Monday, 26 November 2012

Pleistocene Megafauna Extinctions: Hyper-Disease Hypothesis.

Could Disease have caused the extinction of megafauna?

We are almost reaching an end of the various factors which might have contributed to the decline of megafauna during the late Pleistocene. In this blog I will discuss the hyper-disease hypothesis, looking at whether this factor can be seen to be a plausible mechanism for the extinction of megafauna. So far, we have seen that the most likely causes of widespread extinction are climate and man. We have also uncovered other hypothesis such as the occurrence of an extra terrestrial event and the impact of fire.  Without doubt, the impact of disease would have caused some megafaunal decline, but whether this factor was large enough to cause the complete extinction of certain large animals is still unknown.

The hyper-disease is defined as:
"...the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans (MacPhee & Marx, 1997). It Proposes that humans or their commensals introduced once or more highly virulent diseases into vulnerable populations of native mammals, eventually causing extinctions." (Lyons et al 2004: 859).

In historic times, humans have wiped out vast populations of each other during ‘first contact’ scenarios when diseases were accidentally transmitted (2).  Consequently, this mechanism can be seen to be a likely cause for the extinction of megafauna during the late Pleistocene, when two previously isolated species all of a sudden came into close contact (3).The disease hypothesis largely attains credit through lack of evidence supporting the climate or the blitzkrieg theory (1). Some scientists support the idea that megafauna had a weak immune system as they had never been exposed to diseases before and consequently were not able to withstand pathogens. Therefore it was not the influence of a changing climate or man’s overkill that caused extinction, but the pathogens carried by dogs, rat’s, birds, parasites and other living baggage that accompanied the continent’s first human arrival (1). This is reinforced by Dr Ross MacPhee (a mamalogist at the American Natural History Museum in New York) who stated that the overkill hypothesis is far too simple. Furthermore, that lack of kill sites (remains of butchered animals), strengthens the hyper-disease theory. Therefore, whilst there is evidence that disagrees with the hunting hypothesis, man could have caused the extinction of species through bringing disease. Reasons for the demise of megafauna have been controversial, unlike other extinction events, it coincides with periods of climate variability as well as the first appearance of human hunters. Extinction was also rapid, targeting primarily megafauna. Consequently, the disease hypothesis can be seen as a plausible mechanism for such extinction patterns as it could have spread quickly across continents, reducing animal populations to levels which they could not recover.  Dr Preston A. Marx (virologist at the Aaron Diamond AIDS Research Centre) believes that the animals were infected by lethal pathogens unknown to their immune systems (1).

Alternative evidence that supports this hypothesis is that disease would have persisted for many years after it was introduced, being carried by people or organisms that arrived with people. Had any megafauna been able to withstand the disease it would have ultimately infected new generations-leading to eventual extinction. Rothschild & Laub (2006) support the hyper-disease theory by showing the extinction of a particular type of mammoth through human carried tuberculosis. Evidence of this was found in the disease being present in 52% of the 118 skeletons that were surveyed.

Overall, the hyper-disease theory is new and still being developed. Whilst it is plausible in part, it lacks much needed supporting evidence. Furthermore there is lack evidence uncovering a pathogen that has the capability to cause such widespread extinctions. Whilst Rothschild &Laub(2006) were successful in discovering Tuberculosis that killed a type of mammoth, there is no evidence to suggest that this disease was broad enough to kill all the extinct megafaunal species. In spite of this, the hyper-disease hypothesis should not be discredited. Whilst there is lack of evidence, there are many believable aspects of this hypothesis. Perhaps, combining multiple causes of extinction e.g. over-kill, climate, disease etc., we find the most probable cause of megafaunal extinction. 

A basic insight into the 4 main causes of mass extinction:

Mammoth remains in Serbia:

Sunday, 18 November 2012

Extra-Terrestrial Impact Causing Megafaunal Decline: Fact or Fiction?

Could the impact of a Comet cause the extinction of megafauna?
Over resent blogs we have seen an array of factors which could have caused the extinction of megafauna in the late Pleistocene. In this blog I will discuss the impact hypothesis, looking specifically at the evidence for and against the occurrence of an extra-terrestrial event.

The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a comet or meteoric body hit/and or exploded over North America 12,900 years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archaeological culture, and a range of other effects (Printer et al 2011).  The authors that support the impact hypothesis suggest that a comet exploded over the great lakes, destabilizing the Laurentide ice sheet, releasing huge volumes of melt water which subsequently caused the YD re-glaciation and caused intense wildfires that led to the extinction of megafauna. The impact is also claimed to have caused major cultural changes and population decline among the Paleoindians (Buchanan et al 2008). The impact hypothesis gained widespread publicity in 2007, and extensive research has been focused on testing the sources of evidence that support this hypothesis.

