Spillover of organisms from rainforests affects local diversity of land-snail communities in the Akagera savanna in Rwanda

Abstract We investigated how local and landscape variables influence the structure and richness of land-snail communities in a savanna ecosystem. The land-snail species richness in the Akagera savanna in Rwanda is smaller than in nearby rainforests. Generalized linear models indicated that the diversity of land-snail communities in the savanna is most strongly affected by a landscape variable, the distance from the escarpment where remnants of the former rainforests persist. The number and abundance of rare species decline more strongly with increasing distance from the escarpment than that of the frequent species that are supposedly better adapted to the drier habitats in the savanna. Actually, the number and abundance of rare species are also more strongly dependent on precipitation and the distance from the next stream than that of the frequent species. This indicates that the snail fauna of the Akagera savanna represents a metacommunity that depends on the spillover of immigrants from the richer fauna of the adjacent rainforests. Streams are probably the most important pathways for dispersal of land-snails. Given the low dispersal abilities of land-snails the spatial scale of the influence of the rainforests on the savanna is surprising.