A Word on Methodology: The Sporormiella Proxy

In my previous post, I mentioned the Sporormiella proxy used to determine abundance of Pleistocene megafauna in Madagascar. This is an analytical technique that has recently gained prominence in the study of the late Pleistocene megafauna extinction. There is an interesting paper by Feranec et al (2011) about the Sporormiella proxy and the problems associated with using it.

The Sporormiella Proxy

Sporormiella is a fungus that is present on the dung of herbivores. Sporormiella sporulating on dung release spores which adhere to nearby objects (usually plant matter). Herbivores then eat this plant matter and the spores, which pass through their digestive tracts, are released in their dung. The spores of this fungus are preserved readily in lake sediments, and stratigraphic changes in the abundance of this fungus in Pleistocene and Holocene sediment sequences have been used as a proxy to define megafaunal presence, decline and extinction globally.

Sporormiella Spores 

Problems

The presence of Sporormiella is not exclusive to large herbivore dung and has been found in the dung of small herbivores as well, such as hares. Thus, it is difficult to use Sporormiella as a sole and direct proxy for megafauna abundance unless specific species of Sporormiella associated only with large herbivores can be identified.

A stratigraphic decline in Sporormiella does not necessarily indicate a decline in megafauna. For example, Sporormiella is more abundant near lake shores than in the middle of lakes, so a decrease could simply mean a rise in the lake level. Sporormiella may also be preserved to varying degrees depending on type of lake sediment, lake levels, etc. A related point is that the absence of Sporormiella does not indicate the absence of herbivores – some modern day sites with abundant livestock have been shown not to contain Sporormiella in Davis and Shafer’s (2006) study. Thus, Sporormiella needs to be calibrated to other indicators of large herbivore population and is non-conclusive on its own.

Some academic papers must be viewed with some scepticism due to methodological over-reliance on this particular proxy. For example, in a Gill et al (2009) paper, a decline in Sporormiella in a Lake Appleman core in Indiania which starts from 14,800 years ago and which pre-dates a major change in the pollen assemblage is used to conclude that the late Pleistocene megafauna extinction was not caused by (usually climate-linked) vegetation changes. They also show that charcoal frequency increased at that site, indicating that human factors (like vegetation burning) were probably behind the extinctions. However, the tail end of the Sporormiella decline is also associated with a change in lake sediment size, which may reflect changes in the sediment input and hence catchment area of the Sporormiella source, rather than megafauna decline.

Conclusion  

While this analytical technique is certainly promising in contributing to research on Pleistocene megafauna extinction, it still needs to be refined. What is also important is to avoid complete reliance on just one proxy; the conclusions drawn from using this proxy should be calibrated to other indicators of megafauna abundance.

References

Davis, O. K. and Shafer, D. S. (2006) ‘Sporormiella fungal spores, a palynological means of detecting herbivore density’, Palaeogeography, Palaeoclimatology, Palaeoecology, 237, 1, pp. 40-50.

Feranec, R. S. et al (2011) ‘The Sporormiella proxy and end-Pleistocene megafaunal extinction: A perspective’, Quarternary International, 245, 2, pp. 333-338

Gill, J. L. et al (2009) ‘Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America’, Science, 326, pp. 1100-1103

Island Extinctions and the Case of Madagascar

Island Extinctions

Islands which were only inhabited by humans after the late Pleistocene megafauna extinction event offer interesting ‘control experiments’. A compelling argument for human factors driving extinction is the time lag between continent extinctions and those of nearby islands (Martin and Steadman 1999). For example, New Zealand’s moas lasted 30,000 years longer than Australia’s extinct giant bird, the mihirung, which went extinct during a Australia’s wave of rapid megafauna extinction (before the late Pleistocene).

The argument for prey naiveté on early contact with human hunters finds support also in the remarkable tameness of wild birds in remote islands which were undiscovered by prehistoric explorers. These include the Galapagos, Christmas Islands, etc. Galapagos’ avifauna were unafraid of humans, as depicted in historic accounts from 17^th century sailors who discovered the island (Martin and Steadman 1999).

In this blog post I look closer at the island of Madagascar.

