If you travel to Bali, you won’t see a cockatoo, but if you go to the neighbouring island of Lombok, you will. The situation is similar with marsupials: Australia is home to numerous marsupial species, such as the kangaroo and the koala. The further west you go, the sparser they become. While you will find just two representatives of these typically Australian mammals on the Indonesian island of Sulawesi, you will search in vain for them on neighbouring Borneo. Australia, on the other hand, is not home to mammals that you will typically find in Asia, such as bears, tigers or rhinos.
This abrupt change in the composition of the animal world already caught the eye of the British naturalist and co-discoverer of evolutionary theory Alfred Russell Wallace, who travelled through the Indo-Australian Archipelago from 1854 to 1862 to collect animals and plants. He described an (invisible) biogeographical line running between Bali and Lombok and Borneo and Sulawesi that marked the westernmost distribution of Australian fauna.
Fascinating change of wildlife
Biodiversity researchers have long been fascinated by this abrupt change of creatures along the Wallace Line. How these distribution patterns came about, however, has not yet been clarified in detail.
One explanation is plate tectonics. Forty-five million years ago, the Australian Plate began to drift northwards and slid under the mighty Eurasian Plate. This brought two land masses closer together that had previously been far apart. It became easier for land creatures to colonise one continent from the other. Tectonic movements also gave rise to the creation of countless (volcanic) islands between the two continents, which animals and plants used as stepping stones to migrate westwards or eastwards.