Evaluating habitat structural variables as reliable indicators of biodiversity in Mediterranean forests

Sustainable forest management aims to integrate biodiversity conservation while maintaining economic and social functions. However, direct biodiversity monitoring is usually costly and time-consuming, making structural indicators a valuable tool for integrating conservation criteria into forest management. We used the ten habitat structural variables derived from the Index of Biodiversity Potential (IBP), a widely used tool for assessing potential forest biodiversity at the stand scale. This study aims to evaluate the effectiveness of these habitat structural variables as proxy indicators of biodiversity by analysing the relationships between IBP factors and various taxonomic groups, subgroups and ecological guilds in Mediterranean forests. We conducted IBP inventories in 85 plots across 22 stands of different Mediterranean forest types, where we also collected data of six taxonomic groups, four of which were subdivided into forest specialist subgroups. Generalized linear models were employed to test the relationships between habitat structural variables and the richness and abundance of taxonomic groups and subgroups. We found that each habitat structural variable was related with at least one taxonomic group or subgroup. Specifically, the density of logs, the abundance of Tree-related Microhabitats (TreMs), ancientness and the presence of rocky macrohabitats were the habitat structural attributes with most associations. Taxonomic groups showed different responses, ranging from one structural variable (for birds, saproxylic fungi, and bryophytes) and up to four (for forest birds). Forest subgroups of birds, vascular plants and epiphytic bryophytes were more related to structural variables than their corresponding full groups, while forest bats displayed similar associations. However, these relationships differed when focusing on species richness or abundance approaches. Saproxylic ecological guilds, particularly Phloeophagous/ cambiumophagous guild, demonstrated more significant relationships with habitat structural variables than the broader saproxylic beetle assemblage. Our findings highlight that all habitat structural variables contained in the IBP are relevant for biodiversity, but none can be used alone as proxy indicator of forest biodiversity. Therefore, to encompass a wider spectrum of taxa, it is widely recommended to use multiple variables for integrating biodiversity conservation criteria into management planning of the Mediterranean forests. • Habitat structural variables relate to biodiversity in Mediterranean forests. • All structural variables are relevant and linked to at least one taxonomic group. • Taxonomic groups respond differently to variations in habitat structural variables. • Importance of using various structural variables as indicators of biodiversity.