View source for Oceanian realm

{{Short description|Terrestrial biogeographic realm}}
[[Image:Oceanian realm.jpg|thumb|320px|Map of Oceanian realm. It extends further east to include [[Rapa Nui]] and [[Sala y Gomez]].]]

The '''Oceanian realm''' is one of the eight [[biogeographic realm]]s and is unique in not including any [[continent]]al land mass. It has the smallest land area of any of the [[List of terrestrial ecoregions (WWF)|WWF realms]].

This realm includes the  islands of the [[Pacific Ocean]] in [[Micronesia]], the [[Fiji|Fijian Islands]], the [[Hawaiian Islands]], and [[Polynesia]] (with the exception of New Zealand).<ref>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.</ref> [[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, the Solomon Islands and New Zealand are included in the Oceanian realm in the [[List of biogeographic provinces|classification scheme]] developed by [[Miklos Udvardy]] in 1975.<ref name=Udvardy75>Udvardy, M. D. F. (1975). ''A classification of the biogeographical provinces of the world''. IUCN Occasional Paper no. 18. Morges, Switzerland: IUCN.</ref><ref>Udvardy, Miklos D. F. (1975) ''World Biogeographical Provinces'' (Map). The CoEvolution Quarterly, Sausalito, California. [https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~231745~5509023:World----Biogeographical-Provinces link].</ref>

The [[Juan Fernández Islands]] have been included in both the Oceanian and [[Temperate South America]]n realms.<ref name="class"/> Despite only being a few hundred miles removed from the South American coast, the islands have strong Hawaiian and southeast [[Polynesia]]n biogeographic influences, and the presence of an endemic insect and plant family.<ref name="class">{{cite web |last1=Udvardy |first1=Miklos D.F. |title=A Classification of the Biogeographical Provinces of the World |url=https://fnad.org/Documentos/A%20Classification%20of%20the%20Biogeographical%20Provinces%20of%20the%20World%20Miklos%20D.F.%20Udvardy.pdf |publisher=UNESCO |access-date=7 March 2022 |archive-date=18 February 2022 |archive-url=https://web.archive.org/web/20220218131430/http://www.fnad.org/Documentos/A%20Classification%20of%20the%20Biogeographical%20Provinces%20of%20the%20World%20Miklos%20D.F.%20Udvardy.pdf |url-status=dead }}</ref> The uninhabited [[France|French]] territory of [[Clipperton Island]], 1,000 kilometers off the coast of [[Mexico]], has also been associated with the realm.<ref name="clipzoo">{{cite web |last1=Robertson |first1=D. Ross |last2=Allen |first2=Gerald R. |year=1996 |title=Zoogeography of the shorefish fauna of Clipperton Atoll |url=https://repository.si.edu/handle/10088/19435 |publisher=Repository.si.edu |hdl=10088/19435 |accessdate=2022-01-17}}</ref><ref name="class"/>

==Geology==
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 atoll]]s that arose from the sea in geologically recent times, many of them in the [[Pleistocene]]. They were created either by [[Hotspot (geology)|hotspot]] volcanism, or as [[island arc]]s pushed upward by the collision and [[subduction]] of [[tectonic plate]]s. The islands range from tiny islets, [[sea stacks]] and coral [[atoll]]s to large mountainous islands, like [[Hawaii]] and [[Fiji]].

==Climate==

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 [[seamount]]s 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 honeycreeper]]s (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 (ecology)|endemic]] species; Hawaii in particular is considered a global [[center of endemism]], with its forest [[ecoregion]]s having one of the highest percentages of endemic plants in the world.

===Flora===

Land [[plant]]s 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 palm]]s and [[mangroves]], produce seeds that can float in salt water over long distances, eventually washing up on distant beaches, and thus ''[[Coconut|Cocos]]'' trees are ubiquitous across Oceania. [[Bird]]s 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 [[Malesia]]n 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 (plant)|totora reed]].

===Fauna===

[[File:Branta sandvicensis LC399.jpg|thumb|[[Nene (bird)|Nene]] (''Branta sandvicensis''), a native goose from [[Hawaii]]]]

Dispersal across the ocean is difficult for most land [[animal]]s, 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 [[predator]]s and grazing mammals, were entirely absent from the islands of Oceania until humans brought them. [[Bird]]s are relatively common, including many [[seabird]]s 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 [[rallidae|rails]]. A number of islands have indigenous [[lizard]]s, including [[gecko]]es and [[skink]]s, whose ancestors probably arrived on floating rafts of vegetation washed out to sea by storms. With the exception of [[bat]]s, which live on most of the island groups, there are few if any indigenous non-marine [[mammal]]s in Oceania.

==Impact of settlement==

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

[[Malayo-Polynesian]] settlers brought [[pig]]s, [[dog]]s, [[chicken]]s and [[polynesian rat]]s to many islands; and had spread across the whole of Oceania by 1200 [[Common Era|CE]]. From the seventeenth century onwards European settlers brought other animals, including [[cat]]s, [[cattle]], [[horse]]s, [[small Asian mongoose]] (''Herpestes javanicus''), [[sheep]], [[goat]]s, 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 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 (see [[Holocene extinction event]]). [[Easter Island]], the easternmost island in Polynesia, shows evidence of [[ecosystem 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 the [[brown tree snake]] (''Boiga irregularis'') in the 1940s.

==Ecoregions==
[[File:Map of Oceanian Ecoregions.svg|thumb|600px|center|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]]; OC0111: [[Rapa Nui and Sala-y-Gomez subtropical broadleaf forests|Rapa Nui and Sala-y-Gomez tropical 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]]; OC0204: [[Yap tropical dry forests]]; OC0701: [[Hawaiian tropical high shrublands]]; OC0702: [[Hawaiian tropical low shrublands]]; OC0703: [[Northwestern Hawaii scrub]].]]
{{Oceanian tropical and subtropical moist broadleaf forest ecoregions}}
{{Oceanian tropical and subtropical dry broadleaf forest ecoregions}}
{{Oceanian tropical and subtropical grassland, savanna, and shrubland ecoregions}}

==References==
{{commons category|Oceanian realm}}
{{reflist}}

{{Biomes}}
{{Oceania}}

{{DEFAULTSORT:Oceania Realm}}
[[Category:Oceanian realm| ]]
[[Category:Biogeographic realms]]
[[Category:Environment of Oceania]]
[[Category:Flora of Oceania|*]]
[[Category:Natural history of Oceania]]
[[Category:Oceanian ecoregions| ]]
[[Category:Phytogeography]]