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!)

Another video... but this time of the Megalania Prisca

Video 7: A tribute to the Megalania Prisca

http://www.youtube.com/watch?v=g3eaR_finng

So first advice for this video is put your laptop on silent because there is some heavy rock music in the background, but apart from that its quite interesting! The Megalania Prisca is the largest terrestrial lizard known to date and it roamed the lands of Australia before the late Pleistocene extinction where unfortunately it met its end. But this video shows a few photos of reconstructions to demonstrate what it looked like and how big it was. It is predicted to have been at least 20 feet long, with some even saying they could grow to 26 feet! Huuuuuuge, and very scary, wouldn't have liked to come face to face with one of them!

A welcome back after Christmas with a bit of Faith and O'Connell (2011)

It probably would have been more useful to post this blog entry earlier but I’ve only just found it now, so better late than never! I say this because it refers to an article by Prideaux et al (2010) that I covered in a blog entry on the 29^th of November – just in case any of you want to refer back to it!

This study is based at the Tight Entrance Cave (TEC) in south-western Australia where a sequence documenting the extinction of 14 mammals has been carried out (see figure 1). Faith and O'Connell (2011) aim to show that the extinctions in this sequence took place in correspondence with vegetation change, a drying trend and increased biomass burning, predating human arrival on the continent. Prideaux et al (2010) assumed that because there were no extinctions from the Penultimate Glacial Maximum (PGM) at 143ka until the late Pleistocene, the arid conditions couldn’t have caused the mass extinction.

Figure 1: Summary of all of the different species found at TEC.

However, and there is a big however here, 9 of the 14 species known from TEC were known to have survived until the late Pleistocene. So this means that actually 4 to 5 mammals survived to the late Pleistocene, broadly overlapping with the arrival of humans on the continent. Additionally, of the 68 species known in Australia in the Pleistocene, 25 are known to survive to the late Pleistocene and 15 known to overlap with the arrival of humans (Field et al 2008). In this article and others before, this pattern has been put down to long term drying trends towards more arid conditions. Now Prideaux base their argument on the fact that extinctions took place leading up the Last Glacial Maximum (LGM) , however the fact that some species survived human arrival and that some species survived into the late Pleistocene contends this assumption. Hence a lot of these extinctions occurred when the climate begun a transition to open vegetation, a dried climate and increased biomass burning (indicating a drier climate).

Smaller mammals were able to survive this change in climate because of their smaller patches of required habitats, as Horton (1984) contested along with others.  They could survive off less water and had greater range and forage requirements, along with higher reproductive rates to sustain their populations. But Faith and O'Connell (2011) do prove there was also instability in the smaller mammal populations, although obviously not complete extinction. This is indicated by increased chord distances across adjacent pairs of stratigraphic units, which may not mean much to a lot of you, but basically means there was an increased turnover of species and instability within communities. This pattern begins before human arrival and hence must be down to environmental shifts.

So this article shows that Prideaux et al (2010) were rushed in their conclusion of the reason for the extinctions in Australia. Although the majority of the animals at TEC did go extinct before the LGM, some survived until after human arrival. The research on smaller mammals is also very interesting, it is the only reference I have come to in my research of my blog but proves there were changes across all species, its just the changes were obviously more extensive in the larger megafauna.

Interesting study from the Clovis people, or should I say the pre-Clovis people!

So this article was suggested to me by Anson and I think it is really interesting for the late Pleistocene extinction debate in Australia.

This study by Waters et al (2011) (and also supported by Lawler 2011) found the tip of a projectile point made of mastodon bone at the archaeological site of Mansin in Washington. Now I know this isn’t quite in Australia, but what is interesting is that radiocarbon dating and DNA analysis shows the rib (that it was embedded in) was found with other remains that date to 13,800 years ago. So this means these projectile points that are associated with the Beringian Upper Palaeolithic and Clovis were in fact used in the pre-Clovis era. Hence this site shows people were hunting at 800 years before the Clovis. Furthermore, this is backed by evidence of mammoth hunting at sites in Wisconsin, showing hunting occurred way before the once presumed first humans in North America, the Clovis.

This shows that the human co-existence with megafaunal species was even longer than first presumed. Although this is a case study from North America, it just goes to show that actually we don’t have an exact time-line of humans arrival, especially in Australia where there is no clear or accurate dating implementation as of yet. North America has ample more archaeological evidence of both human and megafaunal species remains and even here chronological events have proven false. So the estimation of human arrival varies a lot, from 62-45ka, this is a large time lag, and who’s to say it isn’t longer?! Either way, it can’t be ignored; human co-existence was very prolonged in both Australia and North America it seems. This discounts the ‘blitzkrieg’ model completely, and probably the ‘overkill’ model, well unless the extinction was long and drawn out. But there is more evidence for the climate model applying, surely it can’t be coincidence these extinctions happened as the climate and naturally the environment transformed dramatically?! I don’t think so…

Discovering the first Diprotodon fossils...

