A variety of sources is found to bring stability, says S.Ananthanarayanan.

It is apt that adage about all the eggs in one basket relates to food, as diversity in the food supply chain is found to be effective in preventing stock-outs of the larder.

Michael Gomez, Alfonso Mejia, Benjamin L. Ruddell and Richard R. Rushforth, from the Department of Civil and Environmental Engineering, Pennsylvania State University, and the School of Informatics, Computing, and Cyber Systems, Northern Arizona University, describe in the journal, Nature, their study of what characterises the places, out of over 300 cities and centres in USA, which were best able to avoid disruptions to food supply over a four-year period. They find that the places with the most diverse sources of supply were the ones that had the most resilient food supply line during 2012 – 2015, the years when most of USA faced moderate to severe droughts.

Like eggs in one basket, or the French saying about the mouse that has only one hole to hide in, many crucial networks of supply or service rely on diversity for stability. A well-known example is the Internet, which handles immense and highly variable traffic, but we rarely hear of a breakdown, or even a serious loss of speed. The reason is that a message is not sent out as a load to be carried down a fixed pathway, but the message is broken into fragments and each finds its own way, using widely diverse pathways, equalising the traffic over the best routes and being ready with alternatives if some routes become ineffective.

The other well-known example is the stability of ecosystems. The most luxurious forests, and the ones that can take periods of drought, series of poor summers or severe winters, attack by fungi, or locusts, in their stride, are the ones that have high diversity of species growing together. The importance of insect and animal diversity has been accepted and much of conservation efforts, worldwide, is for conserving diversity.

The constancy of the pressure of a volume of gas is also seen as a result of the range of the speeds of the molecules of the gas. Given the temperature, there is a distribution of the speeds. But, with the large number of molecules, the distribution is unchanging, for practical purposes, and so is the pressure of the gas. The diversity in physical systems has been studied with mathematics, and there is a measure, called the entropy, of the level of uncertainty, which reflects diversity, in the system. The scientist Claude Shannon, who is known for his work on the accidental ‘noise’ that affects information that is sent through telegraph wires, also worked on the diversity in a string of text – measuring how the letter that would turn up next is more uncertain when many different letters are there in the text. The ideas have been extended to ecological systems and a measure to quantify diversity is called the Shannon Index. And in their work on the food supply chains in USA, the authors of the paper in Nature work out this index for the food networks in operation.

The context is the rising dependency of large human settlements on the supply chain of food – cereals, vegetables, milk, meat products. The paper notes that extreme weather events, which are expected to be frequent as a result of climate change, have been the reason for ‘food supply shocks’ – where stores in large cities suddenly find their shelves empty. Geopolitical and policy changes, and events like the pandemic can also affect sources and supply channels. There is a growing risk of ‘global breadbasket failure,’ the paper says. As sources of food supply to a place are now widespread, even international, rather than ‘local’, the result of a crisis can have far-flung consequences. This increases the range of events that could affect supply, the paper says, although having more sources of supply also provides resilience.

The study covered the stability of food supply and the food supply system, that is, the sources of crops, live animals or animal feed and meat, at 284 cities and 45 other centres in the USA. And the consumption centres were classified according to the frequency of ‘food shocks’, or instances when food supply fell by over a given percentage (3% to 15%) for a year, compared to the average over four years. “Thousands of inflows to hundreds of cities” allowed the team to calculate the probability of ‘shock intensity’ growing beyond a limit, over a period of four years. And along with this, was the assessment of the diversity of the supply sources of the cities.

The manner of assessing diversity of sources was to list out each city’s trading partners or neighbours, the food traders-suppliers, of different categories of food. These were classified under physical distance, climate correlation, urban classification, economic specialization and cities that formed clusters. And based of these criteria, a measure of diversity of supply sources, related to the ‘Shannon Index’ was calculated.

Diversity is stability

The chances of food supply shock, in the different cities, were then compared with the level of diversity of the sources of food supply to the cities. As displayed in the figure, the probability of food supply shock falls rapidly with rising diversity in the supply chain. “Analogous to biodiversity buffering ecosystems against external shocks, our results show that cities with a greater diversity of food suppliers have a lower probability of suffering a food supply shock for any reason,” says the paper. The method of study brings together theories of the ecology and network theory to propose a practical food supplies risk management framework, based on actual data, the paper says

More than half the population of the world lives in cities and the proportion is increasing. The factors that affect supply of food materials, in the face pressures from climate change, hence merit careful monitoring. We are now seeing moves in various quarters to modify and ‘bring efficiency’ into established systems of procurement and distribution of farm produce. As the diversity in supply chains proves a powerful deterrent supply failure, whether these changes would affect the level of diversity needs to be ensured. The thing about “highly connected” networks, such as the Internet or ecological systems, or social networks, is that they are not designed and built, but they grow ‘organically’, and are highly optimised, in keeping with stability. They contain redundancies and safeguards, for security, but the characteristic feature is not obvious economy, it is stability.

And so is it with supply chains, which are social networks of agriculture commerce. They are built up over centuries, in step with the growth of cities, trade routes, production regions and markets. After several changes and course corrections, existing systems include efficient and economical alternatives to failure of connections, or rise and fall of production or demand, even crop failure. And several factors, which include the geographic, commercial, logistical and personal have been provided for as the systems evolved.

Under rapidly changing conditions, deficiencies can readily be found. These, however, cannot be grounds to make sweeping changes – it is the deficiencies, to some extent, that may be addressed. But it would be imprudent to give up a system that based on an organic network of mutually dependent agents, in favour of one that has not been proved in the same context.

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