Meta-analysis reveals widespread negative associations between species richness and ecological uniqueness.

Species richness and ecological uniqueness (defined as local contributions of sites to beta diversity of a region) constitute two crucial components of biodiversity. Understanding the relationship between these two components is critical for effective conservation planning, yet global patterns and underlying drivers remain largely controversial. Here, we conduct a comprehensive global meta-analysis to investigate the patterns of the species richness-ecological uniqueness relationship across taxa groups and to evaluate the relative importance of four ecological hypotheses in driving this relationship (i.e. species pool, dispersal limitation, environmental filtering and spatial grain size). We find that negative richness-uniqueness relationships are prevalent across different taxa groups (e.g. terrestrial plants, freshwater macroinvertebrates, birds, and fishes), and such negative association is robust to different data types including presence absence and abundance data. Notably, relationships based on presence-absence data are primarily predicted by species pool attributes (i.e. the proportion of rare species and gamma diversity), whereas abundance-based relationships are more strongly associated with dispersal limitation. In summary, these findings reveal consistent global patterns and mechanistic underpinnings of the richness-uniqueness relationship, offering key insights for biodiversity conservation. We recommend that conservation strategies should prioritize both species-rich and ecologically unique communities to maximize biodiversity protection globally.