Significance The island species–area relationship (ISAR) is a fundamental diversity pattern, best described by the power model. Biogeographic theory assumes predictable variation in power model parameters in relation principally to system isolation, but these assumptions are only weakly supported by previous work, which has been limited in considering the two parameters separately and oversimplistically. By developing and testing a hierarchical (structural equation) model of factors influencing ISAR form, we show that island species diversity patterns are shaped by intra-archipelago processes more strongly than by isolation from mainland source pools. These findings point to a need to quantify the role of differing scales of isolation in influencing propagule exchange among insular systems to develop improved predictive diversity models.