Common species have a bigger impact on species richness patterns than rare ones. A theory was developed to understand how individual species affect overall species richness. The distribution of species richness among sites influences the relationship between species occupancy and species richness patterns. While most species occupancy distributions are right-skewed, biological processes can cause deviations from the expected pattern. By studying European birds, researchers found that some species have a stronger or weaker influence on species richness than predicted. Overall, common species play a bigger role in species richness, but individual species can deviate from this pattern due to biological factors.