The Shift to Sands

By Eric Wagner
October-December 2007 (Vol. 8, No. 4)

Kéfi, S. et al. 2007. Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems. Nature 449(7159):213-217.

Six thousand years ago, the Sahara was as green as any verdant field. Then the weather changed, so to speak, and it became nothing but sand. And for the 2 billion people who live in the arid regions of the world, that shift is important, no matter how old. The specter of climatic upheaval has them worried that it could happen again.
Now, in a recent paper in Nature, scientists from The Netherlands, Spain, Greece, and Morocco write that they have uncovered patterns that could help predict when and how such shifts might occur.

The key to their divining lies in the size and number of vegetation patches. In dry landscapes where water is the limiting resource, plants tend to grow in clumps, which can be small or large. Using field data and models, Sonia Kéfi of Utrecht University in The Netherlands and her colleagues found that the relative number of these clumps is constant with regard to their size—there are far more small than large clumps, as follows the fixed relationship of a power law.

But Kéfi found that there’s a break point at which arid ecosystems can abruptly shift to deserts if, say, rainfall decreases or grazing pressure increases. At this point, there are far fewer large clumps than would be expected—the system has departed from the power law. Or, in more vivid terms, the system is on the verge of sudden, catastrophic, and irreversible collapse. It’s like a ball rolling down a gentle slope that’s green at the top and gets progressively browner—but then the ball comes to the edge of a cliff and sails off. What was a smooth transition is now sudden, and at the bottom of the cliff is a desert.

The recent spate of crippling droughts and increases in grazing pressure has raised fears that arid ecosystems may be speeding toward that edge. Previous work has shown that certain patterns of vegetation patchiness can serve as a signal for an imminent transition to a desert state. Kéfi’s model gives people a better idea of where they are on that slope: when large clumps are nowhere to be found and all that’s left is small tufts of grass, that’s a sign that a place is about to be “desertified.”

Photo: ©Robert Bremec/istock.com

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