Give It Time Experimental ecological studies in recent years have provided a great deal of insight into how species diversify and influence ecosystem properties, but in most cases the experiments have been relatively brief (up to ∼5 years). Reich et al. (p. 589 ; see the Perspective by Cardinale ) performed two 13- and 15-year grassland experiments and found that the effects of plant species richness on community-level processes like biomass production tend to be saturating at early stages but that those impacts grow stronger and more linear as experiments run longer. Stronger influences through time were largely driven by increasing amounts of “complementarity” among species, and these trends were correlated with greater expression of functional diversity in multispecies assemblages. Thus, the effects of diversity grow stronger through time as species gain more and more opportunity to vary in their use of the limiting biological resources in their environment, which emphasizes the functional importance of maintaining diversity in ecosystems.