Intraspecific aggregation alters competitive interactions in experimental plant communities

We tested the prediction from spatial competition models that intraspecific aggregation may promote coexistence and thus maintain biodiversity with ex perimental communities of four annual species. Monocultures, three-species mixtures, and the four-species mixture were sown at two densities and with either random or intraspecifically aggregated distributions. There was a hi erarchy of competitive abilities among the four species. The weaker competi tors showed higher aboveground biomass in the aggregated distribution compa red to the random distribution, especially at high density. In one species, intraspecific aggregation resulted in an 86% increase in the number of flo wering individuals and a 171% increase in the reproductive biomass at high density. The competitively superior species had a lower biomass in the aggr egated distribution than in the random distribution at high density. The da ta support the hypothesis that the spatial distribution of plants profoundl y affects competition in such a way that weaker competitors increase their fitness while stronger competitors are suppressed when grown in the neighbo rhood of conspecifics. This implies that the spatial arrangement of plants in a community can be an important determinant of species coexistence and b iodiversity.