Global relationships between crop diversity and nutritional stability

Ensuring food security for a growing population faces increasing challenges from market volatility, land degradation, pests, and climate change. Traditional approaches focus on total yield or calories, but the growing emphasis on nutrition-sensitive agriculture highlights the inadequacy of this metric. Nutritional diversity, encompassing diet diversity and the range of nutrients needed for a healthy life, is now a critical concern. Crop diversification is considered a promising strategy to improve dietary diversity and nutritional status, although the direct link between crop diversity and nutritional outcomes remains unclear. Existing methods for measuring the impact of agrobiodiversity on nutrient provision lack the capacity to capture the ability of agriculture to sustain nutritious food production through space and time, especially in the face of disturbances. This research addresses this gap by developing a novel approach to quantify "nutritional stability," defined as the food system's capacity to provide sufficient nutrients despite disturbances. This is particularly relevant to Sustainable Development Goal Target 2, which emphasizes the interconnectedness of crop diversity, resilient farming systems, and nutritious diets. While plausible mechanisms exist for crop diversity to positively influence human nutrition, the fragility of this relationship remains largely unexplored due to limitations in tracking stability changes within food systems. The study aims to quantify the extent to which crop diversity underpins nutritional stability over space and time.

The literature reveals mixed evidence on the relationship between crop diversity and nutritional outcomes. Calls for multilevel and systemic approaches that can measure the effect of agrobiodiversity on nutrient provision at various scales (village, region, nation) have emerged. Approaches like potential nutrient adequacy attempt to combine multiple metrics, but none adequately captures a food system’s ability to produce nutritious food despite environmental and economic disturbances. This study builds upon previous work examining the link between crop diversity and nutrition-relevant outcomes at the field or landscape levels, expanding the scope to a national scale by analyzing both production and import contributions to nutritional stability. Previous research has highlighted the increasing homogeneity in global food supplies, posing a potential threat to food security, and it has underscored the critical role of international trade in shaping crop diversity.

This study uses a novel analytical framework to measure the relationship between crop diversity and nutritional stability. The framework links crops and their constituent nutrients into a bipartite network. Nutritional stability is quantified by simulating the effect on nutrient availability when crops are removed from the network, resulting in a unitless metric reflecting the robustness of various crop mixes in providing nutrients. This approach, previously used in fields like information systems and ecological networks, is applied for the first time to food systems. The network's structure is influenced by crop species identity; nutrient-rich crops have more connections. Networks with many nutrient-rich crops are more robust to crop loss. A generalized "robustness curve" is generated through permutations of removal sequences, and nutritional stability (Rs) is defined as the area under this curve. Three removal procedures are performed: random removal, removal from most-to-least connected crops, and removal from least-to-most connected crops. The analysis utilizes over 22,000 crop-nutrient networks constructed using the Global Expanded Nutrient Supply (GENUS) database and 55 years of FAO food production data for 225 crops in 184 countries. Each country has its own network, used to quantify nutritional stability using the three removal procedures. Crop diversity, nutrient diversity, and average crop degree are also calculated. The study evaluates patterns of nutritional stability and its relationship with crop diversity across countries, over time, and for two supply sources: domestic production (P) and production plus imports (PI). The analysis explores the relationship between crop diversity and nutritional stability, examines changes in crop diversity and nutritional stability over time, and investigates regional patterns influencing differences in nutritional stability. Statistical analyses involve non-linear mixed effects models and linear mixed effects models to account for regional and temporal variability, using AR1 correlation structure to account for temporal autocorrelation. To address the issue of negligible contributions from some foods, the study calculates the proportion daily value (PDV) per serving and uses these as link weights, defining a threshold below which links are considered non-existent. This threshold is determined by analyzing the mean and variance of stability across a range of cutoff values, resulting in the choice of a threshold of 0.1. Linear models are used to compare differences in parameter coefficients of nutritional stability and crop diversity between regions.

