Figure 44.8: Species Richness and Number of Functional Groups Affect Primary Productivity (A) In 289 experimental prairie plots, NPP (measured as aboveground plant biomass produced per square meter per year) increased with species richness. (B) When the researchers separated out the effects of the number of functional groups from those of species richness, they found that the number of functional groups was the key influence on production. These results suggest that primary productivity is enhanced by a greater variety of species with complementary niches. Error bars indicate ± 1 standard error of the mean. See Appendix B for review of statistical concepts.