1. Taxonomic sufficiency concerns the use of higher-taxon diversity as a surrogate for species diversity.\ud It represents a fast and cost-effective method to assess community responses to natural and\ud anthropogenic environmental drivers. In spite of the potential applications of using higher taxa as\ud surrogates for species, little research has been carried out to determine the underlying reasons that\ud might make taxonomic surrogacy effective for detecting diversity changes.\ud 2. Here, we determine whether the effectiveness of higher taxa as species surrogates relies mostly on\ud taxonomic relatedness of species (i.e. the relative closeness of species in the Linnaean taxonomic\ud hierarchy) or depends simply on the numerical ratio between species and higher taxa (i.e. the degree\ud of species aggregation). We reviewed the current literature on taxonomic sufficiency to check for\ud any correlation between the effectiveness of higher taxa and the degree of species aggregation across\ud different types of organisms. Tests based on random simulations from diverse marine mollusc\ud assemblages were also carried out to ascertain whether the ability of higher taxa to detect variation\ud in the multivariate structure of assemblages depended on the degree of species aggregation.\ud 3. Mollusc data showed that information loss and the ensuing decrease in statistical power to detect\ud natural or human-driven changes in assemblages at higher taxonomic levels depend on the degree\ud of species aggregation, rather than on the taxonomic resolution employed. Analyses of the literature\ud suggested that such outcomes could be generalizable to a wide range of organisms and environmental\ud settings.\ud 4. Our findings do not support the idea of a direct relationship between taxonomic relatedness and\ud ecological similarity among species. This indicates that taxonomic ranks higher than species may\ud not provide ecologically meaningful information, because higher taxa can behave as random groups\ud of species unlikely to convey consistent responses to natural or human-driven environmental\ud changes.\ud 5. Synthesis and applications. Surrogates of species-level information can be based on the ‘highest\ud practicable aggregation’ of species, irrespective of their taxonomic relatedness. Our findings cast\ud doubt on static taxonomical groupings, legitimizing the use of alternative ways to aggregate species\ud to maximize the use of species surrogacy.\ud Key-words: biodiversity, conservation, higher-taxon approach, impact assessment, marine\ud molluscs, natural environmental variations, phylogenetic relatedness, taxonomic surrogates,\ud taxonom

Characterizing the potential effects of human activities on natural systems is a central problem in applied ecology. This requires the development of analytical procedures able to separate human perturbation from natural spatio-temporal variability displayed by most populations. Beyond-BACI experimental designs provide a framework to address these issues but, to date, their use is limited to the analysis of human impacts on the abundance of single species or other univariate measures. Here, we describe in detail an asymmetrical design that included 1 impact location (I) and a set of 3 controls (Cs), sampled at a hierarchy of spatial scales 4 times over a period of 15 mo. We focused on shallow subtidal assemblages of sessile organisms exposed to sewage discharge along a stretch of coast in southern Italy. The purpose of this paper is to illustrate (1) the comparison of variance components for the assessment of impacts and (2) the use of recently developed multivariate methods (distance-based premutational MANOVA) in the analysis of multivariate species data in response to a complex asymmetrical design. Results indicated that temporal changes in the whole assemblage at I were distinct from those occurring at Cs, and that the nature of this difference (although not its size) was fairly consistent through time. A suite of taxa was identified as important in characterizing the differences found between I and Cs. Some algae (e.g. Colpomenia sinuosa, Gelidium sp. and Pterocladiella sp.), in particular, occurred uniquely at I. Univariate analyses indicated significant Time × I-v-Cs interactions for several taxa, and significantly smaller spatial variation at the scale of quadrats at I compared to Cs. In contrast, the small-scale spatial variation in the number of taxa was significantly greater at I than at Cs. The findings of this study have important implications for future multivariate and univariate analyses in environmental impact assessment.

Marine bioconstructions are biodiversity-rich, three-dimensional biogenic structures, regulating key ecological functions of benthic ecosystems worldwide. Tropical coral reefs are outstanding for their beauty, diversity and complexity, but analogous types of bioconstructions are also present in temperate seas. The main bioconstructions in the Mediterranean Sea are represented by coralligenous formations, vermetid reefs, deep-sea cold-water corals, Lithophyllum byssoides trottoirs, coral banks formed by the shallow-water corals Cladocora caespitosa or Astroides calycularis, and sabellariid or serpulid worm reefs. Bioconstructions change the morphological and chemicophysical features of primary substrates and create new habitats for a large variety of organisms, playing pivotal roles in ecosystem functioning. In spite of their importance, Mediterranean bioconstructions have not received the same attention that tropical coral reefs have, and the knowledge of their biology, ecology and distribution is still fragmentary. All existing data about the spatial distribution of Italian bioconstructions have been collected, together with information about their growth patterns, dynamics and connectivity. The degradation of these habitats as a consequence of anthropogenic pressures (pollution, organic enrichment, fishery, coastal development, direct physical disturbance), climate change and the spread of invasive species was also investigated. The study of bioconstructions requires a holistic approach leading to a better understanding of their ecology and the application of more insightful management and conservation measures at basin scale, within ecologically coherent units based on connectivity: the cells of ecosystem functioning.

Aim  Virtually all studies exploring the use of taxonomic surrogates in assessing patterns of diversity have focused on clear shifts in the location of samples in multivariate space. The potential use of coarser levels of taxonomic resolution to detect patterns of variability in multivariate space, corresponding to β‐diversity in the case of presence/absence data, remains unexplored. Here we considered five ecological data sets of highly diverse marine molluscan assemblages to test the hypothesis that patterns in compositional heterogeneity would be maintained at coarser levels of taxonomic resolution. Location  Italy, Norway, New Zealand and the Arctic. Methods  We used multivariate dispersion based on the Jaccard resemblance measure of presence/absence data as a measure of β‐diversity to test the null hypothesis that patterns of heterogeneity in species composition for molluscs would be maintained at coarser levels of taxonomic resolution. Tests to compare β‐diversities among groups (based on distances to centroids and using 9999 permutations) were carried out separately for each of five data sets at the species level and then for each of genus, family, order and class levels. Results  Differences in multivariate dispersion at the species level (heterogeneity in the identities of species) were maintained for genera and for families, but not at coarser levels of taxonomic resolution (order or class). These results were consistent across all data sets, despite differences in their spatial scale and extent, geographical location, environmental and habitat features (benthic soft sediments, rocky reefs or kelp holdfasts). Main conclusions  These results suggest that either genera or families may be used as effective taxonomic surrogates to detect spatial differences in β‐diversity for molluscs. The use of surrogates can provide considerable sampling efficiencies for biodiversity assessments. We consider, however, that a degree of caution and more work is needed, as heterogeneity at the species level may not be reflected by taxonomic surrogates at smaller spatial scales.