The evidence (see Firestone et al 2007) that supports the impact theory includes:
  1.  Particle tracks in archaeological records
  2.  Magnetic nodules in Pleistocene bones- representing meteorite fragments (cosmic bullets), derived from the YD impactor and directly linking the proposed impact event with the megafaunal demise (Pinter et al 2011)
  3. Impact origin of the Carolina bays
  4. Elevated concentration of radioactivity. Firestone et al (2007) stated that some megafaunal bones in the YD are highly radioactive.     
  5.      Carbon spheres and elongates
  6. Magnetic grains and charcoal/soot (by products of catastrophic wildfire). This is mentioned by Buchanan et al (2008) who stated that the impact was accompanied by a high-temperature shock wave, changes in pressure that would have resulted in hurricane force winds, and extensive groundcover burning from the impact and superheated ejecta.
  7. Nanodiamonds present across North America.
  8.      Extinction of many mammalian and avian taxa occurred abruptly and perhaps catastrophically at the onset of the YD, which is believed to have been caused by a ET event.

         However, whilst there is evidence that supports a ET event, many are skeptical about this. Consequently, such evidence has been largely rejected by the scientific community. The reasons why ET evidence has not been supported is that:

    1.       Peak concentrations of magnetic grains at the start of the YD have yet to be found.
    2.       Impact markers have yet to be found (Pinter et al 2011). Similarly, there has been no evidence for charcoal peaks at the time of the YD.
    3.       None of the evidence supporting the impact hypothesis have been independently reproduced or have met the minimum threshold for scientific credibility (Pinter et al 2011)
    4.       The existence of carbon elongates have been confirmed but are ubiquitous in Pleistocene to modern sediments, and did not originate in catastrophic wildfires.
    5.       Buchanan et al (2008) found no such evidence of a population decline among the   Paleoindians around 12,000 yr BP.

     In conclusion, there is no doubt that the impact of a comet would have been devastating for animals and plants (Firestone et al 2007). However, the impact hypothesis has been controversial. Personally I am skeptical about an extra -terrestrial event triggering the YD, as there has been a lack of reproducible evidence supporting it (see Haynes et al 2010). Whist the majority of the scientific community discredits the impact theory, whether a ET event caused the extinction of megafauna is unknown.

    American Geophysical Union (AGU) Press Conference looking at the key issues surrounding the impact hypothesis:

Wednesday, 14 November 2012

The Impact of Fire

Forest Fire
In previous blogs we have seen the impact of man and climate in causing mega faunal extinction. My past blogs have been quite lengthy so I will try and make this one a bit shorter, looking specifically at a different hypothesis: the impact of fire. Generally most people believe that either climate variability or primitive man was responsible for the collapse of megafauna during the late Pleistocene, however the influence of fire should not be discredited. In fact, the impact of fire forms a good overlap between these two hypotheses as it can be created by both man and climate.

Fire would have had an impact of megafauna in two ways. Firstly, it might have directly caused megafauna to decline as a result of the extreme temperatures. Secondly, it would have altered landscapes resulting in megafauna being unable to sustain themselves, increasing likelihood of extinction.  Gill et al (2009) stated that there might be a causal relationship between the extinction of megafauna, peak rates of vegetation change, and the rise of no-analogy communities in Eastern North America. Consequently, fire can be seen to be a proposed extinction mechanism, and can be used to explain this causal relationship. The presence of charcoal in historic records demonstrates that fire events were common during the Pleistocene. The proposal that fire could have led to the demise of mega fauna is mentioned by Gill et al (2009) who stated that charcoal peaked during the sporomiella (dung fungal spores) decline. Charcoal proxies also reveal that during the late Quaternary there were increased fire regimes (most likely caused by humans), which would have altered the landscape by restructuring plant communities and consequently might have led to the decline of megafauna. Human lit fire allowed people to; live in colder environments (e.g. northern Eurasia), created new technologies, have a source of light, cook, and process food (Rick et el 2012).  Fire was also used by humans to help clear land for cultivations and to provide soil nutrients. The discovery of these advantages meant that human caused fires were more common than lightning events, which reinforces evidence to suggest that early man was responsible for the extinction of megafauna.

