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A long-standing research interest of mine has been the impact of prehistoric people and palaeoclimate on ancient biota (animals, plants, ecosystems). Millennia before the modern biodiversity crisis — a worldwide event being driven by the multiple impacts of anthropogenic global change — a mass extinction of large-bodied fauna occurred.
These end-Quaternary (late Pleistocene and Holocene) extinctions eliminated half of all mammal species heavier than 44 kg (100 lbs) and other large-bodied fauna across most continents (Australia, Eurasia, North and South America) and large islands (West Indies, Madagascar and New Zealand), between 50,000 and 600 years before present. The losses included large mammals (e.g., mammoth, giant wombats), reptiles (e.g. huge monitor lizards about three times bigger than Komodo dragons), and massive flightless birds (e.g. moa, Elephant birds).
The hot debate around this topic has been the relative contribution of people and climate in driving the extinctions. To give you an idea of the state of the play, at least from my ongoing perspective, I’ll cite a couple of abstracts from papers I’ve published on this topic over the last few years. First, one from PNAS, entitled “Explaining the Pleistocene megafaunal extinctions: Models, chronologies, and assumptions“:
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.
You can download the full version of this paper for free. It’s written as a Perspective article, so it should be intelligible to a lay audience. In a later paper, I showed that it’s really tough to use model to distinguish between human and climate impacts, but that it is essentially impossible to cause extinctions of megafauna by overhunting without doing it by blitzkrieg. That is, if it happens by overkill, it axiomatically happens fast. I explain this in a Journal of Biogeography paper entitled: “The uncertain blitzkrieg of Pleistocene megafauna“.
We investigated, using meta-analysis of empirical data and population modelling, plausible scenarios for the cause of late Pleistocene global mammal extinctions. We also considered the rate at which these extinctions may have occurred, providing a test of the so-called ‘blitzkrieg’ hypothesis, which postulates a rapid, anthropogenically driven, extinction event. The empirical foundation for this work was a comprehensive data base of estimated body masses of mammals, comprising 198 extinct and 433 surviving species > 5 kg, which we compiled through an extensive literature search. We used mechanistic population modelling to simulate the role of human hunting efficiency, meat off-take, relative naivety of prey to invading humans, variation in reproductive fitness of prey and deterioration of habitat quality (due to either anthropogenic landscape burning or climate change), and explored the capacity of different modelling scenarios to recover the observed empirical relationship between body mass and extinction proneness. For the best-fitting scenarios, we calculated the rate at which the extinction event would have occurred. All of the modelling was based on sampling randomly from a plausible range of parameters (and their interactions), which affect human and animal population demographics.
Our analyses of the empirical data base revealed that the relationship between body mass and extinction risk relationship increases continuously from small- to large-sized animals, with no clear ‘megafaunal’ threshold. A logistic ancova model incorporating body mass and geography (continent) explains 92% of the variation in the observed extinctions. Population modelling demonstrates that there were many plausible mechanistic scenarios capable of reproducing the empirical body mass-extinction risk relationship, such as specific targeting of large animals by humans, or various combinations of habitat change and opportunistic hunting. Yet, given the current imperfect knowledge base, it is equally impossible to use modelling to isolate definitively any single scenario to explain the observed extinctions. However, one universal prediction, which applied in all scenarios in which the empirical distribution was correctly predicted, was for the extinctions to be rapid following human arrival and for surviving fauna to be suppressed below their pre-’blitzkrieg’ densities. In sum, human colonization in the late Pleistocene almost certainly triggered a ‘blitzkrieg’ of the ‘megafauna’, but the operational details remain elusive.
Sorry if that is bit technical — I’ve bolded the important conclusions.
In the context of the Australian extinctions, the debate over people vs climate has been fierce! One school of thought, centred around a group from University of Sydney, maintains that people played little part in the extinctions. Myself, and a whole bunch of co-authors, respectfully disagreed in Quaternary Science Reviews: “Would the Australian megafauna have become extinct if humans had never colonised the continent?“. I also wrote a popular science version of this technical article, called ‘Megafauna mix-up‘, for Australasian Science magazine. Grab a PDF copy here. There is also a bit of a write up of this kerfuffle on Wikipedia, here.
Perhaps the most iconic of the extinct megafauna is the woolly mammoth. Did climate change knock this on the head? A recent paper by a group of Spanish researchers (one of whom, Miguel Araújo, is a close collaborator of mine in a different area of work [modern climate impacts]), used some neat modelling to show that climate certainly seemed to have played an important role, but humans were still needed.
