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What caused the Pleistocene Mass Extinction?
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Give the attached article a read, "Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover."
Predator prey relationships likely were the main cause of extinction of at least one side of the relationship. Check out the mammoth's range (article attached). It coincides with the cave lion quite well, bar California. Considering the size of the cave lions, mammoth predation probably was in the range of options.
Synergistic roles of climate warming and human occupation in Patagonian megafaunal extinctions during the Last Deglaciation
Abstract: The mechanisms of Late Pleistocene megafauna extinctions remain fiercely contested, with human impact or climate change cited as principal drivers. We compared ancient DNA and radiocarbon data from 31 detailed time series of regional megafaunal extinctions and replacements over the past 56,000 years with standard and new combined records of Northern Hemisphere climate in the Late Pleistocene. Unexpectedly, rapid climate changes associated with interstadial warming events are strongly associated with the regional replacement or extinction of major genetic clades or species of megafauna. The presence of many cryptic biotic transitions before the Pleistocene/Holocene boundary revealed by ancient DNA confirms the importance of climate change in megafaunal population extinctions and suggests that metapopulation structures necessary to survive such repeated and rapid climatic shifts were susceptible to human impacts. 
I have spent some time pondering why the black bear and the grizzly survived in North America when the larger short-faced bears perished. i'm thinking that the surviving species might have had a wider range of food options. Were the short-faced bears diggers and fishers? Also, and perhaps the surviving bears' greatest advantage is the ability to sleep during long months of hard times.
I also believe that the grizzly's size adaptability is a great survival trait. Those living in harsh terrain are small in size, thus in need of less food. Few large mammals of a singular species can range in size from -300 pounds to 1500+ pounds. The puma ( cougar ) was perhaps able to sustain on rabbits and rodents while the big cats had a much more difficult time. I believe that the culprit of the extinction was weather related; affecting the vegetation which in turn affects the animal kingdom. 
I read somewhere entire northern hemisphere had plenty of herbivores in Pleistocene. Thus carnivores too must have flourished. But that cycle or rhythm must have shifted to southern hemisphere. That may be the reason why southern hemisphere with some exceptions has plenty of floral and faunal diversity. Plants must have died first due to change of particular cycle or rhythm followed by mass extinction of animals. Or perhaps it may be due to change in earth's magnetic field. No one knows the true reason. Sorry if I spoiled the discussion.
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In the case of A. simus, I don't think it was humans as hunting it even with a spear would not be easy and there was no good reason to do so. Hunting the prey of A. simus should not have been a major problem as well, as it was omnivorous. My gut tells me size is one factor, as there was a floral turnover due to the climate change event. If I remember correctly, weren't prehistoric American brown bears a bit smaller than today's? Maybe that's why they survived the energy shortage and A. simus did not. Once the niche of gigantic bear opened up and the Earth recovered, modern bears began putting on size and mass.
Yes, I understand that Arctodus simus was an omnivore. But, what I am implying is that he was also a scavenger and kleptoparasite. I know that he also feasted on plants and perhaps the occasional bee hive ( maybe ). But, it is also possible that he lacked the skills of catching fish and digging for roots or ground burrowing rodents. Did the short-faced bear hibernate? No evidence to support this, but then being from a totally different lineage of bears, there is no reason to assume that his habits were exactly like that of the bears of the genus Ursus.
As for the size of the existing black bears and grizzlies, I have no reason to believe that their size was any different from that of their living descendants depending on their available food resources. Even a big grizzly can dig himself a comfortable den for a long winter's sleep.
Coincidentally, neither the Andean bear nor the giant panda hibernate. These two species are not of the Ursus genus. Neither do the sun bears nor the sloth bears, both of which have questionable ancestry. Now, I do realize that due to their environments, this is not strong evidence that neither Arctodus simus nor Tremarctos floridanus hibernated. But, is does give room for thought.
I am interested in why so many huge herbivores and powerful predators went extinct after the last great Ice Age all over the earth.
https://www.researchgate.net/publication...xtinctions Abstract Starting around 50,000 years ago, most large terrestrial animals went extinct in most continents. These extinctions have been attributed either to climatic changes, impacts of human dispersal across the world or a synergy among both. Most studies regarding these extinctions, however, have focused on particular continents or used low-resolution analyses. We used recent advances in fossil dating and past climatic models in a high-resolution quantitative analysis, comparing the explanatory power of the hypotheses at global scale. The timing of human arrival to each region was the best explanation for the extinctions. Climatic effects, where present, were additive rather than synergistic with human arrival. While climatic variation was a contributory cause that helped explaining the process, anthropogenic impacts were the necessary cause that drove it. Bigger kill than chill: The uneven roles of humans and climate on late Quaternary megafaunal extinctions (PDF Download Available). Available from: https://www.researchgate.net/publication/283686166_Bigger_kill_than_chill_The_uneven_roles_of_humans_and_climate_on_late_Quaternary_megafaunal_extinctions [accessed Mar 28, 2017]. 
