Do threatened species occur in species-rich vegetation?

Conservation strategies often assume that the total number of species at a specific location can be used as a proxy for other biodiversity dimensions, such as, the presence of rare and threatened species. However, the validity of this assumption remains unclear, particularly at the plot scale. Here, we used ~17,000 vegetation plots sampled across the Czech Republic to examine the relationship between the occurrence of threatened plant species and species richness in temperate forest and grassland communities. For each individual species, the median, range, and skewness of species richness in the plots in which it occurred were used to define its distribution along the community species richness gradient. These parameters were then compared for threatened and non-threatened species. We also compared the observed values with those obtained under a null expectation to test whether threatened species occurred at random with respect to species richness. On average, threatened species occurred in species-richer plots than non-threatened species. In addition, threatened species assembled non-randomly with respect to species richness, as they occurred more often in species-richer forests but species poorer grasslands than expected by chance. The occurrence pattern of threatened species in relation to species richness was driven by the species-pool sizes of individual habitats. Threatened species associated with low species richness were thus found in extreme habitats, such as bogs, salt marshes, peat forests, and alpine grasslands characterized by small species pools. In contrast, threatened species associated with high species richness were often found in subcontinental semi-dry grasslands and dry thermophilous forests with large species pools. Threatened species also occurred over shorter species richness gradients and were more symmetrically distributed along these gradients than non-threatened species. These patterns may reflect a high habitat specialization of threatened species or strict requirements for habitat quality. We therefore suggest that species richness is a poor indicator of conservation value when comparing habitats and geographic regions. Targeting specific habitats and using the presence or percentage of threatened or specialized species as indicators may provide better assessment of conservation value.