Why are there no kangaroos in Bali, and no tigers in Australia? A new study led by biologists from The Australian National University (ANU) and ETH Zurich in Switzerland is shedding light on why animals in this part of the world are divided by an invisible boundary.
Australia is a sanctuary for a variety of marsupial species, including the iconic kangaroo and the koala. But as you travel westward, these marsupials become scarce. On the Indonesian island of Sulawesi, you will only encounter a handful of Australian mammals, and they are completely absent on the neighboring island of Borneo. By contrast, Australia lacks several animals typically found in Asia, such as bears, tigers, and rhinos.
This abrupt shift in animal species caught the attention of the celebrated British naturalist and co-discoverer of evolutionary theory, Alfred Russell Wallace. During his expedition through the Indo-Australian Archipelago from 1854 to 1862, Wallace discovered a biogeographical separation between Bali and Lombok, and between Borneo and Sulawesi. This invisible boundary is now known as Wallace’s Line.
But how did this distribution pattern emerge in the first place? It turns out that plate tectonics played a significant role. Forty-five million years ago, the Australian Plate began its journey northwards, sliding beneath the mighty Eurasian Plate.
This phenomenon brought the two distant land masses closer, facilitating the migration of land creatures between the two continents. In addition, tectonic movements led to the formation of numerous islands, acting as stepping stones for animals and plants to journey eastward or westward.
Interestingly, more animal species migrated from Asia to Australia, which is documented by the presence of various poisonous snakes, thorny lizards, hopping mice, and flying foxes. This unbalanced distribution of vertebrates along the Wallace Line has been a mystery until now.
To investigate, a team of researchers led by Loïc Pellissier, Professor of Ecosystems and Landscape Evolution at ETH Zurich, developed a comprehensive model. This model combined climate reconstructions, plate displacements from 30 million years ago to the present, and an extensive dataset of about 20,000 birds, mammals, reptiles, and amphibians found in the region today.
The researchers have published their intriguing findings in the latest issue of the journal Science. They found that the climate of the originating areas played a significant role in the uneven distribution of Asian and Australian animal species on both sides of the Wallace Line.
According to the model simulations, Asian-originated animals (which were adapted to a tropically humid climate) found it easier to journey across the Indonesian islands and settle in New Guinea and northern Australia. Conversely, Australian wildlife had evolved in a cooler, increasingly dry climate. These animals found it challenging to thrive on the tropical islands.
“The historical context is crucial for understanding the biodiversity distribution patterns observed today and was the missing piece of the puzzle explaining the enigma of Wallace’s line,” said Alexander Skeels, the study’s first author and a postdoctoral researcher in Pellissier’s group.
Species that evolved in tropical habitats are more competitive, grow faster, and can tolerate a wide range of climates, making it easier for them to settle in new continents. For instance, Australian frogs, adapted to drought and heat stress, bury themselves in the ground and remain dormant for extended periods, a behavior rarely seen in tropical frogs.
“These findings underscore the importance of considering the geological and climatic conditions of prehistoric times to understand today’s biodiversity patterns,” said Pellissier.
The research helps us comprehend why more species exist in the tropics today than in temperate latitudes. It further underscores our need to understand the processes that have shaped our planet’s biodiversity, a fact that is especially relevant given the current rate at which species are being moved between continents due to human activities.
“Knowing the factors that influence exchange on long timescales is crucial to understanding why species can become invasive on more recent timescales,” said Skeels, highlighting the consequences of human-induced invasions in the current biodiversity crisis.
“Our findings could also inform predictions for animal migration in the future and help us predict which species may be better versed at adapting to new environments, as changes to Earth’s climate continues to impact global biodiversity patterns.”