The Wallace Line

 The hidden barrier line between Asia and Australia : Why most animals don’t cross the Wallace Line

Through the middle of Indonesia runs an imaginary line which separates distinct ecosystems, creating a stark division in biodiversity and shaping the lives of countless species. The Wallace Line is an imaginary transitional zone between Asia and Australia which acts a barrier for many species. It represents an abrupt limit of distribution for many major animal groups. Many fish, bird, and mammal groups are abundant on one side of the Wallace Line and largely absent on the other side. In a world where lines define generally the boundaries of countries and regions, one invisible line stands out for its profound impact on the natural world. In an era of unprecedented climate change, scientists are turning to historical phenomena like Wallace’s Line to understand how natural barriers have shaped biodiversity and may influence future species adaptation. Following are the some of the important points:-

 Alfred Russel Wallace identified this boundary in the 19th century, highlighting evolutionary differences.

Wallace’s Line marks a significant biodiversity divide between Asia and Australia.

Australian species evolved uniquely due to isolation and climatic conditions.

The Antarctic Circumpolar Current influenced climate, shaping species evolution on either side of the line.

The Wallace Line is a transitional zone between Asia and Australia that includes what is known as the Malay Archipelago and the Indo-Australian Archipelago. For centuries, the Wallace Line has intrigued scientists and naturalists alike. This imaginary boundary, conceived by Alfred Russel Wallace in the 19th century, separates Asia and Australia. This line was first drawn by the British naturalist Alfred Russel Wallace, who independently came up with the theory of evolution by natural selection at the same time as Charles Darwin, although it was English biologist and anthropologist T.H. Huxley who gave the line its name. The Wallace Line, or Wallace’s Line, runs in the water through Indonesia, separating Asiatic fauna on the West from the mixture of Asiatic and Australian fauna on the East. This has led to it being dubbed “the line that most animals don’t cross.” It marks a stark division in the world’s biodiversity. Species on either side of the line evolved in dramatically different environments, leading to a unique divergence in flora and fauna. Recent research has shed new light on the geological and climatic factors which maintain this invisible barrier. Understanding Wallace’s Line is crucial for comprehending the broader patterns of evolution and the impact of climate on species distribution.

On the Asian side of the line, animals almost exclusively originate from Asia. But on the Australian side, they are a mix of both Asian and Australian descent. Until recently, this asymmetric distribution of species across the Wallace Line puzzled ecologists. What enabled Asian species to move in one direction but prevented Australian species from moving in the reverse direction? Thanks to a recent study, it now appears that scientists may have found the answer. Alfred Russel Wallace first identified this line in the 19th century, recognizing a distinct difference in species between the two regions. Despite their geographical proximity, Asia and Australia host vastly different types of animals and plants. This boundary reflects millions of years of natural evolution, influenced by tectonic movements and climatic shifts. Wallace’s observations laid the groundwork for modern biogeography. His work continues to inform our understanding of how species adapt and evolve in response to their environments. The Wallace Line delineates Australian and Southeast Asian fauna. Wallace’s Line is more than a geographical separator; it represents a historical record of Earth’s evolutionary processes. The line was not drawn randomly but is based on substantial scientific observations. His work highlighted the importance of understanding natural barriers in the study of evolution and migration patterns. Wallace’s legacy remains crucial in the on going study of biodiversity and ecology.

Lead author of that study, Dr. Alex Skeels from the Australian National University, talked about how the Wallace Line came to be. Explaining why the Wallace Line has captured the attention of scientists, Dr. Skeels said, “One of the key goals in the life sciences is understanding what drives the distribution of species around the globe. There are some cases where groups of species end suddenly and are replaced by a new set of species.”  In fact, questions about the distribution of species in this area can be traced back to 1521, when Venetian explorer Antonio Pigafetta recorded stark differences in animals between the Philippines and the Maluku Islands (Spice Islands) on opposite sides of the Wallace Line. Approximately 30 million years ago, the Australian tectonic plate’s drift from Antarctica was pivotal in establishing Wallace’s Line. This separation gave rise to the Antarctic Circumpolar Current (ACC), significantly altering global ocean currents and climate systems. “When Australia drifted away from Antarctica, it opened up this area of deep ocean surrounding Antarctica, which is now where the ACC is,” explained Alex Skeels.

