Understanding biodiversity changes in the Anthropocene (e.g. due to climate and land‐use change) is an urgent ecological issue. This important task is challenging because global change effects and species responses are dependent on the spatial scales considered. Furthermore, responses are often not immediate. However, both scale and time delay issues can be tackled when, at each study site, we consider dynamics in both observed and dark diversity. Dark diversity includes those species in the region that can potentially establish and thrive in the local sites’ conditions but are currently locally absent. Effectively, dark diversity connects biodiversity at the study site to the regional scales and defines the site‐specific species pool (observed and dark diversity together). With dark diversity, it is possible to decompose species gains and losses into two space‐related components: one associated with local dynamics (species moving from observed to dark diversity and vice versa) and another related to gains and losses of site‐specific species pool (species moving to and from the pool after regional immigration, regional extinction or change in local ecological conditions). Extinction debt and immigration credit are useful to understand dynamics in observed diversity, but delays might happen in species pool changes as well. In this opinion piece we suggest that considering both observed and dark diversity and their temporal dynamics provides a deeper understanding of biodiversity changes. Considering both observed and dark diversity creates opportunities to improve conservation by allowing to identify species that are likely to go regionally extinct as well as foreseeing which of the species that newly arrive to the region are more likely to colonize local sites. Finally, by considering temporal lags and species gains and losses in observed and dark diversity, we combine phenomena at both spatial and temporal scales, providing a novel tool to examine biodiversity change in the Anthropocene.

Questions Community assembly in regenerating forests is a key topic in ecology. Most studies examine the assembly process, assessing adult individuals along the forest succession. Although the adult stage is the final outcome of the assembly process, both abiotic and biotic filters can affect community assembly during early ontogenetic stages. Here, we investigate whether the functional composition and trait assembly patterns (trait convergence or divergence) change across ontogenetic stages along regenerating forest stands exposed to precipitation and chronic anthropogenic disturbance (CAD) gradients. Location Caatinga dry forest, Catimbau National Park, Pernambuco, Brazil. Methods Seeds (rain and bank), seedlings and adults of woody plant species were recorded across 15 forest regenerating stands (4–70 years since land abandonment) exposed to precipitation and CAD gradients. Seven functional traits related to species dispersal, establishment and resource use were measured for adult individuals and used to calculate the trait values for seeds and seedlings. Twenty old‐growth forest stands were used as a regional flora. Results Functional composition (community weighted mean, CWM) varied across ontogenetic stages as wood density increased from seeds to seedlings, while seed mass increased from seeds to both seedlings and adults. The CWM also varied along precipitation/disturbance gradients but not along forest regeneration regardless the ontogenetic stage. Among seeds, dispersal distance increased towards wetter forest stands and decreased towards disturbed sites, while adult wood density increased from drier to wetter regenerating forest stands. Traits converged along the precipitation gradient but not in response to time since land abandonment. Conclusions Caatinga dry forest regeneration is functionally structured across ontogenetic stages mediated mostly by water availability, but not over time since land abandonment. The effect of water availability on forest regeneration across ontogenetic stages highlights the complexity of species assembly in human modified landscapes, imposing important consequences considering the expected increasing aridity in the Caatinga.

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Global change drivers (e.g. climate and land use) affect the species and functional traits observed in a local site but also its dark diversity—the set of species and traits locally suitable but absent. Dark diversity links regional and local scales and, over time, reveals taxa under expansion lags by depicting the potential biodiversity that remains suitable but is absent locally. Since global change effects on biodiversity are both spatially and temporally scale dependent, examining long-term temporal dynamics in observed and dark diversity would be relevant to assessing and foreseeing biodiversity change. Here, we used sedimentary pollen data to examine how both taxonomic and functional observed and dark diversity changed over the past 14 500 years in northern Europe. We found that taxonomic and functional observed and dark diversity increased over time, especially after the Late Glacial and during the Late Holocene. However, dark diversity dynamics revealed expansion lags related to species' functional characteristics (dispersal limitation and stress intolerance) and an extensive functional redundancy when compared to taxa in observed diversity. We highlight that assessing observed and dark diversity dynamics is a promising tool to examine biodiversity change across spatial scales, its possible causes, and functional consequences.