Figure 21.7 Effects of nitrogen addition on plant productivity and species richness. Twenty-three experiments at eight locations around the United States added nitrogen to plant communities. (a) All sites experienced increases in primary productivity. (b) All sites experienced a loss in species richness, but the magnitude of the loss varied greatly due to differences in existing nitrogen and differences in the cation exchange capacity of the soil. Error bars are standard errors.

Data from C. M. Clark et al., Environmental and plant community determinants of species loss following nitrogen enrichment, Ecology Letters 10 (2007): 596–607.