Which functional traits promote dark diversity among Southeast European hoverflies (Diptera: Syrphidae)?

In order to better understand and try to alleviate the negative consequences of biodiversity loss, it is important to comprehend different aspects of biodiversity. Therefore, beside using the information about observed diversity, so called dark diversity could bring to light new biodiversity patterns, which were not evident when only observed diversity was investigated. Dark diversity represents portion of species that are not recorded at a particular site, but belong to its species pool, i.e. those species that are currently 'missing' from a site, but have potential to occur there (Pärtel et al. 2011). In our study, we examine dark diversity of hoverflies, important insect pollinator group, across different vegetation types in Southeast Europe and assess which functional traits promote the absence of species from apparently suitable sites (missing species), hence promoting higher levels of hoverfly dark diversity. Dark diversity was calculated as the difference between the species pool (all species available that could potentially colonize and inhabit certain habitat) and observed diversity of each vegetation type. To establish the unique contribution of each trait to the probability of being part of dark diversity, we used Random forest (RF) regression model. Findings show that dark diversity was the lowest in Mediterranean mixed forests (33 species, 11.9%), while the highest value was in Southwest Balkan submediterranean mixed oak forests (162 species, 62.8%). Traits found to have the greatest importance in shaping the patterns of dark diversity were connected with larval food type (saproxylic larvae, phytophagous developing in bulbs and in roots), indicating that specialist species are more susceptible to contribute to the dark diversity. Knowing which functional traits are responsible for dark diversity can help in identifying the processes causing species to be absent from their original species pool. Overall, the assessment of dark diversity enables more precise conservation prioritization, but also can contribute in identification of sites for potential restoration.