Oceania ecozone
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Oceania is the smallest of the world's terrestrial ecozones, and unique in not including any continental land mass. The ecozone includes the Pacific Ocean islands of Micronesia, the Fijian Islands, and most of Polynesia (with the exception of New Zealand). New Zealand and most of Melanesia, including New Guinea, Vanuatu, the Solomon Islands, and New Caledonia, are included, with Australia in the Australasia ecozone. Oceania is the smallest in area of any of the ecozones, and also the youngest geologically; other ecozones include old continental land masses or fragments of continents, but Oceania is composed mostly of island groups that arose from the sea, as a result of hot spot volcanism, or as island arcs pushed upward by the collision and subduction of tectonic plates. The islands range from tiny 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.
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. 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 giantism, 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 dispersed 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. Other plants, notably coconut palms and mangroves, produce seeds that can float in salt water over long distances, eventually washing up on distant beaches. 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 filled with seeds that can pass through the digestive tracts of birds. 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. Metrosideros, Pandanus and Coco are tree genera with a fairly ubiquitous distribution across Oceania.
Dispersal across the ocean is difficult for most land animals, and Oceania has relatively few indigenous land animals compared to other ecozones. Certain types of animals that are ecologically important on the continental ecozones, 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 sea birds 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, although several species have been introduced by humans: the first Malayo-Polynesian settlers brought pigs, dogs, and, inadvertently, rats to the islands; European settlers brought other animals, including cats, mongoose, sheep, goats, and the Norway 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. 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 also caused the human population to implode. 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 a snake, Boiga irregularis, in the 1940's.
Oceania Terrestrial Ecoregions
| Ecozones |
| Afrotropic | Antarctic | Australasia | Indomalaya | Nearctic | Neotropic | Oceania | Palearctic |
External link
- Map of the ecozones (http://www.panda.org/about_wwf/where_we_work/ecoregions/global200/pages/mainmap.htm)ja:オセアニア区