Disharmony of the world’s island floras

Aim: Disharmony is a key concept in island biology that describes the imbalance in the representation of higher taxa on islands compared to their mainland source regions. Although there are strong theoretical arguments for the differential colonization success of different taxa on islands, the empirical evidence for disharmony remains largely anecdotal. Here, we develop a novel method for delineating island source regions and present the first global quantitative assessment of island disharmony. Location: Global. Time period: Recent. Major taxa studied: Vascular plants. Methods: We developed a generalizable method for estimating the source regions of an island flora based on statistical predictions of species turnover. We then designed two metrics to investigate disharmony from an island- and a taxon-centered perspective. First, we used linear mixed effects models to analyse the overall taxonomic bias of 305 island floras (compositional disharmony) as a function of geographical and climatic island features. Second, we applied linear models to examine the over- or under-representation of 450 vascular plant families on islands (representational disharmony) as a function of family size, age, higher taxonomic group and family-specific functional traits. Results: We found that compositional disharmony scales positively with island isolation and negatively with island area, and is strongly modulated by climatic variables. In contrast, the relationship between representational disharmony and family-specific characteristics was weaker. We found significant effects of family species richness and pollination syndrome, whereas family age and all other tested functional traits remained without effect. Conclusions: The taxonomic scope of the disharmony concept has historically limited its wider applicability, because higher taxa are inconsistent ecological proxies. However, our results provide a strong foundation for integrating disharmony with quantitative functional and phylogenetic approaches in order to gain a deeper understanding of assembly processes on islands.