With Numerous Factors being Responsible for Megafaunal decline, why was Africa least affected?

Of all the continents, Africa was the least affected by the Late Pleistocene extinction (Elias &Schreve 2007). Whilst Africa contains 42 genera of megafauna; only 7 died out during the last 100kyr. This blog will explore reasons why African megafauna remained largely untouched by forces that drove extinctions in other continents.

The genera that became extinct in Africa during the last 100 kyr include a genus of Pleistocene elephant (Elephas), a genus of African buffalo (Parmularius) and a genus of modern cattle (Bos). Consequently African megafauna only suffered a loss of about 14% of their genera in the last 100kys (Elias & Schreve 2007).

A number of hypotheses have been proposed to explain why Africa suffered such few losses. Firstly, humans evolved and coincided with megafauna in Africa. Homo sapiens lived and evolved in Africa before they spread to other continents around 200,000 BP. Many believe that this co-evolution was a major factor which ensured the survival of megafauna during this period. It is also believed that megafauna were able to adapt their behaviour to withstand human hunting practices. Similarly, Africa had favourable climatic conditions which could support a high diversity of species. This ensured that species numbers grew to a level which allowed them not become extinct. As well as this, the glacial-interglacial transition was less severe than other continents. This provided greater climatic stability for the species living in Africa, especially those that were unable to adapt to changing environmental conditions,and could easily find refuge spots. Finally, early Homo sapiens in Africa had primitive hunting technology compared to hunter gatherers of North America. This is because the Clovis hunters of North America developed more advanced technology such as stone points, which might explain why extinction was rapid in this continent. This is reinforced by Barnowsky (2004) who stated that sophisticated technology is a key driver of over kill. 

Clovis Hunting Tools

Overall, I believe the combination of these factors explains why African megafauna was least affected by the late Pleistocene extinction, and consequently suffered few losses compared to other continents. 

A Brief Overview of What we have Learnt so Far

My next blogs will be focused on:

   1.      Possible causes of megafaunal collapse in different continents e.g.   Africa, Eurasia etc.

   2.       Specific examples of species e.g. woolly mammoth etc. and their likely cause of extinction.

   3.      Other topics related to the debate e.g. Sporomiella Proxy etc.

       

Before I embark on these, I would like to take this opportunity to briefly review the main findings of what we have learnt so far:

    ·     There are various mechanisms that could have caused the extinction of megafauna during the late Pleistocene (roughly 60,000-11,000 years ago). Mechanisms include: climate change, disease, humans, fire, and the impact from an asteroid/comet.

    ·         Whilst there has been a large degree of research conducted, conclusions remain deeply controversial.

    ·         Human overkill is a likely cause of extinction through hunters preying on large mammals leading to their demise (see Bulte et al 2006 & Roberts et al 2001). However evidence which disagrees with this includes the availability of alternative food sources from agriculture.

   ·         It is plausible that humans could have caused the extinction of megafauna in Australia (see Johnson 2006). The decline of species such as G.newtoni immediately after initial human colonization as well as mild climatic variability in this region, reinforces evidence to support the human overkill theory. However lack of evidence weakens this hypothesis.

·         Climate can be seen to be responsible through exacerbating human impacts (wroe et al 2006), or through habitat modification. Evidence of extreme climatic changes are present, but linking this to megafaunal decline is difficult (Lorenzen et al 2011). Similarly, the demise of species that were able to adapt to climate change weakens this hypothesis.

   ·         Fire can be seen to be responsible for the extinction of megafauna through; extreme temperatures as well as altering landscapes so that megafauna are unable to sustain themselves (see Gill et al 2009). However lack of fossil evidence and uncertainties in dating have made it exceptionally hard to test this hypothesis.

   ·         It is unquestionable that a comet hitting the earth would have ultimately led to megafaunal decline (Firestone et al 2007). However many are sceptical about this due to severe lack of reproducible evidence.

   ·         Disease is credible mechanism as megafauna might have had weak immune systems and were unable to withstand pathogens (see Rothschild & Laub 2006). However there is lack of evidence uncovering a pathogen that has the capability to cause such widespread extinction.

   ·         Poor quality fossil datasets make it exceptionally hard to discover what caused megafaunal collapse. Similarly, it is highly unlikely that extinctions across the globe were a result of a single cause. It is more likely that different species or continents were subject to different forcing mechanisms e.g. continents that experienced dramatic climate variability suffered majority of extinctions because of climate change, whilst the early colinization of man in other continents might have caused megafunal demise.

   ·         In the majority of cases perhaps combining multiple causes of extinction might be the most likely cause of megafaunal extinction.

Some examples of megafauna becoming extinct (going down): woolly mammoth, woolly Rhino, Irish Elk, Diprothodon, Giant sloth, Cave lion, G.Newtoni, Giant Kangeroo, Glytodon, Smilodon.

Some Examples of Extinct Megafauna in relation to Climate and Human Impacts

I recently found this interesting article by Lorenzen et al (2011) that links specific species responses during the late PLeistocene to climate and humans. In this blog I will look at the various suggestions put forward by Lorenzen et al (2011) and assess whether strong conclusions can be made between time of extinction in relation to climate variability and/or human colonization.

As I have stated in previous blogs, the role of climate and Homo Sapiens in driving the dramatic extinctions of large-bodied mammals during the late Pleistocene period remain contentious (Lorenzen et al 2011). In this blog, I will specifically be looking at the demographic history of species such as the woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox. The main findings from Lorenzen et al (2011) show that climate was a major driver of population changes over the past 50,000 years for certain species such as Eurasian musk ox and woolly rhinoceros. However research from ancient DNA and species distribution models also reveal that the combination of climate and anthropogenic impacts seem to be responsible for the extinction of other species such as Eurasian steppe bison and wild horse. Therefore, it is clear that each species responds in a different way to the effects of climate variability and human intrusion, making it even more difficult to predict past responses to various mechanisms of extinction.

Toward the end of the late Quaternary, beginning around 50,000 years ago, Eurasia and North America lost approximately 36% and 72% of their large-bodied mammalian genera (Lorenzen et al 2011). The two most credible causes of extinction include climate and human impact, and these were assessed by Lorenzen et al (2011) in relation to potential ranges of specific megafauna. The dominate role climate played in extinction patterns are shown in a high loss of species in continents that experienced the most dramatic climate variability. The impact of human encroachment is also shown in patterns of megafaunal decline immediately after initial colonization.

The image below (Lorenzen etal 2011) is a very useful illustration modelling specific species and their changes in distribution over time. This illustration is also effective in showing whether extinction was dramatic or occurred gradually over a long period of time. For example, genetic diversity in bison and musk ox declines gradually from 50,000-30,000 kyr BP, whilst other species such as woolly mammoth and woolly rhinoceros loss of genetic diversity occurred suddenly. It is also evident that Reindeer populations remained largely unaffected throughout the late Pleistocene.

Overall, whilst the results for the model (see illustration above) are useful in showing potential ranges of megafauna over time, this research has shown that it is difficult to make a direct link between climatic change and species extinction. Consequently, causes of extinction for certain species such as woolly mammoth are unclear. The results demonstrate that changes in megafauna abundance are idiosyncratic, with each species responding differently to the effects of climate change, habitat redistribution and human encroachment (Lorenzen et al 2011). Therefore it is highly difficult to suggest a single cause of megafaunal extinction during the Late Pleistocene as evidence remains unclear and deeply contested. In later blogs we will look at the possible explanations behind the disappearance of the wooly mammoth.

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:

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:
   
   
   
   

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:

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.