EFFECTS OF FOOD AVAILABILITY AND INTRASPECIFIC AND INTERSPECIFIC INTERACTIONS ON THE DISPERSAL TENDENCY IN THE LAND SNAIL CHONDRINA-CLIENTA

The land snails Chondrina clienta and Balea perversa compete most prob ably for a limited food resource (calcicolous lichens) on rock-faces o n the Baltic Island of Oland (Sweden). Two laboratory and three field experiments were conducted to determine whether food availability and intra- and interspecific interactions affect the dispersal tendency in C. clienta. Under laboratory conditions, individuals of C. clienta sh owed a higher tendency to disperse from previously grazed than from un grazed pieces of limestone (their natural substratum), whereas conspec ific density or presence of B. perversa had no effect. However, when s nails had been kept on the pieces of limestone for 40 days prior to te sting, dispersal tended to increase with increasing density of conspec ifics. In the field, marked individuals of C. clienta were released at different densities on vertical rock walls. Density of conspecifics a t the release points did not influence the distances travelled. Simila rly, intraspecific density did not affect dispersal when the snails' f ood resource (lichens) had been experimentally reduced on the quarry w alls. Neither was the dispersal tendency influenced by different crowd ing conditions experienced for 40 days prior to release. The discrepan cy between the results of laboratory and field experiments suggest tha t additional factors acting on dispersal are of importance in natural populations of C. clienta.