Take a look at this figure from their paper: it’s a climate envelope model of habitat suitability in Eurasia for woolly mammoth (Mammuthus primigenius) at six times over the last interglacial-glacial-interglacial cycle. Red is the highest suitability, green the lowest. Full glacial conditions occurred at 21,000 years before present (kyr BP), warm conditions (as warm or warmer than today) at 126 and 6 kyr BP. The black lines indicate likely northern limit of people.
The model indicates that mammoths survived multiple Pleistocene climatic shifts by condensing their geographic range to suitable climate space during climatically unfavourable times. Finally, however, the new presence of modern humans during the late-Pleistocene and Holocene, at the same time a climatically-triggered retraction of steppe-tundra reduced maximally suitable habitat by some 90%, resulted in extinction. The important message is that mammoth populations’ resilience was weakened by habitat loss and fragmentation, as it may well have been in previous interglacials, but during that last range reduction the mammoths were unable to cope because of the addition of predatory pressure (and possibly other landscape modifications) by human hunters.
So if one insists on a minimalistic answer for what caused the late Quaternary extinctions of megafauna, it seems to be this: the actions of colonising and expanding prehistoric human populations (primarily hunting and habitat modification) seems omnipresent in the past global extinction, but in many cases, species were left much more vulnerable because of climate-induced range contractions and changes in habitat quality. Climate change was the ‘straw that broke the camel’s back’ — much as it will be today (or perhaps a haybale), in an era of already massive global change.
Barry,
One interesting feature is that a lot of the mega fauna were not killed in Africa, and not all were killed in N America 11,000 years ago, for example moose and buffalo.
On the other hand,buffalo were almost driven to extinction in 1800′s when repeating rifles became available, and elephants are being driven to extinction now in Africa. Presumably Clovis points and similar weapons were not adequate to wipe out Elephants in Africa, or Buffalo in N America but could wipe out Woolly Mammoths. Perhaps a better reason is that “buffalo Jumps” such as that in Alberta are very effective at killing large animals, but that mammoths breed too slowly to recover. Why did African mega fauna survive?, larger range? other predators keeping human population in check?
In Australia, aboriginals didn’t wipe out crocodiles, but again European hunters were getting close to exterminating them before they were protected. I would have imagined aboriginals could have wiped out turtles, they seem very sensitive, easy prey when laying eggs, why not?
Neil,
Actually, the giant plains bison, one-and-a-half times as big as the woods bison still found in North America, was wiped out. As were horses, camels, giant sloths, giant beavers, and others. Also, every time I read about another Mastodon find in this area(upstate New York), the remains have spear points in them. In the case of the bison, it seems that the woods bison moved out to the great plains after their predecessors became extinct.
Differential survival is discussed in my Journal of Biogeography paper. Some species with particular behavioural attributes tended to survive better, e.g. forest, arboreal, water-dwelling or nocturnal species had a higher than expected persistence rate whereas savanna and diurnal species did worse. It was also down to intrinsic reproductive rates, as Chris Johnson showed nicely here: http://rspb.royalsocietypublishing.org/content/269/1506/2221.abstract
Africa is a puzzle — there were extinctions there too, but fewer. I posited here that it was due to differences in chronic vs immediate mortality, which relates to an old idea of Paul Martin that megafauna on new continents were ‘naive’ to human hunters, whereas in Africa, they co-evolved and so were ‘predator savvy’ in the face of those clever apes.
Barry,
Maybe it wasn’t hunting but the use of fire. Much of the Canadian short grass prairie is only maintained by human fire activity, it may be the same for the tall grass in US and also Africa?. In Africa , human use of fire would have been gradual, increasing natural fire events, but in N America, S America and Australia , possibly fire would have rapidly modified the environment, allowing buffalo to prosper, but destroying favorable environments for some of the other megafauna. Hunting would have killed of the smaller isolated populations.
Africa also a much larger area of continuous savanna, due to the land mass being in savanna/woodland latitudes.
I think in Australia their is evidence of a very sudden increase in fire adapted species pollen, and charcoal
Thinking out loud while looking at a copy of “Man the Hunted” (Donna Hart and Robert Sussman) … 3 million years ago, there were 10 large carnivores sharing the plains in Africa, 1 went extinct about 2.7 mya and another 5 disappeared about 1.5 mya (I don’t know why).
Still homo in Africa started life as fast food for about 9 large carnivores and would have spent more time avoiding real predators than being one. Life would have been easier when the 5 disappeared.
