Introduction to Wallace Line
The British naturalist Alfred Russel Wallace drew the Wallace line, also known as Wallace’s line, in 1859. English biologist T.H. Huxley gave it its name.
It separates the biogeographical realms of Asia and ‘Wallacea‘, a transitional zone between Asia and Australia formerly also called the Malay Archipelago and the Indo-Australian Archipelago (Present day Indonesia). To the west of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origins is present. Wallace noticed this clear division in both land mammals and birds during his travels through the East Indies in the 19th century.
The line passes across Indonesia, including the Makassar Strait, which separates Borneo and Sulawesi (Celebes), and the Lombok Strait, which separates Bali and Lombok. Although the distances are very short—just around 35 kilometers, or 22 miles—it is sufficient to show the differences in species found on each island.
The Indo-Australian Archipelago’s location at the meeting point of four main tectonic plates, together with additional semi-isolated microplates and ancient sea levels, has resulted in a complicated biogeography. As a result, several taxonomic groupings were isolated on islands that are currently rather near to one another. Since the middle of the 19th century, biologists and naturalists have established several borders, including Wallace’s line, to establish limits on the dispersal of the archipelago’s flora and wildlife.
Historical background
The Venetian explorer Pigafetta documented the biological differences between the Philippines and the Maluku Islands (also known as the Spice Islands), which are on opposite sides of the Wallace’s Line, in 1521 while continuing Ferdinand Magellan’s expedition after Magellan was killed on Mactan. This is one of the earliest accounts of the biodiversity in the Indo-Australian Archipelago. The findings of the English sailor G.W. Earl on the faunal differences among the islands in the Indo-Australian archipelago were later published.
In 1845, G.W. Earl explained how shallow waters linked western islands (like Sumatra, Java, etc.) to the Asian continent and their corresponding fauna, whereas eastern islands (like New Guinea) were linked to Australia and were distinguished by the presence of marsupials.
Wallace’s beliefs on biogeography, which he made public in his 1859 paper after visiting the area extensively, were aided by these early studies. Since “all the islands eastward of Borneo and Java formed part of an Australian or Pacific continent, from which they were separated,” he suggested drawing a line east of Bali.
However, Wallace’s main goal was not to propose the line; rather, he developed his theories of evolution and biogeography in order to better understand the geological phenomena and colonization events that led to the boundaries in the faunal distribution of the area. Wallace’s research in Indonesia illustrated the nascent notion of evolution at the same time as Asa Gray and J.D. Hooker wrote essays endorsing Darwin’s theory. However, Wallace had to contend with numerous conflicting issues regarding his biogeography hypothesis because of his ignorance of tectonic plates and the ambiguity surrounding biodiversity in the Philippines.
In actuality, Wallace left off the Philippines from his 1859 article, and T.H. Huxley (1868) proposed making a line in his honor. Huxley observed that species in the Philippines were significantly different from those in Asia after researching the distribution of gallinaceous birds throughout the archipelago. On the basis of it, he redrawn Wallace’s border, putting it west of the Philippines and calling it “Wallace’s line,” even though Wallace had declined to put the Philippines on the line’s eastern side.
More recent work assessing biodiversity assemblies, phylogeny and utilizing computer-based geospatial tools to analyze former boundaries have elicited patterns of division that were almost identical to the ones proposed by main 19th-century authors. Nonetheless, some special cases not previously elucidated are now observable via modern analyses.
For example, the land mammals, birds, and amphibians in Wallace’s realms were the reference for the boundaries of the theoretical concept of Wallace that always retain relevant and which conveyed the possible relations. Ali et al. (2020) included fauna of Christmas Island in their study and showed that most of the mammals and amphibians that colonized the island are the descendants of the disappeared species from the Lombok Strait. Consequemtly, they recommend a rearrangement of Wallace’s line with Christmas Island being placed on the Australasian side of the biogeographical divide, instead of the oriental side.
The biogeography of Wallace Line
The link between ancient sea levels and continental shelves is crucial to our understanding of the region’s biogeography. Geographically, Wallace’s line may be seen by looking at the outlines of the continental shelf. It functions as a deep-water passage that divides the Sahul Shelf from the southeast side of the Sunda Shelf. While the Sahul Shelf connects Australia to New Guinea and its neighboring islands, the Sunda Shelf connects Borneo, Bali, Java, and Sumatra undersea to the Southeast Asian mainland.
