Friday, 30 December 2011

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!)

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