The Book of Revelations in the New Testament lists the Four Horsemen of the Apocalypse as conquest, war, famine and death, while in the Old Testament’s Book of Ezekiel they are sword, famine, wild beasts and pestilence or plague.
But whatever we call them, they are remarkably close to what we might call the four horsemen of ecology that regulate population size in nature. In his 2016 book The Serengeti Rules, Sean Carroll discusses the work of pioneering ecologist Charles Elton in the 1920s. In thinking about how animal numbers are regulated to avoid over-population, “Elton suggested that, in general, increases in numbers were held in check by predators, pathogens, parasites and food supply.”
Elton’s four regulators are clearly very effective. In one astonishing passage, Carroll tells us that if a single E. Coli bacterium were to double every 20 minutes – the rate found in optimum conditions – it would take only two days (that is, 2144) for the weight of E.Coli to exceed the weight of the Earth – yes, just two days! Clearly, and happily for us, that does not happen, nor does it happen for all the other species – including us.
Nonetheless, we are suffering a population explosion, just as lemmings and other species do from time to time. The human population has more than tripled in the past 70 years, from 2.5 billion in 1950 to 7.8 billion today. So what happened to Elton’s four ecological horsemen? Why are we not controlled? Is there a fifth horseman that will cause our populations to crash at some point, as lemmings do?
Horseman 1: Predators
In The Serengeti Rules, Carroll writes: “Kill the predators and the prey run amok.” Well, we may be running amok but as we humans are apex predators, there is very little that preys on us. Our main predators are crocodiles (about a thousand deaths a year, according to the online World Atlas), lions (about 100), tigers and other big cats, and occasionally wolves, some sharks (about 10 each annually), and a few other species such as bears. The most important large animal that kills humans is us, largely through homicide and war. The World Health Organization (WHO) estimated in 2014 that almost half a million people died from homicide in 2012 and another 200,000 or so directly from war in 2014, with many more dying because of the hunger and diseases that result from war.
Horsemen 2 and 3: Pathogens and parasites
It may be useful to distinguish between what the Old and New Testaments call “plague,” by which I mean the infections we pass on to each other (even though many of them, such as COVID-19, originate in other animals) and pathogens and parasites that we don’t spread directly to each other but are spread to us by other animals, (which might be considered “pestilence” – diseases spread by pests). There is, of course, a wider meaning of pestilence – animals, plants and micro-organisms that harm us indirectly by attacking our crops or herds. Think of locusts (famously called plagues), rats, potato blight and many others, against which we routinely deploy pesticides. In some parts of the world, these have and still can cause famines.
It turns out the most dangerous animals to humans are insect pests. Top of the list is the mosquito, which WHO reports “causes millions of deaths every year” by spreading the malaria parasite (405,000 deaths in 2018) and many other diseases. Altogether, WHO estimates that vector-borne diseases (chiefly via insects) caused by either parasites, bacteria or viruses kill about 700,000 people a year and sicken hundreds of millions more.
But it is plagues that have been and potentially still are the really big killers of humans. The Black Death killed about one-third of Europe’s population in the 14th century while smallpox and other diseases introduced by Europeans killed a huge proportion of the Indigenous population of the Americas – up to 90 percent in some communities. Plagues are still with us. The WHO reports that 1.5 million people died from TB and 770,000 people died from HIV-related causes in 2018. The annual influenza epidemics cause 294,000 to 518,000 respiratory deaths, while in 2018, there were more than 140,000 measles deaths globally. And of course we are in the midst of a Covid-19 pandemic, and even though mortality is relatively low compared to previous serious plagues, it has killed at least 1.1 million people so far.
On the other hand, we have become reasonably adept at controlling most plagues, especially through basic sanitation, hygiene and immunization. So it seems unlikely plagues will control our population, unless we get a pandemic as lethal as the Black Death; imagine, for example, an air-borne version of something like Ebola.
Horseman 4: Food supply
We have one horseman left – food supply: How effective will this be in controlling the human population? So far, food supply has more than kept pace with population growth. A 2019 report from the Intergovernmental Panel on Climate Change (IPCC) notes that there has been more than a 30 per cent increase in the amount of food per person since 1961. But in spite of this, “an estimated 821 million people are currently undernourished” while at the same time, “2 billion adults are overweight or obese.” Clearly, we have a distribution problem, not a supply problem.
The latest 10-year outlook from the OECD and the UN’s Food and Agriculture Organization, looking out to 2029, expects food production to grow 1.4 per cent annually, outpacing growth in demand, and food commodity prices will drop. This in spite of a growth in global population and growing demand for animal products in middle-income countries. But looking further out, the 2019 IPCC report has high confidence that by 2050 climate change will result in up to a 29 per cent increase in cereal prices, up to “183 million additional people at risk of hunger” and growing food system disruptions. Perhaps more troubling is the potential loss of seafood as we hoover up the oceans fisheries. The Worldwide Fund for Nature (WWF) notes that “Eighty-five percent of the world’s fisheries are either fully exploited or overfished,” although experience is beginning to show us that well-managed fisheries can become sustainable.
In summary, then, while food supply is challenging, a loss of food supply of a scale sufficient to create a global population decline or crash does not seem too likely, although local/regional famines may continue to occur.
The fifth horseman: That’s us
Clearly, Elton’s “four horsemen” are not controlling the human population. But something is working: While world population continues to increase – the UN projects it will reach 11 billion in 2100 – the rate of increase has dropped “from 2.2 per cent per year 50 years ago to 1.05 per cent per year,” reports Max Roser in Our World in Data. That, of course, is good news because we still need to get to zero growth – indeed ideally a gradual and well-managed population decline – to preserve the Earth’s natural systems. In discussing this decline, Roser notes, “The three major reasons are the empowerment of women (increasing access to education and increasing labour market participation), declining child mortality and a rising cost of bringing up children.” So it seems that social factors are at work; we are limiting our population ourselves.
However, this requires increased economic development in low-income countries as it takes a certain amount of national wealth to fund the clean water, universal education and other factors that increase social and human development. Our current system of economic and social development already exceeds the Earth’s biocapacity – the overall global ecological footprint is equivalent to 1.7 planet’s worth of biocapacity – and if the whole world lived as we do in Canada, we would need almost four additional planets.
Of course, what low-income countries need is not four planets but a fair share of this one small planet. And that means that we in high-income countries have to take less.
But even with the overall global ecological footprint being only 1.7 planets, we have created a set of global ecological crises, chief among them being climate change and a new Great Extinction, and ushered in a new geologic epoch – The Anthropocene. In doing so, we are undermining the most fundamental ecological determinants of our health – water, food, materials and fuels being chief among them – and vital ecosystem services; a stable climate, UV protection, waste decomposition and recycling, and the cycling of nitrogen and phosphorus being some key examples. And our intrusion into nature increases the risk of encountering and spreading new infectious diseases, as we have seen with Ebola, COVID and other diseases.
We should recall that the WHO has called climate change “the greatest threat to health in the 21st century.” Increasingly, it seems the most likely factor that could lead to a human population crash is a runaway hot-earth scenario combined with a mass species extinction and the other massive and rapid global ecological changes we have triggered.
So although of course an asteroid strike or super-volcano eruption could wipe us out, the greatest threat to the human population, the “fifth horseman” if you like, is us.