Saturday, 7 January 2012

Bit of Webb to bring things to a close!

So this is my penultimate blog entry everyone! In this blog I’m just going to go over an article that nicely sums up the argument that climate caused the mass extinctions in Australia.

Webb (2008) studies the changes in species distribution and the increased fragmentations of habitats. This he points out was due to increased aridity, widespread drought and vegetation change. The study considers 230 fossils sites in Australia and Tasmania composing of 32 species. The fossil site distribution is very patchy in the north and extensive in the east, and a large section is empty in western and central Australia (see figure 1). With a few exceptions, the entire western half of the continent seems devoid of fossil sites, illustrating sparse populations (figure 1). This shows species preferred the wetter semi-tropical climates of the east, rather than the arid conditions of the west. It is logical to assume then that most of the species required a reliable supply of water and a stable environment southeast Australia offered.

Figure 1: Map showing the fossil site distribution in Australia (Webb 2008).
The build up to the Last Glacial Maximum (LGM) was marked by increased aridity, extensive drying in Central Australia, fluctuating sea levels and local extinctions. Each arid phase would see the downturn subjected animals to cumulative population reduction. Each phase would force animals into smaller habitats, moving them further to the coast. As suggested by Horton(1984), animals were pushed into specific refugias and concentric habitat areas until there was nowhere left to go.

Since the climate change was slow, the species adaption must have had a limited response. So basically when environmental change took place in an area bypassing its threshold, the animals would be displaced and moved to another region which could sustain them. This displacement from one area meant the increased reliance in another area, often bypassing its carrying capacity and causing some species to lose their habitats. Increasing distance between habitable areas meant less successful migration, as Horton (1984) also pointed out. Migration corridors would have become shorter and narrower, as fewer animals were able to move. Those species that existed in smaller distance patches would have become extinct. The probability of extinction is inversely related to patch area, but as the larger patches became more fragmented, the smaller they became and the more distant and the less animals were able to migrate. As already mentioned in this blog, smaller animals were more likely to survive because of their higher reproductive capabilities and the fact they could survive in smaller patches of habitats.

Webb (2008) goes on to list various evidence of a drying climate. Evidence from a change to C3 woodland to C4 grassland in northwestern Australia suggests decreasing rainfall. There is also evidence of decreased monsoonal activity in the late Pleistocene from Magela creek in northern Australia, where increased sediment deposition suggests a drying climate (Nanson et al 1993). Paleontological evidence from central eastern Queensland shows the replacement of rainforest adapted species to arid adapted species as long as 180ka ago (Hocknull 2005).

In explaining why there were no extinctions as big in previous glacial transitions. Webb (2008) points out that the increased aridity and environmental stress has actually been occurring over the past 3 glacial episodes. The more advanced it became, the less adaptive the species and ecosystems became and the more thresholds were passed. As it advanced, animals became more marginalised and increasingly pushed to the peripheral coast. The habitats became more fragmented and more species isolated. And even those species that reached larger habitats, the resources would have been minimal due to concentrated species numbers and limited carrying capacities.

Human predation was not a cause. These animals went extinct mainly from 15-20,000 years ago, illustrating a long co-existence with humans. Also considering most of the species were concentrated in southeast where the climate was less arid and humans arrived in the north, they would have been as far as they could be from most species. All hunting would have taken place at low frequency and restricted dispersal of many species. Although they may have developed mosaic burning in Australia, until widespread repetitive anthropogenic burning began (which was much later), it would not have had a massive impact. 

Because not available online:

Hocknull, S. A. (2005) 'Ecological succession during the late Cainozoic of central eastern Queensland: extinction of a diverse rainforest community', Memoirs of the Queensland Museum 51, 39–122

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