Oceania is one of Earth’s eight ecozones. It is the smallest ecosystem found on the planet. Oceania is unique as it is the only ecosystem that does not include any continental land mass. This bioregion includes the Pacific Ocean islands of Micronesia, Fijian Islands, and most of Polynesia (except for New Zealand). This is also the youngest ecozone. While the other ecozones include fragments of ancient continents and land masses, Oceania is composed of volcanic islands and coral atolls that arose from the sea in recent times, as late as the Pleistocene. These islands were created by either volcanic activity or the collision of tectonic plates that helped push the islands upward.
The climate of Oceania’s islands is tropical to subtropical. The climate ranges from humid to dry (seasonally). Wetter regions of islands are covered by tropical and subtropical moist broadleaf forests, while the drier regions are covered with dry broadleaf forests and tropical and subtropical grasslands, savannas, and shrublands. The higher volcanoes of Hawaii (Mauna Kea and Mauna Loa), are home to some very rare tropical mountain grasslands and shrublands.
The flora and fauna of Oceania reached the isolated islands around the times of the last ice age when sea levels were lower and many islands weren’t as isolated as they are today. Once these plant and animal families reached the islands, they adapted to life there and flourished. Multiple species may have evolved from a common ancestor, and then each species adapted to a different ecological area. Adaptations include gigantism, dwarfism, and loss of flight in some birds. While the whole of Oceania has many endemic species, Hawaii itself has one of the most diverse range of endemic plant species in the world.
Many of the plants that are found on the islands of Oceania have arrived by riding the wind. Many plants such as ferns, mosses, and some flowering plants rely on tiny spores or seeds that can remain airborne over long distances. Some New Zealand plants spread to Oceania through the wind. Some plants, such as coconut palms and mangroves produce seeds that can float on salty water for a long time and eventually washed up on distant shores, which is why coconut trees are widespread and common among the islands of Oceania. Birds carry seeds of the fruits of plants in their digestive system and this another way that plants spread. Most scientists agree that the flora of Oceania is derived mainly from Malesia, Indonesia, Philippines, and New Guinea, with some Australia and a few South American species present.
For most land animals, dispersal across the ocean is impossible. Because of this, Oceania has very few indigenous land animals compared to other ecozones. Most animals were entirely absent from these islands until humans brought them. Birds are the most common animals found in Oceania. These include seabirds and some land birds whose ancestors may have been displaced by storms and eventually adapted to life in this new place. Some islands have indigenous lizards (geckos and skinks), whose ancestors probably arrived on floating debris and vegetation washed out to sea by storms. With the exception of bats, there are very few indigenous mammal species in Oceania.
Many plant and animal species were introduced to Oceania by humans. This occurred in two main stages. First, Malayo-Polynesian settlers brought pigs, dogs, chickens and rats to many of the islands which had spread across the whole of Oceania about 1000 years ago. Since the 17th century, European settlers introduced cats, cattle, horses, sheep, goats, and other smaller mammal species (including the brown rat) to Oceania. The arrival of introduced species, over-hunting, and deforestation, have severely altered the ecology of Oceania’s islands and pushed many species to extinction or near extinction.
Many bird species evolved and adapted to laying their eggs directly on the ground due to the absence of natural predators. This made them vulnerable to introduced predators such as cats, dogs, mongooses, and rats. Human settlement has disrupted the ecosystem and has also led to more extinctions of indigenous species. Easter Island shows evidence of an ecosystem collapse that was caused by humans several hundred years ago. This collapse contributed to a 99% decline in the human population of the island. Once lushly forested, Easter Island is now mostly windswept grasslands. Guam’s native bird and lizard populations were wiped out over the last 60 years by the introduction of the Brown tree snake.