|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: