Sizes of hunter-gatherer populations restricted by a high-meat diet

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Populations of hunter-gatherers with a high seasonal dependence on meat for their diet had fewer people per square kilometer than those with abundant plant foods throughout the year.

This new result emerges clearly from a study carried out by researchers from the Institute of Environmental Sciences and Technologies of the Universitat Autònoma de Barcelona (ICTA-UAB), which analyzes how environmental factors have influenced population density. hunter-gatherer societies around the world, and reveals important links between the length of the growing season, diet composition and population density.

About 300 hunter-gatherer societies have persisted in recent history and have been documented by ethnographers. Due to the similarities between their lifestyles and those of our hunter-gatherer ancestors, these contemporary hunter-gatherers have provided a great deal of information about the past of our species. Because they lived on natural food resources through hunting, gathering and fishing, they were heavily dependent on the primary productivity of their local ecosystems and, therefore, on the climate variability of the region. But documented population sizes were often either larger or smaller than one would expect from the productivity of the local ecosystem, for reasons that are not well understood.

The new study, published today in the journal Nature Ecology & Evolution, uses a complex set of calculations to show that differences in the amount of meat in diets may explain why some societies were less densely populated than others. In areas with harsh winters or great dry seasons, the available edible vegetation has been drastically reduced for much of the year, leaving hunter-gatherers heavily dependent on meat consumption. Because animals tend to be less abundant than edible plants, they can only provide enough food for a small human population. On the other hand, populations settled in places where the climate favored the abundance of plant foods throughout the year were able to take better advantage of the overall production of the environment to multiply.

“Basically, if people were to experience long dry or cold seasons when plant food was scarce, they were forced to hunt relatively sparse animals in order to survive,” says Eric Galbraith, a researcher at ICTA-UAB and the McGill University in Canada. “This led to a seasonal bottleneck in the amount of food available, which then set the overall limit on the size of the population, regardless of how much food there was during times of plenty. . “

Previous studies have used statistical methods based on ethnographic recordings of contemporary hunter-gatherers to establish correlations between population density and environmental variables, which allowed them to estimate the spatiotemporal patterns of ancient humans. However, these statistical models are limited to the inputs supplied to them and have not retained the length of the growing season as an important characteristic.

“Going beyond pure statistical correlations, we have developed a process-based hunter-gatherer model that is coupled to a global terrestrial biosphere model. The mathematical model simulates the daily human feeding activities (gathering and hunting) and the resulting carbon (energy) fluxes between vegetation, animals and hunter-gatherers, the result of which determines reproduction and mortality rates human and therefore population dynamics, ”explains Eric Galbraith. , researcher at ICTA-UAB and at McGill University in Canada.

“We were struck by how, despite a long list of unknowns, a very strong result emerged from the model equations: wherever growing seasons were short, hunter-gatherers needed high fractions of meat in their diet. food. As a result, there could be up to 100 times less human population density given the same total annual productivity of vegetation, mainly due to the energy inefficiency of passing through additional predator-prey links, ”explains Dan Zhu, main author of this article. study, former postdoctoral researcher at ICTA-UAB and now assistant professor at Peking University. “When we then returned to detailed ethnographic observations, we found that this result was well supported by contemporary hunter-gatherers, notably the Ache in the rainforest, the Hiwi in the savannah and the Bushmen groups in the Kalahari Desert, but its importance had not been taken into account.

“This study is an exciting example of how thinking about humans as part of the ecosystem, rather than something apart, can help us learn more about ourselves,” says Eric Galbraith. “Then we plan to apply this kind of model to the past, to explore how human populations have changed in different times and in different parts of the world, in the slowly changing climates of ancient times,” Zhu adds.

Reference
Zhu D, Galbraith ED, Reyes-García V, Ciais P. Global hunter-gatherer population densities constrained by the influence of seasonality on diet composition. Nat Ecol Evol. Published online September 9, 2021: 1-10. doi: 10.1038 / s41559-021-01548-3

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