The Effect of Predation on Population Densities of Some Desert Snails

In preliminary investigations it was found that there is a great difference in the population density of two desert snails on southern and northern slopes of a wadi. The density of Sphincterochila boissieri was equal on both slopes, while the density of Trochoidea seetzeni is much lower on the southern slope. It is shown that this difference is due to predation by some rodents, mainly dormice and gerbils, which are more common on the southern slope due to its geomorphologic character. The rodents prefer to eat T. seetzeni because they have thinner shells, while S. boissieri, which have much thicker shells, are seldom eaten. The effect of predation on community stability on the slopes is discussed. It seems that the conclusions of MacArthur and Wynne—Edwards on community stability in extreme regions are valid also in this case.