Madagascar

This paper by Burney et al (2004) discusses how islands can be used to better understand megafauna extinction. Madagascar is interesting because it is the last place on Earth where megafauna went extinct prehistorically – extinctions in which humans had some part to play in most other parts of the world occurred much earlier, during the late Pleistocene or even earlier in Australia. Madagascar offers a relatively fresh record of paleoecological change, since humans only arrived in the late Holocene, about 2,000 radiocarbon years ago. Very little is known about how or why this group of Iron Age people came to Madagascar. Evidence of the first humans can be shown by human-modified megafauna bones, such as cuts on the fossilized bones which show removal of flesh from bone by a sharp object.

Very little is known about the late Pleistocene biota in Madagascar. The amount of data increases greatly for megafauna in the mid-Holocene, where conditions for fossil formation probably became more favourable. For example, most lakes and swamps along the coastline formed only after around 5,000 radiocarbon years ago. Nevertheless, there were major climatic changes in in the late Pleistocene and pre-human Holocene, most of which were survived by most of the megafauna. Although there have been range shrinkages, there were no extinctions. Of the 9 genera of extinct lemurs dated, only one is not securely dated to the human period. Some examples of major climate change are as follows: 

·        

•      20,000 radiocarbon years ago (LGM): widespread dessication occurred. Lake Alaotra, a large lake in humid eastern Madagascar, was dramatically reduced in area if not completely dry during that period. 
•      10,000 calendar years BP: At another site called Trtrivakely, pollen evidence shows the nearly complete replacement of heath vegetation with wooded grassland.

A drastic decline in megafauna, as shown by a huge decrease in Sporormiella in sediments at 1700 radiocarbon years BP (within a few centuries of first human contact), was observed. Sporomiella is a fungus that grows in the dung of large plant-eating mammals, and it releases spores which are preserved in sediments. The presence of these spores is used as a proxy for the presence of megafauna. Humans could have hunted these megafauna or altered their habitat. Before humans arrived, these herbivores had very few predators other than large crocodiles. Although there is very scant evidence for direct human hunting of megafauna, as in many other continents where Pleistocene megafauna extinction has occurred, another way in which humans could have contributed to the decline is through altering the existing fire regime by further increasing fire incidence through burning for settlement and agriculture and through hunting of plant-eating megafauna. The decline of large herbivores such as giant hippos caused ground litter to accumulate, feeding more fires. This can be shown by charcoal peaks above background values, first occurring in the South West where humans first settled, and then spreading outward over Madagascar. Nevertheless, the extinction pattern on Madagascar does not support a Blitzkrieg hypothesis. There is an overlap of around 2,000 years from earliest human evidence to the last occurrence of extinct megafauna.

The chart (Burney et al 2004) shows a summary of events in Madagascar:

My Thoughts

The evidence from Madagascar is indeed intriguing and I feel it does make the argument for human factors in the extinction of megafauna more compelling. Madagascar’s physical geography and vegetation is very similar to Africa such that it is referred to as an ‘Africa in miniature’, and it is probably safe to assume it went through similar climate changes and vegetation responses as Africa. The megafauna on this island certainly survived all these before the humans came, after which they experienced dramatic decline and finally, extinction. The fact that it is an island is important; in a previous post I mentioned the reason for why Africa still has such a large diversity of megafauna left is that it is larger and probably provided more refugia for megafauna. Madagascar probably provided more limited refugia for the stressed populations of megafauna.     

References

Burney, D. A. et al (2004) ‘A chronology for late prehistoric Madagascar’, Journal of Human Evolution, 47, pp. 25-63.

Martin, P. S. and Steadman, D. W. (1999) ‘Prehistoric extinctions on islands and continents’ in MacPhee, R. D. E. (ed.) Extinctions in Near Time: Causes, Contexts and Consequences, New York: Kluwer Academic/Plenum, pp. 17-50

Dissecting the Hyperdisease Hypothesis

I decided to do a post on the disease hypothesis after Josh from http://no-mammoths.blogspot.co.uk/ suggested an interesting paper by Rothschild and Laub (2006). Here is the link to his post on this topic specifically. The hyperdisease hypothesis proposes that humans and their domesticates introduced novel hyperdisease to vulnerable populations of Pleistocene megafauna who had never encountered such diseases before and whose bodies were therefore unable to cope. Since migrations of animals from Europe to North America were not uncommon before the period we are studying, it is more likely that humans and their domesticates were the disease vectors (Lyons et al (2004).

