Feral species

Invasive species are a serious threat to the native biodiversity of Australia and are an ongoing cost to Australian agriculture. Numerous species arrived with European colonisation of Australia and steadily since then.

Management and the prevention of the introduction of new invasive species are key environmental and agricultural policy issues for the Australian federal and state governments. The management of weeds costs A$1.5 billion on weed control and a further $2.5 billion yearly in lost agricultural production.

Both geologic and climatic events helped to make Australia's fauna unique. Australia was once part of the southern supercontinent 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. As Australia drifted, it was isolated from evolutionary pressures in the rest of the world, with its species being more primitive and less competitive than the mainland fauna. This phenomenon is not uncommon in geological history of the world - it resulted in the great American interchange, where more developed fauna of the North America, which was connected to Eurasia via Beringia, replaced many species of the more isolated South American fauna. Other examples of island isolation include Madagascar, New Zealand, Socotra, the Galapagos and Mauritius.

...
Wikipedia