When ‘An ecological theory of changing human population dynamics‘ authored by Kirsten Henderson and Michel Loreau was accepted by the Editors, the model proposed by the authors was so intriguing and held such development potential for the future in human population dynamic modelling, that the reviewers and Associate Editor were keen to explain why the paper is so innovative. Here is what they had to say:

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Miikka Tallavaara, Reviewer

Archaeologists have long recognized human population size as a potentially important factor affecting cultural and social change. Recent methodological advances and the availability of large archaeological databases have now made it possible to reliably reconstruct past human population dynamics over millennial time scales. These studies have revealed a much more dynamic picture than previously assumed, as periods of rapid population growth are often followed by long periods of decline.

With the reconstructions of long-term human population dynamics, archaeologists (including myself) have also started to analyze potential causes of past population fluctuations (usually climate) and the effects of human population size on land cover.

We usually achieve this by simply correlating demographic reconstructions with palaeoclimate records or pollen-based land-cover reconstructions, which can leave the real causal processes ambiguous.

Therefore, a model such as the one proposed by Henderson and Loreau in their newly published paper, can be highly useful to understand the causal mechanisms behind long-term human population dynamics and land-cover changes This allows us to make predictions about potential future population and technological trajectories, and consequently, the destiny of the biosphere. As such, this paper pairs nicely with the recent paper by Weinberger et al.1

Although the ecological and social problems of human population growth are still considered as an unethical research issue in some circles, we need these studies if we want to take informed actions towards a more sustainable future.

Henderson and Loreau’s model, like all models, is a simplification of the reality. For example, as the authors point out, their model lacks the feedback loop between human population size and technology. Although the role of population size in explaining technological evolution is still debated, there is increasing evidence that complex cultural adaptations are better sustained and distributed among larger populations and that a decrease in population size can even result in the loss of such complex adaptations. It is thus necessary that the future versions of the model try to incorporate the feedback between population size and technology, since the resulting dynamics are, most likely, different from the model without such feedback.

I believe that studies like Henderson and Loreau’s open up an important arena for multidisciplinary collaboration between ecologists, archaeologists, and demographers. By seeking even better parameter estimates and by testing the models against archaeological and historical data, it is possible to further improve the predictive ability of such models and, ultimately, to better understand the past demographic changes and predict the future.

Miikka Tallavaara, Department of Geosciences and Geography, University of Helsinki, Finland

1V. P. Weinberger, C. Quiñinao, and P. A. Marquet, 2017. Innovation and the growth of human population, Philosophical Transactions of the Royal Society B: Biological Sciences 372: 20160415.

 

Claire Dooley, Reviewer

I have been disappointed so many times by papers that claim to have taken an inter- or trans- disciplinary approach. Often, these papers scream “here is how my field would solve your problem”, and frustratingly overlook key knowledge outside the authors’ field. This oversight leads to deficiencies in the analysis and results that frequently have little practical use in research and application alike. Reviewing Henderson and Loreau was a refreshing deviation from this trend. Carrying capacities and demographic transitions galore! This paper is an excellent example of true interdisciplinary work.

Henderson and Loreau build an innovative model for human population dynamics that focus on “resource accessibility”, and use this model to demonstrate possible future dynamics under various ecological-technological scenarios. While relatively simple, the approach presented provides a unique framework for linking nature and evolving demographic trends. Resource accessibility is evaluated from fossil pollen records and technological advancement data, and is discussed extensively from the perspective of different fields including ecology, sociology and demography.

This paper boasts a thorough and concise review of the literature in its ‘Human-nature interactions: existing theories and models’ section. Covering ‘Agriculture and resources’, ‘Technology and medical advances’, and ‘Social norms, wealth and education’, this paper will act as an excellent resource to the research community.

Key future extensions outlined by the authors include spatial models and cultural interactions. While the importance of spatial analyses is well established in research, there have been recent calls by the development and humanitarian communities to integrate spatial approaches into practical applications such as health access, development and advocating for policy reform – for example 2018 saw the first The United Nations World Geospatial Information Congress. With this current expansion of applying geographic research, I can’t wait to see how Henderson and Loreau progress their work into the spatial realm!

Dr. Claire Dooley (Research Fellow, University of Southampton)

 

Laura Graham, Associate Editor

In An ecological theory of changing human population dynamics, Henderson and Loreau draw on a wide range of interdisciplinary literatures in order to gain a social-ecological understanding of the drivers of human demography through history and into the future. The fully interdisciplinary nature of this paper made it an ideal fit for People and Nature.

A motivating factor for the model development is that existing theories raise concerns about resource dependency of human population dynamics, but do not incorporate resource availability in terms of land-cover change.

The model developed within the paper combines aspects of existing models of human population dynamics and replaces assumptions of these models which are not supported by data. It makes a novel link between the natural world and changing human populations via accessible resources: availability of ecosystem services, agricultural land, and the technology and innovation required to optimise their outputs.

Although the deliberately simple model cannot predict the impact of dramatic events (for example, epidemics and regime changes), the qualitative trends obtained provide deeper insight into the social and ecological mechanisms driving population trend. They hint towards future work into spatial and stochastic models which may capture cultural interactions and unpredictable events.

Their model was able to capture observed historical changes in human population and land- cover change over the past 12 millennia. In addition to agreement with previous studies, that agriculture and technology are both required to allow the population to expand, their model also allows them to reach the conclusion that ecosystem services and/or continued advances in technology are needed to prevent population collapse.

As well as providing a starting point for incorporating the changing natural world into models of human population dynamics, this paper also serves as a useful overview of the history of human population dynamic modelling. The interdisciplinary review spans literatures including anthropology, economics, evolutionary biology, natural resource management, paleoecology, social-ecological systems, sociology and technological forecasting.