Monday, 3 December 2012

With Numerous Factors being Responsible for Megafaunal decline, why was Africa least affected?

Of all the continents, Africa was the least affected by the Late Pleistocene extinction (Elias &Schreve 2007). Whilst Africa contains 42 genera of megafauna; only 7 died out during the last 100kyr. This blog will explore reasons why African megafauna remained largely untouched by forces that drove extinctions in other continents.

The genera that became extinct in Africa during the last 100 kyr include a genus of Pleistocene elephant (Elephas), a genus of African buffalo (Parmularius) and a genus of modern cattle (Bos). Consequently African megafauna only suffered a loss of about 14% of their genera in the last 100kys (Elias & Schreve 2007).

A number of hypotheses have been proposed to explain why Africa suffered such few losses. Firstly, humans evolved and coincided with megafauna in Africa. Homo sapiens lived and evolved in Africa before they spread to other continents around 200,000 BP. Many believe that this co-evolution was a major factor which ensured the survival of megafauna during this period. It is also believed that megafauna were able to adapt their behaviour to withstand human hunting practices. Similarly, Africa had favourable climatic conditions which could support a high diversity of species. This ensured that species numbers grew to a level which allowed them not become extinct. As well as this, the glacial-interglacial transition was less severe than other continents. This provided greater climatic stability for the species living in Africa, especially those that were unable to adapt to changing environmental conditions,and could easily find refuge spots. Finally, early Homo sapiens in Africa had primitive hunting technology compared to hunter gatherers of North America. This is because the Clovis hunters of North America developed more advanced technology such as stone points, which might explain why extinction was rapid in this continent. This is reinforced by Barnowsky (2004) who stated that sophisticated technology is a key driver of over kill. 

Clovis Hunting Tools
Overall, I believe the combination of these factors explains why African megafauna was least affected by the late Pleistocene extinction, and consequently suffered few losses compared to other continents. 

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.

Some Examples of Extinct Megafauna in relation to Climate and Human Impacts

I recently found this interesting article by Lorenzen et al (2011) that links specific species responses during the late PLeistocene to climate and humans. In this blog I will look at the various suggestions put forward by Lorenzen et al (2011) and assess whether strong conclusions can be made between time of extinction in relation to climate variability and/or human colonization.
As I have stated in previous blogs, the role of climate and Homo Sapiens in driving the dramatic extinctions of large-bodied mammals during the late Pleistocene period remain contentious (Lorenzen et al 2011). In this blog, I will specifically be looking at the demographic history of species such as the woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox. The main findings from Lorenzen et al (2011) show that climate was a major driver of population changes over the past 50,000 years for certain species such as Eurasian musk ox and woolly rhinoceros. However research from ancient DNA and species distribution models also reveal that the combination of climate and anthropogenic impacts seem to be responsible for the extinction of other species such as Eurasian steppe bison and wild horse. Therefore, it is clear that each species responds in a different way to the effects of climate variability and human intrusion, making it even more difficult to predict past responses to various mechanisms of extinction.

Toward the end of the late Quaternary, beginning around 50,000 years ago, Eurasia and North America lost approximately 36% and 72% of their large-bodied mammalian genera (Lorenzen et al 2011). The two most credible causes of extinction include climate and human impact, and these were assessed by Lorenzen et al (2011) in relation to potential ranges of specific megafauna. The dominate role climate played in extinction patterns are shown in a high loss of species in continents that experienced the most dramatic climate variability. The impact of human encroachment is also shown in patterns of megafaunal decline immediately after initial colonization.

The image below (Lorenzen etal 2011) is a very useful illustration modelling specific species and their changes in distribution over time. This illustration is also effective in showing whether extinction was dramatic or occurred gradually over a long period of time. For example, genetic diversity in bison and musk ox declines gradually from 50,000-30,000 kyr BP, whilst other species such as woolly mammoth and woolly rhinoceros loss of genetic diversity occurred suddenly. It is also evident that Reindeer populations remained largely unaffected throughout the late Pleistocene.

Overall, whilst the results for the model (see illustration above) are useful in showing potential ranges of megafauna over time, this research has shown that it is difficult to make a direct link between climatic change and species extinction. Consequently, causes of extinction for certain species such as woolly mammoth are unclear. The results demonstrate that changes in megafauna abundance are idiosyncratic, with each species responding differently to the effects of climate change, habitat redistribution and human encroachment (Lorenzen et al 2011). Therefore it is highly difficult to suggest a single cause of megafaunal extinction during the Late Pleistocene as evidence remains unclear and deeply contested. In later blogs we will look at the possible explanations behind the disappearance of the wooly mammoth.