Increasing evidence indicates that spatial and temporal pattern in ecological systems are not independent from the scale of measurement. In this study we used a hierarchical sampling design to examine spatial pattern in assemblages of algae and invertebrates in midshore and lowshore habitats of rocky coasts in the Mediterranean Sea, increasing the range of scales usually covered by this type of study in marine habitats. To put our results in a broader context, we also conducted a review of the literature targeting studies that explicitly investigated spatial patterns with hierarchical designs. We addressed two main questions: 1- how does small-scale variability compare to large-scale patterns of variation? 2- is there any pattern of variability that can be generalized across species and habitats? The review of the literature indicated that hierarchical analyses of spatial pattern have been limited to a narrow range of habitats and taxa and that very few studies have addressed regional scales of variation (1000s of km). The available data, however, did identify a general pattern: variability was larger at small spatial scales (meters) in almost all habitats, whereas variation over larger spatial scales (10s to 100s of km) depended on the specific habitat and taxa examined. In our case study, we obtained measures of spatial variability through the use of two alternative methods: hierarchical nested design and independent estimate of spatial variance between pairs of quadrats. Both approaches provided further support to the patterns displayed by the analysis of the literature. Most response variables exhibited large variation over small spatial scales, while the significance of mid- to large-scale variability differed between midshore and lowshore habitats and among taxa. Though a proper understanding of large-scale patterns will require additional comparisons across wide geographical areas, small-scale variability emerges as a general property of benthic assemblages in marine coastal habitats. We suggest that this pattern is common to a wide range of natural systems where assemblages are influenced by complex sets of physical and biological processes like those operating in the marine environment

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.

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.