This just struck me while watching a wildlife documentary this evening: why, compared with other continents, does Africa have such a diversity of large mammals? It has a huge breadth of both carnivores {lions, leopards, cheetahs, hyenas} and herbivores {from big 'uns like elephants and giraffes to medium sized 'uns like the vast array of antelope and the like}.
No other continent can match it, as far as I know: North America has bears, cougars, wolves, bison and deer, but nothing like the sheer diversity that Africa exhibits, and no really large herbivores like rhinos or hippos; India and Asia have elephants and tigers, which match the size of African species, but again lack the breadth. Australia has even less, and much smaller species: kangaroos, wombats, and dingoes.
So why the difference? Discounting European arrivals with improved killing technology and more environmentally demanding farming techniques - in other words, back-tracking 500 years or so - and granted that some species may have been hunted to extinction or suffered from habitat loss when humans came on the scene, North America and Australia were still settled relatively late: Australia about 40,000 years ago, America about 15,000 years ago, if I’m recalling correctly.
So given that Africa has been populated from much earlier - indeed, from the outset of modern humanity - shouldn’t there be less species there, not more?
The theory I’ve heard is that you are thinking about it the wrong way around.
When two species (predator and potential prey) evolve together, they fight an evolutionary arms race that tends to balance. You evolve a slightly better offensive mechanism, I evolve a slightly better defensive mechanism and so on.
Humans and big mammals “grew up together” in Africa.
However, where one predator which has evolved really good offensive mechanism moves to an entirely new area, it may come across prey that are a total mismatch.
Hence the big mammals in areas where humans moved late got hammered.
The SDMB’s favorite nonfiction book, Guns, Germs, and Steel touches on this.
As they co-evolved with the ancestors of modern humans (and associated relations), the successful large mammals in Africa adapted behavior that included a healthy fear of the brainy bidepal apes with the sharp sticks and projectiles.
Most of the large mammals in North America did not have time to develop this behavior before fully modern humans arrived on the scene 12-14,000 years ago and rapidly hunted them to extinction.
[grammarNazi] The word you’re looking for here is fewer. [/grammarNazi]
I’ve noticed looking at a globe that a lot of large and exotic wildlife runs along the equator. I think Africas largest wildlife reserves are along the equator as well as South American jungles.
It’s doubtful and highly disputed that humans “hunted them to extinction”. After all, humans have been around in asia for millions of years too, and as the OP pointed out- few large mammals. Next, N. America had billions of Bison, and the natives had hardly put a dent in the population. Youd think that if humans had hunted the giant sloth and the mastodon to extinction, they’d have gone for the easier to kill and more useful bison first.
Next- hunting rarely drives any species to extinction (hunting drives species to *near * extinction, instead). Go ahead- name a few large, successful mammalian species that humans have driven to extinction by hunting in the last few hundred years- after modern firearms made killing so much easier.
Here’s an abstract of an article about Australian megafauna extinction: http://www.blackwell-synergy.com/doi/abs/10.1046/j.1466-822X.1998.00285.x?journalCode=geb
"The worldwide extinction of large terrestrial mammals in the late Pleistocene has been attributed to climate change, overkill by human populations colonizing new continents, or some combination of these two processes. Lack of precision in chronologies for human colonization and mega- faunal extinction suggests that the role of overkill will be difficult or impossible to resolve from the archaeological and fossil record. In this study, we used a simple two species predator-prey model to examine the population density and hunting efficiency of a colonizing Aboriginal population necessary for overkill to have led to extinction (equilibrium density=0), of a range of different sized megafauna (250–1000 kg bodyweight) inhabiting an hypothetical tract of north Australian Eucalyptus savanna. We referred our modelling to this region because estimates of contemporary Aboriginal population density arguably set an upper limit for those occurring during the Pleistocene, and data are available on harvesting rates of large introduced mammals such as buffalo. Modelling results indicated a compensatory trade-off between the effect of Aboriginal population density and hunting efficiency (measured as the rate of effective search for prey) on equilibrium megafauna densities. The modelling also demonstrated that although intrinsic rate of population increase and density at carrying-capacity for mammals increase with declining body size, smaller megafauna would have been more readily exterminated than larger megafauna because they would have been harvested at a higher rate to satisfy the demand for meat. However, while at contemporary Aboriginal densities it appears plausible that over-harvesting could have affected the fate of smaller megafauna (<250 kg), hunting efficiencies required to drive medium (500 kg) and larger (1000 kg) sized megafauna to extinction appear unrealistically high. This conclusion would be reinforced if Aborigines were able to switch prey at low megafauna density, a contingency not considered in our model. Collectively, these results suggest that for overkill to have driven medium to large marsupial megafauna extinct in the northern Eucalyptus savannas, Aboriginal densities must have been considerably higher than contemporary levels, or Aboriginal hunters astonishingly efficient. Alternatively, climate change in the late Pleistocene may have played an important role in the extinction of Australian megafauna."
