The global island species-area relationship for plants

The island species-area relationship (ISAR) is known to be near-ubiquitous, but its properties across the fullest span of island areas globally and how island endemism shapes the ISAR remain poorly understood. We determine the global ISAR for native (Nat_Rich) and for single-island endemic richness (SIE_Rich) of vascular plants, employing data for 1,262 islands, spanning 60.7S to 80.7 N and ten orders of magnitude in area. Using logged species number and area, we compared the power model and four different breakpoint models. For Nat_Rich, a simple power model (slope, z = 0.32, R² = 66%) was best supported. For SIE_Rich, a flat-steep breakpoint model outperforms the power model, with the latter producing a steeper slope (z = 0.48, R² = 0.47) than for Nat_Rich. Rerunning the Nat_Rich power model for subsets of islands of increasing endemism generates increased ISAR slope and improved prediction of continental richness values. Controlling for island area, Nat_Rich declines with isolation, while endemism increases. Semilog analyses show that old, tropical, mountainous continental fragments and landbridge islands, rich in SIE, drive an accelerated increase in Nat_Rich for islands >10,000 km² in size. The global Nat_Rich archipelago species-area relationship was best described by a power model (z = 0.41, R² = 0.54), and there is also evidence of declining richness but higher endemism with increased archipelago isolation. Our findings provide no support for the existence of an upper asymptote in the global plants ISAR, while supporting the application of the power model at a global scale, and highlighting roles for island type, endemism, and isolation as influences on ISAR form.