Eurasian Pleistocene Extinctions & Ecological "Overkill"

Discussions of mass extinctions nearly always give rise to heated debates as to the mechanism(s) behind the disappearance of so many taxa in a short amount of time, and one of the most active debates still surrounds the extinction of Pleistocene megafauna. All over the world the extinction of large animals appears to be correlated with the movements of our own species, Homo sapiens, into new territories. Disease and climate change have their own parts to play, the "Overkill Hypothesis" is perhaps the most popular explanation for the extinction of Pleistocene megafauna. A new study by Pushkina and Raia, however, suggests that in Eurasia the role of our own species in extinction was diminished.

Utilizing data from published literature and databases, the two authors found that while the territories of humans overlapped that of extinct megafauna, the overlap was not total. Indeed, the megafauna appeared to prefer colder regions than humans and other more common species, and a comparison of paleontological sites (sites where there were no human remains or artifacts) and archaeological sites (sites with human remains and artifacts present) across the range of human habitation showed that large, extinct mammals were not very common in temperate areas occupied by humans. In fact, with the exception of one species (the steppe bison, Bison priscus), the fauna most commonly found in association with humans were extant taxa. Indeed, contrary to what might be intuitively expected, humans seemed to prefer medium-sized prey within their range, perhaps causing a co-evolution of the species that were being hunted.

Still, some representatives of the Eurasian megafauna did overlap in range with humans, and there are some important questions as to how those species interacted with earlier representatives of our own. Species naivety is sometimes invoked to explain the Pleistocene megafauna extinctions, the large animals being unfamiliar with earlier humans and their hunting abilities, an ignorance that led to their demise. Recent ecological studies have revealed that prey species can quickly adapt (or re-adapt) to the presence of predators, though, and it appears that humans had a more important relationship with smaller, extant prey than larger species that ultimately went extinct. The researchers explain the implications of the trend this way;

Intuitively, the relative rarity of remains of extinct species in many archaeological sites could simply reflect a situation in which remains of these heavy animals were seldom brought back to any human site. Yet, the extinct large mammals were equally rare in paleontological sites at similar latitudes. This latter observation indicates that the rarity of extinct species within human ranges (and not just in archaeological sites) was a real ecological phenomenon, and strongly argues against the notion that humans were responsible for their demise. Our data suggest that if a direct avoidance of humans by these species via gathering in northern areas was possible, it was not due to human overexploitation in the south.

But what about the steppe bison? They were a large species that did go extinct, and as with the more recent elimination of bison in North America, humans seemed to be a direct factor in the demise of the steppe bison, as well. The prevalence of steppe bison remains seem to increase at archaeological sites and decrease at paleontological sites, increasing human populations and better hunting technologies causing humans to have a bigger effect on bison populations;

In our study too the extinction of the steppe bison appears to be related to humans. Human subsistence activities in Siberia heavily relied on bison, reindeer, and horse during the Late Paleolithic... The commonness of the extinct bison and surviving ungulate species (reindeer, horse, saiga, and red deer) in archaeological sites strongly indicates their intensive utilization by humans, especially during the Weichselian glaciation or the late Middle and Late Paleolithic. This can be related to increased population density (archaeological sites outnumber paleontological sites for the first time during the early and middle Weichselian) and to improving hunting technologies, such as the appearance of the microblade-producing populations of highly mobile hunters across Siberia after the LGM who concentrated on a single prey species of large or medium size... The microblade technology appeared with the exploitation of smaller less gregarious species probably because the big game species were disappearing... Overall, it appears that humans were generalized, albeit effective, predators and efficiently concentrated on the most abundant prey species, most of which survive today.

Still, for the majority of other extinct species mentioned in the paper ("the
woolly mammoth, Mammuthus primigenius; the woolly rhinoceros, Coelodonta antiquitatis; ... ; the musk ox, Ovibos moschatus; the straight-tusked elephant, Palaeoloxodon antiquus; Merck's rhinoceros, Stephanorhinus kirchbergensis; the narrow-nosed rhinoceros, Stephanorhinus hemitoechus; the aurochs, Bos primigenius; the giant deer or Irish elk, Megaloceros giganteus; the Pleistocene ass, Equus hydruntinus; cave hyena, Crocuta crocuta; cave lion, Panthera leo; and cave bear, Ursus spelaeus") humans had a less substantial impact. While humans may have had a more important role in the extinction of megafauna in other parts of the world during the Pleistocene, in Eurasia it seems that something else was at work. The authors of the part posit climate change, but very little about climate change and how it correlates to the data is covered.

The findings of the authors are consistent with the idea that the overkill hypothesis is not a very good fit for the extinction of megafauna in Pleistocene Eurasia, but what exactly did happen still requires resolution. Indeed, I would imagine that the extinctions occurring in the Pleistocene were triggered by varying mechanisms depending on the region, and selecting hunting, disease, climate change, etc. as the answer for global megafaunal extinctions is taking an approach that is far-too narrow to do justice to understand what happened.

[Hat-tip to Greg Laden.]

PUSHKINA, D. (2008). Human influence on distribution and extinctions of the late Pleistocene Eurasian megafauna. Journal of Human Evolution DOI: 10.1016/j.jhevol.2007.09.024