Here’s more:http://www.pnas.org/cgi/content/abstract/99/23/14624
"*Understanding of the Pleistocene megafaunal extinctions has been advanced recently by the application of simulation models and new developments in geochronological dating. Together these have been used to posit a rapid demise of megafauna due to over-hunting by invading humans. However, we demonstrate that the results of these extinction models are highly sensitive to implicit assumptions concerning the degree of prey naivety to human hunters. In addition, we show that in Greater Australia, where the extinctions occurred well before the end of the last Ice Age (unlike the North American situation), estimates of the duration of coexistence between humans and megafauna remain imprecise. Contrary to recent claims, the existing data do not prove the “blitzkrieg” model of overkill. * "
In other words, actual scientific results show that the “overkill hypothesis” is wrong.
http://www.sciencemag.org/cgi/content/abstract/306/5693/70
*One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved. *
Three things led to the megafauna extinction: Climate change (the same climate change that allowed humans to spread), new species (of which humans were one, sure, but many other species came along- including our friend the rat), and new diseases spread by those new species (which I guess you can consider another form of “new species”).
This article puts the nail in the myth of the “Overkill position”
"The arguements that human hunters were responsible for the extintion of a wide variety of large Pleistocene mammals emerged in Western Europe during the 1860’s… Today the overkill position is rejected for wetsern Europe but lives on in Austrailia and North America. The survival of this hypothesis is due almost entirely to Paul Martin… In North America, archaeologists and paleotologist whose work focuses on the late Pleistocene routinely reject Martin’s postiion for two prime reasons: there is virtually no evidence that supports it, and there is a remarkably broad set of evidence that strongly suggests that it is wrong. In response, Martin asserts that the overkil model predicts a lack of supporting evidence, thus turning the absences of empirical support into support for his beliefs. We suggest that this feature of the overkill position removes the hypothesis from the realm of science and places it squarely in the realm of faith. One may or may not believe in the overkill position, but one should not confuse it with a scientific hypothesis about the nature of the North American past."
In other words, you can believe in Overkill/Blitzkrieg just like you can believe in God or the Invisible Pink Unicorn. There is no scientific evidence for any of them.
While I do agree that Aferica has more diversity, you are omitting some whole catagories: Elk (Wapiti, moose (they are related, right?). Wild goats (pronghorn “antelope” & Mountain goats.) Wild pigs (Javalina in the southwest, maybe others elsewhere), Wild sheep (Dahl)
Of the classes you mention, there is more diversity than you credit: There are at least 3 VERY different bear species: Grizzly, Polar and black/brown (The last are pretty closely related). Deer are similarly diverse.: Carabu, Mule deer, whitetales, blacktales. Beyond wolves, there are foxes and coyotes.
I’m sure I’ve missed some, but I have intentionally omitted Ibex and Oryx, as these are exotics imported from africa.
Humans may have been in Asia for millions of years, but Homo sapiens hadn’t. H. erectus had much cruder tools and a less sophisticed brain.
We did this topic in this thread in GD, and the conclusion is that most experts in the field attribute human hunting to the bulk of megafauna extinctions in the time period discussed, although climate change may have been a minor factor.
The abstract you quote states that “Contrary to recent claims, the existing data do not prove the “blitzkrieg” model of overkill.”
Lack of evidence does not mean the hypothesis is wrong. It simply means that the evidence to date is lacking. In addition, the abstract you quote preceding your statement does not present any “actual scientific results,” either. They are merely questioning the assumptions implicit in some of the models used in other studies, and questioning some of the dating.
In other words, they are questioning the conclusions reached by other researchers. This is fine, and is how science works. It does not mean that the hypothesis they are questioning is necessarily wrong, and you cannot properly draw that conclusion from this.
Baloney. Scientists around the world did not come to the conclusion that human pressure was a factor (possibly the major factor) in megafauna extinctions by blind faith.
And based on the cites you have presented, you have no valid case to state unequivocally that there is no scientific evidence for the overkill hypothesis.
What your abstracts are generally stating is that climate change, in addition to human pressure, may have been a factor in megafauna extinctions. This is not the same thing as stating that human pressure was not a factor at all.
I think you are inferring far more from your cites than is there.
I am certainly willing to back off from my statement that human pressure was the sole factor in megafauna extinctions, though.
Nitpick: Black and brown are different species, but grizzly is a subspecies of brown. And for anyone keeping track, Kodiak bears are a regional variation of the grizzly subspecies.
Umm, John? Go back and read the thread. There are just a few experts cited.
We have Wiki, which says this “In broad usage, the Holocene extinction event includes the notable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both.” But that’s Wiki.
Colibri quotes one of the same cites I do, and another very good cite, and concludes:“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.”
Someone else cites Martin, who as pointed out- write a very nice hypothesis, but one without a shred of evidence.
