The Wallace Line is an imaginary line in Indonesia between Borneo and the island to its immediate east, Sulawesi. The line has ecological significance because it separates two zoogeological regions that are respectively associated with Asia and Australia. To the west of the line, fauna is more Asian. To its east, it more closely resembles the Australian fauna. The reason for the line is a deep oceanic trench that has preserved the gulf between these two islands for millions of years. Without it, they would have been periodically connected as Ice Ages caused the sea level to lower by as much as 120 meters (394 ft), connecting together Borneo with the Asian mainland and Sulawesi with Australia.
The region around the Wallace Line is often considered among the most ecologically diverse in the world, alongside the Amazon Rainforest and Congo Rainforest. This region adjacent to the Wallace Line is often called Wallacea. The region is home to over 10,000 plant species, 15% of which are endemic, and 1142 terrestrial vertebrate species, of which about half (529) are endemic. These endemic vertebrate species include seven species of macaque monkey, five species of tarsier, two species of anoa (a rare subgenus of buaffalo), and the babirusa (a pig-like animal with two sets of curved horns).
The entire area within 1,000 miles of the Wallace Line on either side is a huge biodiversity hotspot. On Borneo in the west, there are orangutans, countless birds and monkeys, numerous bats, and the world's only lungless frog. On Sumatra and Java to the west and south of Borneo, there are tigers, rhinos, tapirs, bears, and a leopard found nowhere else. Meanwhile, in New Guinea, the largest island to the east of the Wallace Line, there is an entirely different fauna, absent of all mammals except for rodents and bats. The large mammals are all marsupials (like the arboreal Tree-kangaroos). Due to an absence of large mammalian predators, a few islands are inhabited by the man-sized Komodo Dragon, the largest lizard on Earth, which is an apex predator here.