Sunday, 30 October 2011

A few pictures... (just to liven the mood!)

Found some pictures reconstructing some of the species known to have existed before the mass extinction in Australia...
Figure 1: Propleopus oscillans, a genus of marsupial, much bigger than the Kangaroos living in Australia today!

Figure 2: Protemnodon, from the genus of macropod, based on fossil evidence, it was more similar to the modern day wallaby rather than the kangaroo, but much larger at around 45kg.

Figure 3: Diprotodon, known as the giant wombat and the largest marsupial to have ever lived, predicted to have gone extinct 46,000 years ago.

So these pictures are just to give you an idea of the megafauna around during the Pleistocene in Australia, I'll keep trying to find more to put up on here!

Friday, 28 October 2011

Late Pleistocene climate in Australia and the effect on megafaunal species

So for this blog entry, I am going to now start focusing specifically on Australia (since that's the overall focus of my blog!). Although there has been a lot of recent literature concerning the Late Pleistocene extinction in Australia, I am going to begin with academic articles from a while back. This is just to demonstrate how the overall debate has developed over the past 20 years or so, and to highlight the changes taken place in the past 5 years. The article this blog entry is based on is that of Dodson (1989), who gives a brief overview of the changing environmental landscape at the end of the Pleistocene. During most of the Pleistocene, New Guinea, Timor and Northern Australia were connected by expansive land plains (see figure 1), covered in pockets of rainforest, hummock grassland and shrublands. Across New Guinea, Northern Australia, South-western Australia and Western Tasmania, most of the ecosystems were made up of mesic vegetation, with 25% covered with arid/semi arid shrublands and grasslands (Dodson 1989: 210).
Figure 1: Map of the Australia, New Guinea and Tasmania land mass during the Pleistocene epoch (Dodson 1989: 208).

Some of the most extensive records of the Pleistocene climate come from Lake George, near Canberra (Singh and Geissler 1985) and Darwin Crater, Western Tasmania (Colhoun 1988) dating back 350,000 years. Both indicate the Pleistocene era was cold and dry, inferred from the low water level of the lakes and the predominately herbaceous vegetation. Although these arid conditions were extensive in certain parts of Australia, there were also areas covered in dense forest vegetation, mainly across the plains joining Northern Australia to New Guinea and Tasmania. However, these forestry coverage began to reduce rapidly as the beginning of the Holocene approached. Paleoclimatic studies taken from sediment profiles at Ulungra Springs in New South Wales show that 25,000 years ago, towards the end of the Pleistocene, chenopod shrubland replaced vast forestry ecosystems (Dodson 1989). This indicates persistent arid conditions with up to a considerable 50% reduction in rainfall.

 Although these intensive arid conditions were harsh on some of the already existing species on the continent, they were more than ideal for the grazing animals introduced by Aborigine populations. Dodson (1989) deduces that the arid conditions prevailing in Australia will have been beneficial to growing human populations as their grazing animals thrived. This would have resulted in population growths, and in turn increased pressure on megafaunal prey.

Thinking about it, this would also explain the fact that the extinction in Australia happened approximately 7,000 years before the extinctions in Europe and the Americas. It also explains how Aborigine populations were able coexist with megafaunal populations for so long, during the Glacial Maximum, forest coverage was still vast and the climate was still too cold for human populations to thrive. But with the Late Pleistocene, temperatures rose and dense inhabitable woodland retreated, enabling human populations to prosper. Hence I think looking at the climate is really important when considering the causes of the extinctions, because ultimately, the climate is what most influences human populations and their impacts on the surrounding environment.


Dodson, J. R. (1989) 'Late Pleistocene Vegetation and Environmental Shifts in Australia and their Bearing on Faunal Extinctions', Journal of Archaeological Science, 16, 207-217.

(Because this article isn't accessible on the internet I will just give the reference - Calhoun, E. A., (1988) 'Cuinozoic vegetation of Tasmania', Special Paper, University of Newcastle, New South Wales, Australia).

