Society places value on the multiple functions of ecosystems from soil fertility to erosion control to wildlife-carrying capacity, and these functions are potentially threatened by ongoing biodiversity losses. Recent empirically based models using individual species’ traits suggest that higher species richness is required to provide multiple ecosystem functions. However, no study to date has analyzed the observed functionality of communities of interacting species over multiple temporal scales to assess the relationship between biodiversity and multifunctionality. We use data from the longest-running biodiversity-functioning field experiment to date to test how species diversity affects the ability of grassland ecosystems to provide threshold levels of up to eight ecosystem functions simultaneously. Across years and every combination of ecosystem functions, minimum-required species richness consistently increases with the number of functions considered. Moreover, tradeoffs between functions and variability among years prevent any one community type from providing high levels of multiple functions, regardless of its diversity. Sustained multifunctionality, therefore, likely requires both higher species richness than single ecosystem functionality and a diversity of species assemblages across the landscape.

Consequences of progressive biodiversity declines depend on the functional roles of individual species and the order in which species are lost. Most studies of the biodiversity-ecosystem functioning relation tackle only the first of these factors. We used observed variation in grassland diversity to design an experimental test of how realistic species losses affect invasion resistance. Because entire plant functional groups disappeared faster than expected by chance, resistance declined dramatically with progressive species losses. Realistic biodiversity losses, even of rare species, can thus affect ecosystem processes far more than indicated by randomized-loss experiments.

Increasingly governments and the private sector are using planted forests to offset carbon emissions. Few studies, however, examine how tree diversity - defined here as species richness and/or stand composition - affects carbon storage in these plantings. Using aboveground tree biomass as a proxy for carbon storage, we used meta-analysis to compare carbon storage in tree mixtures with monoculture plantings. Tree mixes stored at least as much carbon as monocultures consisting of the mixture's most productive species and at times outperformed monoculture plantings. In mixed-species stands, individual species, and in particular nitrogen-fixing trees, increased stand biomass. Further motivations for incorporating tree richness into planted forests include the contribution of diversity to total forest carbon-pool development, carbon-pool stability and the provision of extra ecosystem services. Our findings suggest a two-pronged strategy for designing carbon plantings including: (1) increased tree species richness; and (2) the addition of species that contribute to carbon storage and other target functions

The development of a new carbon economy has the potential to offer win-win outcomes for environments and economies. Large-scale tree plantations are expected to play a major role in carbon economies but could have negative ecological and economic consequences when key environmental values such as biodiversity conservation are not considered. We discuss three potential "bio-perversities"-negative outcomes for biodiversity-that could result from inappropriate plantation tree programs aimed solely at reducing atmospheric carbon dioxide and mitigating rapid climate change effects. These are: (1) clearing native vegetation to establish tree plantations, (2) planting trees that become invasive taxa, and (3) tree plantations negatively affecting key ecosystem processes such as fire and hydrological regimes. These bio-perversities may result from common mistakes in environmental management: (1) too narrow a focus on a single environmental value, (2) failing to adequately quantify ecological uncertainty, and (3) failing to anticipate how different groups of people respond to an environmental problem. We highlight ways to prevent possible bio-perverse outcomes in large-scale plantation programs. These include requiring that risk assessments precede project establishment, full carbon accounting is undertaken, incentives used to stimulate tree plantation establishment are rigorously examined, and rigorous compliance and ecological monitoring is undertaken.Bio-perversity-negative biodiversity and environmental outcomes arising from a narrow policy and management focus on single environmental problems without consideration of the broader ecological context.