Case Studies

Snails, rocks and lichens in the Negev Desert

In deserts nitrogen inputs are often low as are soil nitrogen reserves and losses can be high. The losses may come from runoff, erosion, volatilization and denitrification. Limited nitrogen availability puts severe constraints on plant productivity in deserts. Research has shown that rock-crunching snails may be fertilizing deserts with nitrogen derived from the eating of lichens.

The Central Negev Highlands are a desert with about 70% of the surface covered by limestone rocks. In this area three species of snails (of the genus Euchondrus), each less than a centimetre long, feed at night on the endolithic lichens, which grow in small cracks or cavities in the limestone rocks. During the day the snails rest under stones and drop their faeces there. Each year the faeces contribute between 22 and 27 milligrams of nitrogen per square metre, or about 11% of the total nitrogen input to the soil. Moreover, shrub roots are usually under rocks and the faeces deposited there are less likely to be washed away by rain, thereby magnifying the fertilizing effect to account for about 18% of the net nitrogen input.

The snails link two distinct ecosystems. The lichens trap airborne nutrients but stay in the rocks while the vegetation lives in patches of soil. Without the snails the nutrients trapped by the lichens would remain inaccessible to the plants. At least 27% of the nitrogen trapped each year by the lichens is passed to the soil by the snails.

The snails' actions also result in rock weathering at a rate of 0.7 to 1.1 metric tons per year, and such rates of soil formation are comparable to soil deposition by incoming winds.

References

Shachak, M; Jones CG & Granot, Y. (1987). Herbivory in Rocks and the Weathering of a Desert. Science, 236, 1098-1099.

Jones, CG & Shachak, M. (1990). Fertilization of the desert soil by rock-eating snails. Nature, 346, 839-841.

Zaady, E; Shachak, M & Groffman, PM. (1995). Release and consumption of nitrogen by snail faeces in Negev Desert soils. Biology and fertility of Soils, 23, 399-404.