Decline of the Mammals

There has been much debate about the cause of mass extinctions at the last major global climate upheaval: the Holocene glacial to interglacial transition (Lister and Stuart, 2008). Vegetational belts and mammalian communities underwent major reorganisation, with many large mammals becoming extinct, and causality arguments generally revolve around a joint cause of human and climate. Evidence of these extinctions can be found from palaeontology, climatology, archaeology and ecology, using radiocarbon dating to temporally place each piece of evidence (Barnosky et al. 2004). Barnosky et al. (2004) detail how there is strong evidence for human induced population shifts and extinctions, but that climate change also plays a significant if not dominant role in all or some of the scenarios.

From: http://trueunknown.files.wordpress.com/2012/03/mammoths_wooly.jpg

The woolly mammoth (Mammuthus primigenius) is one of the better studied large mammals as an example of mass extinction. Sher (1997) proposed a ‘retreat to the north’ in Eurasia, where distribution of mammoths steadily contracted, until mammoth were restricted to Northern Siberia around 12 ka BP before finally becoming extinct. Stuart et al. (2002) researched further into this, and found this model to be fitting but to overlook the complexity of the pattern of extinction. They suggest, for example, absence and then re-colonization of the Russian Plain and even re-entry to Europe as the Fennoscandian ice sheet retreated (Stuart et al. 2002).

Lima-Ribeiro et al. (2013) studied two Proboscidean (mammoth) species in which they found a large contraction in the geographic range size: Cuvieronius hyodon and Notiomastodon platensis. Both were narrowly distributed on scattered patches habitat refugia around 11 ka, the period in which the earliest humans probably arrived in South America. Under the unsuitable climatic condition at this time, both Proboscideans would be extinct after about 550 years of human hunting, but if climatic conditions were suitable, like in Last Glacial Maximum (LGM), the time-for-extinction would be at least 3 times longer given the same human pressures (Lima-Ribeiro, 2013).

Cortell (2012), in his review of “Driven to Extinction: The Impact of Climate Change on Biodiversity” by Richard Pearson, makes the point that there is a huge amount of uncertainty when discussing climate change.  There is uncertainty in the impacts and extent of anthropogenic climate change, in the future trends in greenhouse gases, in the models we use to predict climate from these trends, and then uncertainties in the impacts of these changes on populations, species, and ecosystems, but that uncertainty should not become the key message (Cortell, 2012). The concluding paragraph of each paper encountered holds the same message: there is a need for more data before can we adequately attempt to answer the question of the cause or causes of extinction: climatic/environmental change or ‘overkill’ by human hunters (Stuart et al. 2002). This is despite many recent studies which have provided significant new finds and radiocarbon dating evidence, as well as other proxy indicators (Stuart et al. 2002), which will help to reduce uncertainty. These debates are ongoing, and it with more reading it becomes clear that the debate is not over choosing human or climate induced shifts, but in the amount of input that each of the factors had.

Barnosky, A., D., P. L. Koch, R. S. Feranec, S., L., Wing and A. B. Shabel (2004) “Assessing the causes of Late Pleistocene Extinctions on the Continents” Science, 306, 70, 70-75

Lima-Ribeiro, M., S., D. Nogués-Bravo, L. C. Terribile , P. Batra and J. A. F.  Diniz-Filho  (2013) “Climate and humans set the place and time of Proboscidean extinction in late Quaternary of South America” Palaeogeography, Palaeoclimatology, Palaeoecology, 392, 546–556

Lister, A., M., and A. J. Stuart (2008) “External Geophysics, Climate and Environment The impact of climate change on large mammal distribution and extinction: Evidence from the last glacial/interglacial transition” C. R. Geoscience 340, 615–620

Richard T. Corlett, (2012) "Climate-driven extinction, now and in the near future", Trends in Ecology and Evolution, 27, 8

Sher, A.V., 1997. "Late-Quaternary extinction of large mammals in northern Eurasia: a new look at the Siberian contribution". In: Huntley, B., Cramer, W., Morgan, A.V., Prentice, H.C., Allen, J.R.M. (Eds.), Past and Future Rapid Environmental Changes: the Spatial and Evolutionary Responses of Terrestrial Biota. Springer-Verlag, Berlin, Heidelberg, New York, pp. 319–339.

Stuart, A., J., L. D. Sulerzhitsky, L. A. Orlav, Y. V. Kuzmin and A. M. Lister, (2002) “The latest woolly mammoths (Mammuthus primigenius Blumenbach) in Europe and Asia: a review of the current evidence” Quaternary Science Reviews, 21, 1559–1569