Higher crop diversity could have a substantial impact on stabilising national food harvests and mitigating sharp declines in yields, according to a paper published in the journal Nature.
For more than a decade, severe droughts and extreme heat have caused crippling drops in grain harvests around the world, including Australia, the US and Russia, heralding a global food crisis.
“Combined with socioeconomic, demographic, and policy conditions, climate-related crop losses have increased poverty and worsened access to nutritious food for many people, especially in developing nations,” says lead author Delphine Renard from the University of California Santa Barbara.
Renard and co-author David Tilman propose a crop diversity-stability hypothesis to complement other efforts to increase crop stability from year to year.
The hypothesis is based on a mathematical theory called the “portfolio effect”, which predicts the conditions under which diversity can lead to greater stability.
Renard explains, “In a diversified system, one crop may fail, but multiple failures are less likely, because when the yield of one crop is negatively affected by high temperatures, the yield of another crop may be less affected and may compensate for the other, thus stabilising the total yield.”
To test the theory, the researchers analysed five decades of data on annual yields of 176 crop species in 91 nations, combined into seven groups: cereals, vegetables, fruits, pulses, oil, sugar, stimulants and spices.
Yield stability was calculated as an index, using probability distribution analysis that measured average yield divided by year-to-year temporal stability of crop yield over 10-year blocks.
They found that assorted crop species and groups improved national yields independently of other variables, including irrigation, fertilisation, rainfall and temperature.
Moreover, the stabilising effect of crop group diversity was comparable in magnitude to the weakening impacts of irregular rainfall and extreme temperatures, considerably reducing the frequency of years with severe harvest deficits.
“The resulting curve implies that even a small increase in stability leads to greatly decreased probability of years with major crop losses,” says Renard.
Specifically, a nation with an average stability index of five had a national yield drop of more than 25% once every eight years, while nations with stabilities of 7.5, 10 and 15 would experience similar declines once every 21, 54 and 123 years.
The researchers write that increasing crop diversity could therefore “complement the benefits of new drought-tolerant crop varieties, increased irrigation, intercropping and greater and more transparent trade”.
They could not yet identify specific combinations of crop species or groups that optimise yield stability. Renard suggests that “it is most probable that these assemblages are place-specific” and are matched to favourable environmental conditions.
From previous research, she says, they know crop rotations provide benefits in terms of productivity because they improve crop nutrition – for instance, legumes increase the availability of nitrogen for cereals.
Renard hypothesises this might help explain their findings and is now investigating further with funding from a program called Make our planet great again, initiated by France’s president, Emmanuel Macron, in 2017.