Dwarfed Giants: Woolly Mammoths on Wrangel Island

Fig 1: Vartanyan et al (1993) Map of Wrangel Island

On the reading list for lecture 2 is an interesting (and conveniently, very short!) article by Vartanyan et al (1993) on Holocene dwarf mammoths on Wrangel Island, which survived long past the consensus extinction date of around 12,000 radiocarbon years BP of the ‘normal’ mammoth. Numerous sets of fossilized teeth 30% smaller than normal mammoth teeth have been found on Wrangel Island, and were dated as young as 7,000 – 4,000 radiocarbon years BP. Based on the relationship between tooth and body size, researchers have concluded that the dwarf mammoth was 180-230cm in shoulder height, at least 30% smaller than woolly mammoths on the mainland. 

Wrangel Island provided an isolated refugia for the mammoth in its dwarf form. In the late Pleistocene, Wrangel Island was part of the land of Beringida, joined up with the lowlands of East Siberia, Alaska and the present-day Arctic shelf. By 12,000 radiocarbon yeas BP, Wrangel Island was separated from the mainland, separating the local population of mammoth from the mainland population. This Arctic island had a much higher diversity of plant types and open vegetation, which supported mammoth populations. Even in the present day, the vegetation on this island is considered to be a poorer relic of late Pleistocene grassland.  Nevertheless, the dwarfing of mammoth size is an adaptation which reflected the severity of the stress that the original ‘normal’ mammoths faced in their original habitat. 

Fig 2: Stuart (2005) Timeline of Mammoth Extinctions in Different Regions

The significance of the Wrangel Island dwarf mammoths is that they provide a better understanding of how megafauna extinction took place, and the role of refugia. Extinction is not a one-off event in which animals are quickly wiped out, but rather, a gradual shrinking of range as these they increasingly found their environments unsuitable. Fig. 2 (above) illustrates the staggered extinction of mammoths, with some populations making their ‘last stands’ in certain places. A useful complementary article to read is Stuart’s(2005) more recent paper on new evidence of mammoths surviving much later than previously thought in other places that provided similar refugia, such as the new mammoth molars found in Estonia, which date to 10,000 radiocarbon years BP. Another example is the continued existence of woolly mammoth in the far north of Siberia, the Taymyr Peninsula, for another 2,000 radiocarbon years after most of them became extinct around the world. As the Holocene period brought warmer climates and forests rather than the open-steppe vegetation which favoured mammoths, such colder refugia allowed mammoths to survive. However, the Wrangel Island mammoths also add more mystery to the megafauna extinction debate. If climates became unsuitable, surely the mammoths were more vulnerable to change on an island, where migration was not possible, rather than on the mainland? Could it be the lack of humans on the island? Could human activities have inhibited the normal migrational responses of megafauna to climate change, thus limiting their range?

While the Wrangel Island mammoths raise even more questions on the megafauna debate than can be answered, it provides important lessons for modern conservation. Wrangel Island highlights the resilience and adaptability of natural ecosystems to change, e.g. through migration and physical adaptation (dwarfing). However, the combination of climate change and human impacts was just too much for the megafauna to bear. Today, as our ability to alter the environment is ever more profound, we need to be increasingly responsible for the consequences of our actions.

References

Vartanyan, S. L. (1993) ‘Holocene dwarf mammoths from Wrangel Island in the Siberian Arctic’, Nature 362, pp. 337-340.

Stuart, A. J. (2005) ‘The extinction of woolly mammoth (Mammuthus primigenius) and straight-tusked elephant (Palaeoloxodon antiquus) in Europe’, Quaternary International 126-128, pp. 171-177.

The Uncertain Blitzkrieg Down Under

By the late Quartenary, Australia had lost 23 out of 24 genera of its megafauna. The 2 main explanations for this are human mediation – overkill and habitat destruction – and climate change. In this blog post I will look at each in turn, and conclude that humans are not the primary cause of megafauna extinction in Australia. While human ‘blitzkrieg’ was previously the consensus, new research has shown increasing evidence that it is likely climate change played a larger role in a majority of extinctions. 

