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|
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.