Photo: Robert DiNapoli
Point-process mapping links Easter Island statuary to freshwater sources.
By Robert DiNapoli, University of Oregon; Carl Lipo, Binghampton University; Tanya Brosnan and Matthew Becker, California State University, Long Beach; Terry Hunt, University of Arizona; Sean Hixon, Pennsylvania State University; Alex E. Morrison, University of Auckland.
Rapa Nui (Easter Island, Chile) is famous for its elaborate ritual architecture: more than 300 monumental platforms (ahu) and nearly 1,000 monumental, multi-ton anthropomorphic statues (moai). To date, however, we lack explicit modeling to explain spatial and temporal aspects of monument construction.
Photo: Steven Sullivan/Shutterstock.com
In a span of only about 500 years, from the 13th century A.D. to European contact in A.D. 1722 and into historic times, the Rapanui islanders sculpted and erected these famous megalithic statues.
Why? And why were they placed where they stand?
For many years, scholars thought that the island must have supported a larger and more complex society under more prosperous environmental conditions that then collapsed following a self-imposed “ecocide.” In recent years, nearly every major component of this narrative has been shown to lack empirical sufficiency.
In this paper, we use spatially explicit point-process modeling to explore the potential relations between ahu construction locations and subsistence resources, namely rock-mulch agricultural gardens, marine resources and freshwater sources — the three most critical resources on Rapa Nui.
Through these analyses, we demonstrate the central importance of coastal freshwater seeps. Our results suggest that ahu locations are most parsimoniously explained by distance from freshwater sources, in particular coastal seeps, with important implications for community formation and inter-community competition in precontact times.
The island’s marginal ecology limited the food options available to the inhabitants. These environmental constraints could be a key factor in the emergence of monuments on Rapa Nui, such as their role as adaptive responses to environmental uncertainty or as territorial signals of control over limited resources.
At top are the locations of image-ahu on Rapa Nui (Easter Island). At bottom are locations of freshwater sources. (Images: Authors)
We quantitatively modeled how the spatial distribution of ahu is explained by different resources thought to be the focus of competition.
Point-process models (PPM) are a wide class of spatially explicit models that facilitate formal analysis of the relationship between point-patterns and a range of spatial covariates. PPM works by fitting a spatial intensity function to the intensity of an empirical point pattern and finding the values of the predictor variables (i.e., parameters) that best fit the data.
The technique is similar to geographically weighted regression or maximum entropy modeling but has a number of strengths, such as its ability to simultaneously model both first-order and second-order properties in the underlying point-pattern and how these properties may be dependent upon a set of underlying spatial covariates.
Our full paper in the January 2019 issue of PLOS One, an open-access scientific journal published by the Public Library of Science, presents a series of formal models that indicate that if Rapa Nui’s monuments did indeed serve a territorial display function (in addition to their well-known ritual roles), then their patterns are best explained by the availability of the island’s limited freshwater.
More info at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0210409.
