Biodiversity is a measure of variety of life forms, and can be assessed at the genetic, species, and landscape levels. Species diversity can be partitioned into its basic components of richness (number of species) and evenness, and into spatial components (alpha, beta, gamma). Local extinction rates are often higher in situations where evenness is low due to low abundances in rare species. Many experimental and observational studies have been done on how ecosystem process rates will be impacted by reductions in biodiversity. The mechanism behind observed positive relationships between diversity and ecosystem process rates can be due to at least four processes: 1) the species sampling effect, 2) the selection effect, 3) complementary resource use, or 4) pest outbreaks in low-diversity plots. Biodiversity is sometimes positively related to biomass stability and resistance to extreme events. The stability of dominant species can also be important in grasslands.