Variation in individual density may explain the unimodal richness-biomass relationship in which species richness peaks at an intermediate level of total biomass. However, it is unclear how individual density is regulated by community thinning (i.e., mortality due to competition with neighbors) as total above-ground biomass increases. We developed a simulation model which demonstrates that the spatial structure of a population can influence the initiation and rate of community thinning and thus the shape of the richness-biomass relationship. Specifically, we found that more clustered populations resulted in a more abrupt initiation and rapid rate of thinning and thus a sharper unimodal richness-biomass relationship. Our simulation also demonstrated that a wide diversity of richness-biomass relationships can be produced by community-thinning.