Interspecific interference competition alters habitat use patterns in two species of land snails

Focus has been placed on the relative importance of environmental heterogeneity and biological interactions on community structure. For land snails, abiotic factors have commonly been assumed to be the most important factors for shaping communities because resources for land snails are commonly not limiting, and because interspecific resource competition would not be strong enough to promote ecological divergence. However, clear divergence in habitat use is often observed among sympatric land snail species. Such an ecological divergence would be promoted not by resource competition but by other interactions such as aggression. To test this hypothesis, laboratory experiments were designed to explore aggressive behaviour in the land snails Euhadra quaesita and E. peliomphala and field surveys were conducted to examine their habitat use. In the laboratory experiments, we examined how the presence of one species affects the growth of the other species by supplying sufficient amounts of food and calcium for both species. Experimental trials showed that adult E. quaesita decreased the growth of E. peliomphala under constant diet conditions. In contrast, E. peliomphala did not affect the growth of E. quaesita. Because E. peliomphala was often attacked by E. quaesita and its shell was often eroded by E. quaesita gnawing, aggression by E. quaesita appears to be the primary factor reducing the growth of E. peliomphala. When the two species coexist in nature, adults and juveniles of E. quaesita are terrestrial, while those of E. peliomphala are arboreal. When these species occurred alone, E. quaesita were still terrestrial, but E. peliomphala were more terrestrial than when they were in sympatry. Our results suggest that habitat use of these species in the natural communities is affected by interspecific interference. These findings further suggest that the divergence of habitat use between the species can occur by aggression even in environments with unlimited resources.