Between 13,000 and 9,000 years ago, a great many “megafauna” – large mammals – went extinct all over the world. This was the sixth known “mass extinction event” in the planet’s history. There are two main theories as to what caused it:
Humans spread out from Africa and hunted the megafauna to extinction, either directly, or by interrupting their food supply.
They died out from natural climate change (the end of the last Ice Age), to which they could not adapt fast enough.
Note: There was no significant extinction in Africa – where large mammals had a change to evolve alongside humans and gradually adapt to their presence, and which still supports more large mammals than any other continent.
It’s not an either/or question. A recent review by Anthony Barnosky et al. concludes that it was the combination of human pressure and climate change that produced the extinction, with climate being more important in some areas and humans in others.
Barnosky, Anthony D., Paul L. Koch, Robert S. Feranec, Scott L. Wing, Alan B. Shabel. 2004. Assessing the Causes of Late Pleistocene Extinctions on the Continents. Science 306: 70-75.
Of course **Colibri **nailed it. This is a subject about which biologists are doing research and publishing papers. I’m not sure I see the point of a bunch of amateurs debating a subject such as this. Although there isn’t strictly a factual answer to the OPs question, this thread would probably be more suited to GQ.
I’d say that the wikipedia article was written by an amateur, or by a pro doing an extremely poor job of talking down.
It is, however, a very interesting question to many of us amateurs, and I’ll be fascinated to follow the argument when those who actually do know what they are talking about pick up this thread. Even if it’s just to say, “Umm, dunno.”
(i.e., I agree. There’s no solid answer, and people who don’t spend their time pouring over statiscal breakdowns of climate data, paleoecology journals, and so on aren’t going to have much to add, or even good groudns to debate a side)
Colibri did not make any arguments of his own, but merely linked to two sources. Do you believe there are no sources/no scientists which/who disagree? Of course not.
If it turns out that Colibri’s links do reflect a concensus of “I dunno”, yes, I will be interested in that, as it overturns what I thought I knew.
Was the climate change at the time of the extinctions siginifcantly greater than the previous climate changes of the Ice Age fluctuations? I was under the impression that the ice had advanced and retreated many times over a long period (40,000 years is the number in my memory, but take it with a grain of salt, please).
If the megafauna did not die out in previous similar climate changes…and large animals are (very generally) well suited to cool climates due to their favorable surface-area-ratio making it easier to retain heat…and man’s arrival DOES coincide with the extinctions…then is there persuasive evidence that climate change was really responsible? The timing of human presence seems to match the extinctions, which would seem to make climate change a far-fetched coincidence, and we know how capable humans are of driving extinctions.
I had been under the impression that the debate was primarily whether human hunting or human disease introduction were responsible, not whether humans did it but how they did it, in other words.
Are these scientists paleontologists, trying objectively to isolate the cause for extinctions, or are the climatologists looking for grant money?
Okay, that was evil of me. But I do wonder about researcher objectivity at times. If your only tool is a hammer, every problem looks like a nail. If you’re a climatologist…
For one thing, there’s a political movement afoot these days to de-emphasize the damaging effects humans have had on the enivornment and say “it’s all a natural climate cycle”: see Bush, Rove et. al.: Everything is Peachy, Go Back to Sleep: God Did It, Volume I, 2000-2006.
Nope. It may have been painted that way by the popular press, or by some Discover Channel show, but scientists have never ruled out climate change as a cause, or one of the causes.
Did you go to the links given? Here are the very first few words of the first link:
This is a multidisciplinary study, but there are plenty of biologists involved.
It’s always good to question things, but research into this issue long predates the present administration and will keep going after Bush is safely retired in Crawford, TX. If you have some tangible info that these particular scientists have a politcal agenda, that would be interesting to see. It’s certainly not out of the question, and we have seen cultural bias slip into anthroplogical research over the years, but I don’t see it in this case.
Yes, the earth has been locked into its current ice age - interglacial cycles for about the past 2 million years. Roughly speaking, the cycle for the last ~million years has been about a 100,000 year cycle (i.e., interglacial periods occur about once every 100,000 years) although in the previous ~million years before that, the dominant cycle period was more like 40,000 years. [There are cycles involving the earth’s orbit found at both these frequencies that are believed to serve as triggers for the climate change…although it is not understood why it primarily followed one frequency for a while and then switched to predominantly following the other frequency.]
