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This is a condensed and modified version of a paper of mine that appeared on “The Journal of Population and Sustainability” this year. The image above is the well known “Four Horsemen of the Apocalypse” by Albrecht Durer – 1498. Yes, I know it is catastrophistic, but it is not my fault if biological populations do tend to collapse! (see also my previous post: “Overpopulation Problem? What Overpopulation Problem?

1. Introduction

“The world has enough for every man’s need, but not enough for every man’s greed.” Gandhi [1]

While Gandhi’s observation about greed remains true even today, it may not be so for the ability of the world to meet every man’s need. Gandhi is reported to have said that in 1947 when the world population was under 2.5 billion, about one-third of the current figure of 7.5 billion. And it keeps growing. Does the world still have enough for every man’s need?

It is a tautology that if there are 7.5 billion people alive on planet earth today there must exist sufficient resources to keep them alive. The problem is for how long: a question rarely taken into account in estimates purportedly aimed at determining the maximum human population that the Earth can support.

The problem of long-term support of a population can be expressed in terms of the concept of “overshoot,” applied first by Jay Forrester in 1972 [2] to social systems. The innovative aspect of Forrester’s idea is that it takes the future into consideration: if there is enough food for 7.5 billion people today, that doesn’t mean that the situation will remain the same in the future. The destruction of fertile soil, the depletion of aquifers, the increased reliance on depletable mineral fertilizers, to say nothing of climate change, are all factors that may make the future much harder than it is nowadays for humankind. The problems will be exacerbated if the population continues to grow.

So, will the human population keep growing in the future as it has in the past? Many demographic studies have attempted to answer this question, often arriving at widely different results. Some studies assume that population will keep growing all the way to the end of the current century, others that it will stabilize at some value higher than the present one, others still that it will start declining in the near future. Few, if any, studies have taken into account the phenomenon of rapid decline that I have termed “Seneca Effect” (or “Seneca Collapse”) [3]⁠, from a sentence written during the 1st century AD by the Roman philosopher Lucius Annaeus Seneca.

The Seneca Collapse is a phenomenon affecting complex systems where strong feedback relationships link the elements of the system to each other. Biological communities where predators and their prey are linked to each other are a good example of these systems. The Seneca Effect describes a situation in which the feedbacks of the system act together to generate a rapid decline of some of the stocks (populations) of the system. The typical “Seneca Curve” (or “Seneca Cliff”)  is shown in the figure below [3] ⁠

Figure 1. A typical “Seneca Curve” calculated by means of system dynamics The x-axis shows the time, the y-axis can be a parameter such as population. It shows how decline can be faster than growth [3]

In the following, I’ll list a series of examples showing that the Seneca Curve is relatively common in biological systems, including for historical human population. The possibility of an upcoming Seneca Cliff affecting humankind in the near future is real

2. Population collapses in natural ecosystems

There are many historical examples of the collapse or rapid decline of biological populations. The causes can be seen as mainly three:

  1. Predation
  2. Resource depletion
  3. Birth control

The first, predation, is the result of the appearance in the ecosystem of a new and highly efficient predator when the prey population has little or no defense against it. There are many examples of this phenomenon in modern times, especially when humans have transported new species to biomes where they didn’t exist before (e.g. hornets as predators of bees). A clear example can be found when the predator is humankind and the prey is the Thylacine species (the “Tasmanian Tiger”) [5]

⁠Figure 2. The population of Tasmanian tigers (Thylacines) before their complete extinction in the 1930s From ref. [5]⁠

These data are not a direct measurement of the size of the Thylacine population but can be reasonably assumed to be proportional to it. When the last Tasmanian tigers were killed, in the 1930s, the species was assumed to be extinct. The obvious origin of this collapse is human hunting, although disease has been sometimes blamed. Whether human or microbial pathogens were the predator, the graph shows how rapidly a biological population can collapse because of high predation rates. Note how the decline is much faster than growth.

Case 2, resource depletion, is often the specular case of efficient predation. It occurs when the predator species is so efficient in using its preys as food that the prey population crashes. It is a classic case of “overshoot” that leaves the predator without food and with the only perspective of a population collapse. A well-known case is that of the reindeer of St. Matthew Island, where the predators are the reindeer and the prey is grass. Obviously, the reindeer were so efficient in removing the grass that the whole population went in overshoot and then collapsed [4].⁠

Fig 3. The Reindeer Population of St. Matthew Island. Image created by Saudiberg.

The third possible case, active birthrate control, doesn’t seem to exist in the wild but we can see it in domesticated populations. Here is the case of horses in the United States.

Figure 4. Horse population in the United States (data source: The Humane Society)

The horse population went down rapidly and abruptly from a maximum of more than 26 million in 1915 to about 3 million in 1960. Today their population has increased again to about 10 million but has not regained the level of the earlier peak.  In this case, horses were simply no longer competitive in comparison to engine-powered vehicles. As a result, horses were not allowed to breed. When old horses died, they were not replaced.

