The Lombok Strait separating the Indonesian islands of Bali and Lombok marks an important boundary known as the Wallace Line, which separates the animal and plant species found in Asia and Australasia.The line is named after the mid-19th century naturalist Alfred Russel Wallace, who noticed a distinct difference in the types of species found on either side of the line.Wallace found that west of the line in Bali, Java and Borneo, species groups are of predominantly Asian origin, such as orangutans and tigers.However, to the east of the line, in Lombok, New Guinea and Sulawesi, species groups are of Australasian origin, including marsupials such as kangaroos and koalas, and egg-laying mammals such as the echidna or platypus.As groups of species evolve, they develop certain characteristics that suit their survival in their environment: their size, type of fur, or shape of their beak might be suited to accessing available food, defence against predators or climate conditions in that area.
These groups of species can spread and colonise other physically similar areas that they have the ability to survive in, such as in Indonesia where, when sea levels were lower, land bridges between the islands connected species groups together.
Several million years ago, Asia and Australia were separated by a huge sea that was impassable to most species, so each continent housed its own species groups with their own characteristics.Today, this sea has been closed by the continents drifting together, as explained by the theory of plate tectonics, and many islands are now very close to each other.However, even with this proximity, many species are still unable to cross this remaining deep ocean trench, impassable even during periods of low sea level, maintaining the ecological divide.A species found in Bali may have similar characteristics to one in Thailand, over 2,000 kilometres away, but may not be found in Lombok, only 35 kilometres away across the Wallace Line.Over time, this distinct line may blur as species fly, swim or float to previously inaccessible islands across the line.