Commentary on Puleston et al (2018): Commentary: Rain, Sun, Soil, and Sweat: A Consideration of Population Limits on Rapa Nui (Easter Island) before European Contact

Carl P. Lipo1*, Robert J. DiNapoli2, Terry L. Hunt3

1Department of Anthropology, Binghamton University, Binghamton, NY, USA.

2Department of Anthropology, University of Oregon, Eugene, OR, USA.

3Honors College and School of Anthropology, University of Arizona, Tucson, AZ, USA.

Keywords: collapse, demography, Easter Island, model parameterization, open science

Constructive criticism is the foundation of sound scientific practice. Problems can be addressed and the design of further research improved in pursuit of the best possible answers. On the other hand, one can simply deny problems and make contradictory statements to give the illusion that one was correct all along. In a recent response to our commentary (Lipo et al. 2018) to their article on Easter Island (Rapa Nui) demography, Puleston et al. (2018) appear to take the latter approach. In their reply to our comments, the authors mischaracterize our criticism and make false claims.

First, the authors assert that they never stated that Rapa Nui’s population size was 17,500, a number not supported by archaeological evidence. Instead, they claim merely their use of this value, or values even higher, “were theoretically possible” (Puleston et al. 2017, p. 12). Yet the authors clearly stated that, “Given our relatively conservative assumptions, we estimate the maximum population of Rapa Nui before European contact would have been closer to 17,500, the approximate average of the high-N scenarios, than the low-N average of 3,500” (Puleston et al. 2017, p. 10, emphasis added). In their reply, they reverse their position by suggesting that, “perhaps our low-nitrogen scenario, which agrees with the population estimates preferred by Lipo et al. (2018) is a better representation of pre-Contact Rapa Nui than is the high-nitrogen scenario…” (Puleston et al. 2018, emphasis added). We note that the authors now alter their original claims, consistent with the point of our commentary.

Second, Puleston et al. (2018) assert that we misread their conclusions as supporting the collapse narrative (sensu Diamond 2005). It is difficult to imagine a scenario in which a population of 17,500, or even larger, that then declined to ca. 3,000 by European contact does not involve some kind of dramatic population decline as envisioned in collapse scenarios. We reiterate that the archaeological evidence on Rapa Nui does not support a pre-European population decline.

Third, Puleston et al. (2018) take issue with our point that soil nitrogen values are not well-understood, saying that they originally acknowledged this as the impetus for exploring two nitrogen scenarios. We agree that choosing a range of parameter values in this case is the best strategy, particularly since empirical values are poorly understood. We reiterate that one of these scenarios, their high-N model yielding a population of 17,500 or higher, clearly contradicts the evidence for Rapa Nui.

Fourth, Puleston et al. (2018) simply ignore our criticism of their failure to incorporate annual variability in productivity within model runs, saying that we instead claimed they included no variability at all. Their statement is false. In the Puleston et al. (2017) model, variability is incorporated between simulation runs but not as annual productivity within individual model runs. The authors are clear in their original paper: “we have limited ourselves in this study to stable seasonal variation, meaning that seasonality is included, but each year is the same as the next. In its effects on productivity and demographic rates the inclusion of environmental stochasticity (i.e., the possibility of periods of drought or above-average rainfall) would be expected to lower population estimates” (Puleston et al. 2017, p. 12, emphasis added).

Finally, Puleston et al. (2018) claim that their agricultural productivity and demographic models are openly available and reproducible. But this is simply not the case. While the CENTURY model has been published and their demographic model has been described elsewhere, the particular versions used in Puelston et al. (2017) are not, nor is any code openly available to reproduce their study as published. We reiterate our original point that the advancement of archaeological science depends critically on openly available data and readily reproducible code (Marwick 2017; Marwick et al. 2017).

 

References

Diamond, J. (2005). Collapse: How Societies Choose to Fail or Succeed. New York: Viking.

Marwick, B. (2017). Computational Reproducibility in Archaeological Research: Basic Principles and a Case Study of Their Implementation. J Archaeol Method Theory 24, 424–450. doi: 10.1007/s10816-015-9272-9.

Marwick, B., d’Alpoim Guedes, J., Barton, C.M., Bates, L.A., Baxter, M., Bevan, A., et al. (2017). Open science in archaeology. The SAA Archaeological Record 17, 8–14.

Lipo, C.P., DiNapoli, R.J., Hunt, T.L. (2018). Commentary: rain, sun, soil, and sweat: a consideration of population limits on Rapa Nui (Easter Island) before European contact. Front. Ecol. Evol. 6. doi: 10.3389/fevo.2018.00025.

Puleston, C.O., Ladefoged, T.N., Haoa, S., Chadwick, O.A., Vitousek, P.M., and Stevenson, C.M. (2017). Rain, sun, soil, and sweat: a consideration of population limits on Rapa Nui (Easter Island) before European contact. Front. Ecol. Evol. 5:69. doi:10.3389/fevo.2017.00069.

Puleston, C.O., Ladefoged, T.N., Haoa, S., Chadwick, O.A., Vitousek, P.M., Stevenson, C.M. (2018). Commentary: Commentary: rain, sun, soil, and sweat: a consideration of population limits on Rapa Nui (Easter Island) before European contact. Front. Ecol. Evol. 6. doi:10.3389/fevo.2018.00072.

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