Snails dine at desert dust depot.

Snails dine at desert dust depot

Winds from the Arabian and Sinaipeninsulas sweep across the limestone rocks of Negev Desert Highlands in Israel, depositing dust in their wake. Long considered a major source of soil formation in the Negev, these winds apparently are not the only significant contributors. Researchers are finding that in the sands below, two species of snails may be equally responsible for soil formation.

Although plant-eating animals havebeen known to affect their ecosystems through overconsumption of a resource, such as overgrazing, these snails have a significant regulatory impact despite the small amount they consume, according to a report in the May 29 SCIENCE by Clive G. Jones of the Institute of Ecosystem Studies in Millbrook, N.Y., and Moshe Shachak and Yigal Granot of the Mitrani Center for Desert Ecology in Israel.

The two species, Euchondrus albulusMousson and Euchondrus desertorum Roch, are found throughout the desert at a rate of 21 snails per square meter and feed seven months out of a year during seasonal dew formation. The lichens they feed on, known for their toughness and low digestibility, grow under the rock surface at depths between 1 to 7 millimeters. Called endolithic lichens, they are cryptogams that don't bear seeds or flowers, and are dominant in limestone in extreme environments. The arid desert terrain these hardy lichens and snails call home is a hilly region some 5,000 square miles large and covered mostly by rock.

Videotapes of lab experiments showedthe snails foraging in short side-to-side motions for about 20 minutes, leaving behind a white trail in the process. Depths of the trails varied, say the researchers, possibly because the snails might rebrowse trails for the lichen that grew there within 48 hours of when they made their first feeding pass. "We don't know how often they rebrowsed a trail,' says Jones. "We expect it's fairly frequently.' The scientists also observed small piles of limestone-colored feces that showed a calcium content similar to that of the upper layers of rock and lichen, supporting the theory that the snails were not only disturbing the rock for food, but also ingesting and redepositing it as waste.

Based on observations of foraging behaviorof 10 snails per stone in the lab, researchers estimated that the snails removed about 7 percent of the surface area of the rock to a depth of 1 mm per year. Field observations produced somewhat lower results: removal of 4 percent of the surface area of the rock to the same depth per year. On the basis of these estimates the researchers conclude that the foraging snails "weather' the desert at a rate of about 0.7 to 1.1 tons per hectare per year.

To find the impact of soil formationcaused by wind deposition, the group measured dust levels from 10 stones each month during the dry season in an area of the highlands with low snail density. Through this method, they estimated about 0.4 ton or more per hectare per year. But dust deposition is difficult to measure, and other estimates place this figure much higher. Either way, the dust deposition caused by snails is at least as great as that produced by wind, according to the study.

Photo: White lines on rock show foraging behavior of Euchondrus desertorum, one of two species of snails found to contribute heavily to soil formation in the Negev Desert Highlands.