Exploring the Indo-Australian Archipelago: The Wallace Line
Image credits: Journal of Biogeography, December 2019
Indonesia has the third largest tract of tropical rainforest in the world, enveloping 17,500 islands in the Malay Archipelago, in the most biodiverse habitat in the world (Simantujak et al.,1997). Tropical Rainforests contain 50% of global biotic species, even though they only cover 6% of the worlds surface (Butler, 2019). This extreme biodiversity is highlighted in Indonesia, where the insular and mountainous nature of the country has led to high levels of endemism. For example, of the 1,531 bird species in Indonesia, 36% are endemic, a rate three times higher than that of the Amazon rainforest (Lambertini, 2000). Furthermore, this exceptionally high biodiversity is only compounded by the fact that Indonesia straddles two of the world’s major zoogeographic regions – the Oriental and Australasian realms, of which the boundary was identified by Alfred Russel Wallace on his expedition to the Malay Archipelago between 1854 and 1862. This Wallace Line has extraordinarily not been significantly altered over the 160 years since conception, but alternatives and alterations reflecting other boundaries have been proposed.
The Wallace Line divides the myriad islands of the Malay Archipelago roughly between the Oriental region to the north west and the Australasian region to the south east. It is no more clearly shown than between the small islands of Bali and Lombok, which are a mere 22 miles from each other, and yet have completely different faunal constitutions. Bali belongs to the Oriental realm, whereas Lombok is part of the Australasian realm. Travelling between these islands is how Wallace realised that there was a biogeographical division in the area. However, this boundary does not just pass between the small islands of Bali and Lombok – it continues north, separating Borneo from Sulawesi (Celebes) through the Makassar Strait, and historically dividing the Philippines from the Moluccas in the south. However, due to the complexities of biogeography (the study of the geographical distribution of plants and animals), there have been alterations to the Wallace Line, and new lines drawn which reflect different faunal boundaries.
- HISTORICAL CONTEXT
Although Wallace is credited with the theorisation of the line, he was not the first to note the differences between faunal communities. In fact, the first reference to this difference is from nearly 350 years before, when Antonio Pigafetta recorded the difference between the Philippines and the Maluku Islands (Moluccas) in 1521, on his journey to circumnavigate the globe under the command of Ferdinand Magellan. Ultimately, the expedition returned to Spain and succeeded as the first circumnavigation of the globe, then under the command of Juan Sebastian Elcano (Cameron, 1973), bringing back the first specimens of the Birds of Paradise. However, when Wallace explored the Malay Archipelago 350 years later between 1854 and 1862, he was able to accurately map the line, and document the differences in species. Moreover, he ultimately sought to explain this perplexing boundary through geographical processes, hence becoming the father of the new science of biogeography (Smith, 2007), the poster child of which were his famed birds of paradise. During his 8-year expedition, Wallace collected an extraordinary 125,000 specimens, of which several thousand were species entirely new to science (Shermer, 2002). The most notable species were the stunning Birdwing butterfly, Ornithoptera croesus, the new bird of paradise species Semioptera walacii, and the flying frog, Rhacophorus nigropalmatus.
His extensive travels through the Malay Archipelago affirmed his long-held view that species evolved from one another as a result of natural selection. Knowing Darwin was a leading contemporary thinker on biodiversity, Wallace sent him a short, 4000-word essay from Ternate in Indonesia, outlining his theory of evolution. Darwin had been working on his own theory of evolution for years, and this essay from Wallace meant Darwin was running out of time to publish his masterpiece, so along with the advice of his friends Charley Lyell and Joseph Hooker, he presented Wallace’s essay along with some of his own at the Linnean Society meeting of the 1st of July 1858. With contacting Wallace taking months due to his location in Indonesia, he was not aware of this rapid publication process, but later expressed his delight at being associated with such an esteemed scientist as Darwin. However, Lyell and Hooker’s plan relegated Wallace to co-discoverer, and ultimately historical obscurity, and years of scholastic debate has attempted to uncover the true motivations for this, although Darwin had likely hypothesised evolution years before Wallace.
