Modelling spatio-temporal changes in the ecological niches of major domesticated crops in China: Application of Species Distribution Modelling

Abstract

This thesis explores how the dispersal of crops in the past related to their ecological niches. In particular, it presents the niche models for five historically significant crops in China: rice, millet, wheat, barley and buckwheat. These models are rooted in the Species Distribution Modelling (SDM) methodology and infer the potential areas suitable for the cultivation of each crop from the present distributional data. Model predictions are extrapolated on the palaeoclimatic reconstructions to obtain suitability estimates extending back to the initial uptake of each crop in China. Those are compared against the patterns of crops’ dispersal deduced from archaeobotanical data to assess the contribution of climatic factors and human niche construction to agricultural trajectories. Specifically, the study draws on ecological niche theory, niche construction theory, cultural evolution and gene-culture co-evolutionary theory to interpret how the observed patterns could result from different pathways to niche construction and distributional expansion. The analysis identified two modes of crop dispersal based on whether those pathways involved more deliberate efforts at spreading and maintaining cultivation or passive responses to the environment. Niche-forming characterised major domesticates which originated in China: rice and millet. It manifested in the gradual broadening of the environmental range occupied by the crops over time and was associated with the pathways that implied prioritisation by farmers. In contrast, a stable environmental range identified the niche following trajectories, expected when the spread of crops was more opportunistic than intentional. Foreign domesticates (wheat and barley) and buckwheat, a comparatively minor crop, exemplify this model. Furthermore, the analysis provided insights into factors that shaped the dispersal patterns of individual crops. Notably, it demonstrated that climate constrained buckwheat movement along major transcontinental dispersal routes, followed by crops such as millet, wheat and barley. In conjunction with the niche-following pattern identified for this crop, this provides an explanation for the delayed westwards spread compared to other East Asian domesticates. Furthermore, the models revealed the discrepancy between the high suitability of environmental conditions predicted in northeast China, and the relatively slow pace of agricultural developments documented in the area, opening up a discussion about possible explanatory factors. Finally, the analysis exposed the climatic shift in the inland southern part of the country. The models demonstrated that this shift opened up a corridor of dispersal for millet and might have facilitated the spread of agriculture in this direction.