 

 Video 6: Diprotodon fossils found!

http://www.youtube.com/watch?v=G3OteN2gAfs 

So I know this video probably isn't too thrilling (especially because there is no men in black in the background!) but I thought it might be interesting to some of you! Its just a compilation of photos and a video at the end recording the discovery of some Diprotodon fossils, which to be honest is quite exciting but this video makes it a bit boring! But yeah, it's still quite interesting if you think of it as you're looking at the bones of a species that lived thousands of years ago...(also for those of you have forgotten a Diprotodon was a species of giant wombat that went extinct during the late Pleistocene in Australia)

Simple timeline

This article is by Trueman et al (2004) and proves the long co-existence of humans and megafaunal species in Australia.

Analysis of faunal assemblages at the Cuddie Springs archaeological site in south eastern Australia found that remains pre-dated the pre-proposed extinction date of 46.5ka. This means the extinction happened over at least 15,000 years, much too long for the ‘blitzkrieg’ or even the ‘overkill’ model to apply. Adding to this particular study, this site contains a secure stratigraphic timeline, with a human overlap enclosed in sediments dating from 30-36ka, which was the period that significant environmental shifts occurred.

So below in figure 1, there is a sum up timeline of the presumed events that coinciding supporting the view that the change in the environment caused the late Pleistocene extinction in Australia. I thought this was quite useful just to sum up the key events and to prove that basic chronology points to the changing environment causing the mass extinction and not human intervention.

Figure 1: Simple time-line of the main events leading up to the mass extinction in Australia at the end of the Plesitocene. Sorry its a bit blurry!

Forster (2003) and the 'self organised instability' hypothesis

I’m quite conscious that I’m coming to the end of my blog now, so I want to make sure that I am tying any lose ends. One of these lose ends is the ‘self organised instability’ hypothesis posed by Forster (2003). I mentioned it in the glossary and I think I should explain it so that I have covered all theories applied to the late Pleistocene mass extinction in Australia.

So basically this theory is saying that the in any given area, the resident species number will reach a certain amount and be maintained through immigration or the breeding of new species (speciation). In these systems, extinction results from an increase in the amount of interaction between species as the number of different species increases, hence instability is acting as a barrier to prevent further diversification. Meaning the feedback of the system incorporating immigration, diversity, interaction and extinction leads to self organised instability of an ecological system.

Signs of intense interaction are seen in the overall ecology with many forms of competition, predation, parasitism and mutulism. In fact most of the interaction is found in the form of co-operation between species and human populations. For example, in the Blue Mountains of Sydney, Aborigines may have consumed no less than 72 plants, 45 mammals, 235 birds, 6 fish, 16 reptiles, 29 frogs and numerous invertebrates (Merriman 1993).

Forster also tries to give evidence of a period of speciation and immigration that had to have occurred prior to the extinction for this theory to stand. Figure 1 visually shows how increased immigration could have caused increased diversity, interaction and finally extinction. To prove immigration levels were high, Forster (2003) gives the example of the Rodentia mammalian, who immigrated from south – East Asia and were the first indications of terrestrial placental mammal in Australia. Many species migrated to Australia during the Pleistocene, increasing speciation and extinction and enhanced diversity. Now the arrival of humans may have pushed the ecological systems into a super-critical state where extinctions occurred quite easily, their broad generality enhanced speciation and interaction and could have led to extinction. Hence Forster (2003) is posing here that actually the mere presence of humans in Australia caused the mass extinction. So this hypothesis has no need to rely on the ‘overkill hypothesis’ or the ‘blitzkrieg model’, it was just the increase in interactions caused increased speciation and hence extinction.

Figure 1: How immigration could have led to extinction in the 'self organised instability' hypothesis.

 

Now to me, this theory seems very vague and I don’t think enough factors have been considered. The physiology and biology of different species has not been considered here, and Forster (2003) says himself that the hypothesis is based on one solitary case study from the Hawaiian avifaunal study of Keitt and Marquet (1996). There is very little solid relevant evidence given here and it seems all a bit too theoretical for me. We have no detailed evidence of the historical, biological or ecological traits of the megafauna that went extinct, or even of those who didn’t, so can we realistically base a theory on such little evidence? I don’t think so. We need far more fossilized and archaeological evidence to even begin to prove this theory, which we definitely don’t have at the moment and possibly will never have in the future. We have no explanations as to the different immigration patterns in and out of Australia in the late Pleistocene and we have no information on the speciation patterns either. We don’t even have reliable dating records to tell us the exact dates of the extinctions in Australia, so to be honest I just don’t think proving this hypothesis is feasible. To me it just seems very ungrounded with very little if any evidence to support it, hence I don’t think it is credible enough to consider as a possible explanation to the late Pleistocene extinctions in Australia.

Because not available online:

Merrimann, J. (1993) 'Difficult rocky, sandy, stoney, flowery: Aboriginal ecology in the Blue Mountains', in Stockton, E. (ed.), Blue Mountains dreaming: the Aboriginal Heritage, Three Sisters Productions: page 82–113.

