The Usual Suspect?

In my last post I discussed whether or not ecocide was the cause for the collapse of the society on Easter Island and as I concluded with a probable 'no' I'm now going to talk about what else could be to blame. Now, the title of this post doesn't refer to the mid 90s neo-noir film starring Kevin Spacey (although I highly recommend it), but instead I'm going to be talking about the common suspect behind environmental issues - climate change.

brrr! 

When I started reading around to understand the argument behind climate change as the cause for deforestation on Easter Island, two issues came up repeatedly - that there is no direct information about climate change on Easter, especially between AD1000-1700 (Diamond 2005, 2007, Mann et. al. 2008) and that Easter's forests survived tens of thousands of years of climatic fluctuations since the Pleistocene (Diamond 2007, Hunt 2007).

These two issues therefore already make the case for climate change a tough one but two articles I read argue the case in spite of this. Mann et. al. (2008) argue that although palaeoenvironmental history of Easter is very poor, some hypotheses can be made. One interesting argument is that latitudinal shifts in subtropical storm tracks could have resulted in changes in intensity and frequency of cyclonic storms in the Pacific in a way that would have deprived Easter of a great deal of rainfall (Mann et. al. 2008). Most of the rain on the island comes from cyclonic storms moving across the Pacific, but the track of these storms can shift depending on the temperature gradient between the pole and equator which varies on an annual, decadal and millenial time scale. It therefore could be that these shifts in storm tracks may have deprived Easter of rainfall that was already fairly low, leading to severe droughts on the island and thus driving deforestation. Although this is an interesting hypothesis, Mann et. al. agree that it is as yet untested.

Strenseth et. al. (2009) argue that periods of greatest deforestation (AD1250-1650) coincide with the most intense El Nino Southern Oscillation activity during the last millennia, suggesting that there could be some correlation. ENSO is known to directly or indirectly cause shifts in Sea Surface Temperatures which can lead to lower biomass production. As a result of this it is possible that the islanders may have put greater pressure on forest resources in order to either build larger canoes in order to reach other fishing grounds or to create more land for agriculture to cope with lower marine food resources (Strenseth et al. 2009). In a Hunt and Lipo paper (2009), evidence suggests this is an unlikely explanation as the palm trees that largely made up Easter's forests were not likely to be used for woods and marine food resources do not disappear from the faunal record at all, with sea mammal bones remaining present in even late pre-historic deposits (i.e. there was no decline in fishing that would have led to a resulting increase on land resources, at least to an extent that would push deforestation alone) (Hunt and Lipo 2009).

Added to the two issues I raised at the start of this post and the lack of convincing arguments elsewhere, I do not think climate change played a hugely significant role in deforestation on the island, although as Cole and Flenley (2007) comment, it is likely forest vegetation recovery may have been constrained by droughts at times, I do not think that any specific climatic events caused it's downfall.