Easter and erosion: Could soil erosion have been a contributing factor?

Now that we’ve looked at the impacts of deforestation on Easter Island, its time to look into a couple of other causes, and whether or not they contributed to Easter’s demise. Using a study by Meith and Bork (2005) in their paper, ‘History, origin and extent of soil erosion on Easter Island (Rapa Nui)’, the existence and contribution of soil erosion on Easter’s demise will be examined. Meith and Bork (2005) follow the view shared by Jared Diamond that:

“The isolated Easter Island (Rapa Nui) is an outstanding example of land degradation caused by land use in a sensitive ecosystem”

Meith and Bork (2005) use a detailed analysis of the causes, chronology and spatial pattern of soil erosion on the Poike peninsula on Easter Island to answer the following questions:
   1. Did erosion of Rapa start with the rise of Polynesian agriculture?
   2. Can phases of different chronological differentiation of soil erosion processes?
   3. Was soil erosion a less significant problem in the prehistory of Rapa or was the loss of fertile soils an important reason for the complete change in way of life and culture about AD1400/1500?

Phases of prehistoric land use and soil erosion were investigated in the most eastern part, northwest and southwestern parts of Poike peninsula, whereby all the investigation sites were all characterised by severe erosion. The method for reconstructing past soil erosion is an entire topic in its own right, and a comprehensive methodology is outlined in the paper, however I won’t go into that right now.

Figure 1. Complex of land use and soil erosion in the most eastern part of Poike peninsula near Cabo Cumming

Figure 2. Soil profile from the most eastern part of Poike peninsula

The two figures above represent the soil profile of the Poike peninsula. Historically, the most interesting part of the bottom of the soil profile is the existence of root channels, which have been identified as the Jubaea chilensis palm found on Easter Island when the Polynesians first settled. Above this layer is yellowish soil that shows evidence for planting pits, which indicated agricultural land use. The absence of soil erosion and quality of the soil in both these layers suggests a phase of successful soil protection and sustainable agriculture up until AD 1300. Meith and Bork propose the reasoning for this is because the palm trees protected the soil against intensive radiation, strong winds and heavy rainfall.

The soil profile shows that this phase ended suddenly with the appearance of charcoal in the soil profile. Meith and Bork state that ‘obviously the palm forest was subjected to an extended woodland clearance’, which (in my opinion) is a rather sweeping statement, as if I have shown anything so far in this blog, the controversy surrounding deforestation and the evidence to support deforestation are certainly not clear, and do not merit a statement such as this one.

However, my opinion aside, Meith and Bork continue to suggest that woodland clearance in AD 1400 had dramatic consequences for soil erosion:
   1. Loss of protection from the palm trees caused sheet erosion
   2. Agriculture under open land conditions might have destabilised slope
   3. Erosion and accumulation process shown in soil layer were caused by heavy rainfall and started at downslope areas

This erosion was thought to be the main cause for the abandonment of agriculture in AD 1500, where after AD 1400/1500, human influence can longer be seen in the soil profile.

So what can we infer from soil erosion, and was a contributing factor for the collapse of Easter Island? Well Meith and Bork propose that the sudden shift from the bottom layers to those influenced by humans suggests that the techniques of land use shifted suddenly from traditional sustainable agro-forestry to intensive non-sustainable land use in open land, however the reasons behind this shift are unknown. They suggest that soil erosion and consequently a decrease in soil fertility caused abandonment of agriculture and moai construction on the Poike peninsula, and the burial of settlements. They conclude that:

‘On the basis of results from Poike peninsula, we hypothesis that soil erosion may have had a dominant role in the break down of Easter’s stone culture’

However, soil erosion has only been investigated in detail on the Poike peninsula. We cannot reconstruct whether soil erosion was a contributing factor in Easter’s demise based on one area, thus much more research on soil erosion on Easter Island should be conducted before we can make such conclusions.

That said, it is an interesting thought that soil erosion may have been a contributing factor to Easter Island’s collapse….