Fig. 2.
Effect of plant species richness on aboveground net primary production (ANPP) in natural and artificial ecosystems. The relative yield total (RYT) is the ratio between species ANPP in mixtures (Oi) and its ANPP in monocultures (Mi), summed for all species in the mixture (N) (RYT = Σi=1N Oi/Mi) (23); we used a corrected version of the RYT (SI Text, RYT). Bars represent the RYT for the maximum richness level averaged among years for different experiments conducted at different localities. Artificial ecosystems were created by sowing different numbers of species in pots or field plots with homogenized soils (i.e., chemically sterilized soils) for the sake of gaining control of the experiment (12); in contrast, the experiment conducted in the natural ecosystem reported here was created by removing species and portions of individuals from established communities in the Patagonian steppe. Dotted lines represent a confidence interval of 99% around the average of artificial-ecosystems. Codes used are as follows: USA1 and USA3, Texas; USA2, North Carolina; USA4, California; USA5 and USA6, Minnesota; GRE, Greece; GER1, Bayreuth, Germany; GER2 and GER3, Jena, Germany; IRE, Ireland; UK1, Silwood Park, United Kingdom; UK2, Sheffield, United Kingdom; POR, Portugal; SWI1, Zürich; SWI2, Lupsingen, Switzerland; NED, The Netherlands; BEL, Belgium; SWE, Sweden; and ARG, Argentina (see SI Table 3 for references).