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What caused the Pleistocene Mass Extinction?
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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. 
This is a very interesting topic.
Previously I believed that humans were the principal factor, based in previous extinctions like the giant sea cow and the Dodo, however, a wider analysis shows that this seems unlikely in many areas. Why mammoths in America and Asia died while elephants in Africa and Asia don't? Both lived with the modern human. Why the dire wolf died and the grey wolf don't? Both shared its habitat with the human in America. Even more, why the large extinction affected all mammals over 50 kg in Central and South America (at exception of the jaguar and the tapir), but not in Europe, Asia and Africa? It is easier to blame the humans, we are incredible destructive in modern time, but we don't know if the early humans were the same. In fact, if we take a view on the oldest cultures of America, Africa and Asia, just to mention a few, most of them have a huge respect for the nature. I think that the problem is more deep than just pointing fingers. 
I think it had to do with the adaption and competiveness.
Many species that didn't know how to adapt the new environment by shifting down their great body mass, they would be probably gone soon after some major climatic change.
I feel different aspects of the mass extinction hypotheses affected different areas of the world. Africa and Asia still kept relatively large animals. The Americas and Europe (bar the bears) lost all the large megafauna, so some differences must've been present.
At least for Asia and Africa, I believe they were a bit warmer than the other areas, as for example, Java was a tropical grassland. Maybe the temperature increase didn't affect the fauna of the areas as much for this reason. Northern Asia/Eurasia and North America seemingly had cooler temperature adapted vegetation. These areas faced the worse of the extinction. With this thought, Kitchener and Dugmore and Luo et al. maybe right in the theory that tigers colonized Manchuria in the last 10,000 years. Tigers may have lived in warmer areas then. Temperature increased, the cave lion was not adapted for the warm temperature, and thus the tiger was the next most suitable successor. The issue with this hypothesis is that South America was likely warmer, so what happened there?
Tiger originated from China, a temperate place where the climate was similar to the Southern Europe and the United States where Panthera fossilis and Panthera atrox used to live.
Since the radical change in the ecosystem from the Europe to the Siberia then to the North America was likely the cause that led to the extinction for the lion-like big cats. PS, I remember that the Mammals of the Soviet Union described that the Manchuria got richer prey base than the Siberia. Except Manchuria, the whole Siberia was always remained frigging cold where lacked of the lush vegetation to support those larger herbivores. Remember Mammuthus Sungari? Although it is actually an invalid species, also a junior synonym for Mammuthus trogontherii, but it is true that the steppe Mammoth got larger in Manchuria than they used to be in Siberia.
Also, remember the Ngandong tiger, it was also a super predator among the Pleistocene megafauna, but its smaller descendant still managed to live until the modern time.
So why other Pleistocene super predators can't have any smaller descendant that represents as its leftover in the modern time?
One possibility is the net production of the ecosystem where the tiger survived. Modern landscape such as rainforest (where the Javan tiger survived) harbor more energy. The problem with this is that South America is an area of one of the greatest net productions.
I still feel that something about the select biomes of each area had to do with the extinctions. Humans can't be the only cause, as tons of other large fauna did survive. The bison of the USA still survived as smaller. Bears, probably smaller than today, still survived. Maybe there was an energy threshold required by the real giants that spelled there doom if the threshold was not met by the new ecosystem. Read this: http://www.geography.wisc.edu/faculty/wi...4final.pdf. Especially the last page, we can discuss from there.
Some time ago, in a documentary, it was mentioned that another possibility for the extinction of the megafauna in America, was caused INDIRECTLY by the humans.
The guy in the program (I forgot the name) stated that it was possible that the new humans arriving to America carried a series of diseases that native animals were unable to survive. The animals in the old world where already used to this and where immune, but all those in the other side were not. The virus spread from herbivores to carnivores and in that form they died. The problem with this is that why it did not affected the "smaller" animals, and the medium size carnivores. Check that although the megaherbivores over 1,000 kg died in North America, those of less than 900 kg survived very well. The American moose is a modern giant with a great population and the bison (that also can reach up to 1,000 in exceptional cases) lived in groups of millions of animals. Wolves, pumas and jaguars survived very well and the bears were very common in all the north, as down as Mexico. This only changed when the Europeans began they holocaust of animals and the killings of the native and TRUE Americans (USA natives, Aztecs, Mayans, Incas, etc....). North America conserved its great ecosystem, only the large mammoths, mastodons, giant sloths, giant vultures, giant bears and the great cats over 150 kg, disappeared; all the other animals survived very well and flourished. In central and south America, it was a different story. All the animals over 50 kg, at the exception of the tapir and the jaguar, simply died, no more giants at all!!! Definitely, I don't believe in the disease case, but a mix of human arrival and the climate could be the real reason. I remember another TV program when it was mentioned that the first USA natives hunted the buffalo in a very stupid way: They attacked the full group of animals and direct it to a cliff and killed the entire group. They wasted tons of meat in this form. Modern natives (still way before the arrival of the Europeans), were more advanced in they hunt form and they religious beliefs, so they only hunted what they need it and the buffalo population was very well. Probably, the first humans were in fact, great killers and wasters, and the native animals, been unable to recognize the humans as menace (they evolved for millions of years without them), simply don't defended in the same way that the old world beasts. This are only hypothesis, but somehow, based in real history and fact, that can help us to found a plausible explanation. Interesting as it is, did you know that all the pumas from the north of America are descendents from a group of originated from South America? What happen with the primitive population of the north?
How long the ancient tiger population had been lasted in Alaska? Just wondering.
That is a good question. The fossils of the "tigers" in Alaska are few, which suggest that they were not very common. I guess that maybe they were just wandering specimens that never managed to establish a colony. The same happen with many other specimens in modern Siberia, recorded by Mazák (1983).
Barnett et al. (2009) stated that his study managed to prove that those tigers were in fact lions, based that he analyzed one of the bones labeled by Herrington (1987), however based in only one bone, if very difficult to state a fact in this form, and most of the Herrington's specimens are still without been studied with DNA.
The Siberia and Manchuria were vastly different domains, the former one is the vast steppe, while the later one is the dense Taiga forest.
I think the most northern outpost for tiger's colony was Manchuria, and the vast steppe was the main reason that tiger never expanded into the depth of Siberia. For the same reason, the Pleistocene lion-like cats never managed to cross into the Manchurian Taiga forest, and same for the modern lions, as they rarely wander into the dense jungle forest in Africa.
For the climate part of the discussion, I'll refer to post #12 from now.
For one, I think the modern bear dodged the extinction event because it is an omnivore. That leaves it with a lot of options, in other words, omnivory results in a very adaptable species. The areas which lost exclusively their giants, North America and Russia on the map, went through significant change. Again, Africa and Asia barely changed. China went through some change, and some mammoth species were lost if I'm not mistaken.
Interesting Tigerluver, I think I most read the document that you post first, in order to understand this.
By the way, GrizzlyClaws, I leave you the link for the study about the Beringia tigers: http://animalsversesanimals.yuku.com/top...-in-alaska Is from the AVA forum, the first time that I post it. Keep it, just in case that I lost it, again. 
(08-22-2015, 01:00 AM)tigerluver Wrote: I think it had to do with the Toba eruption, since the catastrophe used to wipe out many species of the vegetation. The 21kya looks like near the end of an older cycle, that's why the vegetation looked so scarce on the landscape, and the 6kya looks like the prosperous moment in the middle of a newer cycle. |
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