Zoogeographic patterns on very small spatial scales in rock-dwellingPlectostoma snails from Borneo(Gastropoda: Caenogastropoda: Diplommatinidae)

Abstract

We mapped the fine-grained distribution patterns of Plectostoma microsnails on two small isolated limestone outcrops in Malaysian Borneo. On both hills, two species were present (P. simplex and P. concinnum on Tandu Batu; P. fraternum and P. concinnum on Tomanggong Besar), but the patterns we found were different. On Tomanggong Besar, the two species occupy different parts of the hill and meet along a narrow hybrid zone that is characterised by a significantly higher rate of predation by Atopos slugs. On Tandu Batu, the two species broadly overlap and do not form hybrids. The predation rate here is the same in both species, regardless of whether they occur in monospecific localities or in mixed localities. Our results show that even small limestone outcrops of a few hundred m in diameter cannot be considered to be uniformly populated by limestone dwelling snails, and a detailed analysis of intra-hill patterns may reveal information on species differences and maintenance.