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.

Tuesday, 29 November 2011

A study from the other side...(i.e. supporting the human hypothesis)

As I fell I have been maybe a bit bias with my recent entries, I am going to present a study which looks to alternative explanations to the climate change hypothesis for the late Pleistocene extinctions. This study is specifically focusing on a study reporting on the faunal succession from the Tight Entrance Cave in south-western Australia (Prideaux et al 2010)

Archaeological evidence from this study indicates there was a diverse mammal community for at least 100ka up to the earliest evidence of humans at around 49ka. And then within 10 millennia, all larger mammals apart from the gray kangaroo and the thylacine are lost from the regional record. Now this study does acknowledge that there was severe climate change at around 70ka, as determined by stable-isotope, charcoal, and small-mammal records, but it claims this was far before the extinctions. Hence they propose that actually humans played the decisive role in the extinctions, with changes in climate and fire activity only contributing facilitating roles. One of the main reasons for this conclusion is that in the last 5 glacial-interglacial cycles show central southern and south-eastern Australian fauna were identical to their Holocene counterparts and resilient to climate perturbations, so why should this particular transition in climate have wiped out the majority of large mammal populations? Well I think I have probably answered this question before in this blog with Horton’s concentric habitat theory, where a threshold during this climatic transition was reached and hence the mass extinction.

But anyway, Prideaux et al (2010) furthered their investigation by investigating the stratigraphic variation in charcoal concentration, which reflects the fire history of the region and the stable isotopes in aragonitic land snail shells as a proxy for climate change. This evidence found that although the some extinctions between 50-40ka during a dry phase, a lot survived the increased aridity leading to the last glacial maximum. Hence they pose the extinctions cannot be down to climate change alone.
Also the Tight entrance Cave record shows greater bushfire activity in the lead up to the last glacial maximum than in the equivalent climatic phase 100ka before. However, the first indications of regional activity were 20ka before the extinctions, ruling out fire as a main cause of the extinctions. Hence Prideaux et al (2010) turn to human impacts such as hunting and habitat alteration as the primary driver for the extinction, along with the detrimental impacts of the arid climate and the fire regime.

This research is interesting and does point to the other side of the argument over the late Pleistocene extinctions. But it doesn’t change my opinion on the climate change hypothesis being the most likely explanation for the mass extinction. I think most researchers who discount the climate change hypothesis see it in a overly simplistic and linear way. They think because the environmental change in earlier climatic transitions was similar or even more severe than the late Pleistocene extinction that the climate isn’t the answer. But that is simply not the case; the environment is a very complicated system with delayed responses, thresholds and intricate interactions that warrant it a much more complex explanation and understanding as an incredibly powerful mechanism. That’s why I think sometimes the climate is undermined as an explanation for the late Pleistocene extinction in Australia.

Saturday, 26 November 2011

I've got another video for you all... this time a tribute to the Procoptodon goliah and the Simosthenurus!

Video 4: A compilation of reconstructions and models of the Procoptodon goliah and the Simosthenurus!

So this is just a video of all the different drawings, models and skeletal remains of these 2 different species of giant kangaroo. It is a good overview of different medium to give an idea of what these creatures used to look like. Also, a treat to all of you Will Smith fans, the background song is 'Men in Black', which I'm sure you will all enjoy, well most of you anyway! So I hope this is useful and gives you a good indication just how huge these animals were, and how similar they were to modern day kangaroo species.

Wednesday, 23 November 2011

Right back at you Roberts et al (2001)!

So as you may have read from my last blog entry, I wasn’t very happy with the evidence produced by Roberts et al (2001)! And to back me up on my contention, I have come across an article by Field and Fullagar (2001) that basically reinforces what I have already said. 

They use the Cuddie Springs site as a specific example, the oldest human levels that also contain megafaunal remains is dated at 36.4ka years. This was rejected by Roberts et al (2001), because of sediment mixing and the re-deposition of bones from older to younger sites. Field and Fullagar (2001) claim this is not a valid assertion because firstly the human and megafaunal overlap of remains is sealed at its upper and lower limits at the specific archaeological level by consolidated old land surfaces, meaning the movement of material from old sites is impossible. The next reason disputing Roberts et al (2001) rash conclusion is that the archaeological, faunal and geomorphologic data compiled at the site contradict sediment mixing, in particular pollen data can be an indicator of disturbance, yet the pollen records at the site show typical trends of increasing aridity leading up to the Last Glacial maximum.

