Fig. 1. A conceptual diagram illustrating the potential mechanisms behind the positive island species-area relationships, assuming that all islands share the same regional species pool.

The total number of species on an island (gamma diversity) is decomposed to species richness within samples (alpha diversity) and sample-to-sample species turnover (beta diversity) within the island. a Gamma diversity, but not alpha or beta diversity, increases with island area. This pattern is in line with the prediction of the sampling effect, which posits that larger islands contain more individuals and thus accumulate more species by chance. b Gamma and alpha diversity, but not beta diversity, increase with island area. This pattern is often considered as the evidence for the area per se effect, which emphasizes the importance of reduced local extinction risk on larger islands. Alternatively, we suggest that this pattern could also be explained by local habitat quality favoring more species on larger islands. c Gamma and beta diversity, but not alpha diversity, increase with island area. This pattern has often been explained by greater habitat heterogeneity on larger islands. Alternatively, we suggest that this pattern could also arise from stronger dispersal limitation on larger islands.