June 9, 2010

Southeast Asia in the Pleistocene, from grassland to rain forest

ResearchBlogging.orgI’ve been trying to keep up with the Gulf situation, so most of my reading of late has been dominated by those details, and the unread numbers in my RSS folders were a little intimidating, but I finally found some time to read some of the papers I’ve earmarked in the past month or so.

This study from the Journal of Biogeography attempts a new method to assemble the paleoecology and paleoenvironment of Southeast Asia in the late Pleistocene and runs a lengthy comparison against the results of previous studies, corroborating the evidences. The interest in reconstructing these environments is largely generated from more recent discoveries of hominins that lived there in the Pleistocene. Data regarding hominin-mammal interactions is important and can be used to determine evolutionary nuances. If the environments in which these hominins lived can be interpreted, it can give us more details about how they lived, how they continued to disperse and even give scientists better clues as to where remains and artifacts can be found.

Conveniently, the vast amount of fossil data recovered from SE Asia is from medium to large sized mammals. The area seems to have always been rich in mammalian life; at present, 13 percent of all the species of mammals in the world live in SE Asia, which comprises just one percent of the total land mass. The community of mammals hasn’t changed very much since the Pleistocene, boasting relatively few extinctions. Great megafauna like Stegodon, and Pachycrocuta, the giant hyenas, are gone forever, but their remains and others helped the authors piece together a somewhat better look at what the Pleistocene was like so long ago.

Aside from the fossil data, certain ecological variables were established – body size, trophic group (or guild) and arboreality (whether or not the animal lived in trees) – which were used to create a series of categories for the included organisms. The established relationships were calculated using the “synecological” method, which takes such variables and categorizations and is able to calculate fossil sites as open, closed or mixed habitats. Alongside data from previous studies, the researchers were able to paint a very different picture of SE Asia in a very different time.

Instead of the thick, wet rainforest that dominates much of the area, mostly “closed habitats”, stretches of continuous tree cover, the results reveal that much of SE Asia was open or mixed habitat, areas of grassland or a more sparse, heterogeneous cover of trees. Through much of the Pleistocene, the climate was cooler and drier, insufficient precipitation for the extensive forest cover of the Miocene and of today. A site called Trinil in Java was dominated by grassland, an open habitat...

The open environment reconstructed in this analysis is confirmed by the presence of the wide-toothed species of rat (Rattus sp. A), from Trinil (van der Meulen & Musser, 1999), which was presumably a grassland species.

...while Tam Hang, in Laos, the first reconstruction for a site in that country, is most likely a mixed habitat, perhaps an open forest.

Considering studies like this one, which are already seeing entire biomes beginning to shift, it becomes useful to be able to have some impression of how climate has changed certain areas in the past. As I mentioned at the beginning, studies like these can give researchers a better idea of where exactly to look for hominin remains and artifacts by describing the sometimes very different environments they lived in.

Louys, J., & Meijaard, E. (2010). Palaeoecology of Southeast Asian megafauna-bearing sites from the Pleistocene and a review of environmental changes in the region Journal of Biogeography DOI: 10.1111/j.1365-2699.2010.02297.x

Marwick, B. (2009). Biogeography of Middle Pleistocene hominins in mainland Southeast Asia: A review of current evidence Quaternary International, 202 (1-2), 51-58 DOI: 10.1016/j.quaint.2008.01.012

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