Tuesday, 27 December 2011

A welcome back after Christmas with a bit of Faith and O'Connell (2011)

It probably would have been more useful to post this blog entry earlier but I’ve only just found it now, so better late than never! I say this because it refers to an article by Prideaux et al (2010) that I covered in a blog entry on the 29th of November – just in case any of you want to refer back to it!

This study is based at the Tight Entrance Cave (TEC) in south-western Australia where a sequence documenting the extinction of 14 mammals has been carried out (see figure 1). Faith and O'Connell (2011) aim to show that the extinctions in this sequence took place in correspondence with vegetation change, a drying trend and increased biomass burning, predating human arrival on the continent. Prideaux et al (2010) assumed that because there were no extinctions from the Penultimate Glacial Maximum (PGM) at 143ka until the late Pleistocene, the arid conditions couldn’t have caused the mass extinction.

Figure 1: Summary of all of the different species found at TEC.
However, and there is a big however here, 9 of the 14 species known from TEC were known to have survived until the late Pleistocene. So this means that actually 4 to 5 mammals survived to the late Pleistocene, broadly overlapping with the arrival of humans on the continent. Additionally, of the 68 species known in Australia in the Pleistocene, 25 are known to survive to the late Pleistocene and 15 known to overlap with the arrival of humans (Field et al 2008). In this article and others before, this pattern has been put down to long term drying trends towards more arid conditions. Now Prideaux base their argument on the fact that extinctions took place leading up the Last Glacial Maximum (LGM) , however the fact that some species survived human arrival and that some species survived into the late Pleistocene contends this assumption. Hence a lot of these extinctions occurred when the climate begun a transition to open vegetation, a dried climate and increased biomass burning (indicating a drier climate).

Smaller mammals were able to survive this change in climate because of their smaller patches of required habitats, as Horton (1984) contested along with others.  They could survive off less water and had greater range and forage requirements, along with higher reproductive rates to sustain their populations. But Faith and O'Connell (2011) do prove there was also instability in the smaller mammal populations, although obviously not complete extinction. This is indicated by increased chord distances across adjacent pairs of stratigraphic units, which may not mean much to a lot of you, but basically means there was an increased turnover of species and instability within communities. This pattern begins before human arrival and hence must be down to environmental shifts.

So this article shows that Prideaux et al (2010) were rushed in their conclusion of the reason for the extinctions in Australia. Although the majority of the animals at TEC did go extinct before the LGM, some survived until after human arrival. The research on smaller mammals is also very interesting, it is the only reference I have come to in my research of my blog but proves there were changes across all species, its just the changes were obviously more extensive in the larger megafauna.

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