Trevor Hancock: Social, not ecological factors control our population

Social, not ecological factors control our population

In his 2016 book The Serengeti Rules, Sean Carroll tells us Charles Elton, the 1920s pioneering ecologist, identified four factors that control animal numbers: predators, pathogens, parasites and food supply. Two weeks ago I likened these to the Bible’s four horsemen of the apocalypse.

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Elton’s regulators are very effective. In one astonishing passage, Carroll notes 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 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.

In the case of vertebrates (mammals, birds, fish, amphibians and reptiles), it seems humans are the chief cause of population reduction.

Through a combination of habitat destruction, hunting or other interventions, deliberate or otherwise, we have reduced the Living Planet Index (a count of thousands of vertebrate populations around the world) by 60 per cent between 1970 and 2016 (the latest data available).

At the same time, we have vastly increased some vertebrates — our herds of livestock. Oxford University’s Our World in Data reports there were almost four times as many pigs and goats in 2014 as there were in 1950 and more than twice as many cattle.

But the 1.5 billion cattle and around one billion each of sheep, goats and pigs were dwarfed by the 21.4 billion chickens. Clearly, we have controlled the four horsemen for these animals, although of course we end up eating many of them.

At the same time, we have also controlled the four horsemen for ourselves. Our own population has roughly tripled, from 2.5 billion in 1950 to 7.4 billion in 2016; we are not suffering the same fate as most of our fellow vertebrates.

In my previous column I explored the first three “horsemen” and suggested that unless there is a major plague like the Black Death, they were unlikely to control the human population today.

So how effective will be the last of Elton’s four “horsemen” — food supply — 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 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, “two billion adults are overweight or obese.”

Clearly, we have a distribution problem, not a supply problem. But will we continue to have enough food for all?

Well, it depends on whom you ask and how far out you look. The latest 10-year outlook from the OECD and the UN’s Food and Agriculture Organization, looking out to 2029, expects food production will 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.

The latter is offset by “a transition from animal-based protein towards alternative sources” in high-income countries, something the new Canada Food Guide supports.

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.

Even so, this does not seem likely to put a major dent in human population growth. 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.

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 (to which the decline of child labor contributed).”

So for humans today, it is not natural factors that control our population. Instead, it seems, social factors are at work; we are limiting our population ourselves.

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