The European beetle of conservation concern, Osmoderma barnabita, as a surrogate species for conserving tree-related microhabitat richness in forests

Tree-related microhabitats (TreMs) are crucial elements of forest biodiversity, yet their distribution is strongly influenced by tree species, size, vitality, and structural aging. Hollow trees, in particular, are recognized as keystone structures, harboring a high richness of endangered and specialized organisms. Among their most notable inhabitants is the hermit beetle ( Osmoderma eremita species complex), a priority species under the EU Habitats Directive and an established indicator of saproxylic diversity in tree hollows. Despite the recognized role of the hermit beetle in hollow tree ecology, its potential as a broader indicator of TreM richness remains underexplored. We investigated whether the presence of the hermit beetle can also serve as an indicator of TreM richness beyond trunk cavities. Field surveys were conducted in four forest reserves in south-western Poland, comprising 640 habitat trees, of which 193 were hollow. Each tree was assessed for TreMs, species identity, diameter at breast height (DBH), and vitality. Hollow trees were inspected for hermit beetle presence. Hollow trees hosted significantly more TreM types than habitat trees without hollows. Among hollow trees, those occupied by hermit beetles supported a significantly higher richness of TreM types than control trees. TreM richness increased with DBH, was higher in snags, and varied by tree species. Importantly, in living trees, the presence of the hermit beetle was the only significant predictor of TreM richness. These findings highlight the hermit beetle’s potential role not only as an umbrella species for hollow-dependent saproxylic species but also as an indicator of broader TreM diversity. • Hollow trees host more tree-related microhabitats (TreMs) than habitat trees without hollows. • Hollow trees with Osmoderma barnabita show higher TreM richness than other hollow trees. • Hermit beetle predicts TreM richness much better than tree diameter and species. • In living trees, O. barnabita presence is the only predictor of TreM richness. • Hermit beetle may serve as an indicator of TreM richness and associated species.