Oceanian realm

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

The Oceanian realm is one of eight biogeographic realms. It is unique because it contains no continental land mass and it has the smallest land area of the realms.

Where it covers
- It includes many Pacific islands such as Micronesia, Fiji, Hawaii, and most of Polynesia. New Zealand and Australia are not in Oceania; they are in the Australasian realm.
- Some classification systems place certain nearby lands in Oceania instead of Australasia, and a few islands have shifted between realms in different schemes. For example, the Juan Fernández Islands have been listed in both Oceanian and Temperate South American realms, and Clipperton Island is sometimes linked to Oceania.

Geology and landscape
- Oceania is geologically the youngest realm. Its land comes mainly from volcanic islands and coral atolls that rose from the sea only in relatively recent times.
- Islands range from tiny rocks and sea stacks to large, mountainous islands like Hawaii and Fiji.
- Many islands formed where tectonic plates move, collide, or where hotspots push up volcanoes.

Climate and vegetation
- The islands have tropical or subtropical climates, with regions that are humid and others that are drier.
- Wet areas are covered by tropical and subtropical moist forests. Dry areas and many low-lying coral islands have dry forests and grasslands.
- Hawaii’s big volcanoes, Mauna Kea and Mauna Loa, host rare montane grasslands and shrublands.

Endemism and how plants spread
- Oceanic flora and fauna began on the islands without a nearby continent, so many species evolved here in isolation.
- The flora mostly came from the Malesian region (the Malay Peninsula, Indonesia, the Philippines, and New Guinea), with some plants from Australasia and a few from the Americas, especially Hawaii.
- Some Easter Island plants came from South America, such as the totora reed.
- Plants disperse by wind (tiny spores and seeds), by floating in salt water (like coconut palms and mangroves), and by birds that carry seeds or drop them in their droppings.
- Many plants, such as Metrosideros trees from New Zealand, can spread across the ocean by wind.

Animals and what makes Oceania special
- Because the islands are remote, Oceanian land animals are relatively few.
- Birds are common, including seabirds and several land birds that may have reached the islands by chance. Some birds became flightless after arriving.
- There are lizards (geckos and skinks) on many islands, likely arrived on rafted vegetation. Bats are the main non-marine mammals on many island groups.

Humans and their impact
- Humans arrived in two major waves. Malayo-Polynesian settlers brought pigs, dogs, chickens, and Polynesian rats to many islands by around 1200 CE.
- From the 1600s onward, Europeans brought cats, cattle, horses, mongooses, sheep, goats, and brown rats.
- Introduced animals, hunting, and deforestation have dramatically changed island ecologies, causing many species to decline or go extinct. Birds and other native life often struggled because predators and competitors were new to the islands.
- Easter Island, the easternmost island of Polynesia, shows signs of ecosystem collapse after human activity, contributing to a sharp drop in its population several centuries ago.
- In Guam, the brown tree snake introduced in the 1940s caused heavy losses of native birds and lizards.

In short, Oceania is a young, island-based realm with a distinctive set of plants and animals that evolved in isolation. Its ecosystems have been heavily shaped—and often harmed—by human arrival and the introduction of many non-native species.