Friday, 16 December 2011

Johnson and Prideaux... not sure if your evidence supports your hypothesis...

I’ve found another study that is completely against the environmental change hypothesis, and I mean completely!

Johnson and Prideaux (2004) carry out a study considering the extinction rates of both browser and grazer species. Now I think it is important to explain what the difference is between these two groups of species! A browser species feeds mainly off high growing vegetation such as woodlands and forest areas, and a grazer feeds on the lower level growing grasses and herbage. Now Johnson and Prideaux (2004) are comparing their extinction rates because it has been posed that most of the herbivorous species that went extinct were browsers. Browsers would have been more reliant on shrubland and woodland and hence will have been more affected if such habitats contracted rapidly due to human fire regimes and changes in aridity (so actually the climate does come into this as well!)

So the model, after 75 species were analysed from both Australia and New Guinea, found 32 out of the 48 browsers went extinct in the late Pleistocene and only 11 out of the 27 of the grazers went extinct. Hence, it was found that yes more browsers did go extinct over grazers, but not because browsers were more likely to go extinct but because most of the species in the late Pleistocene were in fact browsers.

They pose that directional change in habitat would have caused such a widespread extinction because both sets of species fed of completely different vegetation compositions. But I think this underestimates the power of the climate somewhat. Who’s to say that the climate couldn’t change both of these divergent vegetative habitats? Intense drying and severe arid climate could change the whole face of vegetation in Australia, affecting all the animals in different ways but in the end having the same consequence, extinction. As I’ve already covered in this blog, there was a severe shift in climate in Australia in the late Pleistocene that could explain directly the majority of the extinctions.

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