3. The collapse of human populations in history

This survey of the collapse of biological populations shows three causes for the “Seneca Collapse” to take place: 1) predation, 2) overshoot, and 3) reproductive control. Do the same phenomena take place with human populations? It seems to be possible and let’s see a few historical cases.

Humans have no significant metazoan predator, but they are legitimate prey for many kinds of microbial creatures. In history, diseases are known to have caused human population collapses. A good example, here, is the effect of the “black death” in Europe during the Middle Ages. The data are uncertain, but the “Seneca Shape” of the collapses is clear.

Figure 5 – European Population in history, including the effects of the Great Plague of mid 14th century (from Langer [6])

Regarding overshoot and resource depletion, perhaps the best example is that of the Irish famine that started in 1845. A graph of the collapse is shown in fig. 5

Fig. 6 – Irish population data before and after the great famine of 1845.

The Irish catastrophe has been interpreted in different ways and politically biased interpretations are often invoked. Nevertheless, as discussed in detail in “The Seneca Effect” [3], the Irish famine is a classic case of overshoot-generated collapse. That doesn’t mean that the Irish had overexploited their land in the same way as the reindeer of St. Matthew’s Island, but it is clear that – given the economic, social, and political conditions of the time – the land couldn’t support for a long time the population level reached before the collapse. Then, the parasite of the potato which destroyed the Irish crops was only a trigger for a collapse that would have taken place anyway. After the crash, the Irish population continued to decline for more than half a century and even today it has not reached the pre-crash levels again.

Finally, we can examine cases in which the human population declined mainly because of lower birthrates. There are several modern examples, especially in Eastern Europe after the collapse of the Soviet Union in 1991. An especially evident case is that of Ukraine, shown in figure 7.

Fig. 7 – Ukrainian population – data from the World Bank

There were no widespread epidemic diseases nor famines in Ukraine during the period that covers the recent population collapse. Factors in the decline were emigration and increased mortality due to a declining health care system, but what’s impressive is how the Ukrainian population reacted to the economic crisis with a decline in birthrates. Apparently, Ukrainian families and Ukrainian women thought that they had no benefit in having many children, a reasonable position in a situation of economic decline. The Seneca shape of the population curve is observed for most of the countries which belonged to the Soviet Union.

4. Conclusion

All biological populations need food and are affected by predation. Wild populations have no internal mechanisms to plan ahead and the result is normally what we call “overshoot,” where the population grows over the limits which the resources can sustain over a long time and finally collapses. The result is population curves which take the typical “Seneca Shape” described in [3]

The future of the world’s human population may well be described in similar terms, that is decline caused by overshoot, predation, or birth control. Of the three, predation could take the form of a microbial infection spreading all over the world and killing a substantial fraction of the human population. Another likely effect is overshoot, especially in terms of the decline of the world’s agriculture or, more simply, to the loss of the capability of the globalized economic system to deliver it worldwide.

Unlike in non-human populations, for humans there is also the possibility of birth control. A decline in natality doesn’t necessarily require top-down government intervention to force people to have fewer children. An economic slowdown may be sufficient to convince couples and single women that they have no need and no interest in having many children. In particular, the economic value of human beings is constantly eroded by the development of automated systems that replace them in the workplace. So, if women have access to contraception, we may just see a worldwide expansion of what we call the “demographic transition” and which is commonly observed in the so-called “developed countries” where agriculture ceases to be the main source of wealth.

Will the demographic transition be sufficient to reduce the human population before the evil demons of overshoot and plague intervene? This is hard to say, but it cannot be excluded. Humans are, after all, intelligent creatures and they may still be able to take their destiny in their hands.


  1. Pyarelal. Mahatma Gandhi: the last phase. (Navajivan Publishing House, 1956).

  2. Bardi, U. Jay Write Forrester (1918–2016): His Contribution to the Concept of Overshoot in Socioeconomic Systems. Biophys. Econ. Resour. Qual. 1, 12 (2016).

  3. Bardi, U. The Seneca Effect. Why Growth Is Slow but Collapse Is Rapid. (Springer Verlag, 2017).

  4. Klein, D. R. The Introduction, Increase, and Crash of Reindeer on St. Matthew Island. J. Wildl. Manage. 32, 350–367 (1968).

  5. McCallum, H. Disease and the dynamics of extinction. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 367, 2828–39 (2012).

  6. Langer, W. L. The Black Death. Sci. Am. 210, 114–121 (1964).

(A similar version has appeared in 2017 on “The Journal of Population and Sustainability“)

Ugo Bardi teaches physical chemistry at the University of Florence, in Italy. He is interested in resource depletion, system dynamics modeling, climate science and renewable energy. Contact: ugo.bardi(whirlything)


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  2. “Humans are, after all, intelligent creatures and they may still be able to take their destiny in their hands.”

    The last sentence said it all