There is a fifth horseman of the Apocalypse – and it is us
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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?

Crocodiles kill about 1,000 humans each year.
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.

Mosquitoes cause ‘millions of deaths er year,’ according to the WHO.
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.