Anisotropic Shape of Islands and Species Richness of Land Snail Fauna of the Ryukyus.

Abstract

We introduce a new immigration model in order to derive the famous power law on species-area (SA) relation for anisotropic island. This relation shows that the number of species is constrained not only by the size of the recipient island area or the species' dispersal capacity but also the power index of SA relation. Because this power index depends on island shapes, this means that island shape is one of the factor in constraining the number of species. From this viewpoint, we analyze an SA curve of the land snail fauna in forty-three islands in Ryukyu arc, Japan, and show that most individual species have travelled across the line of the island chain of the Ryukyu islands. Moreover, we reconsider the power index of SA relation in previous studies, and show that species have immigrated along the island chain by stepping-stone dispersal. We also consider the effect of the height of the surface topography on the species richness by using the Hurst parameter. Moreover, we point out that species richness and environmental heterogeneity have the same relationship with the area.