G.W. Earl and other naturalists in the 19th century also observed distinct fauna on opposite sides of the line. “In the early 1800s, Wallace noted such a transition between Borneo, Java and Bali in eastern Indonesia and Sulawesi and Lombok in western Indonesia that separates Asian and Australasian species,” said Dr. Skeels. Alfred Russel Wallace first drew the faunal boundary. The observations made by Pigafetta, Earl and others played a pivotal role in shaping Wallace’s theories about the region’s biogeography, which he presented in 1863 at the Proceedings of The Royal Geographical Society of London. The drift led to cooler global climates, affecting species evolution in both regions. In Asia, tropical conditions supported a diverse array of species, while Australia’s climate became cooler and drier. This divergence in climate facilitated unique evolutionary paths. According to Skeels, “This dramatically changed Earth’s climate as a whole; it made the climate much cooler.” Australian species, accustomed to cooler climates, found it difficult to adapt to the tropical islands compared to their Asian counterparts.

Although the delineation of the line wasn’t Wallace’s primary goal, it emerged from his efforts to understand geological and colonization influences on faunal distribution. His studies in Indonesia also aligned with the his and Darwin’s emerging theories of evolution. Huxley coined the name “Wallace’s Line” in 1868, placing the Philippines on the western side, despite Wallace’s initial hesitation. Wallace’s Line stands as a prominent example of nature’s evolutionary boundaries. On one side, Asia is home to diverse species such as tigers, elephants and monkeys. On the other, Australia boasts a unique array of marsupials, monotremes and reptiles. This division has persisted over millions of years, even as sea levels and climates fluctuated. Despite its apparent rigidity, the line is not completely impermeable. Some species have managed to cross this divide. This rare occurrence highlights the complexities of evolutionary adaptation. “This could help us predict which species may be better versed at adapting to new environments, as changes to Earth’s climate continue to impact global biodiversity patterns,” Skeels noted that understanding these dynamics is increasingly important as the planet faces rapid climate change and shifting ecosystems.

Historically, scientists have been of the view that plate tectonics played a significant role in the formation of the Wallace Line. These events include the emergence and submersion of land bridges due to changes in sea levels during the Pleistocene. The formation of land bridges and barriers due to plate movement has likely facilitated or hindered the dispersal of species, impacting their distribution on either side of the Wallace Line. Dispersal refers to an organism’s capacity to move from one area to another. It is a crucial factor in species colonization. Some organisms possess the capability to traverse open water or other challenging barriers, while others are more limited in their mobility. Among mammals, bats, with their ability to fly, can potentially cross the line. In contrast, larger terrestrial mammals are typically confined to one side or the other. Climate has been a fundamental force shaping the evolutionary paths of species on either side of Wallace’s Line. In Asia, species adapted to tropical, humid environments, promoting diversity and mobility. Conversely, Australia’s drier, cooler conditions demanded different adaptations. The isolation of the continent led to the evolution of species that are distinct from those in Asia.

The Wallace Line, separating the Sunda Shelf (Borneo, Bali, Java, Sumatra) from the Sahul Shelf (Australia, New Guinea), was formed during the Pleistocene when lower sea levels exposed land connections between some islands but not between Asia and Australia. This deep-water channel acted as a barrier for more than 50 million years, keeping the flora and fauna of Australia separate from those of Asia. It also gave rise to the Wallacea region, whose islands remained isolated. These islands were populated only by organisms capable of crossing the straits between islands. Situated to the east of the central region, Weber’s Line marks the tipping point where species of Asian origin intersect with those hailing from Australia. Australian marsupials, such as kangaroos and koalas, evolved unique survival mechanisms suited to their environment. Their Asian counterparts developed traits suited to tropical climates. This evolutionary divergence underscores the role of climate in shaping species’ physical and behavioural characteristics. The on going study of these adaptations provides insights into the broader impacts of climate on biodiversity.

