δ 15 N of biologically independent samples from islands with high or low densities of seabirds for coral host ( n +B = 27, n -B = 25), symbionts ( n +B = 27, n -B = 25), their nitrogen sources (zooplankton ( n +B = 18, n -B = 17) and bird guano ( n = 22)). Macroalgae ( n +B = 55, n -B = 45) from the same reefs are included for comparison. In box plots, the centre line shows the median, the box encompasses 25th and 75th percentiles, and whiskers extend to minimum and maximum values of the dataset. The filled circles display outliers. Significant differences (indicated by different letters) between samples were determined by one-way ANOVA ( P < 0.001) followed by pairwise multiple comparison (HolmSidak method, P < 0.05). The number of biologically independent samples (in the range of n = 1755) are provided in Methods .
