Contribution of geographic and environmental distances, and habitats to beta diversity
We used values of turnover between islands and between cells within islands to test the effect of geographical and environmental distance on beta diversity across scales. The analysis were performed using multiple regression on distance matrices (MRM) with Pearson’s correlation and 10,000 permutations (Lichstein et al., 2007). The MRM were implemented with the function MRM in ecodist package (Goslee & Urban, 2007). At the intermediate and small scales (between cells within islands), we also added a third distance matrix to account the effect of habitats along with the effect of climatic and geographic distance. This distance matrix was computed as follow: two cells belonging to the same habitat were coded as 0, and two cells belonging to different habitat where coded as 1 (following Carvalho & Cardoso, 2014). To determine the relative contributions of geographical and climatic distance, as well as habitats, we further used hierarchical variance partitioning to partition the global R2 into its geographical, and climatic and habitat components using the function hier.part of the R package hier.part (MacNally & Walsh 2004). All statistical analyses were implemented within the R programming environment (R Development Core Team, 2020).