Species richness and evenness as a function of biomass in arable plant communities

P>We evaluated the hypothesis that competitive dominants change the species richness and evenness of arable plant communities. Three field experiments including cool-season crops and unsown short fallows were carried out in Buenos Aires, Argentina. Ground cover, biomass and species richness of crops and weeds were assessed. Evenness was calculated by using species biomass data. Richness and evenness were correlated with total community biomass, and with the biomass of either weeds or the dominant species in the community. Crops, through growth and shading effects, affected the species richness and evenness of arable plant communities. Conversely, the dominant weed in fallows was not as suppressive as crops. Species richness and evenness were constrained by community biomass. Species richness of understory weeds decreased as crops suppressed weed growth. Evenness also decreased as the dominant species became increasingly productive, regardless of their identity (weeds or crops). Our findings provide valuable models to characterise the trajectories that species richness and evenness may follow in different farming scenarios. Community biomass is a major constraint on the maximum diversity of local communities and, consequently, of substantial ecological importance for both biodiversity conservation and weed management purposes.