Anyway, the short answer is that if climate change alone is responsible, you are right that there wasn’t anything particularly special as far as I know about the most recent change from glacial to interglacial climate. On the other hand, I wonder if the relative recentness of this change means we have more fossil data on extinctions than we have for the previous transitions. I.e., is it possible that the previous transitions saw similar levels of extinction but that we just don’t have as detailed a record as one goes further back in time, or is it clear that this last transition had a much higher extinction rate than the previous ones?
It’s extremely clear that the most recent extinction was exceptional in its extent.
From Barnosky et al:
.
Note that this is genera, not species. Genera take longer to differentiate than species, so one would expect that many would be several million years old (and thus predate the Pleistocene itself). For nearly two thirds of genera to become extinct over such a short period is truly astonishing.
What did you think you knew?
I wish I could link directly to the Barnosky article, but I believe it requires a subscription to Science. In any case, here’s a quote that sums up part of the conclusions:
Bolding mine.
The conclusion that more than one factor was involved does not translate into a conclusion that “We dunno.” Instead, as is usual in ecology, it turns out that the actual situation is more complex than simple scenarios can account for.
Lets see: horse, wooley mammoths, sabretooth tigers and giant sloths didn’t survive in the new World. I could understand all of these animals having trouble, except for horse-the feral horses in the Western USA seem to survive very well, even given the human population pressures. Horses seem to thrive in varied climates, and can eat a wide variety of forage. I find it hard to accept that humans wiped them out. Or is it possible that soem didease caused a mass die off? The giant sloth is an interesting case-I’ve heard that a few may have survived 9in remote areas of patagonia) into historic times.
Your conclusion seems to be the opposite of what the evidence suggest. If horses can survive in a variety of climates, and if they today can thrive in an area where they were formerly wiped out (the western US), then how could climate change have caused their extinction? One would think that some other factor must have been involved.
This has been proposed, but there is little evidence for it, and it seems unlikely due to the taxonomic diversity of animals involved (which included not only a great variety of mammals but also large birds and reptiles).
Some researchers continue to disagree. One is Paul S. Martin, who originated the Pleistocene Overkill hypothesis more than 20 years ago, and who still thinks that human hunting is the primary explanation. (Although even Martin, I think, concedes that climate change had some effect.) There are others who continue to argue the primacy of climate change
I would not say that Barnosky et al.'s article represents a consensus, exactly, since there are still a fair number of people with different views on either side. However, it is a review article (rather than original research), published in the most prestigious scientific journal in the US. As such it can be assumed to represent a relatively objective synthesis of the existing state of knowledge on the subject.
Given what you said in response to my question, I don’t see how these folks can stick to this notion (although it may hinge on what they mean by “primacy”). I.e., have these folks identified any way in which the climate change into the Holocene was fundamentally more difficult on flora and fauna than the previous glacial – interglacial transitions? My impression is that there was nothing particularly distinctive about it from a climatological point-of-view, although I could be wrong.
As far as I am aware, there was nothing at all exceptional about the most recent transition between glacial and interglacial conditions compared to earlier ones, except for the presence of advanced humans with a sophisticated tool-using culture. The fact that many taxa that had survived earlier episodes died out in the most recent one is is in fact one of the strongest lines of circumstantial evidence implicating humans.
It would be a compelling case to the (mostly) human caused extintion if it can be stablished that a sizable proportion of the extinct animals fit within the group of animals humans hunted; mammoths, some species of large deer and moas fit the bill in that aspect.
My take on this is that humans and animals at the end of the glaciar period were put on a fragile situation, humans had the edge (more than often the silex edge of a spear) to survive under uncertain conditions by preying on the large mammals at hand. The ecologically strained stocks of megafauna just needed a little poke to spiral into extinction.
And the same point could be made about mammoths, which also were surprisingly adaptable. Their remains have been found as far south as Guatemala, which would have been warm and lush even during glacial periods.
People seem to have the misconception that mammoths only thrived in cold climates. Those critters were all over the place. (Mammoths and mastodons were also wandering around Florida, for example.)
Given their apparent adaptability, my money is on hunting by paleo-Indians as the primary driver of mammoth extinction.
