Global insular leaf size shifts follow the island rule, independently of insect herbivory and macroclimate

The island rule, originally formulated for animals, predicts that small-bodied mainland species evolve larger body sizes on islands (gigantism), but that this effect weakens with increasing mainland body size, ultimately reversing and leading to dwarfism for the largest species. This dynamic is expected to produce a positive, saturating relationship between island and mainland body size, with insular size increases at small sizes and reductions at large sizes. Despite extensive support in animals, this prediction has rarely been tested in plants. Consequently, it remains unclear whether the island rule applies to plants, whether it operates consistently across evolutionary scales, and how biotic and abiotic drivers jointly shape insular size shifts. We tested the island rule in plants by examining leaf size variation—an organ-level analogue of body size—across 48 island species from six oceanic systems and their mainland counterparts. We conducted both conspecific comparisons (same species on islands and the mainland; n = 19 pairs) and congeneric comparisons (island endemics paired with closely related mainland species; n = 29 pairs) to assess patterns across evolutionary scales. We also measured insect herbivory and recorded climatic variables to explore ecological correlates of island–mainland variation in leaf size. Although mean leaf size did not differ significantly between island and mainland populations for either conspecific or congeneric comparisons, we detected a non-linear, positive saturating relationship between mainland and island leaf sizes, consistent with an island rule-like pattern. Small-leaved mainland species tended to evolve larger leaves on islands, whereas this effect diminished for larger leaved species, a pattern observed in both conspecific and congeneric comparisons. Insect herbivory and climate did not explain these relationships. Synthesis: These findings demonstrate that plants follow the island rule for leaf size and suggest that mainland-to-island shifts at opposite ends of the mainland leaf size spectrum offset one another, possibly explaining no overall difference in leaf size between island and mainland populations.