Increasing global food demand, low grain reserves and climate change threaten the stability of food systems on national to global scales1,2,3,4,5. Policies to increase yields, irrigation and tolerance of crops to drought have been proposed as stability-enhancing solutions1,6,7. Here we evaluate a complementary possibility—that greater diversity of crops at the national level may increase the year-to-year stability of the total national harvest of all crops combined. We test this crop diversity–stability hypothesis using 5 decades of data on annual yields of 176 crop species in 91 nations. We find that greater effective diversity of crops at the national level is associated with increased temporal stability of total national harvest. Crop diversity has stabilizing effects that are similar in magnitude to the observed destabilizing effects of variability in precipitation. This greater stability reflects markedly lower frequencies of years with sharp harvest losses. Diversity effects remained robust after statistically controlling for irrigation, fertilization, precipitation, temperature and other variables, and are consistent with the variance-scaling characteristics of individual crops required by theory8,9 for diversity to lead to stability. Ensuring stable food supplies is a challenge that will probably require multiple solutions. Our results suggest that increasing national effective crop diversity may be an additional way to address this challenge.
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The sources of all data used in this study are referenced in the Methods and all raw data are freely accessible at the URLs provided in Extended Data Table 1. The dataset used for the analyses is available from the corresponding author upon request.
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We thank the Bren School of Environment Science and Management of the University of California Santa Barbara for support. This work was also supported by a grant overseen by the French National Research Agency (ANR) as part of the ‘Make Our Planet Great Again’ program (17-MPGA-0004) and by a National Science Foundation grant (LTER-1831944). We thank the FAO and its member countries, the University of East Anglia and the Center for Systematic Peace for data collection, dissemination and guidance on data use.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 Relationship between effective crop species diversity and crop species number per nation.
a, Black dots are mean effective crop species diversities and bars show the σ for nations grouped as planting 1–20, 20–40, 40–60, 60–80 or 80–100 crop species during 2001–2010 (n = 91). Data for each nation are shown as grey dots. Note that for a given number of crop species, there is a wide range in their effective crop species diversity caused by some nations having only a few dominant crops (and thus having a low effective diversity) and other nations having many crops of more similar abundances (and thus a high effective diversity). The two circled dots highlight 2 such nations, both growing 30 crop species but either very unevenly (that is, the dot with low effective diversity) or more evenly (that is, the dot high effective diversity). b, The frequency distribution of the effective crop species diversity values for this same time period.
a–f, Magnitude of the change in national yield stability as dependent on effective crop group diversity (a) and effective species diversity (d), precipitation instability (b, e) and irrigation (c, f). a–c, Values of national yield stability are predictions from the multiple regression model using effective crop group diversity (Extended Data Table 2a). d–f, Values of national yield stability are predictions from the multiple regression model using effective crop species diversity (Extended Data Table 2b). Predicted values were back-transformed from log-transformation, calculated using the observed range of the three predictors and keeping all the other predictors at their mean values. The grey bands represent the regression 95% confidence interval.
Extended Data Fig. 3 Contribution of crop groups to national caloric yield stability for each of six geographical regions.
A positive value of the log-transformed response ratio of yield stability for a crop group indicates that the presence of that crop group has a stabilizing effect. A negative value indicates a destabilizing effect. National log response ratios are represented per geographical region. In most regions, the presence of a given crop group is associated with increased national yield stability (n = 819).
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Renard, D., Tilman, D. National food production stabilized by crop diversity. Nature 571, 257–260 (2019). https://doi.org/10.1038/s41586-019-1316-y
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