Human Overkill and Habitat Destruction

The main argument for proponents of the blitzkrieg hypothesis is that most megafauna were present when humans arrived in Australia, and they were subsequently wiped out by interaction with humans. They point to the evidence that extinction of megafauna occurred around 45,000 calendar years ago, coinciding with human colonization of Australia and predating climate change (Roberts et al 2001). Miller et al (2005) also show in their research that records have shown a decline in food sources for the Australian emu and marsupial wombat, attributed to human modification of the drought- adapted environment of shrubs and grasses into the fire-adapted scrublands of today.

However, Trueman et al (2005) argue that fossil evidence from Cuddie Springs (Southeastern Australia) and other sites refute the consensus that megafauna became extinct about 45,000 calendar years ago. Their research has found that humans and megafauna co-existed for about 15,000 calendar years after the arrival of humans. Besides, Wroe et al (2004) points to the complete absence of any direct evidence implicating human mediation, e.g. kill sites, similar to those that have been found in North America.

Climate Change

Wroe and Field (2006) point to evidence from a variety of climate proxies (pollen, charcoal, hydrology, etc.) that there was a broad trend towards increasing aridification of the Australian climate in the Late Quarternary, which overlaid glacial cycles. Thus, the Holocene interglacial was drier than previous interglacials, disputing the idea that the Pleistocene-Holocene transition was similar to previous transitions. The chart below shows that lake levels and river flow in Northern and Southeastern Australia started declining from around 50,000 calendar years ago, while dust levels increased from around 200,000 calendar years ago. Other evidence from pollen records show that around 200,000 years ago, grasses became more prevalent relative to eucalyptus, indicating increased aridity. 

Fig. 1: Wroe and Field (2006)

Thus, non human-mediated climate change can be shown to have caused the extinction, as these natural processes were in motion long before humans arrived. However, it is certainly possible that humans played a role in further stressing megafauna already stressed by these climatic changes, although they cannot be seen to be a primary cause of the extinctions. 

An Alternative Model: Staggered Extinctions of Australian Pleistocene Megafauna

Wroe and Field (2006) suggest an alternative model of staggered extinctions. Their more recent research shows that at least 65% of the megafauna cannot be shown to have existed beyond 130,000 calendar years ago. Only 13% of megafauna species during the Pleistocene co-existed with humans, and at least half of these species survived 15,000 calendar years after humans arrived. Thus, they argue for an alternative model of repeated range contractions and limitation of refugia for megafauna as Australia’s climate became increasingly arid, resulting in extinctions which predated human contact.

Their model is supported by Cosgrove and Allen’s (2001) study of Tasmanian rockshelters. Since early humans could only have reached Tasmania by 37,000 calendar years ago through the development of a land bridge, if humans caused extinction then megafauna should be shown to survive there until the arrival of humans. However, no megafauna fossil remains younger than 46,000 calendar years ago could be found. 

References

Cosgrove, R. and Allen, J. (2001) ‘Prey choice and hunting strategies in the Late Pleistocene: evidence from Southwest Tasmania’ in Lilley, A. and  O’Conner, S. (Eds.), Histories of Old Ages: Essays in Honour of Rhys Jones. Canberra: Research School of Pacific and Asian Studies, Australian National University, Canberra, pp. 397–430.

Miller et al (2005) ‘Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction’, Science, 309, 5732, pp. 287-290

Roberts, R.G. et al (2001) New ages for the last Australian megafauna continent-wide extinction about 46,000 years ago. Science, 292, 1888–1892.

Trueman et al (2005) ‘Prolonged coexistence of humans and megafauna in Pleistocene Australia’, Proceedings of the National Academy of Sciences of the United States of America, 102, 23, pp. 8381-8385

Wroe, S. et al (2004) ‘Megafaunal extinction in the Late Quaternary and the global overkill hypothesis’, Alcheringa, 28, pp. 291–331.

Wroe, S. and Field, J. (2006) ‘A review of the evidence for a human role in the extinction of Australian megafauna and an alternative interpretation’, Quaternary Science  Reviews, 25,21-22, pp. 2692-2703.