Islands joined together during the Pleistocene, when ocean levels dropped by much to 120 meters (390 feet), although Asia and Australia were never united. As a result, deep water between those two sizable continental shelf sections formed a barrier that kept Australia’s and Asia’s flora and fauna apart for more than 50 million years.
Because the physical characteristics of the separated islands are so comparable, it is reasonable to assume that there was an ocean barrier that prevented species movement. Therefore, only species that can cross the straits between islands populate the islands that make up the Wallacea area, which are separated on their respective continental land masses.
An alternative explanation is that “Weber’s line” passes through this transitional region (east of the center), marking the boundary between Asian and Australian species.
The study of zoogeography
Since many birds avoid crossing even the smallest expanses of open ocean water, their distributions tend to follow the line’s boundaries. Larger terrestrial mammals are often restricted to one side or the other, however, bats can have ranges that straddle the boundary. On the Australian side, native rodents coexist with a variety of marsupial species and some monotremes; however, the presence of rodents in this instance is a result of more recent colonization activities.
On the other hand, placental mammals including apes, cats, elephants, monkeys, rhinoceroses, and others are present on the Asian side, but marsupials are not. On Sulawesi, tarsiers, pigs, and macaques are exceptions to this rule.
The general pattern is remarkable and very persistent, but other groupings of plants and animals exhibit different patterns. Since their colonization events differ in their capacity to move over bodies of water, flora do not follow the Wallace Line to the same degree as animals. With the exception of one species, Eucalyptus deglupta, which grows natively on the Philippine island of Mindanao, the Australasian genus Eucalyptus does not cross the border.
1. What is the Wallace Line, and how did it come to be discovered?
The Wallace Line is a biogeographical boundary that separates the fauna of Australia from that of Southeast Asia. It is named after the British naturalist Alfred Russel Wallace, who first proposed the theory in the mid-19th century. Wallace’s extensive travels in the Malay Archipelago led him to observe that distinct species of animals and plants were present on either side of the line. His observations challenged earlier ideas of the distribution of species, and the line became an important concept in the field of biogeography.
The Wallace Line runs between Bali and Lombok to the west, and between Borneo and Sulawesi to the east, cutting across the Indonesian archipelago. It is a striking example of the way that geographic and ecological barriers influence the evolution and distribution of species. This line is not just a physical feature but also a metaphorical divider between two distinct ecological zones: the Asian zone, which is dominated by species like tigers and rhinoceros, and the Australian zone, which is home to marsupials like kangaroos and koalas.
2. Why is the Wallace Line considered a critical boundary in biogeography?
The Wallace Line is a key feature in understanding the biogeography of the Southeast Asian and Australasian regions. It marks the sharp divide between two distinct ecosystems: the Asian ecozone to the west and the Australian ecozone to the east. On the Asian side, animals like tigers, rhinoceroses, and various primates are prevalent, while the Australian side is home to marsupials, monotremes, and unique bird species like the cassowary.
This distinction is due to the historical development of the region, including the tectonic movements and the varying degrees of land connectivity between the islands of Southeast Asia and the Australian continent. The movement of landmasses over millions of years led to the isolation of species, which subsequently evolved into the distinct fauna seen on each side of the line. The Wallace Line also highlights the impact of past sea levels, which separated landmasses during glacial periods, contributing to the unique biodiversity found in this region.
3. How did Alfred Russel Wallace contribute to the understanding of species distribution?
Alfred Russel Wallace was a pioneering naturalist whose observations and theories laid the groundwork for the field of biogeography. Wallace’s work focused on the geographical distribution of species, particularly in the Malay Archipelago. His expeditions to areas like Malaysia, Indonesia, and the Philippines provided critical insights into how geography, climate, and natural barriers influenced the evolution and spread of species.
In his writings, Wallace noted that the types of animals found in the western part of the archipelago were more similar to those of mainland Asia, while the fauna in the eastern part resembled that of Australia. This led him to propose the existence of a dividing line between the two regions, now known as the Wallace Line. His observations were revolutionary at the time and helped challenge the prevailing views of species distribution, particularly the idea of a global “chain of being” where all creatures were assumed to be related through a singular evolutionary lineage.
4. How does the Wallace Line influence the biodiversity of the region?
The Wallace Line serves as a natural boundary that influences the biodiversity of Southeast Asia and Australasia. On the western side of the line, the flora and fauna share many similarities with those of mainland Asia, including species such as elephants, tigers, and gibbons. This is due to the relatively recent connection of the islands to the Asian continent, allowing for species migration across the land bridge.
On the eastern side of the Wallace Line, the biodiversity is more similar to that of Australia, featuring unique species like marsupials, monotremes, and a wide variety of endemic bird species. This divergence is the result of millions of years of isolation, with the landmasses on the eastern side of the line being separated from the Australian continent. As a result, the Wallace Line plays a significant role in shaping the distinct ecosystems on either side, leading to an extraordinary variety of life.
5. What is the significance of the Wallace Line for evolutionary biology?
The Wallace Line is significant for evolutionary biology because it provides a clear example of how geographic isolation and barriers can influence the process of speciation. The line represents the division between two major ecozones: the Asian and Australian zones. Over millions of years, the isolation of species on either side of the line led to the development of distinct evolutionary paths.
In evolutionary biology, the Wallace Line offers a natural experiment in how physical features like seas, mountains, and other barriers can cause populations to evolve in isolation. The fauna on either side of the line evolved independently, resulting in unique adaptations and the development of species that are highly specialized to their respective environments. The study of these differences has provided valuable insights into the mechanisms of evolution and the importance of geographic and environmental factors in shaping biodiversity.
6. What role does the Wallace Line play in island biogeography?
The Wallace Line plays a crucial role in the study of island biogeography, the field that explores the distribution of species on islands. Islands, due to their isolation, provide natural laboratories for understanding how species evolve in response to limited resources and distinct ecological niches. The Wallace Line marks a clear boundary where different faunal assemblages are found on either side, making it an important case study in island biogeography.
The islands of Southeast Asia and Australasia, divided by the Wallace Line, exhibit striking differences in their flora and fauna. On the western side, the islands are closer to the Asian mainland, and species are more closely related to those found in mainland Asia. On the eastern side, the islands are more isolated from the Australian continent, leading to the development of species that are more closely related to those of Australia. This division underscores the impact of isolation on the evolution of species and highlights the significance of geographical factors in shaping biodiversity.
7. How does the Wallace Line affect the distribution of plant species?
The Wallace Line not only influences the distribution of animal species but also plays a role in the distribution of plant species across Southeast Asia and Australasia. Plants, like animals, are influenced by factors such as climate, soil, and geographical barriers. On either side of the Wallace Line, distinct plant communities have evolved, shaped by the differences in the environment and isolation.
In general, the western islands, which are part of the Asian ecozone, have plant species that are more similar to those found on the Asian mainland. These include tropical rainforests and a variety of hardwood trees. On the eastern side of the line, the flora is more closely related to the Australian continent, with a greater diversity of unique species, including many eucalyptus trees, which are characteristic of Australia. This plant distribution further supports the idea that the Wallace Line acts as a boundary that influences both animal and plant evolution.
8. Are there exceptions to the species distribution across the Wallace Line?
While the Wallace Line is a strong indicator of species distribution, there are exceptions. Some species do cross the line, though these instances are relatively rare. This could be due to factors such as changes in sea levels, which have periodically exposed land bridges, allowing species to migrate between the islands. For example, some birds and reptiles have been found on both sides of the Wallace Line, suggesting that migration across the boundary may have occurred at certain times in history.
Another factor that could account for these exceptions is the mobility of certain species. Birds, bats, and insects, which are capable of flying long distances, might cross the Wallace Line more easily than larger, less mobile species. Similarly, human activity has also played a role in the movement of species across the line, as humans have introduced non-native plants and animals to different regions.
9. How did the geography of the Wallace Line shape the evolution of animals like marsupials?
The geography of the Wallace Line played a critical role in the evolution of animals like marsupials, which are mainly found on the Australian side of the line. Marsupials, including kangaroos, koalas, and wombats, are believed to have evolved in isolation after Australia became separated from other landmasses millions of years ago. The Wallace Line marked the eastern edge of the Asian landmass and the beginning of the Australian continent, creating a barrier that prevented the migration of placental mammals, which dominate the rest of the world.
As a result, marsupials developed and diversified in Australia without competition from placental mammals. This evolutionary path was reinforced by the isolation provided by the Wallace Line, which helped to preserve the distinctiveness of Australia’s fauna. The line thus represents a significant event in the history of marsupial evolution, as it marks the boundary beyond which these animals could not easily migrate.
