Environmental filtering underpins the island species-area relationship in a subtropical anthropogenic archipelago

Numerous mechanisms have been proposed to explain the island species-area relationship (ISAR), including the passive sampling effect, the habitat diversity effect, ecological drift and a group of mechanisms jointly termed environmental filtering (or abiotic selection process). However, determining which of these hypotheses underpins the ISAR in a given system remains an open and controversial question, particularly with regards to the environmental filtering. The effect of environmental filtering on the ISAR for woody plants was tested on 29 islands in a subtropical land-bridge archipelago. Island area had a significant effect on species richness, while isolation was not found to have an effect. After controlling for the passive sampling effect and habitat diversity using a systematic sampling design, we found that a positive ISAR remained. After controlling for island area and isolation, species richness was significantly affected by environmental conditions (e.g. total phosphorus, soil depth and bulk density). When a null model was used that randomly assigned individuals to islands from the regional pool, the difference between the observed and expected richness was greater for smaller islands relative to larger islands. Synthesis. Our results point to an overlooked role of environmental filtering in driving the island species-area relationship (ISAR). Consequently, our study highlights the importance of considering deterministic process in future analyses of the ISAR.