Using dark diversity and plant characteristics to guide conservation and restoration.
Abstract
Dark diversity is a promising concept for prioritising management efforts as it focuses on species that are present in the regional pool, but locally absent even though environmental requirements are met. Currently, we lack knowledge of what characterises species belonging to the dark diversity more often than others, although this is important knowledge for restoration and conservation actions. We applied the concept to a massive national (Danish) plant diversity data base, containing 236 923 records from 15 160 surveys involving 564 species. This enabled the first geographically comprehensive (43 000 km2) assessment of dark diversity, at a spatial resolution relevant for conservation and restoration planning (78 m2) across multiple terrestrial habitats, thereby maximising the practical applications of this concept. The probability for a given plant species to belong to the dark diversity was computed and logistically regressed against variables representing its ecological preferences (e.g. nutrient availability), strategies (competitor, stress tolerant, ruderal), mycorrhizal relationships, establishment capacities (seed mass) and dispersal abilities. Forty-six percent of the species had a high probability (>95%) of being part of dark diversity, whereas for 7% of the species this probability was less than 60%. Typical dark diversity plant species tended to depend on mycorrhiza, were mostly adapted to low light and low nutrient levels, had poor dispersal abilities and were ruderals and stress intolerant. Synthesis and applications. Characterising species that are more often absent from suitable sites than others (dark diversity species) has important implications for the planning and management of natural ecosystems. From our study, practitioners gain insight into the factors triggering the absence of individual plant species in a seemingly suitable habitat. We highlight the need to carefully consider mycorrhizal inoculations with a suitable assemblage of fungi to promote the establishment success of dark diversity plants. Additionally, time-lags in plant species dispersal and establishment as well as spatial connectivity in fragmented habitats are central to consider in nature management although assisted migration might also aid poor dispersers. Finally, nutrient-poor localities are probably important 'islets' allowing nitrophobic dark diversity plant species to thrive within agricultural landscapes that are generally nutrient-rich.