Monday, 12 November 2012

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.

No comments:

Post a Comment