10. What evidence supports the existence of the Wallace Line?
The evidence for the existence of the Wallace Line comes from a combination of observations made by early naturalists like Alfred Russel Wallace, as well as modern studies in genetics, paleontology, and ecology. Wallace’s observations of the fauna of the Malay Archipelago provided the initial clues, showing that species on either side of the line were markedly different.
Modern genetic studies have confirmed these differences, showing that animals and plants on either side of the Wallace Line are genetically distinct. Additionally, fossil evidence supports the idea that the regions on either side of the line were once connected to different continents. The distinct ecosystems on either side of the line, with their characteristic species, provide further evidence that the boundary is not a coincidence, but a key feature in the evolution of the region’s biodiversity.
11. Why is the Wallace Line often referred to as a ‘biological barrier’?
The Wallace Line is referred to as a biological barrier because it separates two distinct biogeographical regions, each with its own unique assemblage of species. This barrier is not a physical wall but rather a result of geological and evolutionary processes that have shaped the biodiversity of the region. The line marks a boundary between the Asian and Australian ecozones, where the species on either side have evolved in isolation over millions of years.
This isolation has led to the development of vastly different ecosystems. For example, the Asian ecozone is home to large mammals like elephants, tigers, and rhinoceroses, while the Australian ecozone is known for its marsupials, monotremes, and unique bird species. The Wallace Line effectively divides the two regions, acting as a barrier that has shaped the evolutionary history of the plants and animals found there.
12. How do tectonic shifts influence the position of the Wallace Line?
Tectonic shifts have played a crucial role in the position and development of the Wallace Line. Over millions of years, the movement of tectonic plates has shaped the geography of the region, leading to the formation of landmasses and the isolation of species. The Wallace Line itself is influenced by the movement of the Indo-Australian plate and the Eurasian plate, which have caused the landmasses to shift and separate.
During periods of lower sea levels, land bridges may have allowed species to migrate across the line, while during higher sea levels, these land connections would have been submerged, reinforcing the separation of the species on either side. The dynamic nature of the Earth’s crust has therefore played a key role in shaping the biodiversity and distribution of species across the Wallace Line.
13. How does the Wallace Line explain the presence of similar species on either side?
The presence of similar species on either side of the Wallace Line can be explained by historical factors like land connections during periods of lower sea levels and subsequent isolation during periods of higher sea levels. During times of lower sea levels, land bridges may have formed, allowing species to migrate across the line and become established on both sides. However, once these land connections were submerged, the species became isolated and evolved independently, leading to the divergence of species over time.
For example, some species of birds and primates found on both sides of the Wallace Line are believed to have originally migrated across the line during periods of lower sea levels. After becoming isolated, these species evolved into distinct forms, adapted to their respective environments. This explains why similar species can be found on both sides, but with noticeable differences due to their evolutionary paths.
14. What impact did the Wallace Line have on the development of early ecological theories?
The Wallace Line had a profound impact on the development of early ecological theories, particularly in the fields of biogeography and evolutionary theory. Wallace’s observations of species distribution helped challenge the prevailing idea that species were evenly distributed across the globe. Instead, Wallace demonstrated that geography played a significant role in shaping the distribution of life.
His theory of the Wallace Line also contributed to the development of the concept of “ecozones” – distinct regions characterized by specific sets of plant and animal species. This idea laid the foundation for modern ecological studies and provided a new way of thinking about the relationship between geography, climate, and biodiversity. The Wallace Line thus played a pivotal role in the evolution of ecological and evolutionary thought.
15. What are the ecological implications of the Wallace Line for conservation?
The Wallace Line has significant ecological implications for conservation efforts in Southeast Asia and Australasia. It highlights the unique biodiversity found in each ecozone, underlining the importance of protecting both the Asian and Australian regions. Conservationists recognize that species on either side of the Wallace Line face different environmental challenges due to their distinct evolutionary histories and ecological adaptations. This means that conservation strategies must be tailored to the specific needs of the species found on each side of the line.
The line also serves as a reminder of the fragility of island ecosystems. Many species on both sides of the Wallace Line are found only in specific areas and are vulnerable to habitat loss and climate change. Protecting the biodiversity across this boundary involves addressing threats like deforestation, habitat fragmentation, and the introduction of invasive species, which can disrupt the delicate balance between native species. Effective conservation efforts must consider the biogeographical importance of the Wallace Line in maintaining ecological integrity.
