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. 

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