Ross Barnett in his book "The Missing Lynx" supports the overkill hypothesis. He outlines that these same megafauna survived climate change previously in inter quaternary periods. And furthermore that the extinctions were selective (Miracinonyx and not acinonyx, stag moose and not moose) and ranged between multiple habitat niches, which cannot be explained by climate change. He also describes how the same species survived much longer in areas that humans had not inhabited (mammoths on Wrangel island for example). He asserts that there is no model which adequately explains worldwide extinctions of slow reproducing species. He contrasts this with multiple examples of humans arriving in an area correlating with megafaunal extinctions, this is true for Australia, Eurasia, North America and South America. He supplements this with the vast amount of evidence of human hunting in the fossil record, disproportionately so.
I used to subscribe to the idea that climate change weakened populations and humans pushed them over the edge, but Barnett makes a very compelling case that it is the fault of migrating humans and it's hard to disagree. The most compelling piece of evidence in my eyes being how different the Holocene is in this case to other periods of ice age warming, and there is one clear added variable which separates them.
A thread refuting the idea that climate change alone is an adequate explanatory factor.
https://twitter.com/DRMegafauna/status/1084896526151942145 Personally I've come to believe that humans are chiefly and overwhelmingly to blame. The evidence is just too strong for any other hypothesis to be true. I think what sells it to me is we have control's in the other inter glacial periods to compare this to, some periods with warming higher than today's holocene by the way. The independent variable being us. (08-17-2020, 06:09 AM)Sully Wrote: A thread refuting the idea that climate change alone is an adequate explanatory factor. I do wonder what causes the distinction between the north and south hemispheres of Eurasia. Extinction wasn’t as brutal near and below the equator of Asia and Africa, at least during the last 10 kya. We know Stegodon disappeared very late and that is linked to vegetation change given how the Asian elephant has more versatile dentition. As such, maybe the northern hemisphere dealt with both the transition from steppe to forest and human pressures. In the south, forests became denser so the changes weren’t as extreme. Maybe forest dwelling species were also more protected from humans as we prefer open habitat. In the case of South America, we also see a very drastic biome and vegetation change. The region went from grasslands to the rainforest rather quickly. All in all, wherever extinctions were commonplace, drastic vegetation shifts coincided.
A new paper has been published advocating for humans as the main cause (available to read in full)
The past and future human impact on mammalian diversity Abstract To understand the current biodiversity crisis, it is crucial to determine how humans have affected biodiversity in the past. However, the extent of human involvement in species extinctions from the Late Pleistocene onward remains contentious. Here, we apply Bayesian models to the fossil record to estimate how mammalian extinction rates have changed over the past 126,000 years, inferring specific times of rate increases. We specifically test the hypothesis of human-caused extinctions by using posterior predictive methods. We find that human population size is able to predict past extinctions with 96% accuracy. Predictors based on past climate, in contrast, perform no better than expected by chance, suggesting that climate had a negligible impact on global mammal extinctions. Based on current trends, we predict for the near future a rate escalation of unprecedented magnitude. Our results provide a comprehensive assessment of the human impact on past and predicted future extinctions of mammals. Here is one of three interesting figures in the paper  *This image is copyright of its original author ig. 1 Different time periods of diversity decline and extinction rate increases between areas and orders. The plots show the declining diversity (black lines, 100 modeled extinction dates for each species) and the magnitude of extinction rate increases relative to the starting rate (red lines, mean values) through time, for all spatial (A to H) and two examples of taxonomic subsets (Ito J) analyzed in this study. Extinction rates were estimated with a Bayesian rate-shift model, inferring the timing, number, and magnitude of shifts in extinction rates from the extinction dates of each subset. We calculated the mean marginal rates (harmonic mean) separately for all shift number models, which were supported by more than 10% posterior probability (table S3). The rate-shift model that was best supported by the data is shown in solid red, while the transparent red lines show the second-best model, if present. All rate estimates are transformed and plotted as the magnitude of extinction rate increase relative to the base value 126 ka ago. Note that the extinction rate axis (right, in red) is plotted in logarithmic space for better visibility. The time axis to the left of the solid vertical black line (0 CE) is plotted in units of ka before present (BP), while the time axis to the right of 0 CE is plotted in years CE in logarithmic space for better visibility of recent rate changes. Vertical columns shaded in green mark the times of first human arrival (if applicable).
Late Quaternary extinctions in the Indian Subcontinent
Abstract Over the last 50,000 years, a majority of megafauna went extinct globally. Only two regions, Africa, and tropical Asia retain megafaunal communities today. Their persistence in Africa has been attributed to co-evolution with hominins, but faunal dynamics are largely understudied in South Asia. Focusing on the Indian Subcontinent and using a novel dataset compiled from the published literature, we report an extremely size biased, but low magnitude extinction during the terminal Pleistocene and early Holocene—approximately 30,000 years after the arrival of Homo sapiens in the region. We document four mammalian extinctions—Palaeoloxodon namadicus, Stegodon namadicus, Hexaprotodon sp., and Equus namadicus; the extirpation of ostriches (Struthio camelus); and a pseudo-extinction of Indian aurochs (Bos namadicus). The per-capita extinction rate is comparable to eastern and southern Africa, but much lower than elsewhere. Given the long history of hominins in the Indian Subcontinent, and similar extinction patterns to those seen in Africa, we provide the first independent test of the co-evolution hypothesis, and further, suggest that robust population networks and climatic refugia may have allowed for the persistence of megafauna in this region. |
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