Wednesday, 30 November 2011

Is human predation the answer...well it might have been in North America, but not Australia!

I thought I should continue my assessment of articles presenting evidence against the climate change hypothesis just so I'm not too bias! This article is by Alroy (2001) and carries out a multispecies overkill simulation of the extinction. Now this study is based on North America but I thought it would be interesting to evaluate the different variables and factors of human predation that could have possibly led to the late Quaternary extinctions.

This computer simulation of the human and large herbivore population dynamics at the end of the Pleistocene predicted 32 out of the 41 prey species correctly (2001). Along with overall extinction rates, human population densities, game consumption rates and temporal overlap of humans and extinct species predictions were close to the observed values taken. Neither climate change nor ‘secondary ecological effects’ such as fire are needed to account for the mass extinction. Sounds promising no? 

Let’s look at this further. This model involves the fast geographic dispersal of prey populations, full competition among prey species and modest rates of human hunting ability. The model also predicts extinction at 1229 years after initial invasion of humans, the earliest being at 80 years ago, not much overlap it seems. Furthermore, the study points out it takes 260 years for a population to exceed 1000 and 410 to exceed 10,000, hence the lack of archaeological remains before those times, because there just wasn’t enough of them (2001). The earliest known species in North America is the Clovis at 13,400 years BP, so actually considering how long the human populations take to augment, a 1200 year overlap between human invasion and species extinction sounds accurate. Also there has been recent evidence pointing out that actually the Clovis populations weren’t the first in North America, but I will address that study in another blog!

The most important parameter of the model was the hunting ability. Relatively high hunting ability could have led to extinctions of all but one or two of the megafaunal species, even with low human population densities.

These diagrams show the relationship between human population size and both extinct and still surviving species population size as predicted by the computer simulation (2001). The graph shows that as the human population size increases, those animals that are now extinct die out or decrease in size dramatically. On the other hand, those species still around today retain their original population size before human interruption. By the way, graph A is just a smaller time frame of 2,500 years after human invasion, whereas graph B spans 14,000 years.

 So with this evidence, how do we apply it to Australia? Well firstly, although it has already been stated that the overlap between human invasion and species extinction in Australia was much longer than that of North America, with estimations ranging from 15,000-20,000 years (Owen-Smith 1987).  So the reasoning of correlating dates does not apply to Australia. The point of lacking human remains, even if human densities were low and grew slowly, there would be some evidence of coalition between the two groups, but there is nothing of the sort in Australia. Well actually I lie, there is one site, of 3 Sthenurus tooth fragments at Seton’s Cave, but still would there not be more evidence, especially if humans co existed with these species for so long, if they hunted them there would be more evidence no (Martin 1984)? So I know this was a bit of a cheat and that you can’t reliably apply one model to another continent, but the main reasons upholding this model do not apply to Australia. So the human predation model may be more likely for North America, but it certainly doesn’t apply to Australia. Models provide very interesting and insightful ways in which to reconstruct past environments and are paramount to paleontology hence why I thought it was useful to address one in this blog entry even though it was directly based on Australia. I have already addressed some key models reconstructing the mass extinction, but I will try and look for more for some up and coming blogs!

Because not available online:

Martin, P. S. (1984)  'Prehistoric Overkill: the global model', in Martin, P. S. and R. S. Klein (eds) Quaternary Extinctions: a Prehistoric Revolution, Arizona, University of Arizona Press, 354-604.

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