Megafauna Extinction in Eurasia

Eurasia (Eurasia and Northern Asia) lost 35% of its megafauna during the late Pleistocene, relatively fewer than North America and Australia. Extinction patterns here differed from those in North America; not all the extinctions occurred synchronously at the end of the Pleistocene (Grayson 2007). For example, mammoths disappeared from many parts of Eurasia at around 12,000 radiocarbon years ago, but lasted as late as 4,000 radiocarbon years ago on Wrangel Island. Similarly, giant deer disappeared from Southwestern France between 12,000 and 11,000 radiocarbon years ago but not from western Siberia until 7,000 years ago Stuart et al 2004).

The consensus is that Man was unlikely to have caused megafauna extinction in Eurasia, as the first modern humans (with sophisticated hunting tools) entered Eurasia around 50,000 radiocarbon years ago, and there were no apparent extinctions then (Grayson 2007). However, Stuart (1999) argues that the human role in Eurasia is not insignificant. For example, according to radiocarbon-calibrated pollen profiles, vegetation able to support mammoth was present more than 1,000 radiocarbon years after these animals disappeared in the region, weakening the argument that environmental change was the sole cause of extinction. Instead, he suggests that the asynchronous nature of extinction in Eurasia could mean that extinctions only occurred when animal populations were already undergoing significant stress from climate change, and human hunting provided the last straw. This probably also explains why there were 2 distinct waves of extinction in Eurasia which coincided with periods of climate change (40,000 to 20,000 radiocarbon years before present and 14,000 to 10,000 radiocarbon years before present), the latter of which is the focus of this blog.

While Stuart’s argument is convincing, I think an interesting counterargument can be found in Anthony Barnosky’s 1986 paper. He argues that Irish deer, which became extinct at around 10,000 to 12,000 radiocarbon years ago, before the arrival of humans in Ireland, were wiped out not by Holocene warming but a brief cold spell just before warming. This cold spell shortened feeding seasons for the Irish elk, which were also unable to migrate to any refugia quickly enough as Ireland is an island. The evidence is in lake sediment layers where pollen records suggest changes in vegetation associated with colder weather and fewer elk bones, during a period called the Nahanagan Stadial. He suggests that the accumulation of many local causes of extinction could have led to the total extinction of megafauna. Nevertheless, more work remains to be done on this fascinating hypothesis.

Figure 1: Irish Elk

Finally, I return to a discussion of the climate change that occurred during the late Pleistocene. In my previous blog post ‘Humans in the Wild West’, I discussed research showing that the late Pleistocene-Holocene glacial transition was unique in the Northern Hemisphere compared to other previous interglacials, both in climate and biological terms. This caused megafauna extinction due to climate unsuitability. Nogues-Bravo et al (2008) constructed a model which shows this for one species – the woolly mammoth in Europe.

Figure 2: Maps of Projected Climatic Suitability for the Woolly Mammoths in the Late Pleistocene and Holocene (Nogues-Bravo et al 2008)

The increasing intensities of red show increasing suitability of mammoth habitat while increasing intensities of green show decreasing suitability. Black dots show mammoth presence while black lines show the northern limit of early humans. The figure shows that climate and habitat suitability for mammoths decreased during the late Pleistocene. Although humans did move Northwards, their presence did not seem to affect mammoth presence as drastically as habitat suitability; even in areas untouched by humans (north of black line), mammoth populations declined as habitat suitability decreased.

The causes of megafauna extinction in Europe are certainly extremely complex. Climate change appears to be the main culprit, since human populations coexisted with megafauna for over 50,000 radiocarbon years while accelerated extinctions only occurred in 2 distinct phases which were periods of distinct climate change. Extinctions in at least some specific geographical locations, such as the Irish elk in Ireland, were certainly distinct from the role of humans.  

References

Barnosky, A. (1986) “Big game” extinction caused by late Pleistocene climatic change: Irish elk (Megaloceros giganteus) in Ireland’, Quaternary Research, Vol. 25, 1, pp. 128-135

Grayson, D. K. (2007) ‘’Deciphering North American Pleistocene extinctions’’, Journal of Anthropological Research, Vol. 63, No. 2, pp. 185-213

Nogues-Bravo, D. et al (2008) ‘’Climate change, humans and the extinction of the woolly mammoth’’, PLOS Biology, 6(4), e79

Stuart, A. J. (1999). Late Pleistocene megafaunal extinctions in MacPhee, R. D. E. (ed.) Extinctions in Near Time: Causes, Contexts, and Consequences, New York: Plenum, pp. 257-269.