Friday, 28 October 2011

Late Pleistocene climate in Australia and the effect on megafaunal species

So for this blog entry, I am going to now start focusing specifically on Australia (since that's the overall focus of my blog!). Although there has been a lot of recent literature concerning the Late Pleistocene extinction in Australia, I am going to begin with academic articles from a while back. This is just to demonstrate how the overall debate has developed over the past 20 years or so, and to highlight the changes taken place in the past 5 years. The article this blog entry is based on is that of Dodson (1989), who gives a brief overview of the changing environmental landscape at the end of the Pleistocene. During most of the Pleistocene, New Guinea, Timor and Northern Australia were connected by expansive land plains (see figure 1), covered in pockets of rainforest, hummock grassland and shrublands. Across New Guinea, Northern Australia, South-western Australia and Western Tasmania, most of the ecosystems were made up of mesic vegetation, with 25% covered with arid/semi arid shrublands and grasslands (Dodson 1989: 210).
Figure 1: Map of the Australia, New Guinea and Tasmania land mass during the Pleistocene epoch (Dodson 1989: 208).

Some of the most extensive records of the Pleistocene climate come from Lake George, near Canberra (Singh and Geissler 1985) and Darwin Crater, Western Tasmania (Colhoun 1988) dating back 350,000 years. Both indicate the Pleistocene era was cold and dry, inferred from the low water level of the lakes and the predominately herbaceous vegetation. Although these arid conditions were extensive in certain parts of Australia, there were also areas covered in dense forest vegetation, mainly across the plains joining Northern Australia to New Guinea and Tasmania. However, these forestry coverage began to reduce rapidly as the beginning of the Holocene approached. Paleoclimatic studies taken from sediment profiles at Ulungra Springs in New South Wales show that 25,000 years ago, towards the end of the Pleistocene, chenopod shrubland replaced vast forestry ecosystems (Dodson 1989). This indicates persistent arid conditions with up to a considerable 50% reduction in rainfall.

 Although these intensive arid conditions were harsh on some of the already existing species on the continent, they were more than ideal for the grazing animals introduced by Aborigine populations. Dodson (1989) deduces that the arid conditions prevailing in Australia will have been beneficial to growing human populations as their grazing animals thrived. This would have resulted in population growths, and in turn increased pressure on megafaunal prey.

Thinking about it, this would also explain the fact that the extinction in Australia happened approximately 7,000 years before the extinctions in Europe and the Americas. It also explains how Aborigine populations were able coexist with megafaunal populations for so long, during the Glacial Maximum, forest coverage was still vast and the climate was still too cold for human populations to thrive. But with the Late Pleistocene, temperatures rose and dense inhabitable woodland retreated, enabling human populations to prosper. Hence I think looking at the climate is really important when considering the causes of the extinctions, because ultimately, the climate is what most influences human populations and their impacts on the surrounding environment.

Dodson, J. R. (1989) 'Late Pleistocene Vegetation and Environmental Shifts in Australia and their Bearing on Faunal Extinctions', Journal of Archaeological Science, 16, 207-217.

(Because this article isn't accessible on the internet I will just give the reference - Calhoun, E. A., (1988) 'Cuinozoic vegetation of Tasmania', Special Paper, University of Newcastle, New South Wales, Australia).

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