The fauna of Australia comprises a huge variety of unique animals; some 83% of mammals, 89% of reptiles, 90% of fish and insects and 93% of amphibians that inhabit the continent are found nowhere else. This high level of endemism can be attributed to the continent's long geological isolation, tectonic stability, and the effects of climate change on the soil and flora over geological time. A unique feature of Australia's fauna is the relative scarcity of native placental mammals. Consequently, marsupials mostly fill the ecological niches that elsewhere are occupied by placental mammals. Australia is home to the two of the four extant monotremes (egg-laying mammals), and has numerous venomous species, which include the monotremes, spiders, scorpions, octopuses, jellyfish, molluscs, stonefish, stingrays. Uniquely, Australia has more venomous than non-venomous species of snakes.

The settlement of Australia by people from the north of the continent more than 40,000 years ago, and by Europeans from 1788, has had a significant impact on the fauna. Hunting, the introduction of non-native species, and land-management practices involving the modification or destruction of habitats, have led to numerous extinctions; unsustainable land use continues to threaten the survival of many species. In recognition of the threats to the survival of its fauna, Australia has passed wide-ranging federal and state legislation and has established numerous protected areas to protect the fauna. However, many people believe that these measures fail to address the immanent threats to the survival of many species.

Origins of Australian fauna

Several reasons for the uniqueness of Australia's fauna have been advanced. Australia was once part of the southern super-continent Gondwana, which also included South America, Africa, India and Antarctica. Gondwana began to break up 140 million years ago (MYA); 50 MYA Australia separated from Antarctica, and was relatively isolated until the collision of the Indo-Australian Plate with Asia in the Miocene era 5.3 MYA. The establishment and speciation of the present-day fauna was apparently shaped by the unique climate and the geology of the continent. As it drifted, Australia was, to some extent, isolated from the effects of global climate change, and species that were present on Gondwana, such as the marsupials, spread throughout the continent. After the Miocene, fauna of Asian origin were able to establish themselves in Australia. The Wallace Line—the hypothetical line separating the zoogeographical regions of Asia and Australasia—also marks the tectonic boundary between the Eurasian and Indo-Australian plates. This boundary prevented the formation of land-bridges, which explains the distinct zoological distribution and limited overlap of most Asian and Australian fauna, with the exception of birds. Following the emergence of the circumpolar current in the mid-Oligocene era (some 15 MYA), the Australian climate became increasingly arid, giving rise to a diverse group of arid-specialised flora and fauna, just as the wet tropical and seasonally wet areas have produced their own adapted species.