Saturday, 7 January 2012

Thank you for reading and I hope you enjoyed it!

The last blog entry!

So as you may have already deduced, I think that the changing climate was the cause of the late Pleistocene mass extinction in Australia.

So humans arrived around 48,000 years ago, some even claiming up to 60,000 years ago, the majority of species disappeared between 26,000 and 15,000 years ago and the LGM set in around the same time at around 26-20,000 years ago. So even looking at these dates, it is obvious there is correlation between the climate and species extinction, not the arrival of humans.

Considering the human hypothesis, there are just too many questions left unanswered. Mainly, how did the megafaunal species and humans co-exist for so long without extinction? Could it just be coincidence the extinctions occurred when the climate saw its most severe transition? Why is there a lack of archaeological evidence with both human and megafaunal remains? Were the small indigenous human populations present really large enough to caused such a large scale extinction? All of the answers to these questions definitely disprove the 'blitzkrieg' model and cast the human hypothesis in much doubt.

Considering the climate change hypotheses, the story seems to make more sense. As I have presented in this blog, there is plenty of evidence for increased aridity. Dodson (1989), Guthrie (1984), Jones and Bowler (1980) and more recently Hesse et al (2004), Mooney et al (2011) and Murphy et al (2011). All of these studies and others from this blog illustrate the continuing aridity in Australia, and the result of deteriorating ecosystems and declining habitat areas.

As I've stated through this blog, I think that Horton's (1984) explanation for the extinction is the most accurate. The concentric habitable areas explains why the extinctions targeted the larger mammals, why they happened over quite a prolonged period but mainly throughout the LGM and why the extinctions were so extensive.

I'll just go over some of the main theories and how they could fit into the climate change hypothesis in Australia. Although it is almost impossible to know which one is completely correct, I think they all provide some additional explanation onto Horton's (1984) hypothesis as for the reason of this extraordinary event. Forster’s (2003) ‘self organised stability’ theory explains why as the habitable areas became smaller as aridity took a steep incline, species composition became less diverse as some were made extinct. Hence the remaining species became more inbred resulting in more extinctions and enhanced vulnerability to environmental changes. In this sense, Guthrie’s (1984) ‘mosaic nutrient hypothesis’ can also be used, as the vegetation within these concentrated areas became more affected by environmental change, the plant diversity decreased. This explains why more and more areas were becoming less inhabitable as the aridity increased. Also even though Ficcarelli’s (2003) hypothesis was applied to Ecuador, the ‘habitat loss hypothesis’ also comes into play here. With increased aridity in Australia, environmental thresholds were crossed and parameters bypassed. This means the demise of many vegetation types and the decline of many habitats leading to extinction. 

And so there you have it, after extensive reading and writing for this blog, that's what I've come up with! Hope you agree!

Bit of Webb to bring things to a close!

So this is my penultimate blog entry everyone! In this blog I’m just going to go over an article that nicely sums up the argument that climate caused the mass extinctions in Australia.

Webb (2008) studies the changes in species distribution and the increased fragmentations of habitats. This he points out was due to increased aridity, widespread drought and vegetation change. The study considers 230 fossils sites in Australia and Tasmania composing of 32 species. The fossil site distribution is very patchy in the north and extensive in the east, and a large section is empty in western and central Australia (see figure 1). With a few exceptions, the entire western half of the continent seems devoid of fossil sites, illustrating sparse populations (figure 1). This shows species preferred the wetter semi-tropical climates of the east, rather than the arid conditions of the west. It is logical to assume then that most of the species required a reliable supply of water and a stable environment southeast Australia offered.

Figure 1: Map showing the fossil site distribution in Australia (Webb 2008).
The build up to the Last Glacial Maximum (LGM) was marked by increased aridity, extensive drying in Central Australia, fluctuating sea levels and local extinctions. Each arid phase would see the downturn subjected animals to cumulative population reduction. Each phase would force animals into smaller habitats, moving them further to the coast. As suggested by Horton(1984), animals were pushed into specific refugias and concentric habitat areas until there was nowhere left to go.

Since the climate change was slow, the species adaption must have had a limited response. So basically when environmental change took place in an area bypassing its threshold, the animals would be displaced and moved to another region which could sustain them. This displacement from one area meant the increased reliance in another area, often bypassing its carrying capacity and causing some species to lose their habitats. Increasing distance between habitable areas meant less successful migration, as Horton (1984) also pointed out. Migration corridors would have become shorter and narrower, as fewer animals were able to move. Those species that existed in smaller distance patches would have become extinct. The probability of extinction is inversely related to patch area, but as the larger patches became more fragmented, the smaller they became and the more distant and the less animals were able to migrate. As already mentioned in this blog, smaller animals were more likely to survive because of their higher reproductive capabilities and the fact they could survive in smaller patches of habitats.

Webb (2008) goes on to list various evidence of a drying climate. Evidence from a change to C3 woodland to C4 grassland in northwestern Australia suggests decreasing rainfall. There is also evidence of decreased monsoonal activity in the late Pleistocene from Magela creek in northern Australia, where increased sediment deposition suggests a drying climate (Nanson et al 1993). Paleontological evidence from central eastern Queensland shows the replacement of rainforest adapted species to arid adapted species as long as 180ka ago (Hocknull 2005).

