Revisiting the ellenberg indicator: A reformulated approach to preserve its ordinal nature

The Ellenberg System of Indicator Values (ESIV) is a widely used for estimating environmental conditions based on the vegetation cover of species. Most methodological papers on the ESIV emphasise the ordinal nature of its scales. From a statistical point of view, therefore, the median should be used for calculations. Nevertheless, the Ellenberg index is often calculated as the mean of its values for the species in a given phytocoenosis, often weighted by species abundance. The manuscript examines the merits of using the weighted median to calculate the Ellenberg index from both theoretical and practical perspectives. To this end, using phytosociological relevés describing a particular phytocoenosis describing the Alopecuretum pratensis association, we calculated the light, moisture, soil reaction, and fertility indices for the relevés using the two methods, and compared the resulting estimates of environmental conditions in the relevés. The findings, supported by a simulation study, reinforce the idea that the median provides a more accurate diagnosis of habitat characteristics and a clearer, more unambiguous interpretation of the index values obtained. We also discuss potential shortcomings of using the weighted median. The R code for calculating the Ellenberg index is presented.