Taxonomic structure and the effectiveness of surrogates in environmental monitoring: a lesson from polychaetes

Multivariate approaches for the quantification of changes in biological assemblages can provide a powerful tool for assessing human impacts on ecological systems. Taxonomic Sufficiency (TS) has been proposed as a short-cut method in environmental monitoring. However, issues about the efficacy of taxonomic surrogates in depicting spatial patterns and taxonomic structure of marine assemblages are still a source of debate. Here, we followed the logic of beyond-BACI (Before After Control Impact) designs and their modifications to deal with cases where no data have been collected before the purported impact. By using, as a case study, the analysis of the effects of sewage pollution on spatial patterns of polychaete assemblages along a stretch of rocky coast in the Ionian Sea (South Italy) this study aims to: (1) investigate whether the assemblage’s taxonomic structure influences the results of the analyses based on coarser taxonomy; (2) test the efficiency of an alternative cost-effective method to TS. The analyses highlighted the impact as a modification of natural spatial patterns of assemblages at the scale of location, site and replicate units. Results were consistent when analysing data at species and genus level. The use of other surrogates failed in fully depicting modifications of natural spatial patterns as a consequence of the impact. The surrogate performance depended on the assemblage’s taxonomic structure, and was related to the information provided by the mono-specific higher taxa (i.e. species distribution). Differences among sites at the impacted location were underestimated after lumping species distributions. Our data suggest that the analysis at species level of a correctly selected taxon (i.e. a single family) can represent a valid cost-effective alternative to using all species based datasets. The results of this study, however, stress that only an a-priori taxonomic screening may address the correct approach for depicting changes in assemblages in relation to human impacts