Modelling the impact of large-scale hydroclimate change on prehistoric Polynesian island life

The South Pacific was one of the last regions on earth to be colonised by humans and offers a unique opportunity to study early climate-human interactions in environments previously untouched by people. Palaeoclimate evidence suggests the South Pacific has experienced shifts between dry and wet periods throughout the past three thousand years, the broad period of colonisation, with extremes in both modes being prevalent. Drought has significant repercussions for small Pacific islands, affecting water and food resources, with potential consequences on the viability of life on these islands leading to internal stress, conflict, collapse or migration. Previously, socio-ecological models have been developed to test mechanisms of change within prehistoric societies worldwide that can lead to migration or societal change, but thus far the connections between past climatic change and prehistoric island life within the tropical South Pacific have not been fully explored. This study utilises palaeoclimatic data alongside a new system dynamics socio-ecological model to explore the relationship between climate, agricultural carrying capacity and population dynamics on the Polynesian island of Mangaia (Cook Islands) in the tropical South Pacific. Model results suggest that as the population density of the island increases, the impact of drought events on population dynamics increases. We also show that the severity of the drought rather than the return frequency drove the largest changes in carrying capacity and population dynamics. Changes in long-term rainfall leading to persistent dry conditions impacted the timing and rate of population growth due to its role as a limiting factor for agricultural productivity. We compare our modelled results with the known history of population stress and societal change from Mangaia and found these corresponded with drought periods and low food availability. We demonstrate the potential for droughts to have impacted on the early colonisation and societal change on Eastern Polynesian islands.