Thursday, 20 December 2012
Sunday, 16 December 2012
In this blog post I look closer at the island of Madagascar.
- 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.
The chart (Burney et al 2004) shows a summary of events in Madagascar:
Friday, 7 December 2012
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.
- It must be able to survive in a carrier state in a ‘reservoir’ species when there are no susceptible hosts to infect.
- It must have a very high infection rate.
- It must be extremely deadly with a 50-75% mortality rate
- 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)|
Wednesday, 5 December 2012
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).
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.
|African Wild Pig|
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.
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
Monday, 19 November 2012
Wednesday, 7 November 2012
Human Overkill and Habitat Destruction
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.
Saturday, 3 November 2012
Figure 1: Irish Elk
Sunday, 28 October 2012
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.
Saturday, 20 October 2012
|Grayson (2007): Table of Extinct Megafauna of the Late Pleistocene|
|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)|
|The Modern Elephant and its Prehistoric Cousins - A Similar Fate?|