Also through optically stimulated luminescence (OSL), they were able to tell ages for Cuddie Springs mixture of grains at the site. They found an increasing age with depth and are consistent with the established radiocarbon chronology, which is exactly what is expected of an archaeological site that has suffered no disturbance or mixing. Also another whole in Roberts et al (2001) study is that they never offered a process by which older megafaunal remains could reach the site.

So in conclusion, the study of Roberts et al (2001) is largely discredited, and although they did find sites with both human and megafaunal remains that were aged at 46.6ka years ago, they ignored a large proportion of them which were significantly younger. Therefore, this study can’t be used to harm the reputation of the environmental change hypothesis, which I still believe is a very strong explanation of the late Pleistocene extinctions in Australia. 

Haha caught you out Roberts and friends!

Monday, 21 November 2011

Welcome to the 21st century and the current debate on the late Pleistocene extinctions in Australia!

So as I promised I am now going to focus on literature that is actually from the 21st century, and gives maybe a better bearing on the current development of the Late Pleistocene extinctions. 

I am starting with an article by Roberts et al (2001), who analysed burial ages for megafauna from 28 sites, and apparently infer the extinction in Australia occurring 46,400 years ago. He therefore rules out extreme climate change and aridity as the cause for the extinction, and poses that actually human induced changes were the main catalyst. Their reasoning for this assumption is that if the last megafaunal existence was probably between 51 to 40 ka years ago, then this was about 20 ka years before the height of the glacial maximum, and hence doesn’t correlate with the changing climate hypothesis. They go on to suggest that the megafauna vanished within 10-5 ka of human arrival, so obviously discounting the ‘blitzkrieg’ model of human extinction, but validating that perhaps humans gradually depleted the number of species leading to their final demise.

However, and trust me this is a pretty big however, this study must be taken with a pinch of salt. This is mainly because actually some of the sites and deposits have been ignored. This Roberts and the other researchers put down to the sites containing disarticulated remains they guessed had been derived from other locations of older units. They also put the emitting of some of the sites down to the fact that sediment mixing has occurred, for example at a site at Cuddie Springs, which has actually been found to have much younger deposits than the rest of Australia. So, in summary, Roberts et al (2001) claim ‘the young ages obtained for disarticulated remains and the indication of sediment mixing at Cuddie Springs are evidence that the remains are not in their primary depositional setting, but have been eroded from older units and re-deposited in younger units with contemporaneous sediment and charcoal.’

Mhmmm to be honest I’m not so sure myself, really can you just ignore certain sites because of supposed disarticulation and mixing. To me it just seems they are ignoring the evidence that doesn’t fit in with their dating methods and their hypothesis, which they probably had in mind before they started the research! But hey, maybe I am being cynical…but all I know is that I don’t see this as a strong argument against the environmental change hypothesis; I think all sites and all evidence must be taken into consideration to come up with a valid conclusion.

Thursday, 17 November 2011

Another video...but this time on the Diprotodon

Video 3: A reconstruction of the Diprotodon

So this is another video of some prehistoric humans hunting the Diprotodon, the largest marsupial to have ever lived! Thought it was quite entertaining and interesting to see actually how massive these marsupial creatures were.

Wednesday, 16 November 2011

Just a little graph for you all!

This is just to give a visual idea of how bad the late Pleistocene extinction was in Australia. In fact relevant to the amount of species already present on the continent, it was the worst affected by the mass extinction across the whole globe.

Martin is blaming the humans...

So as I promised in my last blog, I am going to give an overview of Martin’s general theory, specifically focusing on Australia. 

The first point Martin makes in this article is that the global extinctions at the end of the Pleistocene follow the pattern of the first Homo Sapiens, but ‘the extinction pattern does not obviously track changes in climate throughout the Pleistocene’ (1984). Pointing out that the spread of the first humans was confined to the very end of the Pleistocene when the extinctions occurred, whereas changes in the climate have been continually changing but no extinctions of the amplitude have ever been seen. So basically, the extinctions were following man’s footsteps; hence they are the consequence of man’s actions.

Martin also hypothesises that the first humans would have found the large mammals easy to hunt because the mammals wouldn’t have developed or evolved defensive mechanisms against human predation. For example the lumbering diprotodontids with their small platigrade feet who would not have been able to run fast enough, or the giant kangaroos who were much less gracile than their relatives living today (1984).

