Wednesday, 4 January 2012

Wroe and Field... was it in the late Pleistocene at all?

So this blog is just verifying my reasoning for my clear support for the climate hypothesis!

The first article is by Wroe and Field (2007). They highlight that there is a lot of evidence for increased aridity in the past 400-300,000 years. They also correspond this with one of their studies that demonstrated that 39 species of megafauna (65% of those that went extinct) cannot be placed within 80,000 years of firm evidence for human arrival at 50-43,000 years ago, leaving only 8 species present at human arrival. Only 21 species whose disappearance is put down to human predation are known to have persisted after the penultimate glacial maximum, which was a time of severe climate change. 4 or more persisted until the full glacial conditions at 30,000 years ago. Using this evidence, they claim there was a staggered extinction in which megafaunal species extinction predated human arrival and where humans had a minor role. In figure 1, there is a diagram showing the predicted extinction dates of various species in Australia.

They also make some amendments to Horton’s (1984) theory. Considering that some species may have disappeared before the late Pleistocene, actually disappearing 700-600,000 years ago. They use this with the fact that their evidence showed that lack of evidence for 65% of the species post 130,000 years ago suggests most extinctions occurred during or up until the PGM. The rest of the extinctions then staggered with increased aridity from 100,000 years ago and onwards. They explain the fact most of the species that went extinct were large because increased body size is a common response to low quality food and seasonal conditions (as Owen Smith posed 1988). Furthermore, as habitats declined to minimum thresholds, the larger species will go extinct first because the larger the species, the larger the landmass area needed (Wroe et al 2004).

They also point out that in line with the Vostok ice core, temperatures dropped to on average -8 degrees, obviously illustrating a severe climate (Hesse et al 2004Petit et al 1999). Isolation, low relief, sheer landmass area and low rainfall restrict the distribution of refugia in Australia relative to other continents, increasing the risk of extinction. Since Australia is such an arid continent, relative minor changes in rainfall patterns can impact the environments significantly (Ayliffe et al 1998). Further aridification and stepwise reduction in rainfall would have caused refugias to become too small. As Horton (1984) suggests decreasing refugias meant species had less resources and caused the demise of megafaunal species, especially to those larger ones who needed larger habitat ranges.

So there is contestation here of when the extinctions took place. The evidence here suggests that is was much earlier than the late Pleistocene, but there is mounting evidence against this that most of the extinctions happened in the late Pleistocene, as I have pointed out in the rest of my blog! But the theory they present is alike to Horton’s, declining refugias, depleting resources and environmental thresholds reached are all the mains factors that show the environment caused the mass extinctions.

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