By JOHN NOBLE WILFORD
Have you heard the one about the little termite that could, and did, take on a desert and turn it green? At least a little greener, except for those spots.
The reddish barren spots, thousands of them, are called fairy circles, the name itself an invitation to try to solve the mystery of their origins. They dot a narrow belt of desert stretching from Angola through Namibia into northern South Africa. For no obvious reason, the round patches of sandy soil interrupt the arid grassland, like a spreading blight on the land.
To the Himba people who live in the region, however, there is nothing to explain. That’s just how it is, they tell anthropologists; the circles were made by their “original ancestor, Mukuru,” or more poetically, they are “footprints of the gods.” A just-so story blames a mythical dragon that lives in a crack deep under the earth. The dragon’s poisonous breath kills vegetation to create the circles. Trouble is, some scientists point out, the bad-breath hypothesis apparently originated with fanciful tour guides.
New research may now have yielded a more credible explanation for the fairy circles as examples of natural ecosystem engineering by a particular species of sand termites, Psammotermes allocerus. A German scientist reported on Thursday that most likely these industrious termites were the agents for making much of their desert home an oasis of permanent grassland.
In an article in the journal Science, Norbert Juergens, a professor of ecology at the University of Hamburg, said these termites “match the beaver with regard to intensity of environmental change, but surpass it with regard to the spatial dimension of their impact.”
Over the 1,200-mile length of the Namib Desert, especially in parts of Namibia, Dr. Juergens wrote, “P. allocerus turns wide desert regions of predominately ephemeral life into landscapes dominated by species-rich perennial grassland supporting uninterrupted perennial life even during dry seasons and drought years.”
Last year, Walter R. Tschinkel, a biologist at Florida State University, published an analysis of aerial and satellite photography and other research to describe the number, size and dynamics of these formations. Some are as small as six feet in diameter and never grow much bigger. The largest ones can be at least 40 feet across. It was estimated that the smaller circles have average life spans of 24 years, the larger ones as much as 75 years.
But Dr. Tschinkel had first assumed that termites were implicated and went looking for nests of a different species, harvester termites, without success. He finally concluded that no other termites had been associated with the circles, and seemed resigned to a mystery unsolved.
In a critique, Dr. Tschinkel said he was unconvinced that the termites are the cause of the circles. He said the paper by Dr. Juergens “has made the common scientific error of confusing correlation (even very strong correlation) with causation.”
Scientists at the University of Pretoria in South Africa have also tested hypotheses of escaping natural gases like methane or other toxins rising to the surface and wiping out vegetation at these spots. But the results have been inconclusive.
Dr. Juergens said in a telephone interview that Dr. Tschinkel was “looking for the wrong termites and you could easily overlook the ones that were actually living” deep beneath the surface of the red sandy spots, feasting on grass roots to keep the patches of land free of vegetation. In this way, the soil is better able to absorb rainfall quickly, with little water loss due to evaporation. The absence of vegetation at the site also means that rainwater is not lost through transpiration, the evaporation of water from plants.
The absorbed water, the scientists explained, spreads evenly in the sandy soil all around, which explains the circular patterns. This nourishes the surrounding grassland. And the termites keep chomping the roots of new shoots from beneath the inner circle, preventing new vegetation from disrupting their engineered ecosystem.
Another critical factor, Dr. Juergens said, is that all the circles he and associates examined methodically over 40 field trips in the last six years had two telling characteristics. P. allocerus termites were present in all, and the soil was extremely sandy and porous.
They found strong evidence, Dr. Juergens said, that the species does “things not done by other termites. They are “quite clandestine,” he noted. They build no nests or mounds above ground. Their underground galleries and passages are deep and narrow. “They sort of swim in the loose sand, not leaving tracks,” he said.
The researchers observed that the circles occur only in sandy soil, not where clay predominates. And they studied the presence of the termites in the earliest stages of a circle’s formation, establishing that they were in on its creation, not merely occupying it at later stages. They also were involved in widening the diameter of the circles, as they steadily fed on grasses at their outer margins.
In dry seasons, the termites can remain alive and active by moving out from the circles, still underground, and surviving on roots of the outlying grasses.
David P. Crandall, an anthropology professor at Brigham Young University in Utah who has studied the Himba people closely since 1990, said the fairy circles “are a strange and interesting phenomenon” that is vital to their sparse population spread over an area about half the size of Arizona. Even though the people appear to have little curiosity about why the circles are there, they depend on the grasses around them to graze their cattle, goats and sheep.
The Himba sometimes put the barren spots to new uses. Examining some Google maps, Dr. Juergens was puzzled by what appeared to be black margins to the circles in some pictures. Going to the sites, he found that they had erected temporary wooden fences to corral young cattle overnight, as protection against lions and other predators.