Taxonomic Relatedness of Spider and Carabid Assemblages in a Wetland Ecosystem

Traditional community indices, i.e. Shannon diversity and Simpson's dominance, are generally used to describe biological assemblages in order to infer ecological trends about the effects of disturbance. Such indices are correlated with species richness and abundance and may be strongly influenced by the sampling effort. Conversely, taxonomic relatedness indices show interesting statistical properties and are independent of the sampling effort. Additionally, high values of taxonomical relatedness indices have proven to reflect habitat functionality in marine ecosystems. Their use in terrestrial ecosystems is rare, and despite their good potential, they have never been used for biodiagnostic purposes. In this paper, we present the 1st application of taxonomic relatedness indices to arthropod assemblages (namely spiders and carabids) and their comparison with several traditional community parameters that are generally used to evaluate environmental disturbances. The study was set in a wetland area within the Natural Reserve of Fondo Toce (northwestern Italy). Four different habitats with different degrees of disturbance (a reed bed, a mown meadow, a riparian wood with mesophilous elements, and a transitional mesohygrophilic area invaded by non-native vegetation) were sampled using pitfall traps. The spider and carabid assemblages occurring in each habitat were characterized by means of multidimensional scaling (MDS) and by several community parameters, including abundance, species richness diversity, taxonomic relatedness indices, and some descriptive functional traits. Differences among habitats were tested with generalized linear models. Correlations among community parameters were evaluated with Spearman's rank correlations. According to the MDS plots, both assemblages were clearly separated in relation to habitat type. When considering the parameters describing the assemblages in relation to disturbance, the taxonomic approach was found to be particularly suitable for spiders, for which the less-disturbed habitat was characterized by higher values of average taxonomic distinctness. Furthermore, functional groups of spiders were more evenly distributed in undisturbed habitats. The functional approach was found particularly appropriate for carabids, which responded to disturbance in terms of a decrease in biomass (average body size) and an increase in the proportional abundance of macropterous individuals.