The fauna of Australia consists of a huge variety of unique animals; some 83% of mammals, 89% of reptiles, 90% of fish an insects and 93% of amphibians that inhabit the continent are endemic to Australia. This high level of endemism can be attributed to the continent's long geographic isolation, tectonic stability, and the effects of an unusual pattern 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 the marsupials, a group of mammals that raise their young in a pouch, including the macropods, opossums and dasyuromorphs, occupy many of the ecological niches placental animals occupy elsewhere in the world. Australia is home to two of the five known extant species of monotremes, and has numerous venomous species, which include the Platypus, spiders, scorpions, octopus, jellyfish, molluscs, stonefish, and stingrays. Uniquely, Australia has more venomous than non-venomous species of snakes.