Monday, 3 December 2012

A Brief Overview of What we have Learnt so Far

My next blogs will be focused on:
   1.      Possible causes of megafaunal collapse in different continents e.g.   Africa, Eurasia etc.
   2.       Specific examples of species e.g. woolly mammoth etc. and their likely cause of extinction.
   3.      Other topics related to the debate e.g. Sporomiella Proxy etc.

Before I embark on these, I would like to take this opportunity to briefly review the main findings of what we have learnt so far:
    ·     There are various mechanisms that could have caused the extinction of megafauna during the late Pleistocene (roughly 60,000-11,000 years ago). Mechanisms include: climate change, disease, humans, fire, and the impact from an asteroid/comet.
    ·         Whilst there has been a large degree of research conducted, conclusions remain deeply controversial.
    ·         Human overkill is a likely cause of extinction through hunters preying on large mammals leading to their demise (see Bulte et al 2006 & Roberts et al 2001). However evidence which disagrees with this includes the availability of alternative food sources from agriculture.
   ·         It is plausible that humans could have caused the extinction of megafauna in Australia (see Johnson 2006). The decline of species such as G.newtoni immediately after initial human colonization as well as mild climatic variability in this region, reinforces evidence to support the human overkill theory. However lack of evidence weakens this hypothesis.
·         Climate can be seen to be responsible through exacerbating human impacts (wroe et al 2006), or through habitat modification. Evidence of extreme climatic changes are present, but linking this to megafaunal decline is difficult (Lorenzen et al 2011). Similarly, the demise of species that were able to adapt to climate change weakens this hypothesis.
   ·         Fire can be seen to be responsible for the extinction of megafauna through; extreme temperatures as well as altering landscapes so that megafauna are unable to sustain themselves (see Gill et al 2009). However lack of fossil evidence and uncertainties in dating have made it exceptionally hard to test this hypothesis.
   ·         It is unquestionable that a comet hitting the earth would have ultimately led to megafaunal decline (Firestone et al 2007). However many are sceptical about this due to severe lack of reproducible evidence.
   ·         Disease is credible mechanism as megafauna might have had weak immune systems and were unable to withstand pathogens (see Rothschild & Laub 2006). However there is lack of evidence uncovering a pathogen that has the capability to cause such widespread extinction.
   ·         Poor quality fossil datasets make it exceptionally hard to discover what caused megafaunal collapse. Similarly, it is highly unlikely that extinctions across the globe were a result of a single cause. It is more likely that different species or continents were subject to different forcing mechanisms e.g. continents that experienced dramatic climate variability suffered majority of extinctions because of climate change, whilst the early colinization of man in other continents might have caused megafunal demise.
   ·         In the majority of cases perhaps combining multiple causes of extinction might be the most likely cause of megafaunal extinction.

Some examples of megafauna becoming extinct (going down): woolly mammoth, woolly Rhino, Irish Elk, Diprothodon, Giant sloth, Cave lion, G.Newtoni, Giant Kangeroo, Glytodon, Smilodon.

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