Society places value on multiple functions of ecosystems that are potentially threatened by ongoing biodiversity losses. We use data from the longest running biodiversity-functioning field experiment to date, at Cedar Creek, Minnesota, to test how species diversity affects the ability of grassland ecosystems to provide high levels of up to eight ecosystem functions simultaneously. We hypothesized that more functions, higher minimum thresholds for each function, and longer time scales (more years) would each require higher species richness than would the provision of individual functions in individual years. We also explored the effects of tradeoffs. In communities of interacting species, at least two important types of tradeoffs are likely to affect the relationship between biodiversity and ecosystem multifunctionality. First, certain combinations of functions could be difficult for a single assemblage to support because the functions conflict. For example, productivity and stress tolerance can be negatively related and difficult or impossible to maximize jointly; species best suited for one of these functions often possess traits that lead to low levels of the other function. Second, different functions could be maximized by communities of different species richness and composition. For example, resistance to different invasive species might be maximized by different resident species combinations. If either type of tradeoff proves important, then high levels of multifunctionality likely require not just greater species richness within local assemblages but also a diversity of assemblages through space.

Results/Conclusions

Across three years and every combination of eight ecosystem functions, minimum required species richness consistently increases with the number of functions considered. However, tradeoffs and temporal variability prevented any assemblage from providing high levels of five or more functions, regardless of its diversity. First, tradeoffs among functions, in the form of significant negative correlations between certain pairs of functions, meant that certain combinations of functions such as invasion resistance (maximized at high richness) and plant nitrogen content (maximized in certain monocultures) were never provided at high levels by any single community. Second, the optimal community composition to provide a given bundle of ecosystem functions, and the performance of any given community with respect to each function, varied substantially from year to year.  Sustained multifunctionality in the Cedar Creek experiment therefore depended not only on higher species diversity than single functions, but also on a diversity of community types through either or both of space and time.