Trevor Hancock: The four horsemen of ecology: regulating population size

According to the Book of Revelation in the New Testament, the Four Horsemen of the Apocalypse are conquest, war, famine and death, while in the Old Testament’s Book of Ezekiel they are sword, famine, wild beasts and pestilence or plague. (Sometimes, apparently, conquest is interpreted as pestilence or plague.)

But whatever we call them, they are remarkably close to what we might call the four horsemen of ecology, which regulate population size in nature. In his 2016 book The Serengeti Rules, Sean Carroll discusses the work of the 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.”

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We, of course, are animals, and we are suffering a population explosion, just as lemmings and other species do. But like other animals, we are held in check — or will be — by the same four horsemen of ecology — and some others of our own making, as I will discuss next week. So let’s see how Elton’s four ecological horsemen are working out for humans on Earth today. Why are we not controlled, and what might control us?

The first control is predators, and in The Serengeti Rules, Carroll writes: “Kill the predators and the prey run amok.” But we humans are apex predators, there is very little that preys on us, if by “predator” we mean animals that hunt us to eat us. Our main predators are crocodiles (about 1,000 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.

Animals that kill us somewhat incidentally (they are not preying on us, just protecting themselves) are much more dangerous; snakes kill about 50,000 people each year, scorpions about 3,000. Dogs are not human predators, but are important incidental killers, because in some parts of the world where rabies is widespread they bite and infect us, killing an estimated 25,000 humans annually. But true predators are hardly threats to our numbers today, and anyway — sadly — we have dramatically reduced them.

The most important large animal that kills humans, however, is us, largely through homicide and war, although we are not truly predators. 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.

So let’s turn to Elton’s second and third categories of pathogens and parasites. It turns out the most dangerous animals to humans are insects, and top of the list is the mosquito, which WHO reports “causes millions of deaths every year” by spreading malaria (435,000 deaths in 2015) 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 these are diseases that we don’t spread directly to each other, they require an intermediary. So it may be useful to distinguish diseases spread to us by other animals, (which might be considered “pestilence” — diseases spread by pests) from 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).

“Plagues” have been and are the really big killers in the realm of pathogens. The WHO reports that 1.5 million people died from TB in 2018, while 690,000 people died from HIV-related causes in 2019. The annual influenza epidemics cause 290,000 to 650,000 respiratory deaths, while in 2018, there were more than 140,000 measles deaths globally. But nonetheless, it seems unlikely plagues will control our population, unless we get a pandemic as lethal as the Black Death.

So next week I will deal with the fourth of Elton’s controls — food supply — and the global ecological changes we are creating, as well as the social — rather than ecological — controls we have created for ourselves.

Dr. Trevor Hancock is a retired professor and senior scholar at the University of Victoria’s School of Public Health and Social Policy.

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