On the trail of an enigma in the history of animal dispersal: Why did fewer Australian species make the leap to Asia than vice versa? A study now sheds light on this biogeographical phenomenon. In addition to the continental drift, the climate in which the species had once developed played an important role in the formation of the so-called Wallace boundary line. The findings may also help better understand the spread of invasive species today, the researchers say.
It was the British naturalist Alfred Russell Wallace who first noticed it: When he examined the living beings of Australia and the neighboring islands to the north from 1854 to 1862, he recognized a biogeographical limit in the distribution patterns. Accordingly, a line runs between Bali and Lombok as well as Borneo and Sulawesi, which marks the westernmost distribution area of the typical Australian animals. For example, there are no cockatoos in Bali – but there are on the neighboring island of Lombok. In the case of marsupials, one thing is clear: numerous representatives of this group, such as kangaroo species, live in Australia and New Guinea. However, the further west you go, the rarer marsupials become. On the Indonesian island of Sulawesi there are just two representatives of these typical Australian mammals and one looks for them in vain on Borneo.
Why more Asian animals in Australia than vice versa?
Conversely, the demarcation is less pronounced: the Australian region lacks many mammals typical of Asia, such as bears, rhinos or tigers. But some animals obviously did find their way from Asia to Australia when the land masses once converged by continental drift. Examples of this are numerous poisonous snakes, frilled lizards, bouncy mice or flying foxes. An international team of researchers has now developed a model to track down the causes of this asymmetric propagation along the Wallace line. In doing so, the scientists combined information on plate movements over the past 30 million years and reconstructions of the climate conditions of the past in the Australian and Asian regions. In addition, data on around 20,000 species of modern birds, mammals, reptiles and amphibians from the relevant regions were included in the model.
The team reports that the importance of Australia’s northward shift was once again highlighted, bringing the continent closer to the Eurasian plate. Although there was no unification, the geological processes created volcanic islands between the two land masses. They could then serve as “stepping stones” for animals to spread, the researchers explain. But why didn’t that go the same way on both sides? The model simulations now show that previous adaptations to the living conditions in the different areas of origin were largely responsible for the unequal distribution of Asian and Australian animals.
Adapted to the climate of the Stepping Stones
The researchers explain that the reason why Asian animals were more likely to “jump” across the Indonesian islands to New Guinea and northern Australia is because they found living conditions on these stepping stones to which they were already adapted: There was a tropical, humid climate there – similar to theirs home of origin. However, this was not the case for Australian animals that were “keen on travelling”. This is because they had developed in a cooler and, over time, increasingly drier climate due to Australia’s relocation history. They were therefore less competitive on the warm, humid islands, had a harder time gaining a foothold and were therefore unable to jump any further.
In contrast, characteristics of species that have evolved in tropical habitats include faster growth and strong competitiveness, the researchers say. Because they could make better use of the stepping stones, some Asian species were able to conquer the new habitats, where they could eventually split into new species. “We are thus providing the missing piece of the puzzle that solves the riddle of the Wallace Line,” says lead author Alexander Skeels from the Swiss Federal Institute of Technology in Zurich (ETH). Senior author Loïc Pellisier from the ETH continues: “The results make it clear that we can only understand today’s distribution patterns of biological diversity if we include the geological development and climatic conditions of prehistoric times in our considerations”.
According to the researchers, the information is also important for assessing current developments in biodiversity. “To fully understand the distribution of biodiversity and the processes that sustain it in the present, we need to figure out how it arose,” says Pellisier. In this context, the researchers are also looking at a problem that is currently confusing the living environment in many regions: Humans are responsible for the problematic spread of species that can spread invasively into new regions and thus threaten the traditional ecosystems. “Knowing the factors that influence exchanges on long timescales is important to understand why species can become invasive on younger timescales. In the current biodiversity crisis, this can help to better assess the consequences of human-caused invasions,” concludes Skeels.
Source: Swiss Federal Institute of Technology in Zurich, specialist article: Science, doi: 10.1126/science.adf7122