Was life out of Africa perhaps a little more conducive to becoming a partial agent of extinction?
There is quite compelling evidence for the so-called Clovis comet impacting North America 12,900 years ago. This event ended Clovis culture and appears implicated in the sudden disappearance of all those species of large mammals, other smaller species as well.
Coincidently or casually, Younger Dryas (YD) started right then as best as dating back to that deep time allows. YD has climate impacts in North America, although not as severe as in northern Europe.
On RealClimate there is a thread regarding the possiblity of a casual connection between the Clovis comet and YD initiation.
See also: http://tamino.wordpress.com/2009/01/03/younger-dryas-impact-event/
> Plains bison, woods bison
I’d appreciate a pointer to something recent on that.
I learned it as the woods bison went extinct, but that was in the 1950s, based on measurements of skulls and bones and skins, done long before anyone tested DNA; is there a better report now?
The oldest article here matches my recollection:
http://scholar.google.com/scholar?q=“plains+bison”+”woods+bison” — that also finds more recent articles mentioning controversy about hybridization. All the articles are paywalled.
Current bison are Bison bison, extinct bigger cousins were Bison antiquus.
http://en.wikipedia.org/wiki/Bison
Hi Barry
Thanks for the interesting post – but I disagree with the conclusions of that recent paper modeling woolly mammoth extinction.
The problem with that study is that before the late-Pleistocene extinctions there were elephants and elephant relatives (including mammoths, mastodons, gomphotheres and others) in all environments and climate zones in Europe/Asia and the Americas. This means that a climate change that reduced the range of a cold-adapted elephant like the woolly mammoth, should have increased the ranges of counterparts of the woolly mammoth from temperate or subtropical environments. In the Americas for example, the climate-driven contraction of woolly mammoth habitat coincided with climate-driven expansion of mastodon and gomphothere habitat. The fact that all these animals went extinct at about the same time is pretty convincing evidence that climate had nothing to do with it, neither as a main cause or through interaction with human impact. The extinctions dd coincide closely with the arrival of people.
In the Australian case, there is surprisingly little evidence of an increase in fire when Aborigines arrived, so it’s very hard to make the case that environmental change due to increased human-caused fire caused the extinction of our megafauna.
The question that interests me about the Australian vegetation and the megafauna extinction is: did Aboriginal burning lead to a change in vegetation that caused the extinction of the megafauna, or did it happen the other way around, ie did the extinction of the megafauna lead to vegetation change? There seems to be a fair bit of work around indicating that loss of large fauna can lead to vegetation change (eg the Lago Guri project, numerous papers on the flow-on effects of hunting, trophic cascades, etc), so I don’t think the possibility can be ruled out.
But I agree with Chris Johnson about the influence of climate – it’s hard to se that there really was one. In Australia, nothing particularly dramatic was happening climatically at the time the megafauna went extinct, and animal and bird extinctions on oceanic islands seem to correlate with human arrival, not with climate events. I suspect that if the North American megafauna didn’t dominate the discussion, there wouldn’t be such a large focus on the potential involvement of climate.
A botanical corollary to this concerns Forestry Tasmania’s assurances that iconic tree species such as swamp gum Eucalyptus regnans will regenerate after clear felling. Since these trees mature at 200-400 years of age current stands are not that many generations from the last cold period. I suspect that seedlings will not adapt to warmer conditions so future human generations may not see giant live specimens. Moreover I believe clear felling will amplify feedback effects with reduced transpiration and shade leading to longer dry periods.
Dear CP,
“did Aboriginal burning lead to a change in vegetation that caused the extinction of the megafauna, or did it happen the other way around, ie did the extinction of the megafauna lead to vegetation change?” is a very good question.
I think there is good evidence for the latter: Aboriginals hunted megafauna to extinction, and the vegetation changed as a result of the sudden removal of all the largest herbivores. There is little evidence that increased fire was involved in this, either as cause or effect (at least for the late Pleistocene).
I have a review paper on this topic coming out soon in Proceedings of the Royal Society B: it should be freely available online tomorrow.
Excellent! I look forward to reading it.
[…] Barry Brook put an intriguing blog post on Did climate change kill off woolly mammoths and giant wombats …Here’s a quick excerptThe problem with that study is that before the late-Pleistocene extinctions there were elephants and elephant relatives (including mammoths, mastodons, gomphotheres and others) in all environments and climate zones in Europe/Asia and … […]