diversity Article N90, a Diversity Index Sensitive to Variations in Beta Diversity Components Maria Teresa Farriols * , Francesc Ordines and Enric Massutí   Citation: Farriols, M.T.; Ordines, F.; Massutí, E. N90, a Diversity Index Sensitive to Variations in Beta Diversity Components. Diversity 2021, 13, 489. https://doi.org/10.3390/ d13100489 Academic Editor: Michael Wink Received: 31 August 2021 Accepted: 30 September 2021 Published: 6 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Centre Oceanogràfic de les Balears, Instituto Español de Oceanografía, Moll de Ponent s/n, 07015 Palma, Spain; xisco.ordinas@ieo.es (F.O.); enric.massuti@ieo.es (E.M.) * Correspondence: mt.farriols@ieo.es Abstract: Species diversity in a community is mainly related to the number and abundance of species that form it. N 90 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 N 90 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 N 90 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 N 90 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 N 90 . The sensitivity of N 90 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 N 90 a useful indicator to support the Ecosystem Approach to Fisheries within the current context of global change. Keywords: beta diversity; similarity; species richness; species replacement; species loss; turnover; nest- edness 1. Introduction Diversity is a founding, but at the same time, complex concept in ecology. More than species diversity in the community, understood as a group of interdependent organisms of different species growing or living together in a specified habitat, diversity can be related to genetic diversity within populations or diversity of functional traits. However, for most ecologists, diversity has to do with the number and abundance of species in the community, and a lot of attempts have been made to express this concept numerically. Because of this, a high number of diversity indices have been proposed showing different aspects of the community structure, taking into account factors ranging from the number of species and the relative abundance or biomass of these species, to the taxonomic or functional relationships between them [1]. Although it is generally agreed that diversity is a multidimensional concept and that the use of diversity indices depends on what effect on diversity you want to detect, there is no consensus about the indices that should be used in each case. However, traditional or classical diversity indices such as Species Richness (S), Shannon (H ′ ) or Pielou’s evenness (J ′ ), are usually chosen to describe biological communities because, at least, they are easy to calculate and allow comparisons with previous works. Although in recent years, a new family of diversity indices, known as Hill numbers, have been preferred because they have shown more desired properties than the raw form [2,3]; for example, they obey an intuitive replication principle or doubling property and they are all expressed in units of effective numbers of species [4]. Diversity 2021, 13, 489. https://doi.org/10.3390/d13100489 https://www.mdpi.com/journal/diversity