Cities don’t just appear out of nowhere, and a study by a University of Iowa researcher thinks slime mold might be a good model to learn more about how they grow.

And it has nothing to do with the stuff you saw growing in the subway that time you were in New York.

The study, by David Barker, a member of the Tippie College of Business finance faculty, suggests that individual firms locating near each other is what causes a city to grow. It’s a way that’s similar to how individual slime mold cells combine to look for food, causing a mold colony to grow.

The research follows up on original work by Nobel Prize winning economist Paul Krugman, who made the slime mold analogy in 1998 with a study that suggested business firms build cities by deciding where they locate.

“Individual firms move around looking for profits and they clump together,” Barker says. “When they clump together, that’s a city.”

He pointed out that slime mold builds colonies the same way, with individual slime mold cells clumping together to form structures that increase their chances of finding food and reproducing. But the colony is not hierarchically divided, Barker says, with a chief slime mold or slime mold council directing the entire colony to move this way or that. Instead, each cell acts on its own individual interests, attracting other cells by communicating with a chemical emission and self-organizing into a self-sustaining colony.

“Individual slime mold cells following simple rules without central direction form structures and act as a community,” Barker says. Put another way, he says the mold cells acting individually create a system that builds order out of chaos.

Barker says that cities grow in a similar organic manner, as firms and people act in their own best interest to found settlements that grow because more firms and more people make the individual decision to move there on their own. Only in the rare cases of planned communities does a central authority plan how a city grows.

“Without central direction, people and firms organize themselves into complex systems of cities of different sizes,” Barker says. “Urban systems, like biological systems, are characterized by tension between aggregating and disaggregating forces.”

Krugman’s initial work was done by using a model only on a circle and dividing the population into two groups, farmers and business owners—or firms, which make only one thing. The model assumes that firms aggregate to places where they are near their suppliers. As aggregation continues, two big cities grow on opposite ends of the circle, with smaller cities in between. Some cities keep growing, others level off, and some shrink, eventually disappearing.

Barker’s work takes the model into two dimensions, using the surface of a dodecahedron instead of a line, and also adds geographic features that adds depth to the conclusions of Krugman’s work. His model includes rivers, which act as a natural transportation system, and finds that with the additions, Krugman’s earlier theory is even more realistic. When Barker entered the parameters of U.S. cities into his economic equation, he found the model mirrored the country’s actual growth pattern, showing it closely captures important aspects of city formation and population growth.

Barker’s study, “Slime Mold Cities,” was published recently in the journal Planning and Design. It can be found online at www.envplan.com/abstract.cgi?id=b37078.