Similarity coefficients for classifying relevés

Summary

Some characteristics of the more commonly used similarity coefficients have been discussed. Coefficients with undesirable properties include: the product moment correlation coefficient, some information statistics, the relative homogeneity function, the weighted similarity coefficient, the Euclidean (absolute) distance, Gleason's, Ellenberg's and Spatz's similarity indices, and the absolute value function.

Of the six coefficients that were tested with two sets of phytosociological data, the Canberra measure and the absolute Euclidean distance were the least successful, with at least one data set, in providing classifications that were similar to the classifications obtained by the Braun-Blanquet sorting technique. The standard Euclidean distance and the similarity ratio had intermediate success. The Czekanowski coefficient, especially in its relativized from (= relative absolute value function), was the most successful. This latter coefficient is cover-weighted and therefore the Canberra measure, although it has some undesirable characteristics, may be valuable for investigating relevé similarities that are based on species with low cover.

The transformation of Coetzee & Werger (1973) appears to be appropriate as a conversion of the cover-abundance values. The mean cover percent values corresponding to the cover-abundance values gave poor results.

The relativized Czekanowski coefficient should be suitable at the lower syntaxonomical levels. At higher levels, qualitative coefficients may be sufficient for determining similarities between syntaxa. Qualitative coefficients will not always be successful at the lower levels — this was illustrated with the test data.

In this study, the clustering procedure of group average sorting was used to construct the dendrogram. It gives an average similarity value within the dendrogram groups. These values can be used to give quantitative definitions to syntaxonomic rank.