Integrating functional and trait-based models to explain seed size variation on islands in the New Zealand region

Abstract Seed size is known to evolve predictably on islands, yet the biotic and abiotic factors that filter seed size during the earliest stages of colonisation remain poorly understood. Moreover, most previous studies have relied on trait means rather than full trait distributions, potentially disregarding important aspects of trait variation. In this study, we use functional analyses to test how the seed-size distributions of species dispersed abiotically (by wind or water) and biotically (via attachment or frugivory) vary with island geographic, climatic, and biotic characteristics. Results differed among dispersal modes. Wind and water-dispersed species showed fewer large-seeded plants and more small- and medium-seeded ones on highly isolated islands relative to less isolated islands. For species dispersed via attachment and frugivory, warmer islands housing greater bird diversity and gape widths were associated with higher proportions of medium- to large-seeded plants and fewer small-seeded ones compared to cool, species-poor islands. In conclusion, this study elucidates the geographic, climatic, and ecological filters shaping seed size distributions on islands, highlighting their roles in constraining plant colonisation and driving long-term evolutionary change.