INTRODUCTION
The Woolly Mammoth is an example of a typical megafaunal creature, a large mammal weighing over 1000kg. Over the past 100,000 years, many megafaunal species have been removed from Earth in waves of extinction, Eurasia and North America, for instance, suffered losses of between 36% and 72% 500ka (Lorenz et al. 2011).
The causes of these extinctions are a hotly debated topic, and there is much uncertainty, and many problems associated with reaching a conclusion as to the main driver. While it is key to bear in mind that suggestions of the cause of extinctions are all speculative, the two dominant paradigms of this episode of mass extinction are climate and vegetation change, and human hunting and disturbance, disease may also have been instrumental (Wroe et al. 2004, Lorenz et al. 2011, Stuart 2005, Nicholls 2011, Faith 2011).
CLIMATE
There is little doubt that climate has played a large part in species population change during the past 50,000 years. Fossil and DNA evidence suggests that populations would have shrunk as environmental conditions changed, because of losses of genetic variation and adaptive flexibility (Lister and Stuart 2008, Lorenz et al. 2011).
Building on this idea of changing climate causing environmental shifts, Faith (2011) proposes that the extinction of North American megafauna was caused by an ecological mechanism.
In modern ecosystems, the interaction between plant nutrient content, nitrogen cycling and herbivore-plant relationships produces feedback mechanisms. These vary between modes of nutrient acceleration and deceleration, and are influenced by atmospheric CO2 concentration, temperature and precipitation. Lateglacial climate change, Faith suggests, may have caused increases in atmospheric CO2, and is likely to have shifted ecosystem dynamics away from nutrient acceleration and towards a deceleration mode, causing megafaunal populations to shrink.
HUMANS
Fossil remains indicate that human populations expanded across northern Eurasia 40ka, bringing with them a wave of disturbance.
The Blitzkrieg hypothesis offers an explanation of the variation in rates of megafaunal extinction between global landmasses during the late Quaternary, based on five major observations:
· Rates of megafauna extinction were considerably higher in areas outside of Africa where humans and mammals coevolved.
· Remote island species that had no previous exposure to humans were particularly vulnerable.
· Hunter-gatherers preferentially selected large prey.
· Late Quaternary extinctions had a greater impact on large animals than small ones.
· Climatic fluctuations during the late Quaternary were not reflected in extinctions of megafauna (Nogues-Bravo et al. 2008, Lister and Stuart 2008).
Wroe et al. (2004) and Stuart (2005) dispute this line of thinking, noting that the simplicity of the Blitzkrieg model is appealing, but the exact causes of extinction are still unclear. They argue that:
· Human-related extinctions of both large and small taxa, rather than just large, may have occurred on remote islands.
· Translocated species could have contributed to extinctions.
· The role of humans in megafaunal extinction remains unclear. Perhaps modern hunters, with good technology and organisation did contribute to species loss. But equally, cultural variations such as specialised technologies, diet, and hunting behaviours, suggest that not all human societies would have been to blame for the rapid demise of large mammals.
MULTIPLE FACTORS
Many suggestions point to a multifactoral cause of megafaunal extinctions (Lorenz et al. 2011, Wroeet al. 2004, Stuart 2005, Nicholls 2011, Faith 2011).
Lorenz et al. (2011) acknowledge that it is difficult to fully explain the link between climate change, population size and species extinctions. Changes to climatic conditions are thought to have reduced the carrying capacity of landscapes, making populations vulnerable to extinctions caused by environmental or anthropogenic processes (Faith 2011)
Evidence suggests that individual species responded differently to climatic shifts, environmental changes, and human encroachment. Expansion into different areas in order to increase chances of survival may have been dependent on population density, and the characteristics of the new area, such as predation by other creatures and by humans. It has also been suggested that vulnerability to extinction may have been caused by low fecundity, rather than simply large body size (Lister and Stuart 2008).
MAMMOTHS
The Woolly Mammoth, Mammuthus primigenius was an herbivorous mammal that lived in cool, dry open steppe-tundra in Northern Hemisphere from the late Middle Pleistocene 300ka BP. These creatures vanished from Eurasia and North America during the Holocene, 3.6 ky BP.
CAUSES OF MAMMOTH EXTINCTION
Testing hypotheses of climatic and anthropogenic causes of these extinctions is a challenge, due to complications with gaining quantitative estimates of the relationship between contraction of the geographic range of the mammoth and these two hypotheses.
CLIMATE
DNA research into mammoth behaviour is aided by modern proxy evidence. Modern elephants produce 50 litres of urine each day, so it seems reasonable that mammoths would have done the same. This means that vast land areas will be covered in mammoth DNA. Genetic studies have shown that the range of wooly mammoths had been in decline for several thousand years before their disappearance. 126ky BP, Earth’s climate became progressively colder and drier, until the Last Glacial Maximum, LGM, when conditions became warmer and wetter. These climatic oscillations are thought to be responsible for changes to vegetation and reduced and fragmented geographic range of mammoth habitats (Nicholls 2011, Nogues-Bravo et al. 2008).
