Habitat heterogeneity determines species richness on small habitat islands in a fragmented landscape

Aim The small‐island effect (SIE), as an exception to the species–area relationship, has received much attention in true island systems. However, the prevalence and related patterns of the SIE have not been well evaluated in habitat island systems. Here, we aimed to identify the existence of SIE for habitat islands in fragmented landscapes and determine the key factors influencing species richness on small habitat islands. Location Inner Mongolia Autonomous Region, China. Taxon Vascular plants. Methods Based on 78 grassland fragments in fragmented landscapes of the agro‐pastoral ecotone of northern China, we used piecewise regression, path analysis and null models to investigate the SIE of the species–area relationship. We then used a multi‐model selection to evaluate the impacts of four influencing factors (instability, isolation, habitat heterogeneity and surrounding productivity) on species richness (including habitat specialists and generalists) on small habitat islands within the range of SIE. Results We found an obvious threshold of 5.1 ha in the species–area relationship, below which habitat island area had no direct and indirect effects on species richness. Small habitat islands (