The influence of the Wallace Line is significant in shaping the distribution of various species, particularly birds and mammals. “There are lots of behavioural traits common in animals that prevent them from crossing open water. For example, many birds like to stay protected by dense vegetation to avoid being predated. This makes them unlikely to fly out into the open space,” explained Dr. Skeels. The Australian side of the Wallace Line is characterized by the prevalence of marsupial species like kangaroos, along with the presence of some monotremes and native rodents. Flora, unlike fauna, doesn’t strictly follow this boundary, due to differences in how they colonize new areas. One exception is the Australasian genus Eucalyptus, which mostly stays on its side of the line, except for one species, E. deglupta, found in Mindanao in the Philippines. However, the Wallace Line is less of a barrier to marine life. “The area between Wallace’s Line and the Sahul continental shelf is known as the coral triangle and is the most biodiverse marine environment on the planet. This is likely because of the complexity and abundance of islands and shallow reef systems in this geologically complex region,” said Dr. Skeels, explaining the abundance of marine life in the region. Eucalyptus plants, mostly native to Australasia, typically do not grow beyond this area. The rates of colonization across the Wallace Line can vary depending on several factors which include:-

The number of species available in a region impacts colonization diversity. A larger source pool tends to enhance species interchange potential.

Shorter distances tend to mean easier species colonization.

Influenced by plate tectonics, land bridges can connect isolated landmasses at various time, facilitating species movement. Conversely, when a land bridge is submerged, species are isolated, driving divergence and speciatio

The study of Wallace’s Line continues to offer valuable insights into the complexities of evolution and biodiversity. As climate change accelerates, understanding how natural barriers like Wallace’s Line influence species adaptation becomes crucial. How will these ancient evolutionary boundaries respond to modern environmental changes on our planet.

Dr. Skeels and his team analyzed 20,000 vertebrate species in conjunction with a geoclimate model and the dynamics of biological diversification. They found that paleo-environments profoundly influenced the exchange of terrestrial vertebrates. Kangaroos stay on the Australian side of the line. Of these conditions, two factors emerged in shaping the patterns of vertebrate exchange: precipitation tolerance and dispersal ability. These factors hinged on the intricate interplay between deep-time precipitation gradients and the adaptability of species to navigate the challenging geography of the Indo-Australian archipelago. Dr. Skeels explained their choice of studying vertebrates and the concept of deep-time precipitation gradients, “Deep-time precipitation gradients mean the difference in wet and dry conditions that emerged millions of years ago. Historically, Australia was a lot wetter than it is today, and drying out of the continent over time has uniquely shaped its fauna and limited arid-adapted lineages from dispersing into Asia. Vertebrates have some of the best available information on their distributions and evolutionary history, so they’re a good group to study these patterns across lots of species.” Precipitation tolerance refers to a species’ ability to thrive in a wide range of precipitation conditions. The research revealed that species capable of tolerating diverse precipitation regimes were more successful in colonizing new areas. This adaptability allowed them to bridge the ecological gaps created by shifting climatic conditions over geological time scales. And this is what has led to the asymmetrical colonization patterns observed across the Wallace Line. Dr. Skeels shared his concluding thoughts on their research: ‘Surprisingly, we found that movement across the Wallace Line was quite common in vertebrates. Initially, we thought it would be restricted to only a few vertebrate families, but most vertebrate families have had species cross the line.

The Wallace Line, a boundary etched by nature and scrutinized by scientists for centuries, continues to reveal its secrets about the complex interplay between geography, climate and biodiversity. Dr. Alex Skeels’ recent study sheds new light on the factors influencing vertebrate exchange across this line, emphasizing the importance of precipitation tolerance and dispersal ability in shaping colonization patterns. The Wallace Line remains a testament to the intricate web of life on our planet, where even the subtlest geographical features can hold profound ecological significance for all concerned.