A New, Ecologically Self-Significant Metric of Species-Abundance Unevenness, Reliably Highlighting the Intensity of Interspecific Competition

Abstract

A wide series of commonly used metrics of abundance-evenness (or -unevenness) have been proposed to characterize synthetically the distributions of species-abundances, accounting for the hierarchic-like organization of species within natural communities. Among them, most – if not all–have been relevantly criticized on their serious limitations regarding both their “descriptive” and their “interpretative” capacities. From the descriptive point of view, many authors have already repeatedly emphasized the formal non-independenceof conventional (un-)evenness metrics with respect to species-richness, leading, in particular, to unacceptable bias when comparing communities differing by their species-richness, thus making these metrics unreliable descriptors in this respect. Now, as regards the capacity to provide relevant ecological interpretations, especially in terms of the intensity of competition among co-occurring species, the weakness of conventional (un-)evenness metrics is readily highlighted by the usual absence of any associated interpretation of this kind in the literature: the conventional (un-)evenness metrics beingrestricted to purely descriptive purpose only. Accordingly, a newly designed abundance-unevenness metric – the “standardized abundance-unevenness” index is proposed, positively addressing both kinds of limitations evoked above. By standardizing a conventional measure,U,of abundance-unevenness to the corresponding measure, U’, of the abundance-unevenness in the well-known “broken-stick” model, the resulting “standardized unevenness” index (Istr = U/U’) proves to be efficient against both themajor limitations pointed-out above:indeed,the new index does benefit by being both (i) formally independent from species-richness, thereby allowing reliable,unbiased comparisons of abundance unevenness between species-communities, whatever their difference in species-richness; (ii) able to relevantly quantify the mean intensity of interspecific-competition within community, in term of its direct outcomeuponthe degree of species-abundance unevenness. This double success being, of course, the direct consequences of the properties of the “broken-stick” distribution model, originally putforth in a well-known, yet insufficiently thoroughly exploited paper by the regretted Robert MacArthur.