Glaub and Hall have an interesting analysis of the EROI of hunter-gatherers. Despite humans being generally smaller, slower, and weaker than most large mammals, humans were successful hunters long before the invention of sophisticated weaponry. This was achieved by the practice of endurance running, essentially running an animal to exhaustion before killing the weakened animal by beating, strangling, or spearing. On the one hand, the energy expenditure by the hunters was substantial, but on the other, a successful hunt provided sustenance for a group or family for many days. Using empirical data based on the !Kung, Glaub and Hall estimated a remarkably high EROI of 26:1 to 69:1 for kudu hunting. A similar study from 1969, also of the !Kung, which also included foraging, calculated an EROI of about 10:1. Essentially, the high EROI was ‘spent’ by the !Kung enjoying leisure time and visiting other villages. Of course, these estimates are based on small, localised groups living in arable regions with plentiful herds. In the Pleistocene, roaming bands occupied large tracts of land. Bar-Yosef identified an area of 300 to 2000 square kilometres as the optimum territory for a band of Upper Pleistocene hunter-gatherers.
In the Neolithic transition, beginning around 10,000 years ago, humans began domesticating plants and animals. There are different hypotheses for why humans chose to adopt agriculture in the first place, but it gradually led to specialised crop cultivation, including land clearing and basic irrigation. These advances permitted surplus food production and seasonal food storage for the first time. The surpluses facilitated increased population density, and represented a prelude to the development of specialisation, villages and later, cities. Such societies could be termed organic since the land was the source, directly or indirectly, of all products of use to man. The primary source of energy was the sun, hence the physical and biological limits of production was set by the energy captured and stored by plants, and to a lesser extent, the energy extracted by the wind or flow of water. In most cases, the seasonal cycle defined the production cycle.
The decisive break came from collecting of grain, then the evolution of grain, and cereal farming. The crucial advantage of grains and cereals was that it permitted agricultural surpluses to be converted into seasonal storage. But in the early period at least, the relative calorific return from agriculture may have been less than traditional foraging and hunting, and it is not obvious that farming would have been worth the effort. Agriculture requires intense effort over long periods, often with variable results. The EROI of agriculture must have been much less than hunting and gathering.
The capacity for storage was to prove crucial. On the one hand, grains have several useful properties. They are hard and dry, and do not readily rot and go bad. Since they are dry, they have a high calorific-to-weight ratio, making them both storable and readily transportable. But on the other hand, they are exceptionally difficult to turn into something useful and digestible. Most food gathered or caught by hunter-gatherers is eaten in its raw state or cooked over a fire, but grains require several processing and conversion stages. Wheat must be harvested, threshed, winnowed, and ground. Finally, the flour can be mixed with water and baked to produce unleavened bread. The case of wheat suggests that there must have been a strong evolutionary advantage to adopting agriculture and perhaps derive an easily storable product.
The history of salt provides another example of the importance of food storage. Salt was used since antiquity for curing foods such as beef, pork, fish, and later butter. Such was its importance that it was sometimes a strategic commodity and was used as an early form of currency.
In the Medieval Period, the EROI of farming must have been low. Most people were peasant farmers and worked hard for a subsistence living. Basic survival, the payment of taxes, and caring for the young and elderly, required a minimum EROI for workers of perhaps 3:1, much less than hunter-gatherers. However, once settlements were established, hierarchies and power structures would have made it difficult for peasants to shift to alternative means of survival.
The Green Revolution of the mid 20th century introduced high-yielding varieties, mechanisation, and irrigation, which significantly increased agricultural production. Fossil fuels enabled the production of chemical fertilisers via the Haber-Bosch process. Pimentel estimated the total energy in the food system in developed countries equates to an EROI of approximately 0.25:1. Guillen estimated the EROI of the European fishing fleet averaged 0.11 in 2008. For aquaponics, Atlason et al. calculated an EROI of 0.016:1 to 0.106:1. By traditional standards, these are remarkably low figures What do they mean?
Headline agricultural yields have risen significantly due to the Green Revolution and fossil fuel subsidies. In contrast, traditional hunting-gathering returned a much higher net-energy than farming. However it didn’t permit seasonal storage or the establishment of increased population density, labour specialisation, villages and later, cities.
The entire food production system is now completely reliant on fossil fuels to underwrite the dramatic decline in food production EROI. The lengthened supply chains from farm gate to consumer, including processing, packaging, and distribution, now account for around 60% of the energy cost of food. Some organic food advocates claim it is possible to revert back to more traditional and seasonal forms of food production with shorter supply chains, fewer inputs and less processing. Whether or not this is scalable, it is clear that the low EROI of contemporary food production represents a key vulnerability in an energy constrained world.