Saturday, 15 August 2015

Still no consensus on cause of Late Quaternary mass extinction

Studies reach opposite conclusions on humans vs climate change debate

The cause of the mass extinction of megafauna (land-living species with an adult mass of over 45 kg (100 lb)) during the last Ice Age has been debated since the late eighteenth century. Two main theories have predominated for much of that time: human causation and climate change. Two new studies, published in the journals Science and Ecography respectively, suggest that the debate is set to continue.

In the first study, published in Science, an Australian team compared ancient DNA and radiocarbon data from 31 detailed time series of regional megafaunal extinctions and replacements over the past 56,000 years with standard and new combined records of Northern Hemisphere climate in the Late Pleistocene. The researchers used ancient DNA to identify particular species, claiming that this is more reliable than traditional means of identifying fossil remains at species level. It was found that extinctions peaked during abrupt warm climatic episodes known as Dansgaard-Oeschger events, and the researchers claimed that it was these that had been primarily responsible for the extinctions, with human impact as no more than an exacerbating factor.

The second study, conducted at the University of Exeter and published in Ecography, reached the exact opposite conclusion. In this study, the researchers searched for all published records of dated remains or extinction estimates for terrestrial animal genera potentially present in the past 80,000 years with an adult mass of at least 40 kg (88 lb). In this case, there was no attempt at species-level resolution. By this means, last appearance dates were obtained for megafauna in 14 regions across the Americas, Eurasia, Africa, Australia, Tasmania, New Zealand, New Guinea and Madagascar. These dates were compared with dates of human arrival and episodes of climate change. It was concluded that the extinctions were primarily linked to the former and that the latter was only a secondary factor.

While these studies, with their very different methodologies, make a valuable contribution to our understanding of the Late Quaternary mass extinction, they do illustrate the point that it is very hard to make a convincing case for either climate change or humans being solely responsible on a worldwide basis. Overall, it seems likely that different factors operated in different places and at different times.

Bartlett, L. et al., Robustness despite uncertainty: regional climate data reveal the dominant role of humans in explaining global extinctions of Late Quaternary megafauna. Ecography (2015).
Cooper, A. et al., Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science 349, 602-606 (2015).

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