In explaining why there were no extinctions as big in previous glacial transitions. Webb (2008) points out that the increased aridity and environmental stress has actually been occurring over the past 3 glacial episodes. The more advanced it became, the less adaptive the species and ecosystems became and the more thresholds were passed. As it advanced, animals became more marginalised and increasingly pushed to the peripheral coast. The habitats became more fragmented and more species isolated. And even those species that reached larger habitats, the resources would have been minimal due to concentrated species numbers and limited carrying capacities.

Human predation was not a cause. These animals went extinct mainly from 15-20,000 years ago, illustrating a long co-existence with humans. Also considering most of the species were concentrated in southeast where the climate was less arid and humans arrived in the north, they would have been as far as they could be from most species. All hunting would have taken place at low frequency and restricted dispersal of many species. Although they may have developed mosaic burning in Australia, until widespread repetitive anthropogenic burning began (which was much later), it would not have had a massive impact. 

Because not available online:

Hocknull, S. A. (2005) 'Ecological succession during the late Cainozoic of central eastern Queensland: extinction of a diverse rainforest community', Memoirs of the Queensland Museum 51, 39–122

Wednesday, 4 January 2012

Wroe and Field... was it in the late Pleistocene at all?

So this blog is just verifying my reasoning for my clear support for the climate hypothesis!

The first article is by Wroe and Field (2007). They highlight that there is a lot of evidence for increased aridity in the past 400-300,000 years. They also correspond this with one of their studies that demonstrated that 39 species of megafauna (65% of those that went extinct) cannot be placed within 80,000 years of firm evidence for human arrival at 50-43,000 years ago, leaving only 8 species present at human arrival. Only 21 species whose disappearance is put down to human predation are known to have persisted after the penultimate glacial maximum, which was a time of severe climate change. 4 or more persisted until the full glacial conditions at 30,000 years ago. Using this evidence, they claim there was a staggered extinction in which megafaunal species extinction predated human arrival and where humans had a minor role. In figure 1, there is a diagram showing the predicted extinction dates of various species in Australia.

They also make some amendments to Horton’s (1984) theory. Considering that some species may have disappeared before the late Pleistocene, actually disappearing 700-600,000 years ago. They use this with the fact that their evidence showed that lack of evidence for 65% of the species post 130,000 years ago suggests most extinctions occurred during or up until the PGM. The rest of the extinctions then staggered with increased aridity from 100,000 years ago and onwards. They explain the fact most of the species that went extinct were large because increased body size is a common response to low quality food and seasonal conditions (as Owen Smith posed 1988). Furthermore, as habitats declined to minimum thresholds, the larger species will go extinct first because the larger the species, the larger the landmass area needed (Wroe et al 2004).

They also point out that in line with the Vostok ice core, temperatures dropped to on average -8 degrees, obviously illustrating a severe climate (Hesse et al 2004Petit et al 1999). Isolation, low relief, sheer landmass area and low rainfall restrict the distribution of refugia in Australia relative to other continents, increasing the risk of extinction. Since Australia is such an arid continent, relative minor changes in rainfall patterns can impact the environments significantly (Ayliffe et al 1998). Further aridification and stepwise reduction in rainfall would have caused refugias to become too small. As Horton (1984) suggests decreasing refugias meant species had less resources and caused the demise of megafaunal species, especially to those larger ones who needed larger habitat ranges.

So there is contestation here of when the extinctions took place. The evidence here suggests that is was much earlier than the late Pleistocene, but there is mounting evidence against this that most of the extinctions happened in the late Pleistocene, as I have pointed out in the rest of my blog! But the theory they present is alike to Horton’s, declining refugias, depleting resources and environmental thresholds reached are all the mains factors that show the environment caused the mass extinctions.

The last video everyone! I know sad times...

Video 7: Another compilation of pictures and reconstructions of the Marsupial lion.

So this is the last video and another one taking a look at all of the archaeological remains of the marsupial lions, along with some cartoons and some pictures reconstructing what they looked like. There is a bit of Will Smith in the background, just a treat for you all! Enjoy!

Monday, 2 January 2012

Some recent articles to support the climate hypothesis...

So I'm going to use this blog entry to review the climate changes in Austrailia during the late Pleistocene. I realise I’ve use other blog entries to do this already but I’ve found a few more which have been recently published and I thought they would be good to re-enforce the climate hypothesis!

Cohen et al (2010) carried out a study on the modern Lake Mega- Frome, which is the coalescence, (the merged) Lakes Frome, Blanche, Callabonna and Gregory lakes in souther central Australia. The last time these lakes were connected was at 50-47ka, this final disconnection shows a shift in climate to a more arid and dry climate, which coincided with the arrival of the first humans and the demise of the megafaunal species. So this study shows that acidification could have started as long as 50ka ago, and continued until at least 20ka when the majority of the extinctions took place and the LGM occurred. The transitional shift in climate must have  had a massive impact on the species already inhabiting Australia. This supports the climate hypothesis, although humans arrived at the same time they would have had a much smaller impact than the environment that changed vegetation composition, habitat patterns, fire regimes and overall environmental thresholds.

