The Hidden Barrier: Why Most Animals Don't Cross The Wallace Line PDF

The hidden barrier: Why most animals don't cross the Wallace Line Through the middle of Indonesia runs an imaginary line that separates distinct ecosystems, creating a stark division in biodiversity and shaping the lives of countless species. Tejas...

The hidden barrier: Why most animals don't cross the Wallace Line Through the middle of Indonesia runs an imaginary line that separates distinct ecosystems, creating a stark division in biodiversity and shaping the lives of countless species. Tejasri Gururaj Published: Oct 20, 2023 02:00 AM EST The Gibba dragon (Ctenophorus gibba) from central Australia. Dr. Damien Esquerre The Wallace Line is an imaginary transitional zone between Asia and Australia that 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. But why is this the case? 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. 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. 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." 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. The Wallace Line delineates Australian and Southeast Asian fauna. Altaileopard/Wikimedia Commons In this article, we talk to the lead author of that study, Dr. Alex Skeels from the Australian National University, about how the Wallace Line came to be. Unravelling the Wallace Line Explaining why the Wallace Line has captured the attention of scientists, Dr. Skeels told Interesting Engineering (IE), "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. 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. London Stereoscopic and Photographic Company 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. 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. A biogeographical boundary 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. The original drawing of the line in Wallace's paper. Wallace, Alfred Russel; J. Arrowsmith 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. Distribution of species The influence of the Wallace Line is significant in shaping the distribution of various species, particularly birds and mammals. "There are lots of behavioral 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. Meanwhile, smaller mammals like bats can cross the open water, but larger terrestrial mammals usually stick to one side. 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. The Asian side is populated by placental mammals such as apes, cats, elephants, monkeys, and rhinoceroses, among others. 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. Diana Polekhina/Unsplash Plate tectonics and dispersal ability 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. A map of major global tectonic plates. VectorMine The rates of colonization across the Wallace Line can vary depending on several factors. Source pool size: The number of species available in a region impacts colonization diversity. A larger source pool tends to enhance species interchange potential. Geographic distance: Shorter distances tend to mean easier species colonization. Land bridges: 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 speciation. Deep-time precipitation gradients 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. CraigRJD 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, in this study, 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. The Wallacea region is situated along the Wallace Line. Altaileopard/Wikimedia Commons "We found that the drying of Australia over the past 30 million years has had a strong impact on colonization patterns. Species need a suitable climate to establish in a region once they've dispersed there. If the conditions are too different, they won't survive. As a result, the Australian fauna is generally better adapted to drier and cooler environments than the Asian fauna," explained Dr. Skeels. 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. Conclusion 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. © Copyright 2023 Interesting Engineering, Inc. All Rights Reserved Source: https://interestingengineering.com/science/barrier-animals-wallace-line-indonesia