Tuesday, 25 October 2011

Overview of the global Late Plesitocene extinctions

So to start off with, I want to draw upon some general readings around the Late Pleistocene extinction. With this I hope to give an overall view of the scale of the extinction, and the different theories knocking around to explain its occurrence, and the overall stage of the debate.
Rightio, let's get the all rolling, the late Quaternary extinction was the largest extinction seen in the Quaternary period. The species hit the hardest were those with low reproductive rates, those nocturnal, arboreal and isolated island dwellers survived (Barnosky et al.: 2004). It wiped out 34 genera of megafauna in North America, with the extinction total for those species over 1000kg and 50% for size classes between 1000 and 32 kg (Barnosky and Koch: 2006). In South America, 50 mega fauna lost, around 83% of those already existing and all mammals over 320kg were wiped out (Barnosky and Koch: 2006). Onto Australia, a massive 14 of its 16 genera of Pleistocene mammalian mega fauna were lost along with a complete extinction of mammals over the weight of 100kg (Barnosky and Koch: 2006). Northern Eurasia lost 9 genera, which was 35% of those animals already existing (Barnosky and Koch: 2006). Africa is the anomaly in this sense, only 10 megafauna were wiped out, when if environmental factors are blamed for this, then it should have also had a widespread extinction. All this information alone shows how catastrophic and sudden this extinction was, the biggest extinction in the past 55 million years all bar one (Alroy: 1999).
Such a devastating event! So why? Well there are many contested arguments, the first of which are the ‘environmental extinction hypotheses’, that as can be told by the name blame the transition in the climate for the mass extinction (Guilday: 1967). There are a number of hypotheses that constitute this group, those being the ‘mosaic nutrient hypothesis’ (Guthrie: 1984), the ‘habitat loss hypothesis’ (Barnosky: 1986, Ficcarelli et al.: 2003, King & Saunders: 1984), the ‘co evolutionary disequilibrium’ hypothesis (Graham and Lundelius: 1984) and lastly the ‘self-organised stability’ hypothesis (Forster: 2004). All of the meanings of these hypotheses I have listed in the glossary because their meanings will be useful to know when analysing later articles, but they all see that in different ways the change from glacial to interglacial transition bought the mass extinction, whether this was from changes in vegetative composition, habitat fragmentation, small changes in complex ecosystem networks or flora patterns.
However, this group of hypotheses has been discounted due to a variety of factors. The first being that on a global scale, there is little evidence that there was a complete clearance of stable habitats, in fact in Australia, the extinctions took place in arid conditions when the vegetative composition was stable (Magee et al 2004).  Also there is no explanation as to why slow-breeding arboreal or nocturnal mammals were spared, and large megafaunal were not. The last thing is that paleoclimatic records don’t support a unique glacial-inter glacial transition, no different from other deglaciations (Barnosky et al.: 2004).


 
Another group of theories to explain the extinction are the ‘overkill hypotheses’ (defined in glossary) that notion humans were the main instigator (Martin 1966). Although hyperdisease and forest fires have been highlighted as indirect anthropogenic causes for the extinction, both have been discredited and the ‘overkill hypotheses’ stand as the only direct anthropogenic reason for this event. Martin poses the ‘blitzkrieg’ theory (again defined in glossary) as the main reason to the global extinction because of 3 reasons. One being the prey were na├»ve and lacked behavioural instincts to escape humans. Two because the archaeological evidence shows there was only very brief contact, all that was needed to wipe out whole genera of species and 3 because the extinction was the most extensive in Australia and the Americas because there were fewer modern homo sapien humans.
However, stability analyses of early simulation models coupling human and prey population dynamics found overkill an impossible outcome, with other various multi prey models resulting in the same conclusion (Belovsky 1988). Humans would have relied on a large range of taxa for survival, and will have probably selected their prey randomly as they came across them, or else humans would have gone extinct first, before the megafauna. Also Wroe et al (2004) point out that naivety of animals would only be present on islands where there are no large carnivores, not on the continents where there were plenty. Archaeologists argue there is no “empirical” evidence of overkill in the Americas and Australia, and there is only one site in Australia at Cuddie Springs has lithic artifacts associated with extinct megafaunal mammals (Field & Dodson 1999). And even more to the point, there were no stone spear points and dogs needed to kill the larger megafauna in Australia until the mid Holocene (Barnosky and Koch: 2006).

Considering both of these broad hypotheses, I think that neither of them are the conclusive reason as to why there was a mass extinction in the late Quaternary. The environment showed no unique shifts in climate to serve as a reason for the lack of survival of so many particular species, and the human hunting hypotheses just aren’t enough to explain a large scale global extinction, especially with such little evidence. I think the combined hypothesis is the most accurate, for example Owen Smith (1987) suggested the ‘Keystone Herbivore hypothesis’ that posits climate change and human hunting pushed critical megaherbivores into extinction, leading to shifts in vegetation and detrimental impacts on other species leading to mass demise. Another combined hypothesis theory, is the ‘prey-switching’ hypothesis claiming that hunting and environmental change could have meant the demise of certain herbivore species. This may have disturbed the food web and intensified predation on other megaherbivores, and in turn meant the demise of more species.
So I know this was incredibly long, but I just wanted to give you an insight into the stage of the debate for this historical phenomenon and the reasons thought for its occurrence. I think personally the natural world consists of thousands of different ecosystems at play, presenting a complex network of different species, environments and interactions. Humans contributed to bringing on the extinction, but the climate determined the timing, geography and magnitude of it (Barnosky and Koch: 2006).
Considering Australia specifically, the extinction followed human arrival, but cannot be bracketed within less than 10kyr, making the situation with Australia more complicated. Transitions in the climate in this period had been no different in Australia than they were in earlier phases of the Pleistocene, moreover there is evidence that there was a stable vegetative state, so why extinction at this particular transition? Also humans inhabiting the continent had no efficient weaponry, such as stone spiked weapons, and there is little archaeological evidence of large human populations, so surely human predation couldn’t have played a large role? These are all reasons why the extent of the extinction in Australia seems so surprising, and I hope to answer many of these questions I have in the rest of this blog as I carry out more research!




