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Chewing through the Desert

by Don Smith

The Negev Desert Highlands, Israel, is a hilly, stony desert where 70% of the ground is covered by rocks carrying surface lichens. These are browsed by two species of snails. When feeding they left behind a channel 0.4mm deep with fresh white limestone showing. The radula (tongues) of both species bear cutting teeth with large cusps and curved tips which continually regrow.

The snails are abundant to about 21 per square metre and each eats about 15mm3 of stone daily, removing up to 7% of the rock surface to a depth of 1mm each year. The contribution of powdered rock via snail faeces is about 70-110g/m per year so that clearly, snail consumption of the lichen/rock mixture has a huge effect on the process of rock weathering and soil formation. Each year their munching creates about 1 ton of soil per hectare (2.5 acres). Science (1987) vol.236: 1098-1099. It isnít just foreign snails that chew limestone. They do this, of course, for the purpose of shell building. When examining church masonry for tiny lichen fruits, I have often observed the unmistakeable serrated grooves left by a snail radula. I recall examining the church masonry of St. John at Newton-on-Rawcliffe and discovering at the rear, on a north facing wall, at least a dozen very large snails taking a daytime rest. One can often see their silvery trails leading right up to the guttering.

© Michael Thompson & Ryedale Natural History Society 2010 Back to the Annual Report Contents and Home page