The study reveals a non-linear, saturating relationship between crop diversity and nutritional stability, with regional variations. Nutritional stability increases with crop richness, but additional crops beyond a certain threshold do not significantly improve stability. The average number of crops varies regionally, but most networks included all 17 examined nutrients. Gains in nutritional stability slow after 7-16 unique crops, indicating a threshold effect. Crop diversity increased since 1961 for all regions except Oceania, with considerable regional variability and a significant role of imports. Asia and Europe showed substantial increases in imported crop diversity. Europe demonstrates a large gap between imports and production, with imports driving diversity increases. Spatial variation in nutritional stability exists, with high and stable availability in the US, Brazil, and parts of Europe, contrasted with low and variable stability in the Middle East, Southeast Asia, and Africa. Small island developing states and low-income countries often have low stability. Despite increased crop diversity, nutritional stability remained stagnant or decreased in all regions except Asia. These regional trends remain consistent across different crop removal procedures. Gains in regional nutritional stability are solely associated with imports, not domestic production. For example, Asia's increased stability results from imports, while production-based stability declined. Africa's decrease in production-based stability contrasts with unchanged import-based stability. While crop diversity from production and imports increased for 72% of countries, 87% experienced a decrease in average crop degree (number of nutrients per crop), correlating with decreased nutritional stability. This decline in crop degree occurred across all regions, regardless of supply source. The saturating relationship between crop diversity and nutritional stability highlights diminishing returns from adding more crops, especially when those crops provide fewer nutrients not already available. However, adding crops with few nutrients could be beneficial if those nutrients are currently limited in the food system. The study indicates that developing states and small island states show lower nutritional stability.

The study’s findings highlight a potentially counterintuitive observation: increased crop diversity does not always translate to improved nutritional stability. This is partly explained by the saturating relationship between these two variables and the diminishing returns on nutrient availability from adding more crops, especially crops that do not introduce new nutrients into the food system. The decline in average crop degree further clarifies this, indicating a shift towards less nutrient-rich crops. This underscores the importance of considering not just the number of crops but also their nutritional value and the overall structure of the crop-nutrient network. The significant role of imports in many regions emphasizes the market exposure of crop diversity and nutritional stability. While trade can enhance food security, it also creates vulnerabilities to trade wars, market shifts, and price shocks. The study's results have implications for understanding nutritional measures, reaffirming the importance of crop diversity for agricultural resilience at a national scale, establishing a functional (non-linear) relationship between crop diversity and nutritional stability, and revealing the significant role of imports in maintaining nutritional stability in many regions. The spatial variability in nutritional stability highlights vulnerabilities within and across regions, particularly in Africa. The limitations of the study include its focus on nutrient availability and not food security or nutritional outcomes, limited consideration of animal-based products and variability in crop micronutrient composition, and the assumption of equal removal probability for all crops. Future research could integrate nutrient availability data with nutritional intake data, advance network modeling by considering non-random crop loss, and address the impacts of climate change on crop yields and trade dynamics.

This study advances a method to assess the relationship between crop diversity and nutrient availability globally. The findings highlight the importance of considering both crop diversity and the nutritional value of individual crops for ensuring nutritional stability. The non-linear relationship between crop diversity and nutritional stability suggests that simply increasing the number of crops may not be sufficient; the choice of crops matters. The significant role of imports underscores the vulnerability of many countries to market fluctuations. Future research should explore the links between nutrient availability, consumption, and health outcomes, refine the network approach to incorporate variable crop vulnerabilities, and investigate how climate change will affect crop diversity and nutritional stability.

The study focuses on nutrient availability rather than food security or nutritional status. Animal-based products were not extensively considered, and variations in crop nutrient content are not fully accounted for. The assumption of equal removal probability for all crops is a simplification. Data limitations, especially in low-income countries, also exist. The study's findings should be interpreted considering these limitations.