The late Pleistocene extinction of megafauna in North America was a unique event as previously, species that became extinct were small mammals. In the Pleistocene interglacial, extinction was almost entirely confined to large mammals (>1,000 kg) and medium-large ones (100 – 1,000kg) (Stuart 1991). North America lost 35 out of 45 megafauna genera during the Pleistocene (Grayson 2007: See Table).
|Grayson (2007): Table of Extinct Megafauna of the Late Pleistocene|
|National Geographic (2011): CT scan showing cross-section of the broken tip of a spear embedded in a mastodon's rib (dated 13,800 years ago)|
Grayson and Meltzer (2002) argue against this generally accepted view of human predation driving megafauna extinction. They show that only 15 of the 35 genera that were supposedly killed by Clovis hunters actually survived to 11,000 radiocarbon years ago, which is when they would have encountered Clovis hunters. Besides, megafauna extinction in North America is by no means a unique phenomenon. The entire Northern Hemisphere experienced substantial species extinctions around 10,000 – 11,000 radiocarbon years ago. In Eurasia for example, many megafauna like reindeer, giant deer and mammoth also disappeared around the same time as those in North America did. Woodman et al (1997) support the view that humans cannot be blamed for giant deer extinction in Ireland as there were no people there during that time.
The reasons for why climate change has been rejected as a cause of megafauna extinction are encapsulated in Fiedel’s (2009:30) question: “if Holocene warming was so disastrous for megafauna, why wasn't there a wave of extinction around 125 ka in the last interglacial?” There are 2 arguments proponents of the climate change explanation use. The first is that the last interglacial was much warmer than previous interglacials and had fewer seasonal temperature variations, leading to mosaic vegetation types that supported a richer range of animal species (Graham and Lundelius 1984). When the open grasslands of the late Pleistocene gave way to thicker forests and vegetation which showed strong zoning according to climate during the Holocene, animal extinctions occurred as the less diverse vegetation could no longer support the previous level of biodiversity (Stuart 1991). Graham (1985) has also argued that the Pleistocene – Holocene transition was unique from previous transitions. Not only was Holocene climate and vegetation was very different from that of previous interglacial periods, but also unusually, the last interglacial was warmer, rather than cooler, than the Holocene in both North America and Europe.
The second is that the late Pleistocene was biotically unique. Scott (2010) argues that this assumes a static view of megafauna composition and hence, similar adaptations to climate change. He uses the example of bison becoming more widespread in North America. Guthrie (1980) proposed that since bison survived while other species perished during this period, this may suggest that competition from bison, along with failure to adapt to climate change, drove the megafaunal extinction event. Nevertheless, the key takeway from Scott’s paper is that ‘megafauna’ is not a static concept, and this has implications for evolutionary responses to climate change. This squares with Graham and Lundelius’ (1984) theory of ‘coevolutionary disequilibrium’ – species evolve individualistically and uniquely to climate change in ways that may become detrimental to other species.
Additionally, during 10,000 years of the Holocene very few land mammals became extinct, although human populations certainly increased dramatically (Stuart 1991).
Nevertheless, the extinctions in North America were definitely more sudden and severe than in Europe. The North American case supports the view of a greater, but not dominant, human role in extinction in relation to other regions. The arrival of Clovis hunters (not present in Europe) at a time of more profound climate change was probably the last straw for many species already suffering the stress from a changing environment.
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