Phylogenetic and functional trait-based community assembly within Pacific Cyrtandra (Gesneriaceae): evidence for clustering at multiple spatial scales

Abstract Tropical rainforest communities are often characterized by a small number of species-rich genera that contribute disproportionately to the alpha diversity in these habitats. In the Pacific Basin there are nearly 200 species of Cyrtandra, most of which are white-flowered woody shrubs that are single-island endemics. Within these island communities, multiple Cyrtandra species are commonly observed to occur sympatrically in wet forest understories, forming swarms of what appear to be ecologically similar taxa. The aim of this study was to determine if communities of these plants are randomly assembled with respect to phylogenetic relatedness and traits that are ecologically relevant. Using a combination of ten functional traits and a well-resolved species phylogeny, I examined community assembly within 34 species of Cyrtandra across three Pacific archipelagoes. Coexisting species were generally found to be more closely related and more phenotypically similar than would be expected by chance. This pattern was observed at both broad (island communities) and fine (site communities) spatial scales. The retention of phylogenetic signal in floral traits and the strong influence of these traits on the observed degree of phylogenetic clustering may indicate that pollinators act as a biotic filter for closely related species of Cyrtandra. In contrast, the absence of phylogenetic signal in most leaf traits, coupled with the lower contribution of these traits to niche clustering, suggests that environmental filtering along this trait axis is minimal in the observed communities. This study supports the theory that plant communities are not randomly assembled, and instead, that niche-based processes structure biodiversity at broad and fine spatial scales in diverse congeneric species assemblages.