Size changes in island plants: independent trait evolution in Alyxia ruscifolia (Apocynaceae) on Lord Howe Island

Species that are endemic to isolated islands often differ dramatically in size from their mainland relatives, for reasons that are poorly understood. While decades of research have sought to better understand insular size changes in animals, far fewer studies have investigated insular size changes in plants. Here, I test for changes in plant stature, seed size and leaf area in a woody shrub (Alyxia ruscifolia, Apocynaceae), which inhabits both the continent of Australia, and Lord Howe Island, a subtropical island located 600 km off Australia's east coast. Results showed that island plants became reproductively mature at earlier stages of ontogeny than mainland plants, and that mature plants were taller on the mainland, providing a rare example of dwarfism in plants. Conversely, island plants produced larger seeds, which might make them more competitive as seedlings. Seeds produced by island plants were also less circular and more oblong in shape than their mainland counterparts, perhaps to facilitate their dispersal by avian frugivores with limited gape sizes. Lastly, island and mainland plants had similar average leaf sizes. However, juvenile plants on the mainland produced smaller, more needle‐shaped leaves with larger terminal spines relative to adult plants, which may help protect them against large, ground‐dwelling herbivores. On the other hand, island plants showed weaker ontogenetic shifts in leaf morphology in the absence of large herbivores. When interpreted jointly, results indicate that stature, seed size and leaf area are on separate evolutionary trajectories in A. ruscifolia, which appear to be determined by a complex suite of disparate selection pressures between Lord Howe Island and the mainland.