Tuberculosis and the American Mastodon 
Rothschild and Laub (2006) have suggested that new evidence for the hyperdisease theory has surfaced in the form of bone alterations from infectious tuberculosis found in just over half of 113 mastodon skeletons in the Western Hemisphere. Since not all animals infected with tuberculosis develop this bone alteration, it must follow that probably almost all of the mastodon population must have been infected with tuberculosis. The disease thus qualifies as a pandemic in the sense that it had an extremely high infection rate. Besides, it has a persistent presence in the fossil record from around 34,000 – 10,000 years BP, establishing that it must have been present in the late Pleistocene period. 

However, there is a difference between infection and mortality – the disease was not necessarily fatal. Rothschild and Laub (2006) hypothesize that this disease may have weakened mastodons in the face of climate change and human impacts in the late Pleistocene, further stressing their populations. While the disease could have remained latent, the environmental stresses of that period could have resulted in a loss of latency, increasing mortality. However, it is unlikely that the hyperdisease could have been a major factor in the extinction event. 

The Modern Day West Nile Virus: A Proxy for the Mystery Hyperdisease?

I also found another paper by Lyons et al (2004) which proposes some criteria for the hyperdisease theory to be plausible. 

1. It must be able to survive in a carrier state in a ‘reservoir’ species when there are no susceptible hosts to infect.
2. It must have a very high infection rate.
3. It must be extremely deadly with a 50-75% mortality rate
4. It must be able to infect multiple host species without infecting humans

Lyons et al (2004) use the West Nile Virus in birds, a disease which has seen recent introduction and spread in North America’s bird population, as a proxy to test this hypothesis as it appears to fulfil all of the above criteria of a hyperdisease.

One of the unique features of the late Pleistocene megafauna extinction event was its size-selectivity – smaller and medium-sized animals were largely unaffected. Thus Lyons et al (2004) have tried to test if West Nile virus causes such size-selective infections in birds. It can be shown that it does not, as infection rate increases positively with body size (Fig. 1) and infection occurs across a range of body sizes. This contrasts with the pattern shown by late Pleistocene mammal extinctions. The x-axis of the graph shows the size category of the bird species infected by the West Nile virus and those of the mammals which went extinct during the late Pleistocene. It has been re-scaled for mammals since they contain a much larger range of body masses. Each filled square shows the percentage of species pool in each size category infected by the virus or that went extinct. 

Fig 1 (Lyons et al 2004)

Some have argued that large body size makes species inherently vulnerable to extinction because of life history factors, e.g. low reproduction rates which make it harder for populations to recover from mortality caused by disease. However, Lyons et al (2004) counter-argue that if this is true, then larger species should have high extinction rates relative to smaller species over evolutionary time, which is not the case. 

The Verdict?

I find the hyperdisease hypothesis unconvincing so far and I think it is only considered seriously as a factor in the extinction event because of the general lack of evidence surrounding even the exhaustively-researched hypotheses of climate change and human hunting (e.g. lack of kill sites). However, the even more severe lack of evidence in the hyperdisease hypothesis is even more disturbing. The only known modern disease which fulfils the 4 criteria of a hyperdisease capable of wiping out megafauna during Pleistocene times, the West Nile virus, itself cannot be shown to cause the size-selective extinction pattern in modern day bird populations in North America. Besides, the paper by Rothschild and Laub (2006) only shows a pandemic-scale disease in one type of animal, the American mastodon. It is difficult to find equivalent disease explanations for all other megafauna species killed during the late Pleistocene (mammoth, for example, were not affected by this disease and were close cousins of the mastodon). Therefore, I conclude that hyperdisease is an unlikely explanation for the megafauna extinction event we are studying here.

References

Lyons, S. K. et al (2004) ‘Was a hyperdisease responsible for the late Pleistocene megafaunal extinction?’, Ecology Letters, 7, pp. 859-868

Rothschild, B. M. and Laub, R. (2006) ‘Hyperdisease in the late Pleistocene: validation of an early 20th century hypothesis’, Naturwissenschaften, 93, 11, pp. 557-564

The African Anomaly

Africa is the ‘anomaly’ in the Pleistocene megafauna extinction debate because it only lost 21% of its megafauna species, in contrast to the high levels of extinction in other continents. Today, it also contains a much higher level of megafauna diversity than other continents, including some species which have survived from Pleistocene times, such as the Cape Buffalo. 

Climate Change, Refugia and Disease

Climate change in Africa has been associated with dramatic species extinctions in the whole Pleistocene, not only the late Pleistocene. 59% of the Pleistocene megafauna extinctions occurred in the early Pleistocene, 21% in the middle Pleistocene and 20% in the late Pleistocene. All of the late Pleistocene extinctions happened during the late Pleistocene/Holocene transition (Graham and Lundelius 1984). 