Wallace meticulously documented the biodiversity of the many islands he travelled to, collecting thousands of specimens on each and noting the species, location and habitat information. Wallace also documented the ethnic diversity of the islands and attempted to explain the diversity and features of tribes through their evolution and interracial mixing. He used native Malays and Dayaks to help catch, skin, and transport his specimens and had up to one hundred natives procuring specimens for him (Van Wyhe, Drawhorn, 2015). It was during these travels that Wallace described the first occasion on which he encountered the zoogeographical boundary that would later become the Wallace Line – between Bali and Lombok, which he said differ “as essentially in their animal life as Europe does from America” (Wallace, 2010).
As Wallace observed, in Bali you have the avian families Megalaimidae (Barbets), Turdidae (Thrushes), and Picidae (Woodpeckers) which are typical oriental families, whilst in Lombok you have Cacatuidae (Cockatoos), Meliphagidae (Honeyeaters), and Megapodiidae (Megapodes), typical Australasian families. Furthermore, the mammals also obey this boundary, with for example Sumatra being the only place in the world where Elephants, Rhinocerous, Tigers and Orangutans can be found living together, and Java and Borneo having Civets, Deer, Monkeys and Otters. On the other hand, you have monotremes and marsupials such as the Cuscus on the other side of the Wallace Line. Flora do not obey these delineations (Van Welzen et al., 2011), with the exception of the Australasian genus Eucalyptus, which is not found on the Oriental side of the Wallace Line. In his book, the Malay Archipelago, Wallace attempts to describe the geographical factors which led to the formation of his boundary between the Oriental and Australasian zoogeographic regions, which famously pass through the strait of Lombok.
The ‘line’ can be explained by the two main submarine continental shelves that dominate the region, the Sunda Shelf that links Borneo, Bali, Sumatra and Java together, and the Sahul Shelf which links the island of New Guinea to Australia, as named by Max Carl Wilhelm Weber in 1919 (Ballard, 1993). The latter explains why hundreds of kilometres off the coast of Australia, the shallow Arafura Sea is less than 100 metres deep. During periods of glaciation, sea levels were up to 120 metres lower than they are currently, so during these times, the corresponding islands on both continental shelves were connected by land bridges, allowing free movement of species across them. However, a deep ocean channel marks the south east edge of the Sunda Shelf, and hence a stretch of water would have been present along the Wallace Line, preventing species migration, which explains the vast differences in faunal constitution found between the Philippines and the Moluccas, and Bali and Lombok.
Since Wallace, Lydekker’s Line has been theorised, which is found as a corresponding deep-water channel that marks the boundary of the Sahul Shelf in the south east and defines the boundary of the Australasian realm from Wallacea. Wallacea is the part of Indonesia that is the biogeographical anomaly, consisting of 13,500 islands (Trainor 2007), bordered by the Wallace Line to the north west, and Lydekker’s Line to the south east. Sulawesi is the largest island in Wallacea. Hence being bordered by two deep oceanic trenches that remained even during glacial periods, Wallacea is populated only by animals that were able to cross these deep bodies of water, resulting in the region having low endemism and a relatively low species richness. Weber’s Siboga Expedition from 1899 to 1900, prompted him to theorise a further line – Weber’s Line, an alternative Oriental-Australasian division to the Wallace Line (Mayr, 1944). This runs parallel to the Timor group, and then divides Sulawesi from the Moluccas. He also revealed that the Tanimbar Islands are the most striking boundary between the Oriental and Australasian realms, not Bali and Lombok. The original Wallace Line, however, remains a significant division and has a striking difference in species composition across it.
Without modern oceanographic techniques to aid him, it is extraordinary that Alfred Russel Wallace was able to effectively chart the zoogeographical boundary now known as the Wallace Line, and also correctly postulate that the commonalities on each side were due to the shallow sea that “always intimates a recent land connexion” (Wallace, 2010). However, as has been demonstrated, it is not just the Wallace Line that distinguishes species constitution: centuries of science have contributed two more lines which divide subtly different areas in one of the world’s most biodiverse and highly endemic regions. All of this though can be attributed to the early work of Wallace, who also contributed to evolution, natural selection, warning colouration, amongst other things, and pioneered biogeography.
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