Conservation-relevant plant species indicate arthropod richness across trophic levels: Habitat quality is more important than habitat amount

•Arthropod richness is strongly determined by plant richness indicating protected habitats.•Number of red-listed plant species best predicts red-listed arthropod richness.•Plant richness is more relevant for most arthropod groups than habitat amount.•Semi-natural habitats and edge density promote conservation-relevant plant species.