Wednesday, 2 November 2011

So maybe the climate is the answer?

Soooo I've read an article by Owen-Smith (1987), who highlights the flaws in the arguments that either blame the climate as the main cause of the late Pleistocene extinction or human predation as the major factor. He explains it couldn’t be solely the climate because there were no extinctions on such scale in past glacial-interglacial transitions, and he claims it can’t have been just humans because there were mammalian and avian species that were made extinct that weren’t obvious human prey species.

Guthrie (1984) and Graham (1986) suggest the transition from the Pleistocene to the Holocene was more severe than ever seen, hence the mass extinctions. However, they have been proven wrong (tough luck guys!) and other studies have shown the complete opposite. Shackleton et al. (1983) and Lorius et al. (1985) found that the transition was no more extreme than any past climatic shifts, and that in fact the Sangamon/Eem interglacial appears to have been a lot warmer than the start of the Holocene, and yet there was no elevated extinction rate then (for those of you who don’t know when that was, it was about 130,000 years BP – pretty long time ago). 

Nevertheless Guthrie (1984) came up with an interesting theory, which would actually apply to Australia quite well. He proposed that the transformation of previously mosaic distribution of vegetative communities into more uniform zonal distribution could have reduced habitat diversity, and made it difficult for megafauna to secure their food to survive within their home range. This theory he called the mosaic nutrient hypothesis. I say this is interesting for Australia, because later on in the Pleistocene the environmental conditions were very arid, as Dodson (1989) claimed (my last blog was on his article, don’t worry if you didn’t read it, I don’t blame you!), which could have potentially induced a more homogeneous vegetative community. So could this be a plausible instigation of the disappearance of so many species? Possibly, it would definitely restrict species to certain habitats and areas, making them more susceptible to climate change, creating a continuing viscous circle where the climate prevails? Also, to back this up, the sequence of the main extinctions occurred between 26,000 to 15,000 years ago, coinciding with the driest hyperarid phase of the last Glacial. This means climate must have had a large part to play in the extinction, especially since humans only arrived sometime 40,000-30,000 years ago, much before the extinctions of the species in question. Interesting…

Owen-Smith (1987) also covers the theories solely based on human predation, again he makes another interesting point about the extinction in Australia. He points out that yes, megafauna would have been vulnerable to human hunters because they were large and conspicuous, and yes due to their maximum rate of reproduction at 10% p.a., populations were likely to be driven downwards when hunted. But he also points out that the geographical pattern of the late Pleistocene extinctions is concordant with human occupation. This may actually be true in the cases of the Africa and Asia where humans co-evolved alongside megafauna, and also North America  where the Clovis civilizations are associated with mammoth remains, but it is not the case for Australia (Haynes 1982, Agenbroad 1984, Fisher 1984). Human artifacts are rarely if ever found in association with abundant vertebrate fossils in Australia, and actually humans weren’t really around that much when the extinctions took place in Australia. Now this could have been for a number of reasons, like as I said before the conditions were dry and very cold, so maybe they were too harsh for humans to prosper and expand, or they didn’t have the technique to hunt larger species, but whatever it was there is no evidence they had a significant impact on the species that eventually went extinct.

Owen-Smith (1987) really pushes the ‘Keystone Herbivore Hypothesis’ as the reason for the extinctions in the Late Pleistocene. He uses the example of elephants and rhinos in Africa in the modern day who can transform ‘wooded savanna’ and ‘medium tall grasses’ into, a very nutritious and productive ‘short-grass savannas dominated by rapidly regenerating woody plants and herbs’ through their browsing and grazing of vegetative resources. So he claims that if these megaherbivores were killed and wiped out, by either human or climate agency, then there would have been a massive transformation of the vegetative ecology. This could have isolated animals and reduced their food sources, making them even more susceptible to human hunting and climate change, pushing more species over the edge to extinction. However this was not the case in Australia because there was only one megaherbivore species, and there were delays in the extinction after human arrival, as inferred by the dates I gave before. So I know it may have seemed pointless to explain Owen-Smith’s hypothesis, but I just want to show how this hypothesis, although very applicable elsewhere, isn’t in Australia.

Basically, from reading this, the hyperarid and dry climate of the Australian environment at the end of the Pleistocene seems like a likely explanation for the extinction. Especially since the extinction happened long after human arrival and in the most extreme arid climate, when species were probably isolated to certain areas and driven to extinction. I felt like I have made quite a lot of progress in this blog, and now I’m off to get a cup of tea to reward myself! (and I’m also sorry about the lack of pictures)

Because these articles aren't available online:
Agenbroad, L. D. (1984) 'Hot Springs, South Dakota: Entrapment and Taphonomy of Columbian mammoth', in  Martin, P. S. and R. G. Klein (eds) Quaternary Extinctions, Arizona: University of Arizona Tuscon Press, 113-127.

Graham, R. W. (1986) 'Plant-animal interactions and Pleistocene extinctions', in Elliot, D. K. (ed.) Dynamics of Extinction,  New York: John Wiley and Sons Inc., 131-154.

Haynes, C. V. (1982) 'Were Clovis Progenitors in Beringia', in Hopkins, D. M., J. V. Matthews, C. E. Schweger and S. B. Young (eds) Paleoecology of Beringia, New York: Academic Press, 383-398.

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