Functional traits determine why species belong to the dark diversity in a dry grassland fragmented landscape

A challenge for nature conservation is to know why many species are absent from suitable habitats and whether they might be able to disperse and to establish. Here, we used 272 dry grassland patches within a fragmented landscape to investigate the role of local abiotic conditions and dispersal filtering in determining the likelihood of vascular plants to belong to the dark diversity (i.e. absent portion of the species pool). First, we quantified the species (SD), functional (FD) and phylogenetic (PD) diversity of both observed and dark communities. Second, we determined the roles of abiotic, present-day and historical landscape configuration variables in shaping their patterns. Third, we evaluated the importance of each variable in determining their species. Environmental filtering was assessed as effects of local abiotic conditions and dispersal filtering as the effects of present-day and historical landscape configuration. Dispersal filtering was also estimated by comparing dispersal traits of observed and dark diversity. Finally, we assessed community completeness to determine how much of the species pool was realized within a local community. We found higher SD in the observed compared to the dark communities, but PD did not differ. Contrary to expectations, dark communities resembled higher FD compared to the observed communities. Species with low dispersal capacity, low competitive abilities and high stress-tolerance were more often absent. Observed and dark diversities were mostly affected by local abiotic variables. In the observed communities, present-day landscape configuration variables affected SD while historical landscape configuration variables explained FD and PD. In the dark communities, we found the opposite pattern. Completeness was affected by present-day and historical patch size. Our results explain why dry grassland species may belong to the dark diversity and highlight the importance of local abiotic and dispersal traits of the species to conserve dry grasslands in changing landscapes.