Mystery of Desert ‘Fairy Circles’ Solved, Creators Found
The “artists” behind bizarre, barren, grassless rings dotting the desert of Southwest Africa have been found lurking right at scientists’ feet:termites.
Known as fairy circles, these patches crop up in regular patterns along a narrow strip of the Namib Desert between mid-Angola and northwestern South Africa, and can persist for decades. The cause of these desert pockmarks has been widely debated, but a species of sand termite,Psammotermes allocerus, could be behind the mysterious dirt rings, suggests a study published today (March 28) in the journal Science.
Baby RingsCredit: Image courtesy of N. JuergensA shot out of the open door of a plane showing fully developed “adult” fairy circles, with a few newly established “babies” developing in the interspace between the old ones. (Aerial view of Namibrand, Namibia.)
Scientists have offered many ideas about the circles’ origin, ranging from “self-organizing vegetation dynamics” to carnivorous ants. Termites have been proposed before, but there wasn’t much evidence to support that theory.
Finding patterns in circles
While studying the strange patterns, biologist Norbert Juergens of the University of Hamburg noticed that wherever he found the dirt patches (the barren centers inside fairy circles), he also found sand termites.
Filling in the GapsCredit: Image courtesy of N. JuergensFairy circles in the Marienfluss Valley, Kaokoveld, Namibia, where desert grassland transitions to Mopane savanna. Fairy circles seem to be gaps in the grassland.
Juergens measured the water content of the soil in the circles from 2006 to 2012. More than 2 inches (5 centimeters) of water was stored in the top 39 inches (100 cm) of soil, even during the driest period of the year, Juergens found. The soil humidity below about 16 inches (40 cm) was 5 percent or more over a four-year stretch.
Without grass to absorb rainwater and then release it back into the air via evaporation, any water available would collect in the porous, sandy soil, Juergens proposed. That water supply could be enough to keep the termites alive and active during the harsh dry season, while letting the grass survive at the circles’ rims.
Mature CircleCredit: Image courtesy of N. JuergensA fully developed fairy circle with a green perennial belt (living grass plants) and a yellowish matrix (dead short-lived plants), both formed by the same species of grass (Stipagrostis ciliate). Image of Farm Dieprivier / Namib Desert Lodge, Namibia.
Juergens conducted surveys of the organisms found at fairy circles. The sand termite was the only creature he found consistently at the majority of patches. He also discovered that most patches contained layers of cemented sand, foraged plant material and underground tunnels — telltale signs of sand termites.
The scientist found a few other termite species, as well as three ant species, at fairy circles in areas that get rain during the summer or during the winter, but not at all the sites he studied.
Grassy FoodCredit: Image courtesy of N. JuergensIn a normal dry year only the perennial belt of the fully developed fairy circles provides biomass for herbivores at Giribesvlakte, Namibia.
The termite behavior provides an example of “ecosystem engineering,” Juergens wrote in the Science paper. The insects appear to be feeding on the grass roots to create the characteristic rings, the study suggests. As to why the termites would create circular-shaped patches, Juergens doesn’t say.
“The paper is a useful addition to debating the origin of the fairy circles,” chemist Yvette Naude of the University of Pretoria, South Africa, who was not involved in the study, told LiveScience in an email. But, Naude added, the study “does not address the key question as to what is the primary factor that causes sudden plant mortality, i.e. the birth of a fairy circle.”
On the EdgeCredit: Image courtesy of N. JuergensThe Marienfluss Valley, Kaokoveld, Namibia in a dry year: Plant biomass for herbivores is mainly found at the margin of the fairy circles.
The soil in fairy circles seems to be altered so that plants can’t survive, whereas termites usually enrich soil, making it more hospitable to plants, she said. (Juergens actually thinks the termites chew up the plant roots, and that’s what leads to the barren patches.)
Stickin’ AroundCredit: Mike and Ann Scott of the Namib Rand Nature ReserveAnother study, this one published online June 27, 2012 in the journal PLOS ONE, suggested the small circles stick around for about 24 years, while the larger ones stay put as long as 75 years. Here, a typical fairy circle in Namibia. [See More Images of Fairy Circles]
It is possible the termites don’t cause the fairy circles, but merely live in them. However, Juergens found the insects were present even during the early stages of patch formation, before the grass had died off on the surface. Over the termites’ lifetime, they munch on the grassy borders and gradually widen the circles.
Fairy Circle EvolutionCredit: Mike and Ann Scott of the Namib Rand Nature ReserveThe smallest are about 6.5 feet (2 meters) in diameter, while the largest can be almost 40 feet (12 m) across. Eventually, plants move back in, re-colonizing the circles and leaving only slightly indented “ghost circles” behind.
Going Down?Credit: a href=”http://www.shutterstock.com/gallery-8509p1.html”>Louie Schoeman |ShutterstockOver time as winds scour the bare surfaces the fairy circles form slight depressions, like the one shown here in the Namib Desert.