Relationship between diversity and precipitation
We compiled precipitation data at the plot level using the mean annual precipitation from locally interpolated climate data (Giambelluca et al., 2013). To relate diversity to precipitation, we used generalized linear models with gamma family and log link, as diversity metrics are skewed and always positive. We added an interaction term between mean annual precipitation and island to test the effect of island age on the model. We fitted the models using the base function in R ‘glm’: Diversity ~ log(precipitation) * island, family = Gamma(link = ”log”), where diversity is taxonomic, phylogenetic and functional alpha diversity of the different q orders (0, 1, 2) and precipitation is mean annual precipitation (mm/year). We tested the relationship between betaC diversity and the absolute difference in mean annual precipitation using generalized linear models with gamma family and log link, with an additional interaction term between island and precipitation difference. We determined whether the interaction between island and precipitation was significant for all models in two ways. First by plotting the model estimates and their uncertainty (85% and 95% confident intervals), using the function ‘plot_summs’ in the jtools package (Long, 2022), which reports the results of regression models. Second, we tested the statistical significance of the interaction using the Likelihood ratio test, which compares the goodness of fit of a simpler (no interaction) versus a more complex (interaction) model. We favor the complex model with interaction if the Likelihood ratio test shows a p-value smaller than 0.05. We used the ‘lrtest’ function from the lmtest package (Zeileis & Hothorn, 2002) to compute the Likelihood ratio test. Lastly, because the range of mean annual precipitation across islands differs, we subset precipitation to a common range for all islands (MAP > 800 & MAP < 4000) to test the consistency of the model’s results for all diversity metrics. All analyses were performed in R version 4.1.0 (R Core Team, 2021).