From taxonomic to functional dark diversity: Exploring the causes of potential biodiversity and its implications for conservation

Aims and context: Dark diversity is an emerging and promising concept proposed to estimate the recruitment potential of natural communities and guide conservation and restoration policies. It represents all the species that could be present in a community due to favourable environmental conditions, but are currently lacking. To date, experimental approaches only measured taxonomic dark diversity, mainly based on species coexistence, which relies partly on neutral processes. Thus, these approaches may fail to identify the biodiversity which is lacking for deterministic reasons, and can hence hardly bring out suitable restoration methods. Methods: Here, we propose a novel method to estimate dark diversity, which is based on more deterministic coexistence: the coexistence of functional features. We adapted the Beals co-occurrence index using functional groups, and we estimated functional dark diversity based on coexistence of functional groups. We then made use of functional dark diversity to address a persistent issue of restoration ecology: how does passive rewilding impact the ecological integrity of recovered communities? We compared spontaneous, secondary woodlands with ancient forests, in terms of taxonomic and functional dark diversity of vascular plants and spiders. Results: Our results indicated that functional dark diversity does not equate to taxonomic dark diversity. Considering plants, recent woodlands surprisingly harboured less functional dark diversity than ancient forests, while they had a very similar amount of taxonomic dark diversity. Concerning spiders, recent woodlands harboured a similar amount of functional dark diversity as ancient forests, but more taxonomic dark diversity. Also, the composition of functional dark diversity differed between forest types, shedding light on their past assembly processes and unveiling their potential for conservation and effective restoration. Synthesis: Functional dark diversity brings novel perspectives for ecological diagnostic and restoration. Combined to taxonomic dark diversity, it enables to identify easily the deterministic constrains which limit the re-assembly of ecological communities after land-use changes and to predict the realistic, possible establishments of functional features. Here, we showed that spontaneous woodlands can have very similar, sometimes even higher, ecological integrity than that of ancient forests, and hence may be valuable habitats to be conserved from an ecological perspective.