16. How does the Wallace Line interact with modern climate change?
The Wallace Line continues to be an important factor in understanding how climate change may affect biodiversity in Southeast Asia and Australasia. As climate change impacts the region, shifts in temperature, rainfall patterns, and sea levels could alter the distribution of species, potentially threatening those that are already vulnerable due to the unique conditions of their environment.
For example, rising sea levels could submerge land bridges that once allowed species to migrate between the islands of the region, further isolating species on either side of the Wallace Line. This increased isolation could hinder the movement of species, reducing genetic diversity and making it harder for species to adapt to new environmental conditions. Conversely, changes in climate could also lead to new migration routes or allow species to cross boundaries in unexpected ways, possibly leading to new ecological interactions. Understanding the dynamics of the Wallace Line helps predict how these changes might play out and informs conservation strategies to protect biodiversity under climate change.
17. What role did the Wallace Line play in the development of the theory of natural selection?
The Wallace Line played a crucial role in the development of the theory of natural selection, particularly in the work of Alfred Russel Wallace and Charles Darwin. Wallace’s observations of the distribution of species in the Malay Archipelago helped shape his understanding of how species evolve in response to their environments. His work, alongside Darwin’s, led to the formulation of the theory of natural selection, which explains how species evolve over time through the differential survival and reproduction of organisms best adapted to their environments.
Wallace’s realization that species on either side of the Wallace Line had evolved independently in isolation supported the idea that geographic barriers—such as the line itself—could drive evolutionary change. This insight contributed to the broader understanding that environmental factors, including geographic separation, could lead to the diversification of species and the development of new adaptations. Wallace’s work provided key evidence for Darwin’s theory, highlighting the role of natural selection in shaping biodiversity.
18. Are there other “lines” similar to the Wallace Line in other parts of the world?
Yes, the Wallace Line is not the only biogeographical boundary of its kind. There are several other lines in different parts of the world that represent similar ecological divisions between regions with distinct faunal assemblages. One of the most famous is the “Lydekker’s Line,” which is found in Australia and New Guinea and marks a similar divide between species of the Australian and Oriental faunal regions.
In South America, the “Amazon River Line” separates species on either side of the river, contributing to the development of two distinct ecological zones with different species. Similarly, the “Great American Interchange” refers to the exchange of species between North and South America when the land bridge between them formed, creating another instance of biogeographical division and species migration.
These boundaries all highlight the role of geography and environmental factors in shaping the distribution of species and fostering the development of distinct ecosystems, much like the Wallace Line does in Southeast Asia and Australasia.
19. How does the Wallace Line affect the distribution of marine life?
While the Wallace Line is often discussed in terms of terrestrial fauna, it also has implications for marine life. The boundary extends beyond the land and influences the distribution of marine species, particularly in the waters between the islands of Southeast Asia and Australasia. The marine environments on either side of the Wallace Line are home to different species of fish, coral, and other marine life, which are shaped by the distinct oceanographic conditions of the two regions.
On the Asian side of the Wallace Line, the waters are generally warmer, with more tropical conditions that support a diverse range of species, while the waters on the Australian side are influenced by cooler currents and have different species distributions. These marine differences are also linked to the tectonic activity in the region, as the movement of the earth’s plates affects the ocean currents and the availability of nutrients in the water. The Wallace Line thus has an impact on both terrestrial and marine ecosystems, contributing to the overall biodiversity of the region.
20. What are the future prospects for studying the Wallace Line?
The future of studying the Wallace Line lies in advances in genetics, molecular biology, and climate modeling. As scientists continue to explore the genetic makeup of species on either side of the line, they are uncovering new insights into the evolutionary processes that shaped the biodiversity of the region. By studying the DNA of plants and animals, researchers can better understand the historical connections between species and how they diverged over time.
Additionally, the impact of climate change on the Wallace Line will be an important area of study in the coming decades. Researchers are increasingly focused on understanding how shifts in climate patterns might affect species distributions and whether species will be able to adapt to the changing environment or face extinction. The study of the Wallace Line remains a critical component of biogeography, evolutionary biology, and conservation, offering valuable lessons for the protection of biodiversity in a rapidly changing world.
