In African ‘Fairy Circles,’ a Template for Nature’s Many Patterns

Morgan Kelly, Office of Communications ・ Princeton Environmental Institute

Be it the Mima mounds of Washington state or the famous “fairy circles” of Namibia in southwestern Africa, people are captivated by the regular patterns of plant growth that blanket desert and grassland landscapes, often with mesmerizing consistency.

Scientists have long debated how these phenomena originate and persist. Now, a new theory suggests that instead of a single overarching cause, large-scale vegetation patterns in arid ecosystems could occasionally stem from millions of local interactions among neighboring plants and animals, according to a Princeton University-led study published Jan. 19 on the cover of the journal Nature.

Like Russian nesting dolls, small-scale patterns formed by plants in response to water scarcity lie within a larger polka-dot formation created by the nests of social insects such as termites and ants. The nests in turn appear as circular clusters of vegetation or as gaps of bare soil, depending on how the insects affect plant growth.

Satellite images from four continents showed that insect nests are often remarkably evenly spaced, with each nest having an average of six neighbors. The researchers used mathematical models and computer simulations to show that territorial aggression between adjacent colonies can produce this arrangement, which leads to a large-scale hexagonal, or honeycomb, distribution of the nests. Individual colonies spread outward until they encounter and fight with their neighbors, occasionally killing off smaller colonies. Over time, this leads to a mosaic of six-sided territories. Each of the six sides represents the frontline between a colony and its enemies next door.

Lead author Corina Tarnita, an assistant professor of ecology and evolutionary biology at Princeton, explained that the pattern arises when termite colonies are roughly equal in size and the landscape is homogeneous.