Tendency and distribution indices for ordinal variables applied to Ellenberg indicator values

Abstract

The use of mean value for the analysis of Ellenberg indicator values in plots or plant community types has been criticised by several authors. On the other hand, little attention has been paid to distribution patterns of the respective classes frequencies. Herewith we make a revision of metrics available for the characterization of tendency and distribution patterns of ordinal variables, like in the case of Ellenberg indicator values, and discuss their relation to frequency histogram shapes. We selected three tendencies (mean value, median value and mode) and two distribution metrics (position index and relative dominance). Additionally, we proposed a concentration index, which is analogous to the contagion index commonly used by landscape ecologists. According to our observations, we recommend the use of the median value and the concentration index to quantify respectively the tendency and distribution patterns on indicator spectra (frequency histograms) according to the Ellenberg indicator values. Further studies are required to get better ecological interpretations in the distribution patterns of indicator spectra.