Taxonomic relatedness does not matter for species surrogacy in the assessment of community responses to environmental drivers

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