Integrating Phylogeny and Functional Traits into Evaluations of Nestedness in Island Floras

Questions Understanding the composition and structure of island floras is crucial for making informed conservation decisions. Island floras are often nested, i.e. lower species richness assemblages are frequently subsets of those higher in richness. However, the circumstances under which this occurs on islands are often unclear. Moreover, research in island biogeography rarely integrates phylogeny and functional traits in nestedness studies. Here, we integrated phylogenetic and functional traits in the investigation of relationships between taxonomic nestedness and island characteristics to gain insight into the processes that shape the composition and structure of island floras. We asked whether (i) native plant species assemblages are nested according to their phylogeny and functional traits and (ii) whether nested patterns are related to island characteristics. Location 264 islands offshore from northern Aotearoa New Zealand. Methods We combined field surveys and published data for 775 native plant species across the islands. We compiled information about their phylogeny at the fine (species) and coarse level (i.e. ferns and allies, conifers, monocots, and dicots). Then, for each species, we determined three plant functional traits (growth forms, dispersal modes, and species maximum height). We quantified nestedness by organizing species incidence matrices using the NODF (i.e. nestedness metric based on overlap and decreasing fill) and fixed-fixed null models. Finally, we correlated island nestedness ranks with three island characteristics (area, isolation, and exposure to ocean-borne disturbances). Results All plant categories were nested, except for ferns and water-dispersed species. Nestedness ranks were consistently related to island area, strongly supporting the selective extinction hypothesis. Relationships with isolation and exposure to ocean-borne disturbances were less strong and varied by phylogeny and functional traits, suggesting weaker support for selective immigration and habitat nestedness. Conclusions These findings highlight the importance of nestedness studies and the integration of phylogeny and functional traits for identifying conservation priority areas. The overarching effect of island area in shaping insular plant composition underscores the key role of large islands for conserving plant diversity.