COS 55-9 - Pre-contact intensive agriculture on Rapa Nui (Easter Island): Fine-scale biogeochemical sweet spots?

Wednesday, August 10, 2011: 10:50 AM
6A, Austin Convention Center
Peter M. Vitousek, Department of Biology, Stanford University, Stanford, CA, Oliver A. Chadwick, Department of Geography, University of California, Santa Barbara, CA, Thegn N. Ladefoged, Department of Anthropology, Auckland University, Auckland, New Zealand and Christopher M. Stevenson, Virginia Department of Historic Resources, Petersburg, VA

Intensive agriculture on Rapa Nui supported a population with sufficient surplus labor to create and transport the massive statues for which the island is famous.  However, comparisons with pre-contact agricultural systems in Hawaii – and the soils that supported them – suggest that most Rapa Nui soils are marginal in fertility compared to those on which intensive rain-fed systems were developed and sustained in Hawaii.  We evaluated soil nutrient availability and Nb-based geochemical indices of weathering intensity within and surrounding several rock gardens across Rapa Nui, and analyzed fine-scale variation in soils in the Anamarama region.  Rock gardens there surround outcrops of basaltic rock; both natural and cultural processes have removed rocks from the outcrops and scattered them on the surface nearby. 


Soils on the steep sides of the outcrop were little-weathered and low in resin-available P (~ 20 ppm); those on gentle slopes with surface rock cover were moderately weathered and high in resin P (100 – 120 ppm), and those between outcrops were highly weathered and intermediate in resin P (40-60 ppm).  We conclude that Rapa Nui cultivators intensified agriculture on the gradual slopes where erosion and deposition had created a fringe of higher soil fertility – and that their activities may have improved soil fertility in these “sweet spots”.  In contrast, Hawaiian cultivators intensified agriculture in landscape-level biogeochemical sweet spots tens of square kilometers in size.  

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