Another study I found is by Mooney et al (2011). His study compiled 223 sedimentary charcoal records from Australasia from the late Quarternary period. Fire regimes reflect the overall climatic conditions, with colder climates characterised by less burning biomass burning and vice versa. The records seem to show a clear correlation with the Greenland Interstadials (warm periods) and Greendland stadials (cold periods). Most importantly in these records, there is no change in correlation with the first arrival of humans in Australia 40-50ka ago. There was also an especially low biomass during the Last Glacial Maximum (LGM), with an expansion of drought-tolerant vegetation. During the LGM, there must have been a reduction in fuel availability and severely cold. Dry and arid conditions. This has been supported by a study by Hesse et al (2004) who found there to be stronger winds deduced from dune activity, also demonstrating more arid conditions. This study supports the idea that climate changed significantly during the LGM and the period leading up to it. It also show that fire regimes were controlled by climate and not humans, which I think also shows that the climate had the overriding force in the extinctions of the late Pleistocene, and not human populations.

The last study I found was by Murphy et al (2011). This study specifically focuses on the giant flightless bird, the Genyornis newtoni (don’t worry I’ve never heard of it either!), which disappeared 45-50ka ago. The study found that the preserved eggshells of the extinct emu from Lake Eyre showed an abrupt dietary shift from tropical grasses to temperate grasses and woody browse. This has been interpreted as indicating an ‘ecosystem collapse’. However, this study test the hypothesis that actually it was due to a gradual climatic transition. After re-analysing preserved egg shells from the past 140,000 years, it was found there was no evidence of a sudden climatic shift, but actually a gradual shift from 80-30 ka ago. So this study overrides the theory that a dramtic shift in climate was caused by landscape burning by humans, but that in fact, there was a gradual shift in climate to more arid and dry conditions. This study obviously supports the fact that there was a gradual aridification, changing the environment and its parameters gradually until individual survival thresholds of existing species were reached and were unable to survive any longer.

So I think these studies demonstrate there was a gradual but lethal shift in the climate during the late Pleistocene. The aridification got so severe that the species could no longer survive. The human ‘overkill’ hypothesis just isn’t enough to explain such a large scale extinction of so many species, it just isn’t realistic. Mainly because there simply weren’t enough humans to wipe out so many species, also because there was such a prolonged co-existence of humans and megafaunal species and there is little or no evidence for human impact on the now extinct species.

Friday, 30 December 2011

Co-evolutionary Disequilibrium theory...

So now I'm coming to the end of my blog, I don't want to leave any stone left unturned! So there is one theory that I have not addressed as to explaining the late Pleistocene extinctions, this is the 'co-evolutionary disequilibrium' theory.

Graham and Lundelius (1984) proposed this theory, now it isn't a very popular theory and hence why I have not addressed it before but I will explaining why in a minute. Firstly I just want to briefly explain what the theory proposes. So the meaning of 'co-evolution' is the common evolution of multiple taxa sharing close ecological relationships, in which reciprocal forces make the evolution all taxon dependent upon the evolution of the other. An example is given of the African savannah, which is a highly co-evolved system, where grazing activity on a particular species, plant, stimulates growth and development of other species. This cycle continues as more herbivores migrate to the co-evolved system. To preserve this grazing succession, the co-evolved system must be retained, and changes in climate, migratory patterns of productivity rates will disturb it. So Graham and Lundelius (1984) are suggested there was a change in climate to spoil this well oiled machine, this in turn meant new community patterns, species compo of new communities be completely different to late Pleistocene predecessors. And hence the niche differentiation of herbivorous animals will have lost its clear definition ( the niche differentiation being process by which natural selection drives compeiting species into different patterns of resource use of niche).

This would have led to increased inter-specific and intra-specific competition, new biotic communities and major biotic re-organisation. Hence reducing resistance and predictability in the system, and inducing instability in the equilibrium system. Reducing the niche differentiation and increased competition would have caused complete disruption and hence the species would not work together to provide for each others needs leaning to extinction.

Now the main problems with this theory are, why were smaller species saved from the extinction? Surely if the whole ecological system became unstable, they would also perish? No reason is given for their survival, in fact they even say the extinction was not restricted to certain taxonomic groups or species, it should have killed all of them. Also why hadn't this happened during other glacial to interglacial periods of transition. Surely the climate would have shifted then, so why wasn't there a mass extinction subsequent to the one in the Late Pleistocene. Also this theory very much applies to Africa and the complex co-evolved systems of the savannahs, although tropical forests are also given as an example, no reference i made to Australia. Also there is no evidence supporting this hypothesis, especially in Australia, where there is no evidence of such highly co-evolved systems. But to be honest it would be hard to interpret evidence for such a hypothesis, hence why it has never been taken very seriously. It seems all theoretical and no hard evidence, especially for Australia where evidence of these co-evolved systems just doesn't seem to exist! So all in all, although I should have mentioned this theory before, it doesn't change the overriding debate in this field.

(also I don't know why it has highlighted the last paragraph white, I tried to change it but no luck - sorry!)