But the problem that even Martin himself points out is that there is much uncertainty over the dating of macrofossils of various extinct megafauna. North America has rather rigorous comparable techniques with sloth dung or bone amino acids residues, but no such comparable organic carbon source is available in Australia (1984). Even the famous Lancefield site relies on only 2 charcoal dates that mark the macrofossils as 26,000 years old. There have been other rather more postulating estimates of dating at other sites that are still disputed. Such as a dozen bone points from Devil’s lair in western Australia estimated at 29,500 years old and even an articulated human skeleton at Lake Mungo dating 28,000 years, Jones (1980) even goes as far to say that there are 29 sites with human remains predating 15,000 years (Dortch 1979; Bowler and Thorne 1976). So surely this all points to humans occupying Australia much earlier than the late Pleistocene extinction, an argument truly challenging the ‘overkill hypothesis’ pointed out by Martin.

Another thing that Martin covers is the fact that there are very few sites that contain both extinct macrofossils and human remains. In fact the only evidence found to date of both remains together are of 3 Sthenurus tooth fragments at Seton’s Cave, all of the other sites of human remains are only with existing faunal today (1984). Which again I see as strong evidence that humans didn’t have a large impact on the extinction of the megafauna; surely there would be more evidence of such a huge impact no? Martin puts this down to what he calls ‘Blitzkrieg’ (look to the glossary for meaning), so basically faunal overkill that maximises speed intensity of human impact and minimises time of overlap between the first human invader and disappearance of native fauna (Mosimann and Martin 1975). Something I just can’t agree with, just because something happened quickly doesn’t mean it doesn’t leave any evidence as if it had never happened, if something happened there is always evidence whether it was quick or not. He points to the fact that the North America extinction was much later hence why there is more evidence there are human megafauna interaction, that is a lose hypothesis, but a more solid one would be to blame the climate.

So all in all, I think Martin’s theory doesn’t fit Australia, he seems desperate to fit his hypotheses to the evidence and that shows in the weak connections he makes. He finishes with the need for judicious radiocarbon is imperative, which I definitely agree with, no theory can be rejected until a solid dating foundation is given. But if some of the dating estimations already given are true, then humans were around for a very long time before the extinctions and hence the ‘overkill hypothesis’ is not valid. Even Martin himself admits, ‘Australia may offer the best opportunity for refutation of prehistoric blitzkrieg’ (1984).

Because these references are not available on the internet:

Bowler, J.M and A. G. Thorne (1976) ‘Human remains from Lake Mungo: discovery and excavation of Lake Mungo III’, in Kirk, D. L and A. G. Thorne (eds) The origin of Australians, Canberra, Australian Institute of Aboriginal Studies.

Dortch, C. E. (1979) ‘Devil’s Lair, an example of prolonged cave use in southwestern Australia’, World Archaeology, 10, 258-281.

Jones, R. (1980) ‘Geographical background to the arrival of man in Australia’, Archaeological Physics, Anthropology, 3, 186-215.

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.

Mosimann, J. E. and P. S. Martin (1975) ‘Simulating overkill by Paleoindians’, American Scientist, 63, 304-313.

Sunday, 13 November 2011

Video on the Marsupial lion...

Video 2: A reconstruction of the Marsupial lion

This is a really good video just to give a visual idea of the scale of these huge megafaunal species that went extinct in Australia in the late Quaternary period. It specifically focuses on the Marsupial lion, the biggest marsupial carnivore that ever lived in Australia, and a species which still little is known about. But, in this video, some experts hypothesize their way of living, diet, etc. along with some computer graphics to re-enact  their movements and hunting techniques. These creatures were truly amazing, with no species today that are quite like it, they were obviously very powerful with those large set of incisors and claws!
Shame not more is known about these creatures, it would be really interesting to know more about how they hunted and their general  characteristics. Also I think some of these animals that went extinct were so amazing, such as the Phascolonus gigas, a giant wombat that were as large as a small car, or the Simosthenurus, a giant kangaroo that weighed over 120kg. Its kind of a shame really that they aren't around today, well I mean some of them it's probably good that they aren't around, like the Marsupial lion because I reckon they were pretty viscous! I just couldn't imagine coming face to face with a kangaroo that us nearly 2.5 meters tall, would be amazing!

Saturday, 12 November 2011

A bit more on Horton...

I decided to look at some other work by Horton, just to get a fuller picture of his theory behind the late Pleistocene extinction in Australia. 

