Indicator value is a term that has been used in the ecology of plants for two different indices. The older usage of the term refers to Ellenberg's indicator values from 1974, which are based on a simple ordinal classification of plants according to the position of their realized ecological niche along an environmental gradient.[1] Since 1997, the term has also been used to refer to Dufrêne & Legendre's indicator value, which is a quantitative index measuring the statistical alliance of a species to any one of the classes in a classification of sites.[2]