The grasslands of the Namib Desert in Africa are pock-marked with patches where vegetation just won't grow. Dubbed "fairy circles," these strange structures have sparked debate for years about how they form, with theories ranging from plants competing for limited water, to termite colonies clearing their territories. Now, scientists at Princeton have put forward a new explanation: it's actually both of those ideas, working in unison.
The fairy circles extend for about 1,500 miles (2,414 km) across Africa, with each individual bald spot ranging from 6.6 to 115 ft (2 to 35 m) in diameter. More confoundingly, grass seems to thrive around the edges of the circles, where it often grows thicker and taller than elsewhere on the plains. Looking like little craters and particularly distinctive from above, fairy circles have inspired a variety of mythical origin stories, from deities and dragons of local legend to UFOs – if you listen to certain corners of the internet.
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Scientific opinion on the subject is split into two main camps. One points to the tendency of plants to cluster together to make use of scarce water in the desert environment, a phenomenon known as "scale-dependent feedback." Others believe termites cause the circles by clearing vegetation from the area around their nest, both to feed and to help water pool underground. But both theories have holes in them: scale-dependent feedbacks have never been properly tested, and termites don't seem to be capable of creating such precise, recurring patterns over an area that large.
But nature isn't an either or system, and the Princeton team realized that both phenomena could be at work simultaneously. By creating computer models and comparing the results to real-world observations of the Namibian fairy circles, the researchers found that a combination of plants competing for water and the nest-building habits of termites could explain the formations, where neither could on its own.
"There have long been two leading theories about how these regular patterns, and especially fairy circles, are formed, and those theories have traditionally been presented as mutually exclusive," says Juan Bonachela, co-lead author on the study. "Our findings harmonize both theories and add to the set of possible explanations for regular vegetation patterns observed around the globe."
Starting from satellite images of social-insect nests taken from four continents, the researchers ran computer simulations to determine why those nests took on such uniform arrangements, which look like honeycomb patterns from above. Those models told an interesting story: a colony of insects, like termites, will expand its territory outward in the search for food and water until it encounters another colony. Being fiercely territorial creatures, the two will battle it out and the smaller colony will usually be wiped out.
Eventually, all that's left in an area are the stronger, larger colonies, and when their territories bump up against each other, it's a stalemate. Borders become clearly defined, and the honeycomb pattern of nests emerges.
"Many social insects tend to be territorial and colonies often fight to death," says Corina Tarnita, lead author of the study. "When any incipient mound appears in an existing territory, the established termites eventually find it and destroy it. Over time, large colonies obliterate the smaller ones. But large colonies end up coexisting in a perpetual border war with neither gaining any ground. Eventually you end up with colonies of very similar sizes that are as far from each other as possible, while at the same time not leaving any space unoccupied."
That explains the patches of dirt that make up the fairy circles, and the regular spacing between them on such a large scale. But why are they ringed with taller grass? As it turns out, plants are having their own frantic turf wars over the limited desert resources.
As a result of scarce water, plants tend to organize themselves into clumps, where they end up helping out their immediate neighbors by shading each other and concentrating soil moisture directly beneath them. Plants just outside of a clump will have a hard time competing and they can't encroach further into the circle because foraging termites keep them at bay, resulting in a thin band of healthier plants surrounding each fairy circle.
The computer models predicted that in this case, small-scale clumps of plants should be present in the vegetation between the barren circles. Sure enough, when the team gathered field data from Namibia, they observed exactly that, which had never been reported before. Previous studies missed that vital clue, most likely because they weren't looking for it.
"Our goal throughout this work has been to contribute to a coherent understanding of regular patterns as a set of phenomena that crop up at many different levels in all kinds of systems, both biological and non-living," says Robert Pringle, co-author of the study. "Fairy circles beautifully exemplify the broader category of patterns that we're interested in."
It's been a good week for competing scientific theories coming together to find a middle ground: two conflicting long-term studies finally came to an agreement that caloric restriction does increase the lifespan of monkeys.
The research was published in the journal Nature.
Source: Princeton UniversityView gallery - 3 images