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 probably for a limited food resource (calcicolous lichens) on rock-faces on 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 showed a higher tendency to disperse from previously grazed than from ungrazed pieces of limestone (their natural substratum), whereas conspecific density or presence of B. perversa had no effect. However, when snails had been kept on the pieces of limestone for 40 days prior to testing, dispersal tended to increase with increasing density of conspecifics. In the field, marked individuals of C. clienta were released at different densities on vertical rock walls. Density of conspecifics at the release points did not influence the distances travelled. Similarly, intraspecific density did not affect dispersal when the snails' food resource (lichens) had been experimentally reduced on the quarry walls. Neither was the dispersal tendency influenced by different crowding conditions experienced for 40 days prior to release. The discrepancy between the results of laboratory and field experiments suggest that additional factors acting on dispersal are of importance in natural populations of C. clienta.