Species evenness is a description of the distribution of abundance across the species in a community. Species evenness is highest when all species in a sample have the same abundance. Evenness approaches zero as relative abundances vary.

equanimity, composure, sangfroid mean evenness of mind under stress. equanimity suggests a habit of mind that is only rarely disturbed under great strain. accepted her troubles with equanimity. composure implies the controlling of emotional or mental agitation by an effort of will or as a matter of habit.

Typically, a greater number of species increases an ecosystem's productivity. However, a new study shows that the effects of diversity are not only a matter of species richness. If many species coexist in a forest, even distribution of species abundances is key for high ecosystem diversity and productivity.

Species richness is a measure of the number of different types of species in an ecosystem. A large number of different species in a habitat represents a higher species richness, and an overall more diverse ecosystem. Species evenness is a measure of the relative abundance of each species.