Let the concept of indicator species be functional!

Aims: The identification of diagnostic or indicator species with high fidelity to a given group of sites is an important step for the ecological characterization of habitats or community types. The determination of the degree of fidelity to a target group is traditionally performed by analysing the concentration of species occurrences or abundances in different groups of sites. Surprisingly, although one of the main purposes of indicator species analysis is to give ecological meaning to groups of sites, none of the methods proposed to date take into account the functional ecology of diagnostic species. Therefore, the question we address here is: can we use functional traits of species to improve the diagnostic value of indicator species? Location: Sand dune communities in central Italy. Methods: In this paper we propose a two-step procedure for incorporating the functional traits of a given species in the evaluation of its diagnostic value. For a given set of plots that are classified into different groups, first the indicator species that best characterize each group of plots are identified with the usual statistical tools based on species occurrences. Next, the functional association between the indicator species and the target groups of plots is tested by measuring the functional distance between the indicator species and the centroids of all plots in the target group. A species is positively associated with a group if its mean functional distance from all plot centroids in the group is significantly lower than expected. Results: In this example, we show that the functional association of the indicator species with a given habitat type is represented by less species than the association highlighted solely through species occurrences and/or abundances. This subset of species appears to better characterize the functional ecology of coastal dune plant assemblages and shows a higher diagnostic value in comparison with those obtained through the traditional indicator analysis. Conclusions: As functional traits are the main ecological attributes by which different species influence ecosystem processes, we believe that the methodology proposed here provides a relevant tool for ecological applications as distinct as vegetation science, conservation biology or landscape management.