Quaternary extinctions of Southeast Asia’s megafauna more

Selwyn Symposium 2007 GSA Victoria Division Australia’s megafaunal extinction Geological Society of Australia Abstracts 79 8. Quaternary extinctions of Southeast Asia’s megafauna Biography: Julien Louys is a doctoral candidate at the University of New South Wales. His current research focuses on the ecology and extinction of Quaternary megafauna from Southeast Asia and China. In particular, he is currently working on reconstructing palaeoenvironments of megafauna-bearing sites in Asia using multivariate methods and data from modern nature reserves. His previous research has focused on Miocene koalas from Riversleigh, as well as the recognition of the use of bamboo tools in the archaeological record. Julien Louysa School of Biological, Earth and Environmental Sciences, The University of New South Wales, New South Wales 2052, Australia. louys_julien@hotmail.com a Southeast Asia evokes images of tropical rainforests, magnificently coloured birds and elusive primates, but only rarely does it evoke images of megafauna. Nevertheless, along with Africa, it is one of the few regions in the world where extant megafauna can still be found in a diversity that approaches that present during the Pleistocene. Even so, the extant megafauna of Southeast Asia are depauperate compared to that of the Pleistocene. In particular, many extant megafauna species currently not found in Southeast Asia were once widespread there, for example, the Indian rhino and the giant panda. Many other species became extinct, for example, the giant tapir, the stegodons, the giant pangolin and the giant ape. Within the global megafauna extinction debate, however, little attention has been paid to Southeast Asia until recently (e.g. Louys et al., 2007; Louys, in prep.). This is surprising, as Southeast Asia has had one of the longest continual habitations by early hominins outside of Africa, making it of intrinsic interest within the megafauna debate. In particular, the evidence from Southeast Asia can be used to address the concept of prey naivety to human hunters, as prey species co-evolved with humans in Southeast Asia for only a brief period of time. In addition, Southeast Asia is of significance in our understanding of megafaunal extinction in Australasia, as the first Australasians were almost certainly Southeast Asian in origin. Southeast Asia during the Pleistocene hosted a radically different landscape than present today. This difference was most pronounced during glacial periods, which occurred for up to 90 % of the Pleistocene (Lambeck et al., 2002). In the north of Southeast Asia, the rise of the Qinghai Plateau resulted in a reduction in the intensity of the Asian summer monsoon during these glacial periods (Ferguson, 1993). Because the summer monsoon brings increased precipitation to continental Southeast Asia, during most of the Pleistocene continental Southeast Asia would have been considerably more arid than today. This aridity would have been compounded by the more southern position during glacial periods of the Inter-Tropical Convergence Zone, a low pressure belt in which maximum precipitation tends to occur (Verstappen, 1980). Finally, temperatures were approximately 3-5 °C cooler than today (Heaney, 1991; Chappell et al., 1996). During glacial periods, sea-levels in Southeast Asia were also significantly lower than they are today (Figure 8.1). These lower sea-levels resulted in the exposure of a large, low-lying continental shelf (Figure 8.2). Because of the reduction in the surface area of the Sunda Sea, evaporation, and hence precipitation decreased, resulting in increasing aridity in the region. Heaney (1991) proposed that the low levels of precipitation led to the formation of a central savannah corridor ran through the middle of Southeast Asia, flanked on both sides by tropical rainforest (Figure 8.2). Although Heaney (1991) proposed the presence of the savannah corridor for the last glacial maximum (LGM) only, there is evidence that this corridor existed during other glacial periods (e.g. Batchelor, 1988; de Vos et al., 43 Selwyn Symposium 2007 GSA Victoria Division Australia’s megafaunal extinction Geological Society of Australia Abstracts 79 1994; Verstappen, 1997). The exposed low-lying Sunda shelf also hosted many major rivers and lowlying basins would have formed large fresh-water lakes (Figure 8.2). Relative Sea-level -200 -150 -100 -50 0 0 200 400 kya 600 800 1000 1200 Figure 8.1. Global sea-level for the past 1.2 million years. Dotted line represents the sea-level at the LGM, where Heaney (1991) proposed the existence of the savannah corridor, solid line represents the approximate sea-level where Borneo, Sumatra and Java were all connected to mainland Southeast Asia. Relative sea-level calculated from Chappell and Shackleton (1986), relative ages calculated from Shackelton and Opdyke (1976). The heterogeneous landscape of Pleistocene Southeast Asia enabled many species currently absent from the region to survive there. The loss of the low-lying shelf through rises in sea-level, an event that occurred several times during the Pleistocene, and the concomitant reduction and/or loss of the savannah corridor, would have severely impacted many megafauna species. The Indian rhino (Rhinoceros unicornis), Dubois’ antelope (Duboisia santeng), perhaps the stegodons, and the archaic elephant Palaeoloxodon were all grazers, and as such would have been threatened by the loss of the savannah. In turn, the loss of these large herbivores would have reduced available prey for the savannah carnivores, namely the hyenas (Pachycrocuta and Crocuta). Other megafaunal species in Southeast Asia, although adapted to forest environments, are reliant on freshwater sources. In particular, the rhinoceroses, the Asian hippopotamus, the tapir, the giant tapir, and to a lesser extent 44 Selwyn Symposium 2007 GSA Victoria Division Australia’s megafaunal extinction Geological Society of Australia Abstracts 79 the pigs, all require access to rivers, wetlands or lakes. The extreme reduction of these sources of freshwater through the transgression of the Sunda Sea on the Sunda shelf through the Pleistocene produced a negative affect on the ecology of these species. Figure 8.2. Southeast Asia at the LGM showing the major rivers and lakes present during that time. Heaney’s (1991) proposed savannah corridor is shown in yellow. The dotted line represents the division between Indochina (north) and Sunda (south). Adapted after Bird et al. (2005) (from Louys, in prep.). Whether the effects of sea-level fluctuations, and concomitant losses of grasslands and freshwaters, affected the megafauna gradually throughout the Pleistocene, or affected certain species more rapidly cannot at this stage be determined from the available evidence. However, it is clear that the dramatic environmental changes occurring throughout the Pleistocene detrimentally affected the abundance and distribution of megafauna in the region. Conversely, there is very little evidence to suggest that Pleistocene Southeast Asians negatively affected the megafauna until the Holocene. Direct, unequivocal evidence of human hunting of megafauna in Southeast Asia is non-existent for either Homo erectus or archaic Homo sapiens, and it appears that hunting of megafauna by modern Homo sapiens did not become unsustainable until the past 2000-3000 years (Corlett, 2007). Furthermore, the Pleistocene Southeast Asian toolkit appears to have been unsuited to the hunting of large game. Pleistocene stone tools from Southeast Asia consisted largely of choppers, and did not achieve the sophistication of the European toolkit until the Late Pleistocene/Early Holocene (Reynolds, 1990; Corvinus, 2004). Habitat alteration by humans also appears to have been negligible until the Holocene, and human-induced firing of the landscape appears to have become important only in the past 1400 years (Anshari et al., 2001). 45 Selwyn Symposium 2007 GSA Victoria Division Australia’s megafaunal extinction Geological Society of Australia Abstracts 79 The negative ecological effects of humans in region have, however, escalated since the beginning of the Holocene. Massive deforestation over the past 200 years threatens the existence of the remaining forest megafauna species. Hunting and wildlife trade in the region is also grossly unsustainable, with some researchers estimating it at six times the sustainable rate (Sodhi et al., 2004). Currently, the number of historical extinctions of mammals in Southeast Asia is small. However, a large number of megafaunal species are critically endangered, and unless action is implemented soon, we will witness another megafauna extinction event. 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