Correlations in species richness between taxa depend on habitat, scale and landscape context

Biodiversity indicators are assumed to reflect changes in e.g. species richness of multiple taxa, but correlations in species richness between taxa have often been shown to be weak. However, only few studies are based on data allowing for rigorous tests whether strengths of correlations differ between habitat and landscape factors. We compared strengths of correlations between species richness of butterflies, plants and farmland birds between habitats (semi-natural grasslands, forest verges or field boundaries), spatial scales (0.8 ha, 25 ha and 50 ha) and landscapes differing in heterogeneity and regional land-use intensity. Between habitats, the correlation between butterflies and plants was strongest in semi-natural grasslands. Also concerning butterflies and plants, the correlation was weakest at the 0.8 ha scale, but no consistent scale-dependent patterns were found between plants and farmland birds. In a regional context, butterfly and plant species richness were consistently positively correlated, whereas when involving farmland birds we found correlations between taxa to be weaker and/or not significant in regions with high agricultural land-use intensity and in homogeneous landscapes. In general, species richness was consistently congruent only between butterflies and plants, whereas correlations involving farmland birds were mainly weak and showed contrasting patterns depending on regional context. Increasing landscape heterogeneity thus increased congruence amongst all studied taxa, but in different contexts and due to different underlying mechanisms. Although plants were involved in most of the significant correlations we cannot recommend a particular taxon as a general diversity indicator.