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Pleistocene Extinctions

There are two main theories for the cause of the Pleistocene extinctions, either anthropogenic (i.e. human hunting) or ecological (i.e. climate or vegetation change).

 

Anthropogenic theory

· ‘Overkill’ hypothesis

· New colonisation of humans hunted mammals to extinction

 

Ecological theory

· ‘ Environmental change‘ hypothesis

· Rapid changes in climate/vegetation were the cause of the extinctions

 

Intermediate theory

· ‘ Mixed model’ hypothesis

· Humans and climate had an effect on mammal diversity, but climate was the main driving force, with humans having indirect input, possibly from disease transmission

 

Each of these hypotheses has its uncertainties

· There have been debates over the timing of colonisation of humans and the extinctions in the ‘overkill’ hypothesis, especially in Australia

· It is questionable whether hunting alone could lead to extinction and whether hunting of large carnivores was likely.  Recent simulations suggests that hunting of megaherbivores was more likely than hunting of carnivores.  However, the removal of large prey would adversely cause a decline in large carnivores, causing further extinctions due to vegetation change after the herbivores are eliminated.

· Critics of the ‘environmental change’ hypothesis question why

· Extinctions occurred only during the last ice glacial cycle at the end of the Pleistocene, when there have been approximately 20 other cycles not causing extinctions.

· No marine extinctions occurred and elimination was of large species only

· Problems with the ‘mixed model’ hypothesis includes

· Disease is usually restricted to a single order and if they do affect multiple orders then mortality rates are low and so unlikely to affect large bodied mammals to the point of extinction.

 

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2 A recent study published in Evolutionary Ecology Research¹ evaluated these hypotheses by collecting body size data of mammals on four continents (North America, South America, Australia and Africa)

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4 This paper came to four main conclusions

1. Extinctions of mammals on all continents had similar patterns of size selectivity

· This consistency suggests similar causal mechanism

 

2. Significant differences exist between the Pleistocene extinctions and past extinctions in Australia

· Pleistocene extinction followed the colonisation of aboriginals

· Large mammals were affected

· This did not coincide with climatic change

 

3. Large mammals were thought to be targeted in the past and settlers would be motivated to kill to extinction due to the high food return per unit of effort.

 

4. All continents had similar body size distributions before and after the Pleistocene extinction, again this suggests similar causal mechanisms

· However, although North and South American extinctions coincided with climate change, not enough evidence for extinctions as receding glaciers should have led to mammals extending their range, not causing extinctions.

· All continents showed extinctions shortly after human colonisation, supported by extinctions of large mammals on islands that became inhabited by humans prior to or after the end of the Pleistocene.

 

 

¹ Lyons, S.K. Smith, F.A. Brown, J.H. (2004) Of mice, mastodons and men: Human mediated extinctions on four continents. Evolutionary Ecology Research 6: 339-358

 

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