References:

Barnosky, A.D, P.L Koch, R. S. Feranec, S. L. Wing, A. B. Shabel (2004) ‘Assessing the causes of the Late Pleistocene extinctions on the continents’, Science, 306, 70-75.

Guilday JE. (1967) ‘Differential extinction during late-Pleistocene and recent times’, in P. S. Martin and H. E. Wright (eds) ‘The Pleistocene Extinctions: The Search for the cause’, New Haven: Yale University Press, pp. 121–40.

Koch, P. L and A. D. Barnosky (2006) ‘Late Quaternary Extinctions: State of the Debate’, Annual Review of Ecology, Evolution, and Systematics, 37, 215-250.

Martin, P. S. (1966) ‘African and Pleistocene Overkill’, Nature, 212, 339-42.



Thursday, 20 October 2011

Video 1: Mammoths, Overkill, and a Deep Time Perspective on Pleistocene Extinctions (with Connie Barlow)

I thought this was quite a good video to start off with because Connie Barlow (evolutionist with a keen interest in conservation) gives an overview of the extinctions that happened all over the globe, at different times and to different species. She argues that humans were the main reason for the extinction of mega fauna in the Late Pleistocene, and in fact for some species in the Holocene, up to only 800 years ago. She bases her argument on the late Paul S. Martin’s ‘Overkill theory’, which basically argues that human predation was the main reason for the mass extinctions. Although I haven’t started doing thorough research yet, I must say that many theorists now disagree with the contention that humans were the sole reason for the extinction. More recent studies with newfound radiocarbon and pollen sample evidence have found humans and the now extinct mega fauna to have co- existed for a long time, some in Australia for up to 15,000 years (Lundelius: 1987). Also many studies provide evidence of severe dramatic climatic shifts that could have destroyed animals habitats and tested their adaptability to the point of death. But this is the main issue I will be considering as I develop this blog, was the Late Pleistocene mass extinction in Australia down to the climate or down to humans? We will have to wait and see…(until I’ve done more research!)


Lundelius, Ernest L, Jr. (1987) 'The Pleistocene Mammalian Crisis: Habitat Destruction as an Extinction Mechanism', AnthroQuest,  37: 13-14.


Introductions...

So hello everyone! Now, as you may have guessed from the title, my blog is going to be based on the Late Pleistocene extinctions, specifically focusing on Australia. Now I know it’s not quite as exciting as dinosaurs, but giant kangaroos and wombats will be involved I promise! So can you imagine going back 50,000 years ago, the expansive lands of the earth filled with a diversity of mega fauna species from the sabre toothed cat, the giant ape and the woolly mammoth? No? Me neither. But 11,000 years ago, the biggest extinction of the Quaternary period occurred across the whole globe, most predominately in North America, South America and Australia. Over a period of 40,000 years, 53 mega fauna genera disappeared - that’s a lot, honest. Most of these extinctions happened around 11,000 years ago, during the transition from the Pleistocene to the Holocene epoch. So how did it happen? Well to be honest no one really knows… But there are various theories, the two favourite being environmental causes relating to climate change or as it usually is with everything, us humans, apparently we may have got a bit over excited and hunted certain species to their extinction.
A fierce looking sabre-toothed tiger, maybe a good job they’re not around today...

The debate regarding the Late Pleistocene extinction in Australia is still very active, with more studies focusing on specific paleontological evidence from different parts of Australia. Some of these studies I will be assessing later on in the blog, many of which point to the climate as the main reason for the massive loss in mega fauna. I will also go onto look at case studies based on specific Oceanic islands and maybe look at some of the many studies carried out in Northern America.
During this blog, I hope to not just consider the different academic articles in this blog deliberating this debate (in fear of boring you!), but also post videos and pictures that I think you might find interesting.
But anyway bye for now, I will be posting again soon!