DO INSECTS WEIGH MORE THAN US?

The Music of Science: The other day Oliver Morton came across a niftya neat little fact. It turned out to lead to a neat big thought

From INTELLIGENT LIFE magazine, May/June 2013 

One of the delights of following environmental news is that you are regularly enjoined to eat more insects. Reading one such article recently, I was struck, among the well-worn benefits (low fat, low carbon, widely available, tastes like chicken, etc), by a new one. Lest readers should worry that there were not enough insects to go round, the author wrote reassuringly that there are in fact 40 tonnes of them for every human being on the planet. 

This is what an editor I used to work with called a niftya neat little fact of the sort that might come in handy at, say, a drinks party, but which had a bit more to it than mere neatness. In this case the bit more comes from the way the fact acts as a perspective-shifting micro-homily: hey humankind, you may think you're great, but if the insects got together they could crush you like, well, bugs.

The 40-tonne fly in this ointment, though, was that it was hard to believe. Look around the world. Do you see insects outweighing people by a factor of 1,000? Unless your home is in the process of being eaten by warrior ants, probably not. But while indicative, such an appeal to experience is not sufficient. Science is almost entirely devoted to truths which a cursory look at the world would lead you to deny. So I did what any sensible person in doubt on such a matter would do. I reached for a book by Vaclav Smil.

A professor at the University of Manitoba, Dr Smil writes compendious books that make lovely use of his relentlessly questioning mind, thorough knowledge of the relevant literature, unshakable self-confidence, slightly austere manner and unflagging willingness to deploy his pocket calculator. He ranges widely, but his recurrent theme is the way that energy flows throughand betweennatural and artificial systems on both the grandest scales and the most intimate ones. The result is numerate niftiness galore. 

Here's a quick browse through the subset of Smil’s books in reach from my desk. The power behind the processes of plate tectonics, sufficient over time to raise mountains and pull open oceans, averages just 85 milliwatts for every square metre of the Earth’s surface; all in all, that’s equivalent to about four times the power that industrial civilisation uses. An American draft horse typically uses about 80 megajoules of energy a day at rest, maybe twice that if it is working, which is what you would get burning about five litres of gasoline. To grow the 50m tonnes of forage needed for the 24m horses and mules used in 1910 required 20-25% of America’s farmland.

Nitrogen fixation, a process through which the energies of industry impose themselves on the biosphere with great force, is a favourite Smil topic. For the past 3 billion years such fixationon which all living things rely in order to build protein and the likewas handled almost exclusively by bacteria. For the past 50 years it has been mostly the province of artificial-fertiliser factories fed with air and natural gas. This led Smil to make one of his most widely quoted calculations: roughly 40% of the nitrogen that people get from their dietand thus roughly 40% of the nitrogen in our muscles, our blood cells, our DNA and the rest of uscomes from a fertiliser factory. 

Smil’s latest book, "Harvesting the Biosphere", looks at the amount of matter and energy humans take from the natural world, and how it has changed through history. He digs into the data to show the extraordinary rise in the mass of humanity, a rise made possible only through using more of the planet’s available food. A remarkable amount of that rise took place in the past century, as the mass of humans and their domestic animals increased more than three and a half times. In 1900, the world’s humans outweighed its elephants by a factor of just over four. Many more humans and fewer elephants have now made that factor almost 200. The world’s cattle weigh 16 times as much as all the wild mammals on the planet put together.   

And the insects? Here I came up against Smil’s unwillingness to go too far beyond the data. Estimates of the number of insects, and indeed the number of species of insects, vary by a factor of ten or more, so Smil is unwilling to commit to a mass for them. But he does offer a mass for termitesprobably, given their size and number, an appreciable fraction of the total massof just under 500m tonnes. That seems to sit well with estimates from a bolder expert, Arnold van Huis, an entomologist who expounds on the value of eating the things he studies. In his recent cookbook he puts the planet’s total insect mass somewhere between 1.4 billion tonnes and 14 billion tonnesstill 20 times short of the figure that originally caught my eye.   

Maybe I should have gone to van Huis immediately. But then I would have missed the fun of browsing through the works of Smil, and experiencing the sublime feeling of a world of unseemly vastnessmillions of years, billions of creatures, trillions of wattsthat can, for all its inhuman scale, be circumscribed by numbers and clear thought. The feeling of a sense of proportion brought to a disproportionate, seemingly senseless world. It’s even more nourishing than a cicada fricassée. 

Oliver Morton is the briefings editor of The Economist and author of "Eating the Sun"

Illustration Pete Gamlen