast ofMexico, has also been associated with the realm.[5][4]
Oceania is geologically the youngest realm. While other realms include old continental land masses or fragments of continents, Oceania is composed mostly of volcanichigh islands andcoral atolls that arose from the sea in geologically recent times, many of them in thePleistocene. They were created either byhotspot volcanism, or asisland arcs pushed upward by the collision andsubduction oftectonic plates. The islands range from tiny islets,sea stacks and coralatolls to large mountainous islands, likeHawaii andFiji.
The climate of Oceania's islands is tropical or subtropical, and range from humid to seasonally dry. Wetter parts of the islands are covered bytropical 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 bytropical and subtropical dry broadleaf forests andTropical and subtropical grasslands, savannas, and shrublands.Hawaii's high volcanoes,Mauna Kea andMauna Loa, are home to some rare tropicalmontane grasslands and shrublands.
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 currentseamounts 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 faunaladaptive radiation, whereby multiple species evolved from a common ancestor, each species adapted to a different ecological niche; the various species ofHawaiian honeycreepers (FamilyDrepanididae) are a classic example. Other adaptations to island ecologies include gigantism, dwarfism, and among birds, loss of flight. Oceania has a number ofendemic species; Hawaii in particular is considered a globalcenter of endemism, with its forestecoregions having one of the highest percentages of endemic plants in the world.
Landplants 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 notablyMetrosideros trees from New Zealand spread on the wind across Oceania. Other plants, notablycoconut palms andmangroves, produce seeds that can float in salt water over long distances, eventually washing up on distant beaches, and thusCocos 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 theMalesian Flora of theMalay Peninsula,Indonesia, thePhilippines, andNew Guinea, with some plants from Australasia and a few from theAmericas, particularly in Hawaii.Easter Island has some plants fromSouth America such as thetotora reed.
Dispersal across the ocean is difficult for most landanimals, 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 landpredators and grazing mammals, were entirely absent from the islands of Oceania until humans brought them.Birds are relatively common, including manyseabirds 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 ofrails. A number of islands have indigenouslizards, includinggeckoes andskinks, whose ancestors probably arrived on floating rafts of vegetation washed out to sea by storms. With the exception ofbats, which live on most of the island groups, there are few if any indigenous non-marinemammals in Oceania.
Many animal and plant species have been introduced by humans in two main waves.
Malayo-Polynesian settlers broughtpigs,dogs,chickens andpolynesian rats to many islands; and had spread across the whole of Oceania by 1200CE. From the seventeenth century onwards European settlers brought other animals, includingcats,cattle,horses,small Asian mongoose (Herpestes javanicus),sheep,goats, and thebrown 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 lose the instinct to flee from predators and to lay their eggs on the ground, which makes them vulnerable to predators such as cats, dogs, mongooses or rats. The arrival of humans on these island groups often resulted in disruption of the indigenous ecosystems and waves of species extinctions (seeHolocene extinction event).Easter Island, the easternmost island in Polynesia, shows evidence ofecosystem collapse, probably caused by human activity, 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 grassland. More recently,Guam's native bird and lizard species were decimated after the introduction of thebrown tree snake (Boiga irregularis) in the 1940s.
| Fiji tropical dry forests | Fiji |
| Hawaiian tropical dry forests | Hawaiʻi |
| Marianas tropical dry forests | Guam,Northern Mariana Islands |
| Yap tropical dry forests | Federated States of Micronesia |
