What caused the Pleistocene Mass Extinction?

[tigerluver]

This is one of the most mind boggling questions about the Pleistocene. What could have caused the relatively sudden loss of so many species? At the same time, why didn't we lose just about all large mammalians like Cretaceous catastrophe and its dinosaurs?

I invite everyone, even those not into paleontology, to share their thoughts on this issue.

Here are short summaries of the hyptheses considered by the scientific community:

1. Overkill hypothesis - Humans were mainly responsible for the sudden loss of megafauna. A recent paper (in the Extinct Animal News thread) believes this is answer for sure. A possible caveat to this hypothesis is whether or not the human population at the time was large enough to decimate the great amounts of megafauna, especially considering the limited tools of the time.

A sub-hypothesis to this is the second order predation hypothesis. This hypothesis is more pertinent to the New World.
1) Humans kill off (directly through hunting, indirectly through competition for the same prey base) enough predators to disturb the predator-prey population cycle relationship. 
2) The loss of predatory top-down control allows prey numbers to soar. These species essentially eat themselves to death. They deplete the resources they depend on in the end, as follows:

*This image is copyright of its original author

With the added caveat that the prey doesn't recover, in turn the predator's already dim plight is made worse. The overpopulation of prey would also destroy habitats as a whole.

2. Climate change - Temperatures rose significantly around 15 kya, affecting plants and sea levels to say the least. Change is not helpful to survival. On the other hand, there had been rapid climate change incidents before without such catastrophic effects.

3. Hyperdisease hypothesis - Some virulent pathogen may have decimated populations. We've seen instances of this pathogens wreaking havoc on populations in modern times, such as canine distemper in felids. An argument against this theory is that if such a pathogen existed, it would be unrealistically selective or unrealistically broad as to what species it affected.

4. Comet hypothesis - A sudden burst of comets (Younger Dryas impact) around 12 kya tampered with the climate, and in turn, ecology of the biosphere. An issue with this theory is that not all extinctions coincide with the Younger Dryas impact.

The list does not necessarily end here. Let us know what school of thought you agree or disagree with and any other hypotheses or questions you may have. Reply