What drives phylogenetic and trait clustering on islands?

Context As one primary catchment-scale impacts of dam constructions on ecosystems, fragmentation, resulting in reduced species richness, altered ecological processes and degraded ecosystem functioning, has been increasing intensely. Objectives: Explore the drivers that hinder species co-existence and community assembly would facilitate understanding the fragmentation effect caused by dams. Methods: : We hypothesized that habitat filtering and competitive exclusion can simultaneously drive community assembly processes, such that communities on small islands, where competition for limited space and resources is more intense, would be functionally and phylogenetically less clustered than those on large islands. We used ten functional traits and a phylogeny of 76 woody plant species to assess species diversity and similarity within communities across an island area gradient. Results: As expected, species were more phylo-functionally similar to one another than expected by chance within islands and this underdispersion grew stronger with island area, indicating that while islands contained clustered communities, habitat filtering and competitive exclusion were both likely occurring. By integrating species abundance distributions with community similarity, we found that the most abundant species were phylo-functionally similar to the least abundant species. Species richness increased with island area, as expected, but the additional species found only on large islands tended to have low abundances, providing opportunities for rare species to persist. Conclusions: With habitat filtering narrowing the number of species that can persist, the loss of phylo-functionally closely related rare species on small islands was likely caused by competition or stochastic removals, leading to greater species dissimilarity than on large islands. On large islands, the clustered patterns are likely to be the result of a combination of competitive exclusion caused by resource limitation and from habitat filtering.