Indicator value is a term that has been used in ecology for two different indices. The older usage of the term refers to Ellenberg's indicator values, which are based on a simple ordinal classification of plants according to the position of their realized ecological niche along an environmental gradient. More recently, the term has also been used to refer to Dufrêne & Legendre's indicator value, which is a quantitative index that measures the statistical alliance of a species to any one of the classes in a classification of sites.
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Ellenberg's Indicator Value
Ellenberg's indicator values were the first model of bioindication proposed and applied to the flora of Germany, and they have a long tradition in interpretation and understanding of plant communities and their evolution. The latest edition of Ellenberg's indicator values applies a 9-point scale for each of six gradients: soil acidity, soil productivity or fertility, soil humidity, soil salinity, climatic continentality and light availability. Indicator values also exist in Landolt's Flora of Switzerland and some other floras. A substantial advance on a comparable scale to place a local flora on gradients are the climatic profiles of the French SOPHY online database.
Indicator Value of Dufrêne and Legendre
This indicator value is an integral part of the indicator value , which quantifies the fidelity and specificity of species in relation to groups of sites in a user-specified classification of sites, and tests for the statistical significance of the associations by permutation.
The indicator value of species i for class j is obtained with the equation
Here
Aij is specificity, i.e. the proportion of the individuals of species i that are in class j Bij is fidelity, i.e. the proportion of sites in class j that contain species i