Mammoth habitat range contracted again 12kya, and at this time, mainland populations rapidly moved north. Strangely, this disappearance was not correlated with any pattern of warming and spread of shrub-grassland vegetation over much of Europe. During the cold Younger Dryas phase, on the other hand, when steppe-tundra was allowed to re-establish, the Mammoth moved back into northeast Europe (Stuart 2005).
Woolly Mammoth remains have been unearthed on Wrangel Island, Siberia. C14 data suggest that Mammoths inhabited this area for as long as 6000 years after it vanished from mainland Siberia. It is thought that the species was forced to migrate to Wrangel by the movement of the forest/tundra line, even though the area was perhaps not ideally suitable. (Vartanyan 1995, Nogues-Bravo et al. 2008).
CLIMATE AND HUMANS
It is likely that the collapse of the climatic niche of the mammoth resulted in a drop in their population size, and increase in vulnerability to human hunting pressure.
The hunting intensity model suggests that mammoth extinction was caused by a synergy between collapse of suitable climatic conditions and northward increase in human population densities. This is based on the idea that regardless of cull rate; the percentage of mammoth population that must be killed to drive the species to extinction, hunting intensity has varied through time, so hunting is unlikely to be the only reason for extinctions (Nogues-Bravo et al. 2008).
Like the mammoth, the Straight-Tusked Elephant, Eurasian Musk Ox and Woolly Rhinoceros were forced to retreat because of climatic changes, loss of habitat, and vegetation shifts. While there are arguments suggesting that the extinction of these creatures can be explained by climate alone, anthropogenic effects are thought to be responsible for the loss of the wild horse and steppe bison.
RELEVANCE
These studies demonstrate the complex causes of mass extinction. A greater understanding (Lister and Stuart 2008) of the role of climate in the process of mass extinction may help us to predict the ways in which current species may adapt to future ecosystem shifts.
With regard to the current extinction, arguments that climate is the cause may well be rooted in truth, but, as demonstrated, a variety of other factors also have a part to play. The unprecedented expansion of humans and the resulting climatic warming and destructive activity must surely be responsible for a considerable portion of species loss?
References
Faith J. T. (2011) ‘Late Pleistocene climate change, nutrient cycling, and the megafaunal extinctions in North America’, Quaternary Science Reviews, 30, 1675-1680.
Lister A.M. and Stuart A.J. (2008) ‘The impact of climate change on large mammal distribution and extinction: Evidence from the last glacial/interglacial transition’, Comptes Rendus Geoscience, 340, 615-620.
Lorenzen E.D., Nogues-Bravo D., ORlando L, Weinstock J., Binladen J. Marske K.A., Ugan A., Boregaard M.K., Gilbert M.T.P., Nielsen R., Ho S.Y.W., Goebel T, Graf K.E., Byers D., Stenderup J.T., Rasmussen M., Campos P.F., Leonart J.A., Koepfli K.-P., Froese D., Zazula G., Stafford T.W., Aaris-Sorensen K., Batra P., and Haywood A.M., Singarayer J.S., Valdes P.J., Boeskorov G., Burns J.A., Davydov S.P., Haile J., Jenkins D.L., Kosintev P., Kuznetsova T., Lai X., Martin L.D., McDonald H.G., Mol D., Meldgaard M., Munch K., Stephan E., Sablin M., Sommer R.S., Sipko T., Scott E., Suchard M.A., Tikhonov A., Willerslev R., Wayne R.K., Cooper A., Hofreiter M., Sher A., Shapiro B., Rahbek C. and Willerslev E. (2011) ‘Species-specific responses of Late Quaternary megafauna to climate and humans’, Nature, 479, 359-364.
Nicholls H. (2011) ‘Last days of the mammoth’, New Scientist, 209, 2805, 54-57.
Nogues-Bravo D., Rodriguez J., Hortal J., Batra P., Araujo M.B. (2008) ‘Climate change, humans, and the extinction of the woolly mammoth’, Public Library of Science Biology, 6, 4, 685-692.
Stuart A.J. (2005) ‘The extinction of woolly mammoth (Mammuthus primigenius) and straight-tusked elephant (Palaeoloxodon antiquis) in Europe’, Quaternary International, 126-128, 171-177.
Vartanyan S.L. (1995) ‘Radiocarbon dating evidence for mammoths on Wrangel Island, Arctic Ocean, until 2000 BC’, Radiocarbon, 37, 1, 1-6.
Wroe S., Field J., Fullagar R., and Jermin L.S. (2004) ‘Megafaunal extinction in the late Quaternary and the global overkill hypothesis’, Alcheringa: An Australasian Journal of Palaeontology, 28, 1, 291-331.
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