Scale-dependent shifts in functional and phylogenetic structure of Mediterranean island plant communities over two centuries

Since the Industrial Revolution, the rapid global population and economic expansion have had tremendous impacts on biodiversity across spatial scales, especially for islands. While changes in species richness are easily inferred, the impact of human activity on the underlying community assembly processes has been difficult to ascertain because of lack of long-term community data. Here, we document how the manifestations of plant community assembly have changed over time and space in a Mediterranean archipelago, using a long-term dataset of plant species composition on 16 Tuscan islands sampled across two centuries. The community structure of Mediterranean island plant communities was assessed by integrating species' trait and evolutionary distances. We found that, with increasing island area, the functional and phylogenetic structure of plant communities shifted from clustered early (1830–1950) to overdispersed more recently (1951–2015). On large islands, extirpated species were generally more phylogenetically or functionally similar to remaining residents than expected by chance, while colonists were generally more distantly related to residents. The extinction of similar species and the colonization of dissimilar species drove plant communities towards overdispersion. Synthesis. We provide evidence that plant community assembly on islands has dramatically changed following increased human impacts during the last two centuries, and that this change is shaped by the scale dependency of species extinctions and colonizations. Our results reveal accelerated species replacements of closely related residents by distant colonists on large islands over time, reflecting changes in community assembly and which could alter the functioning of island ecosystems in the future.