The present-day megafauna has been called a ‘living Pleistocene fauna’ (Graham and Lundelius 1984: 240) because of their diversity is almost similar to the diversity of the extinct Pleistocene megafauna. Graham and Lundelius (1984) argue that perhaps the rate and magnitude of climate change was slower in Africa than in other continents, thus explaining the lower rates of megafauna extinction. Savannah environments survived into the Holocene in Africa while they disappeared in other parts of the world. For example in South America, savannah environments were abundant during the late Pleistocene but is today restricted to only a few areas. Other explanations for the African Anomaly have been put forward, although many of these tend to be speculative as Africa is the least studied continent with regards to late Pleistocene megafauna extinction. One possibility is that Africa has a great variety of habitat types which may offer better refugia for megafauna pressured by human activities (Heller 2012). Another explanation could be the existence of diseases that prevented humans or livestock from living in certain areas. This is still true today, as exemplified by locally endemic livestock diseases making large tracts of attractive pasture in Africa unavailable for human settlement. This is a phenomenon unique to Africa (Heller 2012). 

Human Impacts?

Graham and Lundelius (1984) claim that it is unlikely that humans have had much ecological impact on Africa’s megafauna because they have been known to coexist with them for a much longer time than on other continents. Martin (1984) even attributes the lower extinction rates to lower prey naiveté as a result of adapting to the hunting styles of humans. However, the human impact cannot be underestimated. Klein (1984) points to archaeological evidence that the humans of the late Pleistocene/Holocene transition were much more competent hunters than earlier humans. For example, studies of archaeological sites of earlier humans have found that eland (a type of ungulate) remains occur more frequently. Within the archaeological sites of humans who lived in the late Pleistocene/Holocene period (but under similar environmental conditions), remains of wild pigs, which were more dangerous to hunt and therefore required more sophisticated hunting techniques such as traps for example, were more prevalent than those of eland. 

Eland

African Wild Pig

The Cape Buffalo Study

A recent paper by Heller et al (2012) using genetic sequencing of African Cape buffalo, a species which has survived from the late Pleistocene period to the present-day, has found evidence of benign human – megafauna interaction during the late Pleistocene. Cape buffalo began a population expansion from 80,000 radiocarbon years ago and reached a peak at 8,000 radiocarbon years ago, which shows that humans and climate change had relatively little impact on the population. This study provides further evidence of benign human-megafauna co-existence during the late Pleistocene. To the extent that Cape buffalo is representative of the ecological dynamics facing other African megafauna, this new research also supports the Graham and Lundelius’ (1984) finding that most of African Pleistocene extinctions occurred in the early Pleistocene. If Klein is correct, this was at a time when human hunters were very poorly technologically developed. Thus, climate change would be the larger factor impacting megafauna populations in Africa. 

Cape Buffalo

References
Klein, R. G. (1984) ‘Mammalian extinctions and Stone Age people in Africa’ in Martin, P. S. and Klein, R.G. (eds.) Quaternary Extinctions, Arizona: Arizona University Press, pp. 553 – 573

Graham, R. W. and Lundelius, E. L. (1984) ‘Coevolutionary disequilibrium and Pleistocene extinctions’ in Martin, P. S. and Klein, R.G. (eds.) Quaternary Extinctions, Arizona: Arizona University Press pp. 223 – 249

Heller, R. et al (2012) ‘Cape buffalo mitogenomics reveals a Holocene shift in the African human–megafauna dynamics’, Molecular Ecology, 21, pp. 3947–3959

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.

All Creatures Great and Small (are important considerations in the megafauna extinction debate)

I came across an interesting journal article the other day about introducing a behavioural economics perspective to megafauna extinction. This journal is interesting not only to me as an Economics and Geography student, but also because it suggests the value of an inter-disciplinary perspective to environmental geography.   In the 2005 paper ‘Megafauna Extinction: A Paleo-economic Theory of Human Overkill in the Pleistocene’, Bulte et al argue that previous scientific models of overkill concentrated only on megafauna as prey for humans, while ignoring the entire opportunity set facing the human hunters, which is the presence of substitute foods and behaviours. One of these is hunting small animals or minifauna.