Thus, the conclusion was not “the conclusion is that most experts in the field attribute human hunting to the bulk of megafauna extinctions in the time period discussed, although climate change may have been a minor factor.” In fact, Colibris’ cites (the only decent ones) support what I have said here.
I must have watched the same documentary. I wondered the same thing, and considered posting the same question. My addition to the thread is this question:
Why didn’t North America develop creatures like the hippo or rhino? Environmental differences (temperature, etc.)?
Dr. Deth: Read how **Colibri **summarized the current thinking. You claim his cites aren’t good, but that’s BS. Frankly, you have consistently posted info in GD regarding human evolution that is 20 to 30 years out of date. Please take this as constructive crticism and acquaint yourself with the current research.
BTW, **Colibri **is an expert in the field, and is one of the few posters here who can say “my post is my cite” and be taken seriously.
If a Hypothesis had no valid data to back it up, it is “not valid”. I suppose “wrong” may be too strong a word, I concede. But it (the Overkill Hypothesis) has *no valid data * to verify it.
Sure, I have plenty of cites that show that the “Overkill Hypothesis” has no data. The “Overkill Hypothesis” does not state that human pressure was one of several factors- it states “that human hunters were responsible”. Period. And, that has been shown to be without any valid data, and several experiments/models have shown it is very likely wrong- that climatological changes were also very important.
I stated very clearly “Three things led to the megafauna extinction: Climate change (the same climate change that allowed humans to spread), new species (of which humans were one, sure, but many other species came along- including our friend the rat), and new diseases spread by those new species…”, thus I agreed that humans were a definate factor. Just not THE only factor.
And, it’s not “scientists”- it’s one dude. Martin. He wrote a very exciting argument, one that many laymen found convincing. Unfortunately, real scientists need data, not just arguments. I find Martins hypothesis no better than the “Aquatic Ape” Hypothesis. Both sound good, neither have any real data to back them up. (I enjoyed the AA hypothesis myself, it’s just that it’s without any data. )
John? I said this about Colibri’s cites “quotes one of the same cites I do, and another very good cite”. Hardly saying his cites aren’t good. In fact- just the opposite.
Next, this OP isn’t about human evolution, and your statement is an *ad hominem * argument. However, I cited 4 new up to date cites. Can you refute those cites instead of giving us an ad hominem argument?
I agree Colibri is an excellent source, in fact Colibri & I agree here to a very large extent. In fact, he debunks Martin also "One scenario, however, that has not been confirmed is Paul Martin’s extreme “blitzkrieg” hypothesis for the extinction event in the Americas due to the rapid expansion of Clovis hunters. It’s now pretty well established that the human presence in the Americas predates Clovis, and that humans co-existed with the megafauna for longer than had initially been supposed. The process may have been “sitzkrieg” (attrition) rather than blitzkrieg."
Colibri in that thread posted two slightly different statements:
“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.” and "I agree with you on that. I should perhaps have been more explicit in my first few posts that the key factor was humans, with climate being involved mainly in determining some of the regional details."
BOTH of which agree with my cites and statement that hunting by humans was NOT the only factor, and that climatological change was a factor.
My cites say that climatological change was a factor perhaps as strong as human hunting, if not more so. That’s exactly what I say. Thus **Colibri ** and my cites disagree only on the matter of degree. However, I’d like Colibri to come here and review my cites, and see what he says. I think those cites present some interesting new information on this subject.
However, note that my opinion isn’t really important. What is important is my cites. You can refute my opinion with ad hominem arguments all you want, however, I’d like this to stay in GD, and have you refute my cites.
The extinct Irish Elk had antlers that were as much as 10’ wide. Were these antlers simply too big and heavy, or did prehistoric humans kill them off? Man, these elks are ONE species I’d like to have clones made of!
It ceratainly falls in that category as well. But note your satement, in this thread, about humans being in Asia for millions of years. That is both inaccurate and misleading (misleading, if it had been true).
I wasn’t making an argument, I was offering advice.
The difference between those two scenarios is irrlelavent wrt the question: was it climate or humans as the major factor?
Let’s see, just off the top of my head megafauna included: Toxodons (rhino-hippo like deals from South America), camels, Glyptodons, armadillos, ground sloths, mammoths, mastodons, musk oxen, saiga, steppe bison, lions, smilodons, dire wolves, short-faced bears, brown bears, black bears, polar bears, Teleoceras (a hippo-like rhino), horses, pronghorns, Miracynonyx (the “american cheetah”), giant capybaras, giant badgers, moose, elk (wapiti), moose-elk (Cervalces), deer, caribou, giant beavers, peccaries, mountain goat, sheep, jaguars, cougars, lynx, giant skunks…
Uh, John, I’m not sure where you get this. DrDeth is claiming that Colibri’s cites in that thread were the only good ones, i.e., that everyone else’s cites weren’t good. Nowhere do I see DrDeth criticizing Colibri’s expertise or his cites.