Computing species compositional turnover across the
environmental gradient
To estimate the magnitude of species turnover across the precipitation
gradient and evaluate whether there was an influence of island age on
the magnitude of this turnover from drier to wetter sites, we used a
metric of beta diversity that captures the degree to which species are
non-randomly distributed across space using individual- and
coverage-based rarefaction and extrapolation (called betaC sensuEngel et al., 2021). We used pairwise comparisons among plots for each
island, comparing the betaC for a pair of plots with the absolute
difference in mean annual precipitation (hereafter ‘precipitation
difference’) among that pair. If the precipitation difference is large,
plots are quite distinct in their precipitation, whereas when the
difference is small, plots are in similar precipitation habitats.
Because plot sizes varied considerably in this compiled dataset, we used
a rarefaction approach to control for the plot size differences by
randomly sampling 13 individuals (the lowest number of individuals
observed in a plot) from each plot of a pair to compute betaC. We used
sample completeness of 60% to compute betaC, which was the maximum
possible coverage given the number of individuals (13) used in the
randomization. Each randomization was repeated 100 times and we
estimated the mean value of betaC for each pair of plots. Finally,
tested for the effect of a different level of coverage on the betaC
estimates using a lower level of sample completeness, i.e., 30%. We
calculated betaC using the function ‘betaC’ from (Engel et al., 2021).