Ecological drivers of hunter-gatherer abundance

Although ecological approaches in anthropology and archaeology have a history as long as these disciplines themselves, little is known about the influence of environmental conditions on the abundance of pre-industrial humans. Our new study reveals that net primary productivity, biodiversity, and pathogens have strongly influenced the global pattern of population densities of ethnographically documented hunter-gatherers.

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Our paper “Productivity, biodiversity, and pathogens influence the global hunter-gatherer population density” in PNAS is here:

The background story of our paper is pretty boring. It involves neither data collection in exotic places nor careful experiments in the laboratory. However, it is a good example of what can come out as a result of interdisciplinary collaboration. The mixture of backgrounds of an archaeologist (me), an ecologist (Miska Luoto), and a palaentologist (Jussi Eronen) enabled us to analyse old data with fresh ideas and new perspectives – although, eventually, we had to admit that even our fresh ideas were not completely new.

Few years ago, our group in Helsinki started to develop species distribution modelling of population dynamics of prehistoric human hunter-gatherers as a complement to more established archaeological and genetic methods. The idea is to use the information about how environment influences hunter-gatherer population density in modern or historical ethnographic data to hind-cast prehistoric population densities. Such hind-casting requires information about past environmental conditions, which is usually derived from climate model simulations.

In our pioneer study about human population dynamics in Europe over the Last Glacial Maximum, we modelled hunter-gatherer population range and density using potential evapotranspiration, water balance and mean temperature of the coldest month. We took these three variables pretty much as given, because they are ecologically sound predictors of population density in the cold and dry Ice Age conditions. However, we did not explore any alternatives.

After the first paper, it was clear to me that we needed to dig deeper into global-scale hunter-gatherer population ecology. The resulting knowledge would not only make the simulations of prehistoric dynamics more robust, but it would be extremely interesting in itself: Hunter-gatherers, and pre-industrial human populations in general, provide a rare opportunity to investigate the drivers of the distribution and abundance of a single species along global environmental gradients.

Previous studies have shown that primary productivity and other measures of resource abundance have influenced hunter-gatherer density. However, we assumed that not only the total amount of available energy, but also the diversity of potential resources may play an important role by stabilising hunter-gatherers’ subsistence variability. Thus, and given the hypothesised role of species richness in stabilising ecosystem productivity in general (diversity-stability hypothesis), we wanted to explore the effect of biodiversity as well.

Relationship between hunter-gatherer population density and net primary productivity.

Maybe the most intriguing feature in the response of hunter-gatherer population density to environmental variability is the peaking of density in temperate and subtropical climates, where net primary productivity is around 1,400 g/m2/year, and the subsequent decline and levelling-off of density when productivity increases above 1,500 g/m2/year. We would have assumed that population density shows a monotonic positive response to resource availability. Instead, some factors appear to counter this positive response in high-productivity environments and we were fascinated by our idea that pathogens would cause the observed pattern.

The results appeared to confirm our hunch about the role of pathogens: Pathogen stress was the strongest predictor in the high-productivity environments. Our results also showed that productivity had significant positive effect on hunter-gatherer population density throughout its gradient when the effect of pathogens was held constant – something, which we had assumed, but wasn’t evident in the bivariate relationship between productivity and population density. What we didn’t anticipate was threshold effects. Biodiversity turned out to be the strongest predictor in the low productivity environments of arctic, boreal and temperate biomes, but non-significant in high-productivity of tropics, whereas pathogens showed the opposing pattern. It took the expertise of the two ecologists in our team to acknowledge that the observed pattern resembled the pattern in the factors affecting global biodiversity: Resource availability is important in the high- and mid-latitudes whereas biotic interactions become more dominant in the tropics. It is tempting to speculate that these similarities in the patterns of limiting factors of a single species’ abundance and the global biodiversity might suggest similarities in the underlying mechanisms as well.

Net primary productivity, biodiversity, and pathogens constrain the global hunter-gatherer population density, but their effects vary in different parts of the globe. Biodiversity affects population density mostly in the low-productivity environments, whereas pathogen stress is a crucial constraint on population density especially in the tropics. Blue, red, and grey arrows indicate positive, negative and statistically insignificant effect of a variable, respectively. Thickness of an arrow indicate the strength of the effect of a variable.

When we were making final tuning of our manuscript, we wanted to highlight the result that temperate and subtropical biomes appeared to provide the most suitable conditions for hunter-gatherers due to the optimum between the positive effects of resource availability and the negative effects of pathogens. Knowing that eminent anthropologist Lewis Binford had previously observed that temperate and subtropical biomes provide optimal conditions for hunter-gatherers, I went back to his writings1 to find out, if he had suggested any mechanism for this optimum. Surprisingly, I found out Binford proposing that resource availability and pathogens would play an important role in driving hunter-gatherer abundance! I was a little confused – our idea was not as novel as we had thought. Luckily for us, however, Binford hadn’t quantified these relationships. It was only a proposition – an idea that we had now corroborated with data. Most likely, I had read Binford’s proposition about the interplay of resources and pathogens years ago, forgot where that idea came from, and started to think that it was our own development. This was a good reminder that novel ideas in science are rarely completely new.

The commentary of our paper by Joseph R. Burger and Trevor S. Fristoe in PNAS is here:

1Binford, L.R., 2001. Constructing Frames of Reference: An Analytical Method for Archaeological Theory Building Using Ethnographic and Environmental Data Sets. University of California Press, Berkeley.

Miikka Tallavaara

Postdoctoral researcher, Department of Geosciences and Geography, University of Helsinki