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.



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