Johnson and Prideaux... not sure if your evidence supports your hypothesis...

I’ve found another study that is completely against the environmental change hypothesis, and I mean completely!

Johnson and Prideaux (2004) carry out a study considering the extinction rates of both browser and grazer species. Now I think it is important to explain what the difference is between these two groups of species! A browser species feeds mainly off high growing vegetation such as woodlands and forest areas, and a grazer feeds on the lower level growing grasses and herbage. Now Johnson and Prideaux (2004) are comparing their extinction rates because it has been posed that most of the herbivorous species that went extinct were browsers. Browsers would have been more reliant on shrubland and woodland and hence will have been more affected if such habitats contracted rapidly due to human fire regimes and changes in aridity (so actually the climate does come into this as well!)

So the model, after 75 species were analysed from both Australia and New Guinea, found 32 out of the 48 browsers went extinct in the late Pleistocene and only 11 out of the 27 of the grazers went extinct. Hence, it was found that yes more browsers did go extinct over grazers, but not because browsers were more likely to go extinct but because most of the species in the late Pleistocene were in fact browsers.

They pose that directional change in habitat would have caused such a widespread extinction because both sets of species fed of completely different vegetation compositions. But I think this underestimates the power of the climate somewhat. Who’s to say that the climate couldn’t change both of these divergent vegetative habitats? Intense drying and severe arid climate could change the whole face of vegetation in Australia, affecting all the animals in different ways but in the end having the same consequence, extinction. As I’ve already covered in this blog, there was a severe shift in climate in Australia in the late Pleistocene that could explain directly the majority of the extinctions.

A model considering the overkill hypothesis...

I have found rather an interesting study by Choquenot and Bowman (1998) that applies a predator-prey model to the late Pleistocene extinction in Australia.

The two species predator prey model examines the population density and hunting efficiency of an aborigine population needed for overkill to have led to mass extinction. Now this model highlights some rather large flaws in the overkill hypothesis. Primarily, the model demonstrates the smaller megafauna would have been more readily killed over larger ones because they would have reproduced at a faster rate to satisfy meat demand. As we already know, this was not the pattern at the end of the Pleistocene, and in fact many smaller species survived. The Aborigine population densities and hunting efficiencies would have had to be unrealistically high to kill out the majority of large megafauna. The authors go on to highlight the fact there are few co-existing human and megafaunal remains found at archaeological sites and dating is very precarious. This is very true on both counts, meaning it is hard to prove solidly any theories in Australia. However, since archaeological evidence has been found on other continents, mainly North America where humans were known to have hunting megafaunal species, then why isn’t there in Australia?

A few more details about the mathematical model used in this study reveal the downfalls in the overkill hypothesis. The model showed that to exterminate all of the megafaunal species 500kg and more, the population of Aborigines would have had to be in excess of 0.07 aborigines per km and a rate of effective search of 100 ha per person per day, now that is a lot! 100 ha, blimey I would be knackered (slash dead) if I had to cover that every day! 

Figure 1 shows that, as expected, the larger the species, the higher the aborigine population density and hunting efficiency needed.

Figure 1:

·         Extinction isoclines for megafaunal of 1000kg (dotted line), 500 kg (dashed line) and 250kg (solid line), the isoclines are divided by hunting efficiency and Aborigine population density, obviously showing that the larger mammals would have taken more efficiency and larger population densities to hunt. Estimates of aborigine density from areas in northern Australia are indicated by the lines crossing the lower axis from 2 estimates.

 

Now another model that I think is worth mentioning is that designed by Mosimann and Martin (1975). They made the process of species reproduction spatially significant by having human populations colonise a certain geographical area only supporting unexploited megafauna. Once all of the megafaunal species are exterminated, the humans will move to new areas containing less exploited species. But because these species were indefensible to these human populations, soon the complete extinction of megafaunal species was inevitable. Hence the extermination of these species was dependent on the concentrated formation of a colonising human population, precipitating regional extinction. Wherever the human population density and per capita offtake exceeded the capacity of regional megafaunal populations to sustain breeding, extinction occurred. However, although this facilitates the overkill hypothesis, it is inaccurate to presume per capita offtake remains the same ignores the effect of declining prey and hence the need to hunt further afield. This means that fact that as the prey population declines as it is hunted; the proportion of prey removed thereafter is ignored, dramatically increasing chances of hunters to exterminate species. They also predict a high level of wasted meat and an exaggerated population growth among humans to make the demand as high as it would need to be to eliminate all the larger species.

So all in all, unrealistically high hunting efficiencies and even idealistically large Aborigine populations would have been needed to cause the mass extinction of megafaunal species in the late Pleistocene. To be honest, the figures given by this model prove how implausible the overkill hypothesis is. The archaeological evidence found to date in Australia doesn’t account for such large Aborigine populations and there is no way one individual could hunt over a 100ha area every day! The model by Mosimann and Martin (1975) proves the improbable parameters needed for the overkill hypothesis to apply and this in itself is proof that climate obviously had a key role to play in the mass extinction.