Why are there no kangaroos in Bali or tigers in Australia?

The movement of species across new continents formed millions of years ago continues to determine the flora and fauna of today.

If you travel to Bali you won’t see any cockatoos, but if you go to the neighboring island of Lombok you will. The situation is similar with marsupials: Australia is home to many species of marsupials, including the kangaroo and koala. The further west, the rarer they are. While on the Indonesian island of Sulawesi you will find only two representatives of these typically Australian mammals, on neighboring Borneo you will look in vain. On the other hand, Australia is not home to typical Asian mammals such as bears, tigers or rhinos.

This abrupt change in the composition of the animal world already caught the attention of the British naturalist and co-discoverer of evolutionary theory Alfred Russell Wallace, who traveled through the Indo-Australian archipelago from 1854 to 1862 to collect animals and plants. He described an (invisible) biogeographical line between Bali and Lombok and Borneo and Sulawesi, marking the westernmost distribution of Australian fauna.

Fascinating change in wildlife

Biodiversity researchers have long been fascinated by this abrupt shift in creatures along the Wallace Line. However, it has not yet been clarified in detail how these distribution patterns occurred.

One explanation is plate tectonics. 45 million years ago, the Australian Plate began to move northward and slipped under the mighty Eurasian Plate. This brought two landmasses that were previously far apart together. Land creatures found it easier to colonize one continent from another. Tectonic movements also led to the creation of numerous (volcanic) islands between the two continents, which animals and plants used as springboards to migrate west or east.

More Asian animals in Australia than vice versa

But until now it was a mystery why there were more species from Asia to Australia – countless venomous snakes, spiny lizards (Moloch horridus), jumping rats (Notomias sp.) or flying foxes attest to this – than vice versa.

To better understand this asymmetrical distribution of vertebrates along the Wallace Line, researchers led by Loïc Pellissier, professor of ecosystems and landscape evolution from ETH Zurich, created a new model. It combines reconstructions of climate, plate movements between 30 million years ago and the present, and an extensive dataset of nearly 20,000 birds, mammals, reptiles and amphibians recorded in the region today.

The climates of the areas of origin are decisive

In the latest issue of Science, the researchers now show that adaptations to the climates of the home areas are partly responsible for the uneven distribution of Asian and Australian fauna on either side of the Wallace Line.

In addition to tectonic plates, the environmental conditions prevailing millions of years ago were decisive for the exchange between the two continents. Based on simulations, the researchers found that animals coming from Asia were more likely to “hop” across Indonesian islands to reach New Guinea and northern Australia.

These islands had a humid tropical climate, which they were comfortable with and to which they had already adapted. The Australian fauna was different, having evolved in a colder climate that became increasingly drier over time, so it was less successful in gaining a foothold on tropical islands than the fauna migrating from Asia.

Thus, the Asian climate favored the creatures that arrived in Australia via the tropical islands of the faunal region known as Wallacea, especially those that tolerated a wide variety of climates. This facilitated their settlement on the new continent. “Historical context is crucial to understanding the patterns of biodiversity distribution seen today and was the missing piece of the puzzle that explained Wallace’s line puzzle,” says first author Alexander Skeels, postdoctoral researcher of the Pellissier group.

Competitive advantages for tropical species

Characteristics of species that have evolved in tropical habitats include faster growth and greater competitiveness that allows them to withstand the pressure of coexistence with many other species. In more severe climates, such as the coldest and driest regions of Australia, organisms often have to evolve special adaptations to deal with drought and heat stress. Among them, behavioral adaptations, such as nocturnal activity, and physiological adaptations, to minimize water loss. “Many Australian tree frogs burrow into the ground and remain dormant for long periods of time for this reason,” notes Skeels. “Something that is rare in tropical frogs.”

The findings are important for researchers: “They make it clear that we can only understand current patterns of biodiversity distribution if we include prehistoric geological development and climatic conditions in our considerations”, says Pellissier.

The heritage of past eras has shaped biodiversity patterns to the present. It also helps us understand why more species are found in the tropics today than in temperate latitudes. “To fully understand the distribution of biodiversity and the processes that maintain it today, we need to know how it emerged”, says the researcher.

Learning to understand invasive species

This is especially true in biogeography, because the exchange of species between continents continues today with regularity and at an alarming rate, as humans move animals and plants across the planet. These species can become invasive on other continents and harm ancestral fauna and flora. ‘Understanding the factors that influence switching over long time scales is important for understanding why species can become invasive over more recent time scales. In the current biodiversity crisis, this can help us to better assess the consequences of human-induced invasions”, emphasizes Skeels.


Paleoenvironments shaped the exchange of terrestrial vertebrates through the Wallace Line

Source: ETH Zurich

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