The effects of food and density of the movement of Arion ater and Ariolimax columbianus (Pulmonata: Stylommatophora) between habitats

Summary Experiments were conducted to determine if slug density, or food quantity and quality could cause dispersal by slugs. Arion ater dispersed quickly from overcrowded areas, where high mortality was incurred by this species. Poor food conditions, caused more dispersal than good food conditions, but food had no effect on the mortality of A. ater . Neither food nor density factors had significant effects on the dispersal of Ariolimax columbianus . However, more A. columbianus died in the high‐ than low‐density treatments. A. columbianus mortality was unaffected by the food factor. Dispersing Arion were heavier than non‐dispersers, but there was no difference between the weights of dispersing and non‐dispersing Ariolimax . There were significant seasonal changes in the dispersal rate, mortality and weight of dispersing Arion , whereas only the mortality of Ariolimax changed significantly during the experiments. Since Arion ater did not markedly reduce its activity during unsuitable weather it was able to leave areas of poor food and high slug density fairly rapidly. Ariolimax columbianus' large size allowed it to remain inactive during unfavourable weather and permitted this species to be relatively unresponsive to the experimental treatments.