Diversity patterns in the land-snail fauna of Afromontane forest in the Rwenzori Mountains in Uganda

Abstract

We investigated the land-snail fauna in Afromontane forest along three transects on the eastern slopes of the Rwenzori Mountains in Uganda. Forty plots at altitudes between 1,770 and 2,932 m were studied. A total of 4,823 specimens were collected and were assigned to 91 land-snail species. Considering additional literature records, the land-snail fauna of the Rwenzori Mountains includes at least 174 species (142 on the Ugandan side). Currently, no area in Africa of a similar size is known to harbour such high regional land-snail species richness. The high species richness and endemism are partly the result of radiations of several Afromontane groups within the Rwenzori Mountains. The regional richness in the Albertine Rift is mainly affected by the distance from Pleistocene forest refugia. The majority of the species recorded in the Rwenzori Mountains extends from the lowest investigated altitudinal levels to different upper altitudinal levels, but there are also groups of species that are restricted to higher altitudes. Thus, the change of the composition of the land-snail communities along the altitudinal gradient is not completely gradual, but there is a rapid turnover in parallel with the transition from the montane rainforest zone to the bamboo zone. Whereas minute Afromontane species often have wide, disjunct distributions across East Africa, larger sized Afromontane snail groups are usually represented by endemic species in different mountain ranges. The better passive dispersal abilities of microsnails probably resulted in a higher gene flow between mountain ranges, resulting in coherence of the populations and a reduced likelihood of speciation.