Will they fit? How many can you put into one space? These are questions of density. Sometimes the answer is easy to measure such as with determining the number of glass marbles that can fit within a box. Or, we can predict population densities over time with the Lotka Volterra predator-prey model. In all density calculations, it is a defined space that is key to the calculation.

Let’s set the space as the land surface of Earth and the question is, “How many people will fit?” For the record, some believe that at one time there were less than a hundred thousand hominin living on the surface. These easily fit and the density barely registers. Yet we predict the population this century to crest at over 10 billion humans. The resulting density is 83 people per square kilometre. It rises to 117 people per square kilometre when we locate people only on viable land cover, i.e. excluding deserts and such. This will be the average global density.

For comparison, at the end of the last glacial period about 12,000 years ago when humans lived as primitive hunter-gatherers, the estimated population was 2 million or about 0.02 people per square kilometre. Contrarily, in the city of Manila today, there’s a local density of 46,178 people per square kilometre. This increased density speaks to our prolificacy.

Density is important as it relates to the amount of resources humans consume. For instance, to maintain current density, we use a third of all Earth’s land surface for agriculture so as to produce food. What do you expect of the future as the human population continues to increase while the land surface remains constant? How can we use local density values to predict sustainability and future survival?