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.
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.