which restricts kangaroos and co

On the trail of a puzzle in the history of animal dispersal: Why did fewer Australian species jump into Asia than the other way around? A study is now shedding light on this biogeographical phenomenon. In addition to continental drift, the climate in which species once evolved played an important role in the formation of the so-called Wallace boundary. The researchers say the findings may also help better understand the prevalence of invasive species today.

The British naturalist Alfred Russel Wallace was the first to notice this: when he examined the organisms of Australia and neighboring islands to the north from 1854 to 1862, he recognized a biotic geographic limit in distribution patterns. Accordingly, a line runs between Bali and Lombok as well as Borneo and Sulawesi, which represent the westernmost distribution area of ​​the typical Australian fauna. For example, there are no cockatoos in Bali – but they are found on the neighboring island of Lombok. In the case of marsupials, one thing is clear: many representatives of this group, like kangaroo species, live in Australia and New Guinea. However, the further west you go, the marsupials become rarer. On the Indonesian island of Sulawesi, there are only two representatives of these typically Australian mammals and someone is looking for them in vain in Borneo.

Why are there more Asian animals in Australia than the other way around?

On the contrary, the demarcation is less clear: the Australian region lacks many mammals typical of Asia, such as bears, rhinos or tigers. But apparently some animals found their way from Asia to Australia when land masses were once brought together by continental drift. Examples include the many venomous snakes, frilled lizards, bouncy mice or flying foxes. An international team of researchers has now developed a model to track the causes of this asymmetric spread along the Wallace line. In doing so, the scientists combined information about plate movements in the last 30 million years and reconstructed past climatic conditions in the Australian and Asian regions. In addition, data on about 20,000 species of modern birds, mammals, reptiles and amphibians from relevant regions are included in the model.

The team reports that the importance of Australia’s northward shift has been highlighted again, bringing the continent closer to the Eurasian plate. Although there is no consolidation, geological processes have created volcanic islands between the two landmasses. They could then serve as a “stepping stone” for the animals to spread, the researchers explained. But why didn’t it go the same way on both sides? Simulations now show that earlier adaptations to living conditions in different regions of origin were largely responsible for the unequal distribution of Asian and Australian animals.

Adapted to the climate of Stepping Stones

The researchers explained that the reason that Asian animals may have “jumped” across the Indonesian islands to New Guinea and northern Australia is because they found living conditions in these ungulates for which they were already adapted: there was a humid tropical climate there – similar to their native habitat. However, this was not the case for the Australian animals that were “eager to travel”. This is because they have evolved in a cooler climate and, over time, have become drier due to their resettlement history in Australia. So they were less competitive on the warm and humid islands, and had a hard time gaining a foothold and so couldn’t jump any further.

In contrast, the researchers said, characteristics of species that evolved in tropical habitats include faster growth and strong competitiveness. Because they were able to make better use of their starting points, some Asian species were able to invade new habitats, where they could eventually split into new species. “We thus provide the missing piece of the puzzle that solves the mystery of the Wallace line,” says lead author Alexander Skeels of the Swiss Federal Institute of Technology Zurich (ETH). “The results show that we can only understand the current distribution patterns of biodiversity if we include geological evolution and prehistoric climatic conditions in our considerations,” continues senior author Loïc Pellisier from ETH.

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 support it at present, we need to know how it came to be,” says Pellizier. In this context, the researchers also investigate a problem that currently confounds 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 traditional ecosystems. “Knowing what factors influence exchanges over long time scales is important for understanding why species can become invasive on smaller time scales. In the current biodiversity crisis, this can help improve assessment of the consequences of human-caused invasions.”

Source: Swiss Federal Institute of Technology Zurich, specialized article: Science, doi: 10.1126/science.adf7122

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