1. Despite the fact that insects are suffering a global decline, little is known about the extent to which species loss affects functional diversity. Thus, to understand the relationship between taxonomic and functional diversity metrics, we focused on saproxylic beetles, which perform vital functions in forest ecosystems.2. Beetles were collected monthly for a year using flight interception traps placed in three protected areas of mature evergreen Mediterranean Quercus forests. Five morphological traits, one phenological trait, and one physiological surrogate trait were measured, and trophic guild identities were determined. Two taxonomic and six functional metrics were assessed to determine their relationships.3. Taxonomic and functional diversity patterns, as well as trait patterns, differed depending on the protected areas, as they were influenced by habitat and microhabitat characteristics. Generally, a high number of species tended to support high values of functional richness, but did not ensure high values of functional redundancy, thus affecting the resilience of communities.4. A 5% of species loss strongly affected the functional redundancy of communities, making them more vulnerable. Functional richness, however, was not affected, since it was associated with a high number of redundant species. Thus, the way species loss affected communities is highly dependent on the presence of rare or redundant species.5. Taxonomic and functional diversity metrics should be used as complementary tools for conservation purposes. In this way, the comparative analysis of taxonomic and functional patterns may help to predict the vulnerability of saproxylic assemblages to changes or disturbances.

Understanding how biodiversity is distributed across geographical and environmental gradients is a main goal of diversity sciences. However, since ecosystem processes are linked to variation in functional traits of the biota, examining functional beta diversity is particularly important. Our objective was to analyze the taxonomic and functional beta diversity patterns of saproxylic beetle assemblages in evergreen Quercus forest of Spain. We tested whether environmental or geographical distance had a greater influence on taxonomic and functional beta diversity, and if both measures of beta diversity were affected by the same environmental variables. We used 45 flight interception traps distributed in three protected areas over a 12-mo period to sample saproxylic beetles. We measured 13 environmental variables around each trap and the geographical distance between traps. For functional composition, we used 12 functional traits from four functional groups (morphological, phenological, trophic, and a surrogate of physiological). Our results showed that environmental differences between areas influenced the taxonomic and functional beta diversity components (replacement and loss/gain) but in different ways. While replacement components (higher for taxonomic composition) increased with environmental distance, the loss or gain components (higher for functional composition) remained constant, indicating that species replacement mostly involved functionally redundant species. Besides, environmental variables influencing both taxonomic and functional composition were strongly dependent on each area. In conclusion, in well-preserved Mediterranean forests, environmental filtering determines the taxonomic and functional composition of saproxylic beetle assemblages, by favoring species replacement but filtering species traits.