Monday, 7 November 2011

Horton and his insistence that climate is the one and only answer...

So this blog is based on some of the work by Horton. His basic argument is that the extinctions in Australia were influenced by the climate and not human arrival, and that in fact the only impact humans could have had is killing animals that were already destined for extinction. In this article I have read, he focuses on the red kangaroo species because out of all the megafauna, they were the best adapted and lived until the beginning of the Holocene, the longest out of all the now extinct species (Horton 1984).

Jones and Bowler (1980) have shown from paleoclimatic reconstructions that from 30,000 – 18,000 years BP (before present), there was a wet ‘mega lake phase’. But at around 18,000 years BP, nearly all the lakes disappeared and ‘dune building’ extended south to north across the continent. And this is what Horton bases his hypothesis on, well specifically on the availability of free water. He poses that larger megafaunal species had to live within the travelling time it took for them to survive without water, so they could get to water before they died from thirst. During the arid phases, water dried up, and they were unable to find alternative sources as they became fewer and far between, meaning they couldn’t survive the journey in between. But the critical factor in this is food. Because they had to stay within a critical radius of the water source, they had to obtain all their food within this radius, so once the food ran out within the area, they starved. So even those species in the southern fringes of the continent, the ‘refuge’ areas, where water remained, there were still extinctions. So, Horton presents, that it wasn’t actually the arid expansion that directly killed the megafauna, but actually the fluctuations in water availability in the fringe areas on Australia. Thinking about it, this would explain why smaller species were exempt from the extinctions. They could survive from the water they derived from their food alone, hence they didn’t have to rely on water sources, and they had smaller refugias. 

So this theory that Horton claims proposes that animals were forced into concentric habitat arrangements. They were reduced to peripheral areas, away from the inland arid core of the country and restricted in their food range. This theory also explains why the larger mammals went extinct first (we may have a winning theory here!) He claims that preceding arid phases happened in small magnitudes, and left some areas with water supplies, but between 26,000 to 15,000 years BP, a threshold was crossed, and the water availability reduced so much so that a mass extinction occurred. This is supported by various studies showing that water literally disappeared and no refuges were left for species to survive, in southwest Australia (Wyroll 1979), northern Australia (Webster and Streten 1972) and Tasmania (Colhoun 1978).

This hypothesis could also correlate with the extinctions on a global scale. The extinctions were more extensive in North America and Australia, the former because of the ever expanding glacial that pushed species southwards, and the latter because of the increasing aridity as discussed before. But on the other hand, Africa had less extinction, this could have been because of the country’s situation astride the equator, providing a range of refuges, and although there were extinctions, relatives of the extinct species survived, so maybe these extinctions were expected for the transition from the Pleistocene to the Holocene. But species in the West Africa still survived, this, Horton poses, is because of the wet forest there in the Pleistocene that provided a refuge for various megafauna (1984).

So I think this reading was really useful, and actually gave a really in depth and very plausible explanation as to why the climate was actually the cause of the mass extinction. I am really starting to think that the changing climate and its influence on both habitat and water sources was the catalyst for this bizarre event. Decreasing refugias and declining resources caused the extinction of these species. I think most of the literature agrees with this as well, if only with small differences on emphasis and cause and effect. But if I had to judge, this hypothesis posed by Horton, is completely possible, and correlates well with the little evidence we have. I think the changes in climate are much better documented than the interactions between humans and the megafaunal species, and perhaps this may be a bias. But, then again, considering this on a more basic level, climate will always have the overriding influence on humans and species alike, it has and always will control our food, our water, our safety and so our survival no? So surely was the case then, such a large scale change could not have only been down to humans, and really with such small populations, could we have really made an impact great in any way to even tip the balance to this extinction? With so many species lost across the whole globe, I just don’t think it would be possible, and so yes from my reading so far I do think that Horton is correct, climate and climate alone had to be the cause.

Because these artciles aren't available online:

Colhoun, E. R. (1978) 'Recent Quaternary and Geomorphological studies in Australia', Australian Quaternary Newsletter, 12: 2-15.

Jones, R. and J. M. Bowler (1980) 'Struggle for the Savannah: Northern Australia in ecological and prehistoric perspective', in R. Jones (ed.) Northern Australia, Options and Implications, Research School of Pacific Studies, Australian National University, 3-31.

Webster, P. J. and N. A. Streten (1972) 'Aspects pf late Quaternary climate in tropical Australasia',
in D. Walker (ed.) Bridge and Barrier, Canberra, Australian National University, 39-60.

Wyroll, K. H. (1979) 'Late Quaternary Climates of Western Australia: evidence and mechanisms', Journal of the Royal society of Western Australia, 62: 129-142. 

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