Wallace’s Line is the biogeographical boundary that occurs between the islands of Bali and Lombok in the Indonesian archipelago. Alfred Russel Wallace came to the conclusion in 1860 that ‘the Strait of Lombok…separates two of the great zoological regions of the world'(Riddle and Hafner 2010). Wallace’s line separates the predominately Asian fauna to the west and the Australasian faunas to the east, however the boundary does have crossover, with some species occurring on both sides of the boundary (Lohman et al. 2011). As a biogeographical boundary, it highlights the discontinuities in faunal dispersal in the region. Wallace’s Line also traces one edge of the transition zone known as ‘Wallacea’, referencing the grouping of islands between the continental shelfs that divide the region. The Wallace line traces the Asian Sunda shelf and ‘Lydekker’s Line’ traces the Australian Sahul shelf (Lohman et al. 2011). The current hypothesis of the geological formation of Wallacea is that of oceanic islands, with no terrestrial connection to the surrounding land masses. Thus, the biota of Wallacea arose predominantly through dispersal of flora and fauna and not through vicariance. Wallace’s Line also played a role in the further development of the hypothesis of the origin of species and the importance of biological, geological and geographical factors when developing and understanding about faunal regions and species origin (Camerini 1993)
The complicated geological history of the region supported the production of this intense faunal divide, with the movement and breakup of various landmasses producing the Malay archipelago (Whitmore 1982). The West Malay Archipelago and most of south east Asia consists mainly of fragments that broke off from Australia and collided with the Eurasian plate. This occurred before the evolution of most recent plant and animal taxa, and thus most biota in this region will be of Southeast Asian origin. The East Malay Archipelago was also created by fragments of Australia and New Guinea, but at a much later date, and are thus populated by more Australasian biota (Turner et al. 2001). The changing global climates have also impacted this biogeographical boundary, with the areas on each side of ‘Wallacea’ – the biogeographical region of Wallace’s Line, having an ever-wet climate, and the area of Wallacea having a dry monsoonal climate (Bacon et al. 2013). This could in turn support the divide, with species suitable to climates only on one side of the Wallace Line unless there is a climatic change, as has happened in the past during dispersal events.
There are genus and families which have crossed the boundary on multiple occasions, such as the avian family Campepehaigdae, and many groups of animals which have had a single dispersal event across Wallace’s Line (Lohman et al. 2011). Wallace’s Line allows for scientific investigation into the migration of species and rates of diversification with a biogeographical region. In investigating migration and diversification over biogeographical barriers, this can further the understanding of patterns of diversity and the diversity that occurs within clades (Bacon et al. 2013). Wallace’s Line and the understanding of biogeographical transition zones has proved a basis for further models in evolutionary and ecological biogeography (Riddle and Hafner 2010). The biota in transition zones provides insight into Earth’s geological history, with biogeographical boundaries allowing for a deeper understanding of continental shifts and tectonic plate migrations. The study of biogeographical boundaries such as Wallace’s line are further advanced with the modern possibilities of molecular genetics, providing timeframe estimates of isolation, interchange and diversification (Riddle and Hafner 2010). In studying Wallace’s Line, we can better understand biogeographical boundaries in other areas of the globe, allowing for further investigation into the diversification across the Indian subcontinent and Eurasia.
Wallace’s Line reveals patterns in dispersal and diversification of species, in both species that have crossed the boundary and those that have not.
Some Aves genera do reflect the same pattern on both sides of Wallace’s Line, with migratory bird species obviously moving across large areas of land and ocean. As these migratory and shorebird species are long distance migrators, crossing multiple continents, they are not restricted to a side of the Wallace Line. (Carstensen and Olesen 2009). Another example is the Kingfisher family, which occurs on both sides of the Wallace Line, with the Indo-Australian region being a major diversity centre for the group. However, bird families such as Honeyeaters, which occur only on the Australian side of the Wallace Line and Woodpeckers, which occur only on the Asian side of the Wallace Line, demonstrate different patterns of evolution (Carstensen and Olesen 2009). This supports the hypothesis that a major clade of birds, the Corvida, evolved from Gondwanan ancestors, in Australia. Molecular genetics research has produced further evidence to this hypothesis (Riddle and Hafner 2010). Overall, there is very few avian species that migrate across Wallace’s Line, with most being migratory shorebirds, and only ten species being land birds from Eurasia (McCallum et al. 2008). This is believed have had a significant effect in protecting Australia from H5N1 avian influenza in past years, as the low number of bird species crossing Wallace’s line diminished the possible number of infected birds that could transmit the virus. Obviously, the advent of human colonisation and long distance migration both over water and land has moved many species into areas in which they did not originally occur.
Angiosperms are the most diverse group of land plants, having existed on earth since the Early Cretaceous period and now existing world-wide. Some genera, such as eucalypts, occur on both sides of the Wallace Line, however only a small number of species occur naturally outside Australia, with only 9 species exclusively non-Australian (Sellers 1910). In contrast, the genus Begonia is wide spread on both sides of the Wallace Line, colonising from the Asian mainland and down towards and across the Wallace Line, into Sulawesi and the Philippines (Thomas et al. 2012). The drivers behind this diversification of Begonia is understood to be the ongoing climate changes and intense geological shifts that have occurred in the region in recent geological time (Thomas et al. 2012). Wallace’s Line is shown to be a stopping barrier for species, with twice as many species stopped on one side or other of the barrier as those that pass (van Welzen et al. 2011). It is also shown that the most easterly variant of Wallace’s Line, Lydekker’s Line, stopping the most number of species from crossing out of Wallacea. In a paper by van Welzen et al. (2011) they divide Malaysia into 3 areas based on climate, which is shown to also have a distinct impact upon the movement of angiosperm species across the Wallace Line and its variants. Some species that once existed east of Wallace’s Line, in what was once Gondwana, are now exclusively found west of Wallace’s Line, in parts of southeast Asia, but have not crossed back to the eastern side of the Wallace Line (Coiffard et al. 2014). Comparatively, various clades of Vireya rhododendrons show a distinct separation between the phylogenetic analysis of the clade found on the western side of the Wallace Line and the clade found on the eastern side, which indicates that there was a past major divergence and the clades have not come back into contact, which supports the understanding that single dispersal events occur, but there is not a continual dispersal between the two sides of the Wallace Line (Brown et al. 2006) This allows for deeper investigation of past climatic and geological changes in relation to dispersal events.
Wallacea and its extensive islands are recognised as extensively diverse ecosystems, with Borneo and Sulawesi as sites of extensive in situ speciation. Sulawesi contains biota of both Asian and Australasian origin, allowing for ecosystems that exist nowhere else on the planet. Borneo’s mountain ranges are areas of rainforest refugia, with the surrounding lowlands going through periods of dryer and wetter climates over time, helping to preserve genetic diverse populations of organisms (Lohman et al. 2011). Wallace’s Line is an area of intense diversity, with a multitude of endemic species that exist nowhere else on the planet, and the convergence of two very different biological backgrounds, with Asian fauna, Australian fauna and the mixing of the two occurring in this area. In protecting and conserving these areas of biographical boundaries, we allow for greater future understanding of how geological and climatic changes will affect varying biomes and how speciation can occur over restricted areas. In the context of diversification rates, Wallace’s Line and the biogeographical boundary can provide information about the speed and direction of movement of diversification factors, both intrinsic and extrinsic (Bacon et al. 2013).