Graphical abstract. Credit: DOI: 10.1038/s41559-021-01548-3
People per square kilometer of hunter-gatherer communities that were heavily dependent on meat during the season had less people than those who had plenty of plant foods all year.
This is the result of a new study by the Institute of Environmental Science and Technology (Universitat Autnoma de Barcelona, ICTA-UAB), that analyzes how environmental factors influence the population density of hunter/gatherer societies around world. It reveals important connections between the length of the growing season, diet composition, and population density.
Ethnographers have documented 300 hunter-gatherer communities that survived into recent times. These hunter-gatherers provide a lot of information about the past because they share many similarities with our foraging ancestors. They were dependent on local ecosystem productivity and climate variability to survive on their natural food sources of hunting, fishing and gathering. The population numbers were not always as large or small as would have been expected based on local ecosystem productivity. This was due to reasons that are still poorly understood.
This complex study published in Nature Ecology & Evolution today uses complicated calculations to explain why certain societies are less densely populated. The availability of edible plants in regions that have harsh winters and long dry seasons was reduced significantly, making hunter-gatherers heavily dependent on meat consumption. Due to the fact that animals are less plentiful than edible plants, it is difficult for them to provide sufficient food for a small population. The population settled in areas with a high number of edible plants throughout the year was able to reap the benefits of the climate to become more numerous.
Eric Galbraith is a researcher at McGill University, Canada, and the ICTA UAB. He says that if people had long, dry, or cold seasons where food was scarce, then they were forced to hunt relatively few animals to survive. "This caused a seasonal shortage in food, which led to an overall limit on population size, regardless of how plentiful the food was.
In order to determine spatiotemporal patterns in ancient humans, previous studies used statistical methods that were based on ethnographic records from contemporary hunter-gatherers. These statistical models only take in the information they have and do not consider the length of the growing season as an important feature.
"We developed a process-based hunter/gatherer model, which is coupled with a global terrestrial biosphere model. This goes beyond statistical correlations. Eric Galbraith is a researcher at McGill University, Canada, and at the ICTA UAB. The mathematical model simulates daily human hunting and gathering activities and the carbon (energy), flows between plants, animals, and hunters. This determines human reproduction rates and population dynamics."
"Despite a lot of unknowns, we were surprised to see that the model equations produced a strong result: hunter-gatherers needed high amounts of meat wherever there were short growing seasons. This could lead to a 100-fold drop in human population density for the same amount of annual total vegetation productivity. It is mainly due to the energetic inefficiency that passes through additional predator-prey connections," Dan Zhu, the study's lead author, is a former postdoctoral researcher at ICTA/UAB and assistant professor at Peking university. "We then re-examined the ethnographic data and found that the result was supported by current hunter-gatherers, including the Hiwi in savannah and the Ache in tropical forest. However, its significance had not been fully realized.
Eric Galbraith says, "This study is a fascinating example of how considering humans as part of an ecosystem rather than something separate can help us learn about ourselves." Zhu says, "Next, we will apply this model to the past to examine how human populations have changed over time and in different places of the globe, under the slowly changing climates from ancient times."
Continue reading Cultural evolution has not liberated hunter-gatherers of environmental forcing
More information: Dan Zhu et.al, Global hunter/gatherer population density constrained by the influence of seasonality diet composition, Nature Ecology & Evolution (2021). Information from Nature Ecology & Evolution Dan Zhu et., Global hunter/gatherer population density constrained by seasonality on diet composition. Journal (2021). DOI: 10.1038/s41559-021-01548-3
