Essay on Biogeography of Malesia
Malesia has extremely rich flora. This region containing high concentrations of endemic flora (Sodhi et al, 2004). Furthermore, this region also overlaps with three biodiversity hotspots for conservation. Biodiversity hotspots are areas of high priority for conservation and may be selected on the basis of their local species richness, degree of concentration of rare species or the two measures combined with some assessment of urgency for conservation action (Araujo, 2002). The three biodiversity hotspots in Malesia are the Sunda Shelf, Wallacea, and Philipines. In this biodiversity hotspots, there are more than 7.4% endemic plants from 300,000 of global plants (Myers et al, 2000).
Malesia region is phytogeographic region which stretches the whole length of Malay archipelago and beyond to Bismarck archipelago east of New Guinea (Whitmore, 1984). It comprises three sub-unit region i.e. the Sunda Shelf, Wallacea, and the Sahul Shelf. The Sunda Shelf comprises Malay Peninsula, Sumatra, West Java, and Kalimantan. The Wallacea –the transition zone between the Sunda Shelf and the Sahul Shelf- comprises Java, The Lesser Sunda Islands, Sulawesi, the Moluccas, and Philipines. The Sahul Shelf only consists of New Guinea.
Why does Malesia region has different endemic plants in each sub-unit region? There are two main factors that affect this region. That factors are global geology and climate.
The major events of global geology that affect the biogeography of this region for example in the Wallacea’s region are the progressive break up of Gondwanaland from about 140 million years ago and the drifting north of the Indian fragment to collide with Laurasia at about 55 million years ago and of the Australia/New Guinea fragment to collide with the southeastern extremity of Laurasia at only about 15 million years ago. Before that the Malay archipelago did not exist. Plants could have reached modern Malesia from one of three sources i.e. Laurasia, Gondwanaland via Australia, or Gondwanaland via India followed by southeastwards migration (Whitmore, 1982).
Another factors that affect the biogeography is climate. Climate are expected to has strong effects on plant communities (Randin et al, 2009). In Malesia region, there is also a climatic difference between west Malesia, central Malesia, and east Malesia. West Malesia (the Sunda Shelf) has an everwet climate. Central Malesia (Wallacea) has a dry monsoon (though there are everwet areas). East Malesia (the Sahul Shelf) is also everwet and only contains New Guinea (van Welzen, 2005).
The combinations of two factors i.e. geology factor and climate factor causing Malesia region has rich flora and between the three sub-unit region in Malesia have different endemic plants. This statement base on the estimated number of species in Malesia (estimated 42,000 spp.) and endemic numbers in each sub-unit regions. In Malesia itself, base on Flora Malesiana books, there are 6616 species, 70% of the flora or 4599 are endemic to Malesia. The numbers of endemic plants are the Sunda Shelf comprises 50% endemic species or 1813 species; Wallacea has 31% endemic species or 844 species; and the Sahul Shelf has 54% endemic species or 1419 species (van Welzen, 2005). Nevertheless, endemic species–area relationship in Malesia was significant for all endemic species taken together. This indicates that concentrations of endemics do not occur at the island level but occur in sub-unit region level in Malesia (Roos et al, 2004) and the flora richness of each island group correlates significantly with the size of the areas (McArthur and Wilson, 1967 in van Welzen et al, 2005).
References
Araujo, M.B. (2002). Biodiversity hotspots and zones of ecological transition. Conservation Biology 16 (6): 1662-1663.
Myers, N., R.A. Mittermeier, C.G. Mittermeier, G.A.B. da Fonseca, and J. Kent. (2004). Biodiversity hotspots for conservation priorities. Nature 403: 853-858.
Randin, C.F., R. Eengler, S. Normand, M. Zappaz, N.E. Zimmermann, P.B . Pearman, P. Vittoz, W.Thuiller & A. Guisan. (2009). Climate change and plant distribution: local models predict high-elevation persistence. Global Change Biology 15: 1557-1569.
Roos, M.C., P.J.A. Kepler, S.R. Gradstein, and P. Baas. (2004). Species diversity and endemism of five major Malesian Islands: diversity-area relationships. Journal of Biogeography 31: 1893-1908.
Sodhi, N.S., L.P. Koh, B.W. Brook, and P.K.L. Ng. (2004). Southeast Asian biodiversity: an impending disasters. Trends in Ecology and Evolution 19: 654-660.
van Welzen, P.C., J.W.F. Slik, & J. Alahuhta. (2005). Plant distribution patterns and plate tectonics in Malesia. Biol. Skr. 55: 199-217. ISSN 0366-3612. ISBN 87-7304-304-4.
Whitmore, T.C. (1982). Wallace’s line: A result of plate tectonics. Annals of the Missouri Botanical Garden 69 (3): 668-675.
Whitmore, T.C. (1984). A Vegetation map of Malesia at scale 1 : 5 million. Journal of Biogeography 11: 461-471.