Determination of diagnostic species with statistical fidelity measures

摘要

Statistical measures of fidelity, i.e. the concentration of species occurrences in vegetation units, are reviewed and compared. The focus is on measures suitable for categorical data which are based on observed species frequencies within a vegetation unit compared with the frequencies expected under random distribution. Particular attention is paid to Bruelheide's it value. It is shown that its original form, based on binomial distribution, is an asymmetric measure of fidelity of a species to a vegetation unit which tends to assign comparatively high fidelity values to rare species. Here, a hypergeometric form of it is introduced which is a symmetric measure of the joint fidelity of species to a vegetation unit and vice versa. It is also shown that another form of the binomial it value may be defined which measures the asymmetric fidelity of a vegetation unit to a species. These it values are compared with phi coefficient, chi-square, G statistic and Fisher's exact test. Contrary to the other measures, phi coefficient is independent of the number of releves in the data set, and like the hypergeometric form of it and the chi-square it is little affected by the relative size of the vegetation unit. It is therefore particularly useful when comparing species fidelity values among differently sized data sets and vegetation units. However, unlike the other measures it does not measure any statistical significance and may produce unreliable results for small vegetation units and small data sets. The above measures, all based on the comparison of observed/expected frequencies, are compared with the categorical form of the Dufrene-Legendre Indicator Value Index, an index strongly underweighting the fidelity of rare species.