N90, a Diversity Index Sensitive to Variations in Beta Diversity Components
M. Teresa Farriols | Francesc Ordines | Enric Massutí
Species diversity in a community is mainly related to the number and abundance of species that form it. <i>N</i><sub>90</sub> is a recently developed diversity index based on the results of the similarity percentage (SIMPER) analysis that represents the number of species contributing up to ninety percent of within-group similarity in a group of samples. The calculation of <i>N</i><sub>90</sub> is based on the Bray–Curtis similarity index and involves the number of species and abundances in a group of samples. We have explored the properties of <i>N</i><sub>90</sub> compared to other alpha, beta and gamma diversity indices and to beta diversity measures accounting for nestedness and turnover. We have used a non-real data set to compare the values of all indices with <i>N</i><sub>90</sub> and two real data sets of demersal fish communities along large and short depth gradients with higher influence of turnover and nestedness, respectively, to correlate the same indices with <i>N</i><sub>90</sub>. The sensitivity of <i>N</i><sub>90</sub> to reductions in the frequency of occurrence and the evenness of the distribution of species abundances among samples allows the detection of diversity loss due to the fishing-induced retreatment of species populations to localities presenting the most favorable ecological conditions. This property, both in the identification of species replacement and species loss through SIMPER analysis, make <i>N</i><sub>90</sub> a useful indicator to support the Ecosystem Approach to Fisheries within the current context of global change.
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