The Impact on Vegetation
Conversely, past proxy evidence reveals that fire intensities increased dramatically after the extinction of megafauna. This is because fire would have burned both live biomass and litter untouched by herbivores (Gill et al 2009).  This is reinforced by Rule et al 2012 who stated that a relaxation of herbivory directly caused increased fire, presumably by allowing the accumulation of fine fuel. Further evidence that disagrees with the fire hypothesis is that all sorts of organisms would have become extinct if fire was the driving factor. Other arguments suggest that some plants are fire-tolerant, having adaptive mechanisms of coping with extreme temperatures. This might have sustained mega herbivores leading to the strengthening of alternative extinction hypothesis. Fire also supplies a valuable source of nutrients to the soil, which would have eventually increased the productivity of the land and would have sustained megafaunal populations.

Re-growth after forest fire
In conclusion lack of fossil evidence and  uncertainties in dating has made it exceptionally hard to test the fire hypothesis. Fire caused landscapes to be modified and habitats to be fragmented, but whether this was enough to drive extinction is still unknown. 

Click here to learn more about firestick farming practices:

Monday, 12 November 2012

Can Climate be Been to be Influential in the Extinction of Australian Megafauna?

In past blogs I have questioned whether humans were responsible for the death of megafauna in Australia. In this blog I will discuss the impact of climate in this controversial debate. I will also discuss that whilst there is evidence of human causation (see earlier blogs), there is mounting evidence to suggest that the last 400-300 ka, Australia has been characterised by escalating aridity and climatic variability (Wroe 2006)

Aridification in Australia
Only 35% of Australian megafauna have been recognized to have survived the Penultimate Glacial maximum and whose death is likely as a result of human hunting. Therefore 65% cannot be reliably placed within 85,000 years of firm evidence for human arrival (Wroe 2006). As a result, the reasons behind the extinction of megafauna are still not 100% known. Whilst scientists like to identify the exact cause of mega fauna collapse  it is not clear whether extinction was driven by a multitude of processes. For example the late survival of West Indian sloths was suggested to support anthropogenic causation in North America, but this does not demonstrate that continental extinction would not have taken place in the absence of climate change (Wroe et al 2006). Therefore, it is incredibly difficult to tell whether extinctions in various continents was a result of climate variability and/or the colonization of early humans.  Australia is an example of a continent with an unknown ‘primary’ causation of megafaunal extinction. This is because there is absence of direct evidence for either predation or habitat modification (Wroe et al 2004). Furthermore there is little information known as to the timing of extinction and human colonization. Changes to arid conditions might have caused vegetation shifts (Habitat modifications), which would have decreased the amount of suitable areas for megafaunal populations to survive. 

Wast Indian Sloth
Genyoris Newtoni

Evidence of anthropogenic impact in Australia has been largely based on remote island studies, with humans being undeniably the cause of megafaunal extinction in these regions. However these studies cannot justify why megafauna became extinct on the whole continent of Australia. Similarly, evidence of megafauna such as Genyornis newtoni disappearing before climate change took place, urges some to believe the human causation hypothesis. However, there is some evidence of past climatic variability. For example, sea level data highlights around ca700 ka there was a greater shift to greater glacial-interglacial amplitudes. Similarly there is evidence for increased levels of aridification in Australia (from ca 400 ka) which would have subsequently modified landscape patterns. Higher levels of pollen and charcoal related to eucalypts also suggest increased levels of aridity. High concentrations of continental dust from the eastern seaboard demonstrates how climate variability would have caused ecosystem alterations, which might have accounted for megafaunal extinction. Dodson (1998) reinforces this belief by stating how Australia suffered contractions in the cool drier periods of the glacial maxima casing an expansion of arid environments. The loss of a significant number of species by 80ka would predate known human arrival, and coincide with significant climatic events.

Other examples of climate having an impact on the extinction of mega fauna are shown in:
1 Questions whether mega fauna were able to learn anti predator behaviours.
2 Hunting technology such as spear throwers, and butchery tool only appeared    after the LGM.
3 The lack of evidence for the survival of species after the PGM.
4 The hydrological threshold was breached in the course of the last glacial cycle (Wroe etal 2006). This would have reduced supplies of water which would result in the extinction of large fauna.
5 Lack of knowledge as to megafauna responses to glacial-interglacial cycling (Prideaux etal 2007)

Australian climate variability: Aridification
Overall, Australia underwent the worst extinctions of all the continents, losing 90% of its mega fauna by ca. 45 ka (Roberts et al 2001). This blog has highlighted the influence of climatic variability in causing widespread extinction. However, opinions remain strongly contested as to the most influential driver of megafaunal collapse. Therefore, it is fair to say that the combination of climate variability and anthropogenic forces are likely causes of extinction in Australia.

