Mean ecological indicator values: use EIVE but no cover-weighting

Aims: To test the predictive power of mean ecological indicator values (EIVs) based on different EIV systems and weighting approaches. Study area: Preda in Grisons, Switzerland. Methods: We used three regional datasets of vegetation plots accompanied with measured soil pH values or mean annual near soil air temperature. We calculated mean EIVs for each plot with four EIV systems that cover the region, namely “Ellenberg”, “Landolt”, “Tichý” and the Ecological Indicator Values for Europe (EIVE), combined with four weighting approaches (unweighted, cover-weighted, square-root cover-weighted, inverse niche width weighted). We correlated the mean EIVs of each combination with the measured environmental variables and compared the mean Pearson r values. Results: No cover-weighting (0.78) was slightly better than square-root cover-weighting (0.75) and clearly better than full cover-weighting (0.68). In the two EIV systems providing a niche width measure (EIVE and “Landolt”), inverse niche weighting gave similar results than no weighting. Mean EIVE values (0.76) had significantly stronger correlations than “Tichý” (0.73), while the differences to “Ellenberg” (0.75) and “Landolt” (0.71) were not significant. Conclusions: The results suggest that even within the definition areas of two long-established EIV systems (“Ellenberg”, “Landolt”), EIVE gives at least as good predictions as these and significantly better than “Tichý”. Likewise, any type of cover-weighting reduces the predictive power of mean EIVs. Both findings could be the consequence of the statistical principle “wisdom of the crowd”, according to which the average estimate of several sources (be it regional scientists or plant species) is usually better than the estimate of one or few experts. Accordingly, EIVE currently is the best choice for mean EIVs in Europe, and no cover-weighting should be applied. We recommend that similar studies should be undertaken in other regions and for other niche dimensions. Syntaxonomic reference: Mucina et al. (2016). Abbreviations: EIV = ecological indicator value; EIVE = Ecological Indicator Values for Europe 1.0; R = ecological indicator value for soil reaction; T = ecological indicator value for temperature.