Island area is widely known to affect taxonomic richness across trophic levels. However, the effect of island size on species evenness, which quantifies the species abundance distribution, has yet to be explored across taxa, especially for tropical island ecosystems. In this study, we surveyed 20 tropical islands with areas ranging from 2 ha to 406 ha and minimal human disturbance, then measured the taxonomic evenness of both aboveground plant and belowground soil microbial communities. We found that, like the positive area-richness relationship, the taxonomic evenness of the plant community also increased with island area, indicating more stable plant communities on the larger islands. However, the island area did not affect the taxonomic evenness of soil bacterial or fungal communities. Furthermore, the effects of island area on the taxonomic evenness of the plant and soil bacterial communities were mediated through soil factors (e.g., soil pH and salinity). Overall, the contrasting area-evenness relationships among taxonomic groups highlight the importance of dissecting potential mechanisms underlying multitrophic community dynamics; more comprehensive explanations of biodiversity patterns will have critical implications for biodiversity conservation.

Our dataset was collected from 20 tropical islands in the South China Sea.

On each island, we established two to eight permanently marked 20 m × 20 m plots, with the number of plots approximately proportional to the island area on a logarithmic scale (Fig. S1) and also dependent on the island vegetation, resulting in a total of 84 plots across all islands. To minimize spatial autocorrelation, the distance between any two adjacent plots on a given island exceeded 100 m, with the exception of the smallest island, where a minimum distance of 40 m was maintained between adjacent plots.

To describe the plant community, we inventoried all woody plants rooted within the study plots with a diameter at breast height of ≥ 1 cm. We determined species identity based on morphology and recorded the abundance of each species. A total of 272 plant species were found across all islands. To census soil bacteria and fungi, we established three evenly distributed 2 m × 2 m quadrats along the diagonal of each sample plot. We collected four soil cores (10 cm in depth) from random locations within each quadrat; soil samples from a single plot were combined into one composite sample. Samples were sieved through a 2 mm mesh to remove large particles before then subdividing each sample into two subsamples. One subsample was kept at 4°C for the determination of soil physical and chemical properties, while the other was stored at −20°C for later DNA extraction.