Birds and plants as indicators of arthropod species richness in temperate farmland

Agricultural intensification has led to severe declines in biodiversity. Arthropods in particular have suffered from agricultural intensification but are rarely incorporated in large-scale biodiversity monitoring. Instead, a few established indicator taxa such as birds or plants are often used as a cost-and time-efficient assessment of overall biodiversity. However, recent studies provide varying results regarding correlations between these indicators and other taxa. The strong variability in cross-taxa relationships has been attributed to the spatial scale of the analyses, the studied habitat types, and the trophic interactions between taxa. Here, we studied cross-taxa relationships of birds, spiders, butterflies and moths, leaf-and plant-hoppers, and plants across different spatial scales in 21 landscapes. We first compared species richness of birds with the species richness of different arthropod taxa and plants at the landscape scale. In addition, we compared the species richness of plants with the species richness of the different arthropod taxa at the landscape and the plot scale. Therefore, we distinguished between three different habitat types and between trophic interactions of varying degrees of specialization. Positive relationships between the studied taxa were scarce and depended on the spatial scale of the analysis. Spiders were the only taxon that was positively related to the species richness of birds. At the plot scale, the species richness of plants was positively related to all studied arthropod taxa across different habitat types. Relationships between adjacent trophic levels and those with a high degree of specialization were stronger compared to less specialized species, with some exceptions. Given the inconsistency of positive cross-taxa relationships, we suggest that the use of indicator taxa involves a high degree of uncertainty concerning predictions on the species richness of arthropods. We therefore encourage conservationists to incorporate multiple arthropod taxa in large-scale monitoring in order to achieve a more comprehensive and reliable measurement of biodiversity.