Global relationships between crop diversity and nutritional stability

Abstract

Nutritional stability - a food systems capacity to provide sufficient nutrients despite disturbance - is a critical feature of sustainable agriculture, especially in light of ongoing climate change. Yet, measuring nutritional stability has proven challenging. Addressing this challenge will help identify resilient food systems, detect shortcomings in nutrient availability, and evaluate if stability-focused interventions actually work. We develop a novel approach that uses 55 years of crop data across 184 countries to assemble over 22,000 bipartite crop-nutrient networks. We then quantify the tolerance of these networks to disturbance simulated via sequential crop loss (Fig. 1) and evaluate patterns of crop diversity and nutritional stability across countries, over time and between crop supply scenarios (imports versus in country production). We observe a positive, saturating relationship between crop diversity and nutritional stability across countries; however there is substantial variability between countries over time. Next, despite crop diversity gains since 1961, nutritional stability has remained stagnant or decreased in all regions except Asia. A decline in the average number of nutritional links per network (range: -3 to -18% across regions) and the aforementioned saturating relationship explain this counter-intuitive finding. Finally, we find that imports increase crop diversity and improve or sustain stability, indicating that nutrient availability is market exposed in many countries, particularly developing states. Although applied globally, our approach is applicable across levels of organization, from household intake to sub-national production, and provides a way forward for understanding the contributions of crop diversity to the stability of nutrients available for human consumption. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=113 SRC="FIGDIR/small/227520v1_fig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@d6712forg.highwire.dtl.DTLVardef@a0cc45org.highwire.dtl.DTLVardef@c2d6bforg.highwire.dtl.DTLVardef@1a0a459_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 1C_FLOATNO A new approach for understanding nutritional stability. The robustness of networks, such as a crop-nutrient network (a), can be gauged through assembling attack tolerance curves (b & c), whereby as nodes (i.e. crops) are eliminated constituent nutrients are lost. Of course, removal order matters (b vs c), therefore permuted combinations of crop loss generate average loss curves (see Methods). We use the area under this curve as our measure of nutritional stability (RN). Crop-nutrient networks with few redundant connections are susceptible to crop removal and can experience rapid nutrient loss (e.g. Maldives; d), whereas countries with a diverse food supply are more robust (e.g. China; e). This unitless metric is generalizable across different levels of aggregation (e.g. individual, household, community, national food system). C_FIG