Plant species diversity has declined rapidly in grasslands, and it is poorly known how to establish and maintain diverse mixtures containing grasses, forbs, and legumes. Each of these groups can be important. For example, grasses and forbs can produce forage and increase resistance to weeds, and legumes can produce forage and maintain high fertility by fixing nitrogen. There is some evidence that exotic (introduced) plant species and changes in land use are contributing to declines in diversity. Exotic species could be causing declines in diversity in situations where they differ from native plant species in growth rates and other plant traits. However, previous studies were based on correlative data, and we have a poor understanding of whether exotic species themselves are causing direct declines in diversity, or if associated land use changes are causing the declines in diversity.