Australia is a sovereign transcontinental country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. Australia is the oldest, flattest, and driest inhabited continent, with the least fertile soils. It is a megadiverse country, and its size gives it a wide variety of landscapes and climates, with deserts in the centre, tropical rainforests in the north-east, and mountain ranges in the south-east.
Although most of Australia is semi-arid or desert, the continent includes a diverse range of habitats from alpine heaths to tropical rainforests. Fungi typify that diversity—an estimated 250,000 species—of which only 5% have been described—occur in Australia. Because of the continent's great age, extremely variable weather patterns, and long-term geographic isolation, much of Australia's biota is unique. About 85% of flowering plants, 84% of mammals, more than 45% of birds, and 89% of in-shore, temperate-zone fish are endemic. Australia has at least 755 species of reptile, more than any other country in the world. Besides Antarctica, Australia is the only continent that developed without feline species. Feral cats may have been introduced in the 17th century by Dutch shipwrecks, and later in the 18th century by European settlers. They are now considered a major factor in the decline and extinction of many vulnerable and endangered native species. Australia is also one of 17 megadiverse countries.
Australian forests are mostly made up of evergreen species, particularly eucalyptus trees in the less arid regions; wattles replace them as the dominant species in drier regions and deserts. Among well-known Australian animals are the monotremes a host of marsupials, including the kangaroo, koala, and wombat, and birds such as the emu and the kookaburra. Australia is home to many dangerous animals including some of the most venomous snakes in the world. The dingo was introduced by Austronesian people who traded with Indigenous Australians around 3000 BCE. Many animal and plant species became extinct soon after first human settlement, including the Australian megafauna; others have disappeared since European settlement, among them the thylacine.
Many of Australia's ecoregions, and the species within those regions, are threatened by human activities and introduced animal, chromistan, fungal and plant species. All these factors have led to Australia's having the highest mammal extinction rate of any country in the world. Numerous protected areas have been created under the National Strategy for the Conservation of Australia's Biological Diversity to protect and preserve unique ecosystems; 65 wetlands are listed under the Ramsar Convention, and 16 natural World Heritage Sites have been established. Australia was ranked 21st out of 178 countries in the world on the 2018 Environmental Performance Index. There are more than 1,800 animals and plants on Australia's threatened species list, including more than 500 animals.
Paleontologists discovered a fossil site of a prehistoric rainforest in McGraths Flat, in South Australia, that presents evidence that this now arid desert and dry shrubland/grassland was once home to an abundance of life.