Bulte et al have designed a model which suggests that counter intuitively, hunting minifauna was essential to the overkill hypothesis. Minifauna supported human populations and allowed humans to reach critical densities which were large enough to wipe out megafauna. Besides, hunting minifauna enabled more chance encounters with megafauna. Complementing last week’s post, they suggest that the harsh environmental conditions of the late Pleistocene may have triggered humans to engage in minifauna hunting rather than more benign activities like agriculture.  Minifauna did not go extinct because they breed much more quickly than slow-breeding megafauna.

This echoes modern day poaching behaviour in Africa. Poachers often hunt both rhinos and elephants, and not only the more valuable rhino ivory alone, because the probability of encountering rhinos is much smaller, making sole rhino hunting a loss-making activity. Thus, poachers also hunt the relatively more abundant elephants, taking rhinos as a bonus. Thus, elephant hunting is what economists call a ‘complementary’ activity to rhino hunting, in the same way as hunting minifauna was complementary to hunting megafauna for early humans.  

Although the authors of this journal are economists rather than paleo or megafauna experts, they do make an important point about considering the behavioural incentives facing early human hunters and not simply portraying them to be the mechanistic 'superpredators' of most overkill models.

The First Outlaws? Megafauna Murderers of the Wild West: Clovis Hunters

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

Proponents of the ‘overkill’ or ‘blitzkrieg’ hypothesis emphasize that the megafauna extinctions of 11.5 - 10,000 radiocarbon years ago coincided with the arrival of humans with advanced hunting techniques in North America, called the Clovis hunters. This could explain why megafauna extinctions were concentrated in the late Pleistocene rather than previous inter-glacial transition periods, as the only differentiating element of the late Pleistocene was the presence of the first ‘anatomically modern’ humans. Since it was the animals’ first encounter with them, they did not yet have the behavioural skills to avoid predation. Paul Martin (1967) was one of the earliest and most enthusiastic proponents of this view. Haynes (1967) showed that the earliest known appearance of Clovis hunters was 11.5-11,000 years ago, although more recent evidence suggests that an earlier people before Clovis existed as early as 13,800 years ago and was already developing the tools that would later evolve into those of Clovis hunters (See photograph). Barnosky et al (2004) points out that most large-bodied survivors of the late Pleistocene extinction event were nocturnal, alpine or deep forest dwellers, making them more difficult targets for humans. Furthermore, Graham and Lundelius (1984) suggest that the killing of megaherbivores like mammoths and mastodons (the 2 mammals with the largest number of recorded kill sites) was especially damaging and upset the entire ecosystem, driving other animals which were not directly hunted by humans to extinction. Mammoths and mastodons, like modern day elephants, tore down trees and bushes while feeding, making them accessible to other herbivores who would not otherwise be able to reach them. The disappearance of herbivores would then result in the starvation of their predators. This could be a counter-argument to Grayson and Meltzer’s (2002) argument that only 2 megafauna genera – the woolly mammoth and mastodon – have been shown to be the prey of Clovis humans, and this is supported by just 14 kill sites. 

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.

The Modern Elephant and its Prehistoric Cousins - A Similar Fate?

In the Beginning...

Woolly mammoths, sabre-toothed tigers and other Pleistocene megafauna roamed the Earth, forming a vital part of natural ecosystems. Today, they occupy no more than a CGI-manipulated place in the popular imagination, with appearances in natural history museums, animated movies and even in the hit TV series Game of Thrones as the dire wolf. How these mighty beasts have fallen!

Natural climate change was long thought to be behind their extinction, but recent studies have pointed to a new culprit: Man. Although Man cannot be solely held responsible, evidence suggests that early humans played a precipitating role in megafauna extinction which complemented the effects of climate change, at least in a fair number of continents like North America, Eurasia and Australia. Indeed, even in his less-than-glorious past, with his primitive weapons and small numbers, Man has managed to sound the death knell for these powerful creatures. With Man’s impact on biodiversity tremendously larger today, the prospects for today’s megafauna - elephants for example – are dire.

That said, important lessons abound for modern conservation from the unfortunate fate of these vanished giants. Human-induced climate change and human impacts on the natural environment are ever greater today, and the past has shown that these two factors have decimated two thirds of megafauna genera between 50,000 and 10,000 years ago – 40,000 years is merely the blink of an eye in geological time.

This blog explores the role of Man in Pleistocene megafauna extinctions and bridges the lessons of the past with the implications for contemporary conservation efforts.