Flower species richness is an effective indicator of aboveground grassland arthropod biodiversity

To curb biodiversity loss, reliable and scalable indicators that represent a broad range of species groups are urgently needed. Habitat quality metrics may provide more broadly representative and more efficiently measurable biodiversity indicators than single species groups. However, few studies have critically evaluated the applicability of such indicators across multiple arthropod groups. Flower abundance and diversity may reflect multiple aspects of habitat quality in grasslands, including resource availability, vegetation structure and microclimate. Therefore, we test if the species richness and total cover of open flowers can indicate local species richness of plants and multiple arthropod groups in grasslands. In April, June and July 2022, flower, plant and arthropod data were collected along a 150 m 2 transect in 41 Dutch grasslands, ranging from low to high land-use intensity levels. Flower richness showed consistent linear relations with plant and arthropod species richness, and resembled both the seasonal increase of arthropod richness and the exponential declines of plant and arthropod richness with increasing land-use intensity. By contrast, flower cover was strongly driven by a few dominant flowering species, decreased over the sampling period and showed more variable relations with plant and arthropod richness. Flower richness qualifies as a broadly representative biodiversity indicator in grasslands that can efficiently be surveyed at site level, and is directly relatable to local land use practices. Flower richness measurements can therefore complement inventories of species composition and steer biodiversity restoration efforts such as result-based agri-environmental schemes on farmland, in which there is increasing emphasis on improving habitat quality. • The flower richness of grasslands robustly indicates plant and arthropod diversity. • Relations with flower richness persisted along the entire land use intensity gradient. • Flower surveys can cover large areas and detect seasonal trends in habitat quality. • Flower surveys can inform restoration management at a field-specific basis.