Extinction of Megafauna: The Impact of Climate

Could Climate cause the decline of suitable areas that sustained megafaunal populations?  
After discussing the importance of early humans in the extinction of megafauna, I will now discuss the role of climate. The late Quaternary period saw the rapid extinction of the majority of the world’s terrestrial megafauna (Prescott  et al2012). The various causes that led to the demise of megafauna is highly controversial as there is conflicting opinion as to whether humans and/or climate were responsible.  Prescott et al (2012) analyses this through looking at the distribution and timing of all megafaunal extinctions in relation to climatic variables and human arrival on five landmasses. His main findings conclude that extinctions can be best explained by models combining anthropogenic and climatic forces. This is reinforced by Grund et al (2012) who stated how ‘the cause of the terminal Pleistocene extinctions in North America is debated but is most commonly ascribed to climate change and anthropogenic overkill’.

The impacts of climate
There are various different reasons why climate can be responsible for the extinction of megafauna. During the Late Pleistocene, climate was subject to rapid oscillations between glacial (colder) and inter-glacial (warmer) conditions. Resulting changes in climate might exacerbate human impacts (Wroe et al 2006), resulting in increased levels of hunting for survival. Secondly, significant cooling events would have caused landscape modifications and altered ecosystems, resulting in megafauna lacking a viable habitat in which to survive. Such environmental pressures may have caused megafauna that are unable to adapt to die out, or species with small geographic ranges to face extinction. This is a significant issue with small landmasses as they would be less likely to provide refuge from climatic changes. The more likely individuals have access to ‘refuge’ habitats, the greater the species potential to support a larger population size, enabling species survival and re-colonization after localized extinction events (Grundet al 2012). The late Pleistocene transition 130,000 yrBP was a time of extreme climate change, with average temperatures decreasing by 80C or more. Prescott et al 2012 stated that of the climatic variables, the strongest predictor of extinction rate is associated with the most rapid rate of temperature decrease within a time period. Wroe et al (2009) stated that of the twenty middle Pleistocene species of mega fauna found in Naracoorte, only four persisted beyond this crucial point. Furthermore, in North America, only 45% of the genera persisted to within 1000 years of human arrival (Wroe et al 2006). An example of a continent that suffered altered conditions is Australia, which has been subject to progressively increased level of aridification causing ecosystem instability (see next blog). Therefore, climatic variability may be seen to be responsible in causing megafaunal collapse. Another example is the expansion of the Scansinavian and Alpine ice sheets as a result of falling temperatures during the last glacial maximum. Iice advancement converted wooded areas into treeless regions, thus having severe impacts on species such as the Megaloceros giganteus (the ‘Irish elk’). Consequently, climate change would have reduced the growing season of plants, which would have severe impacts on megafauna that relied on them. 
Scaninavian Ice Sheet
Megaloceros giganteus
Evidence against the impact of climate is stated my many researchers who believe that increased human population densities and the introduction of many non-human predators are more important causes of extinction. Strong evidence supporting the overkill hypothesis is that large fauna became extinct and not smaller animals. This reinforces evidence suggesting that early man selectively hunted larger prey. Wroe et al (2006) states that there is an inherent supposition that previous glacial maxima was similar to the last Glacial Maximum (LGM) and caused no major losses. The fact that some mega fauna appeared to persist until the start of the last glacial maximum 30,000 yr BP reinforces evidence to suggest that climate change was not a main driver of megafaunal extinction. Evidence that disagrees with climate causing shifts in vegetation is mentioned by Grund et al (2012) who stated that the hunting of mega herbivores may have altered habitat conditions as megafauna were no longer present to alter vegetation on a large scale.

Overall, it is clear to see that the impact of climate can be seen to have significant influence over the extinction of megafauna during the late Pleistocene. Whilst the impact of climate varies for each continent, it is important not to overlook its importance in driving megafaunal extinction. Whilst climate might not be the cause of extinction in all continents, it might account for the loss of particular species. In conclusion, whilst other factors such as the overkill hypothesis may be more important than climate in faunal collapse, the influence of climate should not be discredited. In the majority of circumstances, it was the combined influence of both man and climate which caused such widespread extinction.