In another paper of his, he studies the possible impacts of Aborigine populations and their fire regimes (1980). In this paper, he considers the ‘fire stick farming’ model, which suggests that Aborigines changed the frequency and nature of fires in order to manipulate animals and plant resources. He various evidence to disclaim this theory, one piece of evidence being from Kangaroo Island, who lived on the island until 2,500 year BP, but their living there had little effect on the vegetation, and the only change in vegetation was at 4,800 years BP, which can be attributed to the climate. Horton blames the fire starting on thunder storms, Cheal, Day and Meredith (1979) prove that at least 54% of the fires in North West Victoria are started by lightening, and give figures as high as 97% for New South Wales between 1974-75, seeming a possible cause of large scale fires, rather than human intervention.

Also he points to the fact that under the ‘fire stick farming’, most of the vegetation early stages of growth, not having reached anywhere near maturity, which would in turn have meant the mass extinctions of small species, which did not occur (1980). He also looks at the human side of it as well and points out that if they did keep relighting these fires it would have in fact been very bad for the soils and been detrimental to the Aborigine populations who relied heavily on these soils for food and livelihood. But the main point he highlights that shows that this model is obsolete, is that this model requires Aborigine populations to change the nature, season and frequency of the fires in Australia, but this would have been impossible because the natural fire potential of Australian vegetation cannot be changed. Although Aborigine populations may have provided an ‘alternative of fire’ they didn’t change the fire regime, these interacting fire cycles had continued for at least 2 million years and were probably unaffected by the human arrival (1980). I have to agree with Horton here, and think that nature is a lot stronger than people give it credit for, and these people couldn’t have changed the vegetative community in Australia, unless they were large groups of settlers with growing populations of both livestock and populations, which we know they were not.

In another one of his papers, based on evidence from Southeast Australia, he presents evidence from excavations of the Australian swamp, Lancefield. Here, a bone bed dating about 26,000 years was found containing possibly 10,000 giant extinct megafaunal species (1978). This, in line with the first human appearance in Australia, highlights that animals and humans co-inhabited Australia for 7,000 years, reducing the human predations theory credibility. If human predation was the main cause of the extinction, then surely the extinctions would have happened much earlier, when humans first arrived, not 7,000 years later. And remember, the timing of the extinctions was perfectly in line with the most dramatic shift in climate in the late Pleistocene; this could not surely just be coincidence?

I think people forget about the fact that humans were also restricted during the arid times in Australia. Humans rely on free water as well so would have followed the same concentric distribution around water sources as megafauna did (see last blog for more information). Humans would have had an economy based on principle resource, determined by balance between economic return and effort required (Jones 1980) – catching megafauna would have been high return and high effort, something humans would not have risked when there was so little water. Hence another reason why human predation did not have a big impact, humans probably lived more off fish and small mammals. And this is proven in the fossils is it not? There are very few, in fact only one site where human and megafaunal fossils are found together, discrediting the both the 'blitzkrieg' and 'overkill hypothesis' somewhat!

So I think these other studies of Horton's re-enforce the  hypothesis that actually humans had very little impact on the extinctions and certainly didn't cause them. However, I feel like I have been a bit biased with articles I'm picking, so on my next blog I will review an article by Martin, who was actually the one who first proposed the 'blitzkrieg' model of human predation. Then after that as I've promised, I will go onto more recent research and see how the argument has developed and if there are any new or updated theories knocking around.

Because they're not available online:

P. D. Cheal, Day, J. C. and Meredith, C. W (1979) 'Fire in the National Parks of North West Victoria', National Parks Service: Victoria.

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, Canberra: Australian National University, 3-31.

Thursday, 10 November 2011

Change of background...

Just to let everyone know, I've decided to change my background because as Anson rightly pointed out, woolly mammoths weren't around in Australia in the Pleistocene era, or in fact ever. So I've replaced it with a picture of Thylacoleo carnifex (a marsupial carnivore about the size of the modern lioness) hunting a Procopotodon goliah (hoof-toed giant short-faced kangaroo), both of which were a common species in Australia before the Late Pleistocene extinction. Enjoy!

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. 

Friday, 4 November 2011


So I know that woolly mammoths weren't actually in Australia before the Late Pleistocene extinction, but they were in Eurasia and North America, and I thought these cartoons were quite funny! Also I'm not sure why I've got mammoths as my background either...oh well, here are the cartoons anyway...

Hope you enjoyed them! I might try and find some more, but to be honest, cartoons relevant to the Late Pleistocene extinctions are quite hard to find...

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.