When you visit Bali, you won’t come across a cockatoo, but if you venture to the nearby island of Lombok, you will spot one. A similar situation applies to marsupials: Australia harbors a variety of marsupial species like kangaroos and koalas. However, as you head westward, their presence diminishes. While Sulawesi, an Indonesian island, is home to only two representatives of these distinctively Australian mammals, you’ll be disappointed in your search for them on neighboring Borneo. On the other hand, Australia doesn’t host mammals that are typically found in Asia, such as bears, tigers, or rhinos.
The striking shift in the animal population’s composition in this region was noted by the renowned British naturalist Alfred Russell Wallace, who not only co-discovered the theory of evolution but also embarked on a scientific expedition from 1854 to 1862 across the Indo-Australian Archipelago to study and gather specimens of animals and plants. During his travels, Wallace observed an intriguing biogeographical boundary, imperceptible to the naked eye, that separated Bali from Lombok and Borneo from Sulawesi. This invisible line delineated the furthest extent to the west where Australian fauna could be found.
Fascinating change of wildlife
The abrupt transition of species along the Wallace Line has captivated biodiversity researchers for a significant period of time. Despite their fascination, the exact mechanisms that led to these distinct distribution patterns have yet to be fully elucidated.
One plausible explanation for these distribution patterns is the phenomenon of plate tectonics. Approximately 45 million years ago, the Australian Plate initiated a northward drift, eventually subducting beneath the massive Eurasian Plate. This convergence brought two land masses closer together that were previously separated by considerable distances. As a result, it became more feasible for land-dwelling organisms to colonize one continent from the other.
The tectonic movements also led to the formation of numerous volcanic islands in the region, acting as stepping stones for the migration of animals and plants. These islands provided opportunities for organisms to disperse either eastwards or westwards, further influencing the distribution patterns we observe today.
More Asian animals in Australia than vice versa
However, the mystery of why a greater number of species migrated from Asia to Australia, such as numerous venomous snakes, spiky lizards (Moloch horridus), leaping mice (Notomys sp.), and flying foxes, compared to the opposite direction, has remained unsolved until this point.
To gain a deeper insight into this uneven distribution of vertebrates across the Wallace Line, a team of researchers, headed by Loïc Pellissier, a Professor specializing in Ecosystems and Landscape Evolution at ETH Zurich, has developed a novel framework. This model incorporates climate reconstructions, plate movements spanning from 30 million years ago to the present, and an extensive dataset encompassing approximately 20,000 birds, mammals, reptiles, and amphibians that are presently documented within the region.
Climates in areas of origin decisive
In the most recent edition of Science, the researchers present findings indicating that the uneven distribution of Asian and Australian fauna across both sides of the Wallace Line can be partially attributed to adaptations to the respective climates in their areas of origin.
Apart from plate tectonics, the prevailing environmental conditions of millions of years ago played a crucial role in facilitating the interchange between the two continents. Through simulations, the researchers discovered that creatures originating from Asia had a higher probability of “hopping” across the Indonesian islands to reach New Guinea and northern Australia.
These islands exhibited a tropical and humid climate, which the Asian fauna was accustomed to and had already adapted to. In contrast, Australian wildlife had evolved in a cooler climate that had gradually become drier over time. Consequently, they faced challenges in establishing themselves on the tropical islands, making the migration from Asia more successful in comparison.
The climate in Asia was advantageous for organisms that reached Australia through the tropical islands of the faunal region called Wallacea, particularly those capable of adapting to diverse climates. This adaptability facilitated their successful colonization of the new continent. Alexander Skeels, the first author of the study and a postdoctoral researcher in Pellissier’s group, emphasizes the significance of historical context in comprehending the current patterns of biodiversity distribution. It serves as the missing element that elucidates the mystery surrounding Wallace’s Line.
Competitive advantages for tropical species
Characteristics exhibited by species that evolved in tropical environments encompass accelerated growth and heightened competitiveness, enabling them to thrive amidst the challenges posed by coexistence with numerous other species. Conversely, organisms residing in harsher climates, such as the colder and drier regions of Australia, typically undergo specialized adaptations to endure drought and heat stress. These adaptations include behavioral traits like nocturnal activity and physiological adjustments to reduce water loss. Skeels highlights the fact that many Australian frogs bury themselves in the ground and enter prolonged periods of dormancy as a means of survival, a behavior that is uncommon among tropical frogs.
The researchers recognize the significance of these findings, emphasizing that comprehending the present-day distribution patterns of biodiversity necessitates the inclusion of geological transformations and climatic circumstances from prehistoric eras in our analysis. Pellissier stresses the importance of integrating historical context into our considerations to achieve a comprehensive understanding of the subject matter.
The influence of bygone epochs has left a lasting impact on the current patterns of biodiversity. This historical legacy further aids in comprehending the reason behind the greater abundance of species in tropical regions compared to temperate latitudes. The researcher emphasizes that unraveling the origins of biodiversity is crucial to gain a complete understanding of its distribution and the underlying mechanisms that sustain it today.
Learning to understand invasive species
This holds particularly true in the field of biogeography, as the ongoing and accelerated movement of animals and plants by humans across continents leads to regular and concerning species exchanges. Such exchanges can result in invasive species that pose a threat to the native fauna and flora. Understanding the factors that influence long-term species exchange becomes crucial in comprehending the reasons behind the invasiveness observed in more recent timeframes. In the context of the present biodiversity crisis, this knowledge can assist in more accurately assessing the consequences of human-induced invasions, as highlighted by Skeels. (Newswise)
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