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The identification of priority areas for biodiversity conservation is a cornerstone of systematic conservation planning. However, biodiversity, or even the distribution of all species, cannot be directly quantified, due to the inherent complexity of natural systems. Species indicator groups may serve as important tools for the identification of priority areas for conservation. Yet, it is unclear which factors make certain indicator groups perform better than others. In this study, using data on the Danish distribution of 847 species of plants, vertebrates and insects, we assessed whether the taxonomic diversity in species indicator groups influence their effectiveness in the identification of priority areas for species conservation. We tested whether indicator groups comprising a higher taxonomic diversity (i.e. indicator groups consisting of species from many different taxonomic groups) perform better than indicator groups comprising a lower taxonomic diversity. The performance of the indicator groups was evaluated on target groups, which, in terms of species composition, were independent of the indicator groups. Overall, we found that using taxonomically diverse indicator groups did not consistently increase the coverage of target species than when using less taxonomically diverse indicator groups. However, indicator groups chosen solely from one taxon were outperformed by taxonomically diverse groups. Second, we found that taxonomic diversity of the indicator did not influence the taxonomic diversity of the covered target species (in terms of number of represented families, orders, etc.) Thus, taxonomic diversity per se only had a marginal influence on the performance of indicator groups. Distributional characteristics, such as the proportion of species with narrow range sizes, had a larger impact on indicator performance. Our results suggest that it might be of minor importance to consider taxonomic diversity when acquiring species data for future conservation projects using indicator groups.

The higher-taxon approach may provide a pragmatic surrogate for the rapid identification of priority areas for conservation. To date, no continent-wide study has examined the use of higher-taxon data to identify complementarity-based networks of priority areas, nor has the influence of spatial grain size been assessed. We used data obtained from 939 sub-Saharan mammals to analyse the performance of higher-taxon data for continental priority-setting and to assess the influence of spatial grain sizes in terms of the size of selection units (1 ◦ × 1 ◦ ,2 ◦ × 2 ◦ and 4 ◦ × 4 ◦ latitudinal–longitudinal quadrates). Independent of spatial grain size, the selection of priority areas based on genus data is more effective than the random selection of these areas, while the selection of priority areas based on family data is less effective than random selection. Compared to selection based on species data, genus-based priority areas represent between 5–14% fewer species, but represent even fewer of the threatened species (6–31%) and range-restricted species (7–42%) that are normally considered most valuable for conservation. While genus-based priority areas for the 4 ◦ grain size represent species almost as effectively as species-based priority areas, genus-based areas perform considerably less effectively than species-based areas for the 1 ◦ and 2 ◦ grain size. Thus, our results favour the higher-taxon approach for continental priority-setting only when large grain sizes (≥ 4 ◦ )a re used.

Identification of reliable surrogate indicator taxa and effective shortcuts for the preservation of overall biodiversity is a major challenge. Encouragingly, complementarity-derived priority sets of focal taxa were found highly effective in representing non-target taxa in a study from Ugandan forests. It has been proposed cross-taxon congruence in complementarity is less distinct in temperate regions than in the tropics, because temperate regions have fewer species, and hence focal taxa may be less effective as they are less diverse. To test this, we used all available atlas data (10 × 10 km scale) in Denmark (butterflies, birds, amphibians, reptiles, large moths, bats and click beetles, n = 434 species), and looked at whether conservation priority areas based on one taxonomic group represent species richness of other groups in a highly fragmented landscape. We apply a new approach (based on G-statistics) that enables statistical evaluation of cross-taxon congruence in complementarity-derived priority sets. We found indications that the same taxa represented other groups effectively and exhibited cross-taxon congruence in complementarity-derived priority sets. Birds performed significantly, but relatively poorly, butterflies somewhat better and bats relatively well in representing species richness of other groups. Large moths performed best and may be an overlooked predictor of diversity in other animal taxa.

More biodiversity could be protected in situ if the few species that attract the most popular support (the ‘flagship’ species) had distributions that also covered the broader diversity of organisms. We studied how well different groups of mammals performed for representing the diversity of mammals and breeding birds among 1° areas of sub-Saharan Africa. We demonstrate that choosing areas of sub-Saharan Africa using either conservationists' six primary flagship mammals, or the six ‘Big Five’ mammals popular with wildlife tourists, is not significantly better for representing the diversity of mammals and birds than choosing areas at random. Furthermore, neither of these groups is significantly better for representing the diversity of mammals and birds than groups of the same number of species chosen at random. We show that in order to succeed in representing many mammals and birds in area selection, it is not sufficient for the groups used for selection to occur in many different eco-regions, they must also have low overlaps in distribution, so as to provide high ecological complementarity (a similar pattern of ecological complementarity must be shared by the larger group of species to be represented). Therefore there may be a need for an explicit policy to balance the requirements of flagship conservation and broader biodiversity conservation, which will have implications for the distribution of resources.