In Mortal Engines, based on the book series by Philip Reeve, in a post-apocalyptic wasteland ravaged by nuclear war, the surviving settlements have become “traction cities,” putting themselves on wheels to roam what was once Eurasia (an area known in the books as “the Great Hunting Ground”) and sustain themselves by literally eating each other. This strange lifestyle is known as “Municipal Darwinism”.
Could this ridiculous premise actually work? Let’s set aside the engineering problem of putting something that’s half the size of Manhattan on wheels, let alone getting it moving. (If you want to know more, see Because Science’s YouTube video on the subject.) Could London, the film’s “villain” city, maintain this lifestyle for hundreds of years? The answer, weirdly, is maybe yes, and the clues come from real predators and prey on Earth. There is an environment on Earth that eerily resembles the world of Mortal Engines, and “life, uh, finds a way” there.
The important things to understand here is trophic levels. In simple terms, this is a description of an organism’s place in the food chain: plants, herbivores, and carnivores, for example, but there are often bigger predators that eat the smaller predators, making for more levels than the simple three—usually four or five.
When you look at the organisms that are in different trophic levels, you start to see a pattern. There is ten times as much biomass in plants as there is in herbivores, and there is ten times as much biomass in herbivores as there is in carnivores. This is often expressed that there are ten times as many prey animals as predators (notably in Disney’s Zootopia), but it’s more complicated. Some predators, like weasels, eat animals smaller than themselves, while others, like lions, eat animals larger than themselves. It works out better to just talk in terms of biomass.
There’s a good reason for this. Eating other organisms, be they plants or animals, is very inefficient. Most of the biomass is processed for energy, not used for growth, so the higher trophic levels can’t afford to be very heavily populated. Cities aren’t much better off. They need some durable goods for repairs. They might have some population growth. But they mostly need food for their people and fuel for movement, and indeed, in the film, we see that most of Salthook is simply burned by London as fuel.
But it turns out this only works on land. There’s another way for trophic levels to work. In the ocean, the biomass in each trophic level increases as you move up the food chain. There’s very little algae and other phytoplankton, which form the base of the food chain. There’s more biomass in small fish, and the largest amount in large predators like sharks and whales. (At least in parts of the ocean where fishing stocks haven’t been depleted.)
How can this work with so little biomass at the bottom of the food chain? The answer is that the bottom levels have very high turnover. The smallest organisms breed very quickly and are eaten just as quickly, so they produce a lot of biomass over the course of a year, but very little at any one time.
The open ocean is also like the world of Mortal Engines in other ways. It’s mostly empty, at least to the eye. Resources are sparse; there’s no soil to grow on and light only near the surface. The ocean floor is even more barren, the only rich sources of nutrients being hydrothermal vents and whale carcasses, and yet life thrives there too.
But this is only by analogy. What do the trophic levels actually look like in Mortal Engines? Well, cities tend to follow a rule call Zipf’s law in terms of population. Zipf’s law says that on average, the second-largest city in a country will be half the population of the largest city. The third-largest city will be a third the population of the largest city, and so on. You can see this in the United States. Los Angeles is half the size of New York; Chicago is a third the size of New York; Houston is a fourth the size, and so on. (More precisely, this refers to the populations of the cities proper.) This means that there will be ten times as many small (prey) cities that are a tenth the size of a big (predator) city, but the total population (biomass) in each trophic level will be…equal. Halfway between land and sea.
But we can do better than this. Mortal Engines involves mostly British cities, so we can look at this list of the 1,000 largest cities and towns in the United Kingdom by population, down to a population of just 1,300. You’ll see that there are 64 towns with populations between 100,000 and 200,000, but there are only 205 towns with populations between 10,000 and 20,000—far fewer than Zipf’s law predicts. This is probably because of economies of scale. Very tiny villages are hard to run efficiently, lacking the expertise and resources needed to do so. Most of the people live in medium-to-large cities.
So the traction cities are top-heavy and resemble the ocean food chain. Can they work? The ocean food chain’s solution where the smallest organisms breed quickly and are eaten quickly obviously doesn’t work with people, but maybe it could work with houses and villages.
In the books, Municipal Darwinism is said to extend all the way down to small stationary settlements. Small farmsteads on the plains seem like the only way to grow crops reliably to produce food for the cities. This system could theoretically work if there is a farming underclass of a few percent of the population (fewer than 2% of Americans are farmers) that are being literally chewed up and spat out every harvest season by the smallest traction towns, which are themselves eaten and rebuilt fairly regularly, while the farmers keep just enough crops to survive themselves. (In the film, it’s not clear where the small towns get their food, but London definitely gets it from the towns it eats.)
Remember that the Great Hunting Ground is largely depopulated, and in the books, the cities move much slower than the frenetic pace of the movie. In the film, we see London moving at something like 100-150 miles per hour, while in the books, its top speed is 40 miles per hour, and only in short bursts. You can also see it coming from many miles away, and chases can last for days. The pace of life is far slower in the books, so a farmstead providing enough food for a town and yet only falling prey to one once a year isn’t so crazy.
Of course, there is another option. The small towns like Salthook are probably small enough to do their own farming without tearing up their fields. (For that matter, there’s nothing to stop London from farming, either, but any small vehicles it deploys for the purpose would be much more vulnerable than the massive city.) In this way, they could cut out the middleman and get food from the land directly instead of from farmsteads that might not have a stable enough existence to be viable.
Incidentally, whales do something like this in the ocean food chain. While sperm whales eat large animals like giant squid, they are outliers. Most large whales are baleen whales, which cut out the middleman, as it were, and eat krill, very small animals that themselves eat mostly algae. This unusually short food chain allows krill to be among the most numerous animals on the planet and whales to be the biggest. So ironically, London might be better off going after the small towns it has to settle for at the time of the film.
Is it a stretch? Absolutely, but the whole point of the original books was that the traction cities are unsustainable. They offer short-term gains to their residents in a harsh world, but eventually, they run out of resources to plunder. So as absurd as it sounds, I’m putting Municipal Darwinism in the realm of just barely possible.