The Oceania ecozone encompasses all the islands of the Pacific except New Zealand. It is the smallest and youngest of the ecozones, as many of these islands were formed recently, in geological terms.
Oceania is composed mostly of volcanic high islands and Coral atolls that arose from the sea in geologically recent times. Because the islands of Oceania have never been connected by land to a continent, the flora and fauna of the islands have all reached the islands via the ocean. On reaching the islands the flora and fauna evolved to life on the islands; for example a number of birds evolved into flightless species due to the lack of predators.
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. However, amphibians who are intolerant to salt water are absent. With the exception of bats, which live on most of the island groups, there are few if any indigenous mammal species in Oceania.
Rainforests are the world's powerhouses, the most vital habitats on the planet. Characterised by high rainfall, they only cover 6% of the Earth across the tropical regions, but they contain more than half of its plant and animal species.
Tropical dry forests, in contrast to rainforest, have to survive a long dry season each year, so the predominantly deciduous trees shed their leaves to cope with it. Sunlight can then reach the ground, so the season that's bad for the trees is good for the forest floor.
Tropical grasslands include the savanna usually associated with Africa, and savanna-type grasslands found in India, Australia, Nepal and the Americas. They are characterised by drought-resistant shrubs and grasses, dotted with trees such as acacias or baobabs.